ALSA: doc: ReSTize writing-an-alsa-driver document

Another simple conversion from DocBook to ReST.
This required a few manual fixups and reformats, but the most of
contents are kept as is.

Signed-off-by: Takashi Iwai <[e-mail]>

Documentation/DocBook/Makefile

···
 	    kernel-api.xml filesystems.xml lsm.xml usb.xml kgdb.xml \
 	    gadget.xml libata.xml mtdnand.xml librs.xml rapidio.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
-	    debugobjects.xml sh.xml regulator.xml \
-	    writing-an-alsa-driver.xml \
+	    80211.xml debugobjects.xml sh.xml regulator.xml \
 	    tracepoint.xml w1.xml \
 	    writing_musb_glue_layer.xml crypto-API.xml iio.xml
 

Documentation/DocBook/writing-an-alsa-driver.tmpl

···
-<?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
-	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
-
-<!-- ****************************************************** -->
-<!-- Header  -->
-<!-- ****************************************************** -->
-<book id="Writing-an-ALSA-Driver">
-  <bookinfo>
-    <title>Writing an ALSA Driver</title>
-    <author>
-      <firstname>Takashi</firstname>
-      <surname>Iwai</surname>
-      <affiliation>
-        <address>
-          <email>[e-mail]</email>
-        </address>
-      </affiliation>
-     </author>
-
-     <date>Oct 15, 2007</date>
-     <edition>0.3.7</edition>
-
-    <abstract>
-      <para>
-        This document describes how to write an ALSA (Advanced Linux
-        Sound Architecture) driver.
-      </para>
-    </abstract>
-
-    <legalnotice>
-    <para>
-    Copyright (c) 2002-2005  Takashi Iwai <email>[e-mail]</email>
-    </para>
-
-    <para>
-    This document is free; you can redistribute it and/or modify it
-    under the terms of the GNU General Public License as published by
-    the Free Software Foundation; either version 2 of the License, or
-    (at your option) any later version. 
-    </para>
-
-    <para>
-    This document is distributed in the hope that it will be useful,
-    but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the
-    implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A
-    PARTICULAR PURPOSE</emphasis>. See the GNU General Public License
-    for more details.
-    </para>
-
-    <para>
-    You should have received a copy of the GNU General Public
-    License along with this program; if not, write to the Free
-    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
-    MA 02111-1307 USA
-    </para>
-    </legalnotice>
-
-  </bookinfo>
-
-<!-- ****************************************************** -->
-<!-- Preface  -->
-<!-- ****************************************************** -->
-  <preface id="preface">
-    <title>Preface</title>
-    <para>
-      This document describes how to write an
-      <ulink url="http://www.alsa-project.org/"><citetitle>
-      ALSA (Advanced Linux Sound Architecture)</citetitle></ulink>
-      driver. The document focuses mainly on PCI soundcards.
-      In the case of other device types, the API might
-      be different, too. However, at least the ALSA kernel API is
-      consistent, and therefore it would be still a bit help for
-      writing them.
-    </para>
-
-    <para>
-    This document targets people who already have enough
-    C language skills and have basic linux kernel programming
-    knowledge.  This document doesn't explain the general
-    topic of linux kernel coding and doesn't cover low-level
-    driver implementation details. It only describes
-    the standard way to write a PCI sound driver on ALSA.
-    </para>
-
-    <para>
-      If you are already familiar with the older ALSA ver.0.5.x API, you
-    can check the drivers such as <filename>sound/pci/es1938.c</filename> or
-    <filename>sound/pci/maestro3.c</filename> which have also almost the same
-    code-base in the ALSA 0.5.x tree, so you can compare the differences.
-    </para>
-
-    <para>
-      This document is still a draft version. Any feedback and
-    corrections, please!!
-    </para>
-  </preface>
-
-
-<!-- ****************************************************** -->
-<!-- File Tree Structure  -->
-<!-- ****************************************************** -->
-  <chapter id="file-tree">
-    <title>File Tree Structure</title>
-
-    <section id="file-tree-general">
-      <title>General</title>
-      <para>
-        The ALSA drivers are provided in two ways.
-      </para>
-
-      <para>
-        One is the trees provided as a tarball or via cvs from the
-      ALSA's ftp site, and another is the 2.6 (or later) Linux kernel
-      tree. To synchronize both, the ALSA driver tree is split into
-      two different trees: alsa-kernel and alsa-driver. The former
-      contains purely the source code for the Linux 2.6 (or later)
-      tree. This tree is designed only for compilation on 2.6 or
-      later environment. The latter, alsa-driver, contains many subtle
-      files for compiling ALSA drivers outside of the Linux kernel tree,
-      wrapper functions for older 2.2 and 2.4 kernels, to adapt the latest kernel API,
-      and additional drivers which are still in development or in
-      tests.  The drivers in alsa-driver tree will be moved to
-      alsa-kernel (and eventually to the 2.6 kernel tree) when they are
-      finished and confirmed to work fine.
-      </para>
-
-      <para>
-        The file tree structure of ALSA driver is depicted below. Both
-        alsa-kernel and alsa-driver have almost the same file
-        structure, except for <quote>core</quote> directory. It's
-        named as <quote>acore</quote> in alsa-driver tree. 
-
-        <example>
-          <title>ALSA File Tree Structure</title>
-          <literallayout>
-        sound
-                /core
-                        /oss
-                        /seq
-                                /oss
-                                /instr
-                /ioctl32
-                /include
-                /drivers
-                        /mpu401
-                        /opl3
-                /i2c
-                        /l3
-                /synth
-                        /emux
-                /pci
-                        /(cards)
-                /isa
-                        /(cards)
-                /arm
-                /ppc
-                /sparc
-                /usb
-                /pcmcia /(cards)
-                /oss
-          </literallayout>
-        </example>
-      </para>
-    </section>
-
-    <section id="file-tree-core-directory">
-      <title>core directory</title>
-      <para>
-        This directory contains the middle layer which is the heart
-      of ALSA drivers. In this directory, the native ALSA modules are
-      stored. The sub-directories contain different modules and are
-      dependent upon the kernel config. 
-      </para>
-
-      <section id="file-tree-core-directory-oss">
-        <title>core/oss</title>
-
-        <para>
-          The codes for PCM and mixer OSS emulation modules are stored
-        in this directory. The rawmidi OSS emulation is included in
-        the ALSA rawmidi code since it's quite small. The sequencer
-        code is stored in <filename>core/seq/oss</filename> directory (see
-        <link linkend="file-tree-core-directory-seq-oss"><citetitle>
-        below</citetitle></link>).
-        </para>
-      </section>
-
-      <section id="file-tree-core-directory-ioctl32">
-        <title>core/ioctl32</title>
-
-        <para>
-          This directory contains the 32bit-ioctl wrappers for 64bit
-        architectures such like x86-64, ppc64 and sparc64. For 32bit
-        and alpha architectures, these are not compiled. 
-        </para>
-      </section>
-
-      <section id="file-tree-core-directory-seq">
-        <title>core/seq</title>
-        <para>
-          This directory and its sub-directories are for the ALSA
-        sequencer. This directory contains the sequencer core and
-        primary sequencer modules such like snd-seq-midi,
-        snd-seq-virmidi, etc. They are compiled only when
-        <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel
-        config. 
-        </para>
-      </section>
-
-      <section id="file-tree-core-directory-seq-oss">
-        <title>core/seq/oss</title>
-        <para>
-          This contains the OSS sequencer emulation codes.
-        </para>
-      </section>
-
-      <section id="file-tree-core-directory-deq-instr">
-        <title>core/seq/instr</title>
-        <para>
-          This directory contains the modules for the sequencer
-        instrument layer. 
-        </para>
-      </section>
-    </section>
-
-    <section id="file-tree-include-directory">
-      <title>include directory</title>
-      <para>
-        This is the place for the public header files of ALSA drivers,
-      which are to be exported to user-space, or included by
-      several files at different directories. Basically, the private
-      header files should not be placed in this directory, but you may
-      still find files there, due to historical reasons :) 
-      </para>
-    </section>
-
-    <section id="file-tree-drivers-directory">
-      <title>drivers directory</title>
-      <para>
-        This directory contains code shared among different drivers
-      on different architectures.  They are hence supposed not to be
-      architecture-specific.
-      For example, the dummy pcm driver and the serial MIDI
-      driver are found in this directory. In the sub-directories,
-      there is code for components which are independent from
-      bus and cpu architectures. 
-      </para>
-
-      <section id="file-tree-drivers-directory-mpu401">
-        <title>drivers/mpu401</title>
-        <para>
-          The MPU401 and MPU401-UART modules are stored here.
-        </para>
-      </section>
-
-      <section id="file-tree-drivers-directory-opl3">
-        <title>drivers/opl3 and opl4</title>
-        <para>
-          The OPL3 and OPL4 FM-synth stuff is found here.
-        </para>
-      </section>
-    </section>
-
-    <section id="file-tree-i2c-directory">
-      <title>i2c directory</title>
-      <para>
-        This contains the ALSA i2c components.
-      </para>
-
-      <para>
-        Although there is a standard i2c layer on Linux, ALSA has its
-      own i2c code for some cards, because the soundcard needs only a
-      simple operation and the standard i2c API is too complicated for
-      such a purpose. 
-      </para>
-
-      <section id="file-tree-i2c-directory-l3">
-        <title>i2c/l3</title>
-        <para>
-          This is a sub-directory for ARM L3 i2c.
-        </para>
-      </section>
-    </section>
-
-    <section id="file-tree-synth-directory">
-        <title>synth directory</title>
-        <para>
-          This contains the synth middle-level modules.
-        </para>
-
-        <para>
-          So far, there is only Emu8000/Emu10k1 synth driver under
-        the <filename>synth/emux</filename> sub-directory. 
-        </para>
-    </section>
-
-    <section id="file-tree-pci-directory">
-      <title>pci directory</title>
-      <para>
-        This directory and its sub-directories hold the top-level card modules
-      for PCI soundcards and the code specific to the PCI BUS.
-      </para>
-
-      <para>
-        The drivers compiled from a single file are stored directly
-      in the pci directory, while the drivers with several source files are
-      stored on their own sub-directory (e.g. emu10k1, ice1712). 
-      </para>
-    </section>
-
-    <section id="file-tree-isa-directory">
-      <title>isa directory</title>
-      <para>
-        This directory and its sub-directories hold the top-level card modules
-      for ISA soundcards. 
-      </para>
-    </section>
-
-    <section id="file-tree-arm-ppc-sparc-directories">
-      <title>arm, ppc, and sparc directories</title>
-      <para>
-        They are used for top-level card modules which are
-      specific to one of these architectures. 
-      </para>
-    </section>
-
-    <section id="file-tree-usb-directory">
-      <title>usb directory</title>
-      <para>
-        This directory contains the USB-audio driver. In the latest version, the
-      USB MIDI driver is integrated in the usb-audio driver. 
-      </para>
-    </section>
-
-    <section id="file-tree-pcmcia-directory">
-      <title>pcmcia directory</title>
-      <para>
-        The PCMCIA, especially PCCard drivers will go here. CardBus
-      drivers will be in the pci directory, because their API is identical
-      to that of standard PCI cards. 
-      </para>
-    </section>
-
-    <section id="file-tree-oss-directory">
-      <title>oss directory</title>
-      <para>
-        The OSS/Lite source files are stored here in Linux 2.6 (or
-      later) tree. In the ALSA driver tarball, this directory is empty,
-      of course :) 
-      </para>
-    </section>
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Basic Flow for PCI Drivers  -->
-<!-- ****************************************************** -->
-  <chapter id="basic-flow">
-    <title>Basic Flow for PCI Drivers</title>
-
-    <section id="basic-flow-outline">
-      <title>Outline</title>
-      <para>
-        The minimum flow for PCI soundcards is as follows:
-
-        <itemizedlist>
-          <listitem><para>define the PCI ID table (see the section
-          <link linkend="pci-resource-entries"><citetitle>PCI Entries
-          </citetitle></link>).</para></listitem> 
-          <listitem><para>create <function>probe()</function> callback.</para></listitem>
-          <listitem><para>create <function>remove()</function> callback.</para></listitem>
-          <listitem><para>create a <structname>pci_driver</structname> structure
-	  containing the three pointers above.</para></listitem>
-          <listitem><para>create an <function>init()</function> function just calling
-	  the <function>pci_register_driver()</function> to register the pci_driver table
-	  defined above.</para></listitem>
-          <listitem><para>create an <function>exit()</function> function to call
-	  the <function>pci_unregister_driver()</function> function.</para></listitem>
-        </itemizedlist>
-      </para>
-    </section>
-
-    <section id="basic-flow-example">
-      <title>Full Code Example</title>
-      <para>
-        The code example is shown below. Some parts are kept
-      unimplemented at this moment but will be filled in the
-      next sections. The numbers in the comment lines of the
-      <function>snd_mychip_probe()</function> function
-      refer to details explained in the following section. 
-
-        <example>
-          <title>Basic Flow for PCI Drivers - Example</title>
-          <programlisting>
-<![CDATA[
-  #include <linux/init.h>
-  #include <linux/pci.h>
-  #include <linux/slab.h>
-  #include <sound/core.h>
-  #include <sound/initval.h>
-
-  /* module parameters (see "Module Parameters") */
-  /* SNDRV_CARDS: maximum number of cards supported by this module */
-  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
-  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
-  static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
-
-  /* definition of the chip-specific record */
-  struct mychip {
-          struct snd_card *card;
-          /* the rest of the implementation will be in section
-           * "PCI Resource Management"
-           */
-  };
-
-  /* chip-specific destructor
-   * (see "PCI Resource Management")
-   */
-  static int snd_mychip_free(struct mychip *chip)
-  {
-          .... /* will be implemented later... */
-  }
-
-  /* component-destructor
-   * (see "Management of Cards and Components")
-   */
-  static int snd_mychip_dev_free(struct snd_device *device)
-  {
-          return snd_mychip_free(device->device_data);
-  }
-
-  /* chip-specific constructor
-   * (see "Management of Cards and Components")
-   */
-  static int snd_mychip_create(struct snd_card *card,
-                               struct pci_dev *pci,
-                               struct mychip **rchip)
-  {
-          struct mychip *chip;
-          int err;
-          static struct snd_device_ops ops = {
-                 .dev_free = snd_mychip_dev_free,
-          };
-
-          *rchip = NULL;
-
-          /* check PCI availability here
-           * (see "PCI Resource Management")
-           */
-          ....
-
-          /* allocate a chip-specific data with zero filled */
-          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
-          if (chip == NULL)
-                  return -ENOMEM;
-
-          chip->card = card;
-
-          /* rest of initialization here; will be implemented
-           * later, see "PCI Resource Management"
-           */
-          ....
-
-          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
-          if (err < 0) {
-                  snd_mychip_free(chip);
-                  return err;
-          }
-
-          *rchip = chip;
-          return 0;
-  }
-
-  /* constructor -- see "Constructor" sub-section */
-  static int snd_mychip_probe(struct pci_dev *pci,
-                              const struct pci_device_id *pci_id)
-  {
-          static int dev;
-          struct snd_card *card;
-          struct mychip *chip;
-          int err;
-
-          /* (1) */
-          if (dev >= SNDRV_CARDS)
-                  return -ENODEV;
-          if (!enable[dev]) {
-                  dev++;
-                  return -ENOENT;
-          }
-
-          /* (2) */
-          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                             0, &card);
-          if (err < 0)
-                  return err;
-
-          /* (3) */
-          err = snd_mychip_create(card, pci, &chip);
-          if (err < 0) {
-                  snd_card_free(card);
-                  return err;
-          }
-
-          /* (4) */
-          strcpy(card->driver, "My Chip");
-          strcpy(card->shortname, "My Own Chip 123");
-          sprintf(card->longname, "%s at 0x%lx irq %i",
-                  card->shortname, chip->ioport, chip->irq);
-
-          /* (5) */
-          .... /* implemented later */
-
-          /* (6) */
-          err = snd_card_register(card);
-          if (err < 0) {
-                  snd_card_free(card);
-                  return err;
-          }
-
-          /* (7) */
-          pci_set_drvdata(pci, card);
-          dev++;
-          return 0;
-  }
-
-  /* destructor -- see the "Destructor" sub-section */
-  static void snd_mychip_remove(struct pci_dev *pci)
-  {
-          snd_card_free(pci_get_drvdata(pci));
-          pci_set_drvdata(pci, NULL);
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
-    </section>
-
-    <section id="basic-flow-constructor">
-      <title>Constructor</title>
-      <para>
-        The real constructor of PCI drivers is the <function>probe</function> callback.
-      The <function>probe</function> callback and other component-constructors which are called
-      from the <function>probe</function> callback cannot be used with
-      the <parameter>__init</parameter> prefix
-      because any PCI device could be a hotplug device. 
-      </para>
-
-      <para>
-        In the <function>probe</function> callback, the following scheme is often used.
-      </para>
-
-      <section id="basic-flow-constructor-device-index">
-        <title>1) Check and increment the device index.</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int dev;
-  ....
-  if (dev >= SNDRV_CARDS)
-          return -ENODEV;
-  if (!enable[dev]) {
-          dev++;
-          return -ENOENT;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-
-        where enable[dev] is the module option.
-        </para>
-
-        <para>
-          Each time the <function>probe</function> callback is called, check the
-        availability of the device. If not available, simply increment
-        the device index and returns. dev will be incremented also
-        later (<link
-        linkend="basic-flow-constructor-set-pci"><citetitle>step
-        7</citetitle></link>). 
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-create-card">
-        <title>2) Create a card instance</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  struct snd_card *card;
-  int err;
-  ....
-  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                     0, &card);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          The details will be explained in the section
-          <link linkend="card-management-card-instance"><citetitle>
-          Management of Cards and Components</citetitle></link>.
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-create-main">
-        <title>3) Create a main component</title>
-        <para>
-          In this part, the PCI resources are allocated.
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  struct mychip *chip;
-  ....
-  err = snd_mychip_create(card, pci, &chip);
-  if (err < 0) {
-          snd_card_free(card);
-          return err;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-
-          The details will be explained in the section <link
-        linkend="pci-resource"><citetitle>PCI Resource
-        Management</citetitle></link>.
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-main-component">
-        <title>4) Set the driver ID and name strings.</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  strcpy(card->driver, "My Chip");
-  strcpy(card->shortname, "My Own Chip 123");
-  sprintf(card->longname, "%s at 0x%lx irq %i",
-          card->shortname, chip->ioport, chip->irq);
-]]>
-            </programlisting>
-          </informalexample>
-
-          The driver field holds the minimal ID string of the
-        chip. This is used by alsa-lib's configurator, so keep it
-        simple but unique. 
-          Even the same driver can have different driver IDs to
-        distinguish the functionality of each chip type. 
-        </para>
-
-        <para>
-          The shortname field is a string shown as more verbose
-        name. The longname field contains the information
-        shown in <filename>/proc/asound/cards</filename>. 
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-create-other">
-        <title>5) Create other components, such as mixer, MIDI, etc.</title>
-        <para>
-          Here you define the basic components such as
-          <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>,
-          mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>),
-          MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>),
-          and other interfaces.
-          Also, if you want a <link linkend="proc-interface"><citetitle>proc
-        file</citetitle></link>, define it here, too.
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-register-card">
-        <title>6) Register the card instance.</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  err = snd_card_register(card);
-  if (err < 0) {
-          snd_card_free(card);
-          return err;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          Will be explained in the section <link
-        linkend="card-management-registration"><citetitle>Management
-        of Cards and Components</citetitle></link>, too. 
-        </para>
-      </section>
-
-      <section id="basic-flow-constructor-set-pci">
-        <title>7) Set the PCI driver data and return zero.</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-        pci_set_drvdata(pci, card);
-        dev++;
-        return 0;
-]]>
-            </programlisting>
-          </informalexample>
-
-          In the above, the card record is stored. This pointer is
-        used in the remove callback and power-management
-        callbacks, too. 
-        </para>
-      </section>
-    </section>
-
-    <section id="basic-flow-destructor">
-      <title>Destructor</title>
-      <para>
-        The destructor, remove callback, simply releases the card
-      instance. Then the ALSA middle layer will release all the
-      attached components automatically. 
-      </para>
-
-      <para>
-        It would be typically like the following:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void snd_mychip_remove(struct pci_dev *pci)
-  {
-          snd_card_free(pci_get_drvdata(pci));
-          pci_set_drvdata(pci, NULL);
-  }
-]]>
-          </programlisting>
-        </informalexample>
-
-        The above code assumes that the card pointer is set to the PCI
-	driver data.
-      </para>
-    </section>
-
-    <section id="basic-flow-header-files">
-      <title>Header Files</title>
-      <para>
-        For the above example, at least the following include files
-      are necessary. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  #include <linux/init.h>
-  #include <linux/pci.h>
-  #include <linux/slab.h>
-  #include <sound/core.h>
-  #include <sound/initval.h>
-]]>
-          </programlisting>
-        </informalexample>
-
-	where the last one is necessary only when module options are
-      defined in the source file.  If the code is split into several
-      files, the files without module options don't need them.
-      </para>
-
-      <para>
-        In addition to these headers, you'll need
-      <filename>&lt;linux/interrupt.h&gt;</filename> for interrupt
-      handling, and <filename>&lt;asm/io.h&gt;</filename> for I/O
-      access. If you use the <function>mdelay()</function> or
-      <function>udelay()</function> functions, you'll need to include
-      <filename>&lt;linux/delay.h&gt;</filename> too. 
-      </para>
-
-      <para>
-      The ALSA interfaces like the PCM and control APIs are defined in other
-      <filename>&lt;sound/xxx.h&gt;</filename> header files.
-      They have to be included after
-      <filename>&lt;sound/core.h&gt;</filename>.
-      </para>
-
-    </section>
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Management of Cards and Components  -->
-<!-- ****************************************************** -->
-  <chapter id="card-management">
-    <title>Management of Cards and Components</title>
-
-    <section id="card-management-card-instance">
-      <title>Card Instance</title>
-      <para>
-      For each soundcard, a <quote>card</quote> record must be allocated.
-      </para>
-
-      <para>
-      A card record is the headquarters of the soundcard.  It manages
-      the whole list of devices (components) on the soundcard, such as
-      PCM, mixers, MIDI, synthesizer, and so on.  Also, the card
-      record holds the ID and the name strings of the card, manages
-      the root of proc files, and controls the power-management states
-      and hotplug disconnections.  The component list on the card
-      record is used to manage the correct release of resources at
-      destruction. 
-      </para>
-
-      <para>
-        As mentioned above, to create a card instance, call
-      <function>snd_card_new()</function>.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_card *card;
-  int err;
-  err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The function takes six arguments: the parent device pointer,
-        the card-index number, the id string, the module pointer (usually
-        <constant>THIS_MODULE</constant>),
-        the size of extra-data space, and the pointer to return the
-        card instance.  The extra_size argument is used to
-        allocate card-&gt;private_data for the
-        chip-specific data.  Note that these data
-        are allocated by <function>snd_card_new()</function>.
-      </para>
-
-      <para>
-	The first argument, the pointer of struct
-	<structname>device</structname>, specifies the parent device.
-	For PCI devices, typically &amp;pci-&gt; is passed there.
-      </para>
-    </section>
-
-    <section id="card-management-component">
-      <title>Components</title>
-      <para>
-        After the card is created, you can attach the components
-      (devices) to the card instance. In an ALSA driver, a component is
-      represented as a struct <structname>snd_device</structname> object.
-      A component can be a PCM instance, a control interface, a raw
-      MIDI interface, etc.  Each such instance has one component
-      entry.
-      </para>
-
-      <para>
-        A component can be created via
-        <function>snd_device_new()</function> function. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        This takes the card pointer, the device-level
-      (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the
-      callback pointers (<parameter>&amp;ops</parameter>). The
-      device-level defines the type of components and the order of
-      registration and de-registration.  For most components, the
-      device-level is already defined.  For a user-defined component,
-      you can use <constant>SNDRV_DEV_LOWLEVEL</constant>.
-      </para>
-
-      <para>
-      This function itself doesn't allocate the data space. The data
-      must be allocated manually beforehand, and its pointer is passed
-      as the argument. This pointer (<parameter>chip</parameter> in the
-      above example) is used as the identifier for the instance.
-      </para>
-
-      <para>
-        Each pre-defined ALSA component such as ac97 and pcm calls
-      <function>snd_device_new()</function> inside its
-      constructor. The destructor for each component is defined in the
-      callback pointers.  Hence, you don't need to take care of
-      calling a destructor for such a component.
-      </para>
-
-      <para>
-        If you wish to create your own component, you need to
-      set the destructor function to the dev_free callback in
-      the <parameter>ops</parameter>, so that it can be released
-      automatically via <function>snd_card_free()</function>.
-      The next example will show an implementation of chip-specific
-      data.
-      </para>
-    </section>
-
-    <section id="card-management-chip-specific">
-      <title>Chip-Specific Data</title>
-      <para>
-      Chip-specific information, e.g. the I/O port address, its
-      resource pointer, or the irq number, is stored in the
-      chip-specific record.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct mychip {
-          ....
-  };
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        In general, there are two ways of allocating the chip record.
-      </para>
-
-      <section id="card-management-chip-specific-snd-card-new">
-        <title>1. Allocating via <function>snd_card_new()</function>.</title>
-        <para>
-          As mentioned above, you can pass the extra-data-length
-	  to the 5th argument of <function>snd_card_new()</function>, i.e.
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                     sizeof(struct mychip), &card);
-]]>
-            </programlisting>
-          </informalexample>
-
-          struct <structname>mychip</structname> is the type of the chip record.
-        </para>
-
-        <para>
-          In return, the allocated record can be accessed as
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  struct mychip *chip = card->private_data;
-]]>
-            </programlisting>
-          </informalexample>
-
-          With this method, you don't have to allocate twice.
-          The record is released together with the card instance.
-        </para>
-      </section>
-
-      <section id="card-management-chip-specific-allocate-extra">
-        <title>2. Allocating an extra device.</title>
-
-        <para>
-          After allocating a card instance via
-          <function>snd_card_new()</function> (with
-          <constant>0</constant> on the 4th arg), call
-          <function>kzalloc()</function>. 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  struct snd_card *card;
-  struct mychip *chip;
-  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                     0, &card);
-  .....
-  chip = kzalloc(sizeof(*chip), GFP_KERNEL);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          The chip record should have the field to hold the card
-          pointer at least, 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  struct mychip {
-          struct snd_card *card;
-          ....
-  };
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          Then, set the card pointer in the returned chip instance.
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  chip->card = card;
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          Next, initialize the fields, and register this chip
-          record as a low-level device with a specified
-          <parameter>ops</parameter>, 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static struct snd_device_ops ops = {
-          .dev_free =        snd_mychip_dev_free,
-  };
-  ....
-  snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
-]]>
-            </programlisting>
-          </informalexample>
-
-          <function>snd_mychip_dev_free()</function> is the
-        device-destructor function, which will call the real
-        destructor. 
-        </para>
-
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_mychip_dev_free(struct snd_device *device)
-  {
-          return snd_mychip_free(device->device_data);
-  }
-]]>
-            </programlisting>
-          </informalexample>
-
-          where <function>snd_mychip_free()</function> is the real destructor.
-        </para>
-      </section>
-    </section>
-
-    <section id="card-management-registration">
-      <title>Registration and Release</title>
-      <para>
-        After all components are assigned, register the card instance
-      by calling <function>snd_card_register()</function>. Access
-      to the device files is enabled at this point. That is, before
-      <function>snd_card_register()</function> is called, the
-      components are safely inaccessible from external side. If this
-      call fails, exit the probe function after releasing the card via
-      <function>snd_card_free()</function>. 
-      </para>
-
-      <para>
-        For releasing the card instance, you can call simply
-      <function>snd_card_free()</function>. As mentioned earlier, all
-      components are released automatically by this call. 
-      </para>
-
-      <para>
-      For a device which allows hotplugging, you can use
-      <function>snd_card_free_when_closed</function>.  This one will
-      postpone the destruction until all devices are closed.
-      </para>
-
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- PCI Resource Management  -->
-<!-- ****************************************************** -->
-  <chapter id="pci-resource">
-    <title>PCI Resource Management</title>
-
-    <section id="pci-resource-example">
-      <title>Full Code Example</title>
-      <para>
-        In this section, we'll complete the chip-specific constructor,
-      destructor and PCI entries. Example code is shown first,
-      below. 
-
-        <example>
-          <title>PCI Resource Management Example</title>
-          <programlisting>
-<![CDATA[
-  struct mychip {
-          struct snd_card *card;
-          struct pci_dev *pci;
-
-          unsigned long port;
-          int irq;
-  };
-
-  static int snd_mychip_free(struct mychip *chip)
-  {
-          /* disable hardware here if any */
-          .... /* (not implemented in this document) */
-
-          /* release the irq */
-          if (chip->irq >= 0)
-                  free_irq(chip->irq, chip);
-          /* release the I/O ports & memory */
-          pci_release_regions(chip->pci);
-          /* disable the PCI entry */
-          pci_disable_device(chip->pci);
-          /* release the data */
-          kfree(chip);
-          return 0;
-  }
-
-  /* chip-specific constructor */
-  static int snd_mychip_create(struct snd_card *card,
-                               struct pci_dev *pci,
-                               struct mychip **rchip)
-  {
-          struct mychip *chip;
-          int err;
-          static struct snd_device_ops ops = {
-                 .dev_free = snd_mychip_dev_free,
-          };
-
-          *rchip = NULL;
-
-          /* initialize the PCI entry */
-          err = pci_enable_device(pci);
-          if (err < 0)
-                  return err;
-          /* check PCI availability (28bit DMA) */
-          if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
-              pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
-                  printk(KERN_ERR "error to set 28bit mask DMA\n");
-                  pci_disable_device(pci);
-                  return -ENXIO;
-          }
-
-          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
-          if (chip == NULL) {
-                  pci_disable_device(pci);
-                  return -ENOMEM;
-          }
-
-          /* initialize the stuff */
-          chip->card = card;
-          chip->pci = pci;
-          chip->irq = -1;
-
-          /* (1) PCI resource allocation */
-          err = pci_request_regions(pci, "My Chip");
-          if (err < 0) {
-                  kfree(chip);
-                  pci_disable_device(pci);
-                  return err;
-          }
-          chip->port = pci_resource_start(pci, 0);
-          if (request_irq(pci->irq, snd_mychip_interrupt,
-                          IRQF_SHARED, KBUILD_MODNAME, chip)) {
-                  printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
-                  snd_mychip_free(chip);
-                  return -EBUSY;
-          }
-          chip->irq = pci->irq;
-
-          /* (2) initialization of the chip hardware */
-          .... /*   (not implemented in this document) */
-
-          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
-          if (err < 0) {
-                  snd_mychip_free(chip);
-                  return err;
-          }
-
-          *rchip = chip;
-          return 0;
-  }        
-
-  /* PCI IDs */
-  static struct pci_device_id snd_mychip_ids[] = {
-          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
-            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
-          ....
-          { 0, }
-  };
-  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
-
-  /* pci_driver definition */
-  static struct pci_driver driver = {
-          .name = KBUILD_MODNAME,
-          .id_table = snd_mychip_ids,
-          .probe = snd_mychip_probe,
-          .remove = snd_mychip_remove,
-  };
-
-  /* module initialization */
-  static int __init alsa_card_mychip_init(void)
-  {
-          return pci_register_driver(&driver);
-  }
-
-  /* module clean up */
-  static void __exit alsa_card_mychip_exit(void)
-  {
-          pci_unregister_driver(&driver);
-  }
-
-  module_init(alsa_card_mychip_init)
-  module_exit(alsa_card_mychip_exit)
-
-  EXPORT_NO_SYMBOLS; /* for old kernels only */
-]]>
-          </programlisting>
-        </example>
-      </para>
-    </section>
-
-    <section id="pci-resource-some-haftas">
-      <title>Some Hafta's</title>
-      <para>
-        The allocation of PCI resources is done in the
-      <function>probe()</function> function, and usually an extra
-      <function>xxx_create()</function> function is written for this
-      purpose.
-      </para>
-
-      <para>
-        In the case of PCI devices, you first have to call
-      the <function>pci_enable_device()</function> function before
-      allocating resources. Also, you need to set the proper PCI DMA
-      mask to limit the accessed I/O range. In some cases, you might
-      need to call <function>pci_set_master()</function> function,
-      too.
-      </para>
-
-      <para>
-        Suppose the 28bit mask, and the code to be added would be like:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  err = pci_enable_device(pci);
-  if (err < 0)
-          return err;
-  if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
-      pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
-          printk(KERN_ERR "error to set 28bit mask DMA\n");
-          pci_disable_device(pci);
-          return -ENXIO;
-  }
-  
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-    </section>
-
-    <section id="pci-resource-resource-allocation">
-      <title>Resource Allocation</title>
-      <para>
-        The allocation of I/O ports and irqs is done via standard kernel
-      functions. Unlike ALSA ver.0.5.x., there are no helpers for
-      that. And these resources must be released in the destructor
-      function (see below). Also, on ALSA 0.9.x, you don't need to
-      allocate (pseudo-)DMA for PCI like in ALSA 0.5.x.
-      </para>
-
-      <para>
-        Now assume that the PCI device has an I/O port with 8 bytes
-        and an interrupt. Then struct <structname>mychip</structname> will have the
-        following fields:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct mychip {
-          struct snd_card *card;
-
-          unsigned long port;
-          int irq;
-  };
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        For an I/O port (and also a memory region), you need to have
-      the resource pointer for the standard resource management. For
-      an irq, you have to keep only the irq number (integer). But you
-      need to initialize this number as -1 before actual allocation,
-      since irq 0 is valid. The port address and its resource pointer
-      can be initialized as null by
-      <function>kzalloc()</function> automatically, so you
-      don't have to take care of resetting them. 
-      </para>
-
-      <para>
-        The allocation of an I/O port is done like this:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  err = pci_request_regions(pci, "My Chip");
-  if (err < 0) { 
-          kfree(chip);
-          pci_disable_device(pci);
-          return err;
-  }
-  chip->port = pci_resource_start(pci, 0);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        <!-- obsolete -->
-        It will reserve the I/O port region of 8 bytes of the given
-      PCI device. The returned value, chip-&gt;res_port, is allocated
-      via <function>kmalloc()</function> by
-      <function>request_region()</function>. The pointer must be
-      released via <function>kfree()</function>, but there is a
-      problem with this. This issue will be explained later.
-      </para>
-
-      <para>
-        The allocation of an interrupt source is done like this:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  if (request_irq(pci->irq, snd_mychip_interrupt,
-                  IRQF_SHARED, KBUILD_MODNAME, chip)) {
-          printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
-          snd_mychip_free(chip);
-          return -EBUSY;
-  }
-  chip->irq = pci->irq;
-]]>
-          </programlisting>
-        </informalexample>
-
-        where <function>snd_mychip_interrupt()</function> is the
-      interrupt handler defined <link
-      linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>.
-      Note that chip-&gt;irq should be defined
-      only when <function>request_irq()</function> succeeded.
-      </para>
-
-      <para>
-      On the PCI bus, interrupts can be shared. Thus,
-      <constant>IRQF_SHARED</constant> is used as the interrupt flag of
-      <function>request_irq()</function>. 
-      </para>
-
-      <para>
-        The last argument of <function>request_irq()</function> is the
-      data pointer passed to the interrupt handler. Usually, the
-      chip-specific record is used for that, but you can use what you
-      like, too. 
-      </para>
-
-      <para>
-        I won't give details about the interrupt handler at this
-        point, but at least its appearance can be explained now. The
-        interrupt handler looks usually like the following: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
-  {
-          struct mychip *chip = dev_id;
-          ....
-          return IRQ_HANDLED;
-  }
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        Now let's write the corresponding destructor for the resources
-      above. The role of destructor is simple: disable the hardware
-      (if already activated) and release the resources. So far, we
-      have no hardware part, so the disabling code is not written here. 
-      </para>
-
-      <para>
-        To release the resources, the <quote>check-and-release</quote>
-        method is a safer way. For the interrupt, do like this: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  if (chip->irq >= 0)
-          free_irq(chip->irq, chip);
-]]>
-          </programlisting>
-        </informalexample>
-
-        Since the irq number can start from 0, you should initialize
-        chip-&gt;irq with a negative value (e.g. -1), so that you can
-        check the validity of the irq number as above.
-      </para>
-
-      <para>
-        When you requested I/O ports or memory regions via
-	<function>pci_request_region()</function> or
-	<function>pci_request_regions()</function> like in this example,
-	release the resource(s) using the corresponding function,
-	<function>pci_release_region()</function> or
-	<function>pci_release_regions()</function>.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  pci_release_regions(chip->pci);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-	When you requested manually via <function>request_region()</function>
-	or <function>request_mem_region</function>, you can release it via
-	<function>release_resource()</function>.  Suppose that you keep
-	the resource pointer returned from <function>request_region()</function>
-	in chip-&gt;res_port, the release procedure looks like:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  release_and_free_resource(chip->res_port);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-      Don't forget to call <function>pci_disable_device()</function>
-      before the end.
-      </para>
-
-      <para>
-        And finally, release the chip-specific record.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  kfree(chip);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-      We didn't implement the hardware disabling part in the above.
-      If you need to do this, please note that the destructor may be
-      called even before the initialization of the chip is completed.
-      It would be better to have a flag to skip hardware disabling
-      if the hardware was not initialized yet.
-      </para>
-
-      <para>
-      When the chip-data is assigned to the card using
-      <function>snd_device_new()</function> with
-      <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is 
-      called at the last.  That is, it is assured that all other
-      components like PCMs and controls have already been released.
-      You don't have to stop PCMs, etc. explicitly, but just
-      call low-level hardware stopping.
-      </para>
-
-      <para>
-        The management of a memory-mapped region is almost as same as
-        the management of an I/O port. You'll need three fields like
-        the following: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct mychip {
-          ....
-          unsigned long iobase_phys;
-          void __iomem *iobase_virt;
-  };
-]]>
-          </programlisting>
-        </informalexample>
-
-        and the allocation would be like below:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  if ((err = pci_request_regions(pci, "My Chip")) < 0) {
-          kfree(chip);
-          return err;
-  }
-  chip->iobase_phys = pci_resource_start(pci, 0);
-  chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
-                                      pci_resource_len(pci, 0));
-]]>
-          </programlisting>
-        </informalexample>
-        
-        and the corresponding destructor would be:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int snd_mychip_free(struct mychip *chip)
-  {
-          ....
-          if (chip->iobase_virt)
-                  iounmap(chip->iobase_virt);
-          ....
-          pci_release_regions(chip->pci);
-          ....
-  }
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-    </section>
-
-    <section id="pci-resource-entries">
-      <title>PCI Entries</title>
-      <para>
-        So far, so good. Let's finish the missing PCI
-      stuff. At first, we need a
-      <structname>pci_device_id</structname> table for this
-      chipset. It's a table of PCI vendor/device ID number, and some
-      masks. 
-      </para>
-
-      <para>
-        For example,
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static struct pci_device_id snd_mychip_ids[] = {
-          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
-            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
-          ....
-          { 0, }
-  };
-  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The first and second fields of
-      the <structname>pci_device_id</structname> structure are the vendor and
-      device IDs. If you have no reason to filter the matching
-      devices, you can leave the remaining fields as above. The last
-      field of the <structname>pci_device_id</structname> struct contains
-      private data for this entry. You can specify any value here, for
-      example, to define specific operations for supported device IDs.
-      Such an example is found in the intel8x0 driver. 
-      </para>
-
-      <para>
-        The last entry of this list is the terminator. You must
-      specify this all-zero entry. 
-      </para>
-
-      <para>
-        Then, prepare the <structname>pci_driver</structname> record:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static struct pci_driver driver = {
-          .name = KBUILD_MODNAME,
-          .id_table = snd_mychip_ids,
-          .probe = snd_mychip_probe,
-          .remove = snd_mychip_remove,
-  };
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The <structfield>probe</structfield> and
-      <structfield>remove</structfield> functions have already
-      been defined in the previous sections.
-      The <structfield>name</structfield>
-      field is the name string of this device. Note that you must not
-      use a slash <quote>/</quote> in this string. 
-      </para>
-
-      <para>
-        And at last, the module entries:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int __init alsa_card_mychip_init(void)
-  {
-          return pci_register_driver(&driver);
-  }
-
-  static void __exit alsa_card_mychip_exit(void)
-  {
-          pci_unregister_driver(&driver);
-  }
-
-  module_init(alsa_card_mychip_init)
-  module_exit(alsa_card_mychip_exit)
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        Note that these module entries are tagged with
-      <parameter>__init</parameter> and 
-      <parameter>__exit</parameter> prefixes.
-      </para>
-
-      <para>
-        Oh, one thing was forgotten. If you have no exported symbols,
-        you need to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  EXPORT_NO_SYMBOLS;
-]]>
-          </programlisting>
-        </informalexample>
-
-        That's all!
-      </para>
-    </section>
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- PCM Interface  -->
-<!-- ****************************************************** -->
-  <chapter id="pcm-interface">
-    <title>PCM Interface</title>
-
-    <section id="pcm-interface-general">
-      <title>General</title>
-      <para>
-        The PCM middle layer of ALSA is quite powerful and it is only
-      necessary for each driver to implement the low-level functions
-      to access its hardware.
-      </para>
-
-      <para>
-        For accessing to the PCM layer, you need to include
-      <filename>&lt;sound/pcm.h&gt;</filename> first. In addition,
-      <filename>&lt;sound/pcm_params.h&gt;</filename> might be needed
-      if you access to some functions related with hw_param. 
-      </para>
-
-      <para>
-        Each card device can have up to four pcm instances. A pcm
-      instance corresponds to a pcm device file. The limitation of
-      number of instances comes only from the available bit size of
-      the Linux's device numbers. Once when 64bit device number is
-      used, we'll have more pcm instances available. 
-      </para>
-
-      <para>
-        A pcm instance consists of pcm playback and capture streams,
-      and each pcm stream consists of one or more pcm substreams. Some
-      soundcards support multiple playback functions. For example,
-      emu10k1 has a PCM playback of 32 stereo substreams. In this case, at
-      each open, a free substream is (usually) automatically chosen
-      and opened. Meanwhile, when only one substream exists and it was
-      already opened, the successful open will either block
-      or error with <constant>EAGAIN</constant> according to the
-      file open mode. But you don't have to care about such details in your
-      driver. The PCM middle layer will take care of such work.
-      </para>
-    </section>
-
-    <section id="pcm-interface-example">
-      <title>Full Code Example</title>
-      <para>
-      The example code below does not include any hardware access
-      routines but shows only the skeleton, how to build up the PCM
-      interfaces.
-
-        <example>
-          <title>PCM Example Code</title>
-          <programlisting>
-<![CDATA[
-  #include <sound/pcm.h>
-  ....
-
-  /* hardware definition */
-  static struct snd_pcm_hardware snd_mychip_playback_hw = {
-          .info = (SNDRV_PCM_INFO_MMAP |
-                   SNDRV_PCM_INFO_INTERLEAVED |
-                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
-                   SNDRV_PCM_INFO_MMAP_VALID),
-          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
-          .rates =            SNDRV_PCM_RATE_8000_48000,
-          .rate_min =         8000,
-          .rate_max =         48000,
-          .channels_min =     2,
-          .channels_max =     2,
-          .buffer_bytes_max = 32768,
-          .period_bytes_min = 4096,
-          .period_bytes_max = 32768,
-          .periods_min =      1,
-          .periods_max =      1024,
-  };
-
-  /* hardware definition */
-  static struct snd_pcm_hardware snd_mychip_capture_hw = {
-          .info = (SNDRV_PCM_INFO_MMAP |
-                   SNDRV_PCM_INFO_INTERLEAVED |
-                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
-                   SNDRV_PCM_INFO_MMAP_VALID),
-          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
-          .rates =            SNDRV_PCM_RATE_8000_48000,
-          .rate_min =         8000,
-          .rate_max =         48000,
-          .channels_min =     2,
-          .channels_max =     2,
-          .buffer_bytes_max = 32768,
-          .period_bytes_min = 4096,
-          .period_bytes_max = 32768,
-          .periods_min =      1,
-          .periods_max =      1024,
-  };
-
-  /* open callback */
-  static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          struct snd_pcm_runtime *runtime = substream->runtime;
-
-          runtime->hw = snd_mychip_playback_hw;
-          /* more hardware-initialization will be done here */
-          ....
-          return 0;
-  }
-
-  /* close callback */
-  static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          /* the hardware-specific codes will be here */
-          ....
-          return 0;
-
-  }
-
-  /* open callback */
-  static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          struct snd_pcm_runtime *runtime = substream->runtime;
-
-          runtime->hw = snd_mychip_capture_hw;
-          /* more hardware-initialization will be done here */
-          ....
-          return 0;
-  }
-
-  /* close callback */
-  static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          /* the hardware-specific codes will be here */
-          ....
-          return 0;
-
-  }
-
-  /* hw_params callback */
-  static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
-                               struct snd_pcm_hw_params *hw_params)
-  {
-          return snd_pcm_lib_malloc_pages(substream,
-                                     params_buffer_bytes(hw_params));
-  }
-
-  /* hw_free callback */
-  static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
-  {
-          return snd_pcm_lib_free_pages(substream);
-  }
-
-  /* prepare callback */
-  static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          struct snd_pcm_runtime *runtime = substream->runtime;
-
-          /* set up the hardware with the current configuration
-           * for example...
-           */
-          mychip_set_sample_format(chip, runtime->format);
-          mychip_set_sample_rate(chip, runtime->rate);
-          mychip_set_channels(chip, runtime->channels);
-          mychip_set_dma_setup(chip, runtime->dma_addr,
-                               chip->buffer_size,
-                               chip->period_size);
-          return 0;
-  }
-
-  /* trigger callback */
-  static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
-                                    int cmd)
-  {
-          switch (cmd) {
-          case SNDRV_PCM_TRIGGER_START:
-                  /* do something to start the PCM engine */
-                  ....
-                  break;
-          case SNDRV_PCM_TRIGGER_STOP:
-                  /* do something to stop the PCM engine */
-                  ....
-                  break;
-          default:
-                  return -EINVAL;
-          }
-  }
-
-  /* pointer callback */
-  static snd_pcm_uframes_t
-  snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          unsigned int current_ptr;
-
-          /* get the current hardware pointer */
-          current_ptr = mychip_get_hw_pointer(chip);
-          return current_ptr;
-  }
-
-  /* operators */
-  static struct snd_pcm_ops snd_mychip_playback_ops = {
-          .open =        snd_mychip_playback_open,
-          .close =       snd_mychip_playback_close,
-          .ioctl =       snd_pcm_lib_ioctl,
-          .hw_params =   snd_mychip_pcm_hw_params,
-          .hw_free =     snd_mychip_pcm_hw_free,
-          .prepare =     snd_mychip_pcm_prepare,
-          .trigger =     snd_mychip_pcm_trigger,
-          .pointer =     snd_mychip_pcm_pointer,
-  };
-
-  /* operators */
-  static struct snd_pcm_ops snd_mychip_capture_ops = {
-          .open =        snd_mychip_capture_open,
-          .close =       snd_mychip_capture_close,
-          .ioctl =       snd_pcm_lib_ioctl,
-          .hw_params =   snd_mychip_pcm_hw_params,
-          .hw_free =     snd_mychip_pcm_hw_free,
-          .prepare =     snd_mychip_pcm_prepare,
-          .trigger =     snd_mychip_pcm_trigger,
-          .pointer =     snd_mychip_pcm_pointer,
-  };
-
-  /*
-   *  definitions of capture are omitted here...
-   */
-
-  /* create a pcm device */
-  static int snd_mychip_new_pcm(struct mychip *chip)
-  {
-          struct snd_pcm *pcm;
-          int err;
-
-          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
-          if (err < 0) 
-                  return err;
-          pcm->private_data = chip;
-          strcpy(pcm->name, "My Chip");
-          chip->pcm = pcm;
-          /* set operators */
-          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
-                          &snd_mychip_playback_ops);
-          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
-                          &snd_mychip_capture_ops);
-          /* pre-allocation of buffers */
-          /* NOTE: this may fail */
-          snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
-                                                snd_dma_pci_data(chip->pci),
-                                                64*1024, 64*1024);
-          return 0;
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
-    </section>
-
-    <section id="pcm-interface-constructor">
-      <title>Constructor</title>
-      <para>
-        A pcm instance is allocated by the <function>snd_pcm_new()</function>
-      function. It would be better to create a constructor for pcm,
-      namely, 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int snd_mychip_new_pcm(struct mychip *chip)
-  {
-          struct snd_pcm *pcm;
-          int err;
-
-          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
-          if (err < 0) 
-                  return err;
-          pcm->private_data = chip;
-          strcpy(pcm->name, "My Chip");
-          chip->pcm = pcm;
-	  ....
-          return 0;
-  }
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The <function>snd_pcm_new()</function> function takes four
-      arguments. The first argument is the card pointer to which this
-      pcm is assigned, and the second is the ID string. 
-      </para>
-
-      <para>
-        The third argument (<parameter>index</parameter>, 0 in the
-      above) is the index of this new pcm. It begins from zero. If
-      you create more than one pcm instances, specify the
-      different numbers in this argument. For example,
-      <parameter>index</parameter> = 1 for the second PCM device.  
-      </para>
-
-      <para>
-        The fourth and fifth arguments are the number of substreams
-      for playback and capture, respectively. Here 1 is used for
-      both arguments. When no playback or capture substreams are available,
-      pass 0 to the corresponding argument.
-      </para>
-
-      <para>
-        If a chip supports multiple playbacks or captures, you can
-      specify more numbers, but they must be handled properly in
-      open/close, etc. callbacks.  When you need to know which
-      substream you are referring to, then it can be obtained from
-      struct <structname>snd_pcm_substream</structname> data passed to each callback
-      as follows: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_pcm_substream *substream;
-  int index = substream->number;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        After the pcm is created, you need to set operators for each
-        pcm stream. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
-                  &snd_mychip_playback_ops);
-  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
-                  &snd_mychip_capture_ops);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The operators are defined typically like this:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static struct snd_pcm_ops snd_mychip_playback_ops = {
-          .open =        snd_mychip_pcm_open,
-          .close =       snd_mychip_pcm_close,
-          .ioctl =       snd_pcm_lib_ioctl,
-          .hw_params =   snd_mychip_pcm_hw_params,
-          .hw_free =     snd_mychip_pcm_hw_free,
-          .prepare =     snd_mychip_pcm_prepare,
-          .trigger =     snd_mychip_pcm_trigger,
-          .pointer =     snd_mychip_pcm_pointer,
-  };
-]]>
-          </programlisting>
-        </informalexample>
-
-        All the callbacks are described in the
-        <link linkend="pcm-interface-operators"><citetitle>
-        Operators</citetitle></link> subsection.
-      </para>
-
-      <para>
-        After setting the operators, you probably will want to
-        pre-allocate the buffer. For the pre-allocation, simply call
-        the following: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
-                                        snd_dma_pci_data(chip->pci),
-                                        64*1024, 64*1024);
-]]>
-          </programlisting>
-        </informalexample>
-
-        It will allocate a buffer up to 64kB as default.
-      Buffer management details will be described in the later section <link
-      linkend="buffer-and-memory"><citetitle>Buffer and Memory
-      Management</citetitle></link>. 
-      </para>
-
-      <para>
-        Additionally, you can set some extra information for this pcm
-        in pcm-&gt;info_flags.
-        The available values are defined as
-        <constant>SNDRV_PCM_INFO_XXX</constant> in
-        <filename>&lt;sound/asound.h&gt;</filename>, which is used for
-        the hardware definition (described later). When your soundchip
-        supports only half-duplex, specify like this: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-    </section>
-
-    <section id="pcm-interface-destructor">
-      <title>... And the Destructor?</title>
-      <para>
-        The destructor for a pcm instance is not always
-      necessary. Since the pcm device will be released by the middle
-      layer code automatically, you don't have to call the destructor
-      explicitly.
-      </para>
-
-      <para>
-        The destructor would be necessary if you created
-        special records internally and needed to release them. In such a
-        case, set the destructor function to
-        pcm-&gt;private_free: 
-
-        <example>
-          <title>PCM Instance with a Destructor</title>
-          <programlisting>
-<![CDATA[
-  static void mychip_pcm_free(struct snd_pcm *pcm)
-  {
-          struct mychip *chip = snd_pcm_chip(pcm);
-          /* free your own data */
-          kfree(chip->my_private_pcm_data);
-          /* do what you like else */
-          ....
-  }
-
-  static int snd_mychip_new_pcm(struct mychip *chip)
-  {
-          struct snd_pcm *pcm;
-          ....
-          /* allocate your own data */
-          chip->my_private_pcm_data = kmalloc(...);
-          /* set the destructor */
-          pcm->private_data = chip;
-          pcm->private_free = mychip_pcm_free;
-          ....
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
-    </section>
-
-    <section id="pcm-interface-runtime">
-      <title>Runtime Pointer - The Chest of PCM Information</title>
-	<para>
-	  When the PCM substream is opened, a PCM runtime instance is
-	allocated and assigned to the substream. This pointer is
-	accessible via <constant>substream-&gt;runtime</constant>.
-	This runtime pointer holds most information you need
-	to control the PCM: the copy of hw_params and sw_params configurations, the buffer
-	pointers, mmap records, spinlocks, etc.
-	</para>
-
-	<para>
-	The definition of runtime instance is found in
-	<filename>&lt;sound/pcm.h&gt;</filename>.  Here are
-       the contents of this file:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-struct _snd_pcm_runtime {
-	/* -- Status -- */
-	struct snd_pcm_substream *trigger_master;
-	snd_timestamp_t trigger_tstamp;	/* trigger timestamp */
-	int overrange;
-	snd_pcm_uframes_t avail_max;
-	snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
-	snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
-
-	/* -- HW params -- */
-	snd_pcm_access_t access;	/* access mode */
-	snd_pcm_format_t format;	/* SNDRV_PCM_FORMAT_* */
-	snd_pcm_subformat_t subformat;	/* subformat */
-	unsigned int rate;		/* rate in Hz */
-	unsigned int channels;		/* channels */
-	snd_pcm_uframes_t period_size;	/* period size */
-	unsigned int periods;		/* periods */
-	snd_pcm_uframes_t buffer_size;	/* buffer size */
-	unsigned int tick_time;		/* tick time */
-	snd_pcm_uframes_t min_align;	/* Min alignment for the format */
-	size_t byte_align;
-	unsigned int frame_bits;
-	unsigned int sample_bits;
-	unsigned int info;
-	unsigned int rate_num;
-	unsigned int rate_den;
-
-	/* -- SW params -- */
-	struct timespec tstamp_mode;	/* mmap timestamp is updated */
-  	unsigned int period_step;
-	unsigned int sleep_min;		/* min ticks to sleep */
-	snd_pcm_uframes_t start_threshold;
-	snd_pcm_uframes_t stop_threshold;
-	snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
-						noise is nearest than this */
-	snd_pcm_uframes_t silence_size;	/* Silence filling size */
-	snd_pcm_uframes_t boundary;	/* pointers wrap point */
-
-	snd_pcm_uframes_t silenced_start;
-	snd_pcm_uframes_t silenced_size;
-
-	snd_pcm_sync_id_t sync;		/* hardware synchronization ID */
-
-	/* -- mmap -- */
-	volatile struct snd_pcm_mmap_status *status;
-	volatile struct snd_pcm_mmap_control *control;
-	atomic_t mmap_count;
-
-	/* -- locking / scheduling -- */
-	spinlock_t lock;
-	wait_queue_head_t sleep;
-	struct timer_list tick_timer;
-	struct fasync_struct *fasync;
-
-	/* -- private section -- */
-	void *private_data;
-	void (*private_free)(struct snd_pcm_runtime *runtime);
-
-	/* -- hardware description -- */
-	struct snd_pcm_hardware hw;
-	struct snd_pcm_hw_constraints hw_constraints;
-
-	/* -- timer -- */
-	unsigned int timer_resolution;	/* timer resolution */
-
-	/* -- DMA -- */           
-	unsigned char *dma_area;	/* DMA area */
-	dma_addr_t dma_addr;		/* physical bus address (not accessible from main CPU) */
-	size_t dma_bytes;		/* size of DMA area */
-
-	struct snd_dma_buffer *dma_buffer_p;	/* allocated buffer */
-
-#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
-	/* -- OSS things -- */
-	struct snd_pcm_oss_runtime oss;
-#endif
-};
-]]>
-            </programlisting>
-          </informalexample>
-	</para>
-
-	<para>
-	  For the operators (callbacks) of each sound driver, most of
-	these records are supposed to be read-only.  Only the PCM
-	middle-layer changes / updates them.  The exceptions are
-	the hardware description (hw) DMA buffer information and the
-	private data.  Besides, if you use the standard buffer allocation
-	method via <function>snd_pcm_lib_malloc_pages()</function>,
-	you don't need to set the DMA buffer information by yourself.
-	</para>
-
-	<para>
-	In the sections below, important records are explained.
-	</para>
-
-	<section id="pcm-interface-runtime-hw">
-	<title>Hardware Description</title>
-	<para>
-	  The hardware descriptor (struct <structname>snd_pcm_hardware</structname>)
-	contains the definitions of the fundamental hardware
-	configuration.  Above all, you'll need to define this in
-	<link linkend="pcm-interface-operators-open-callback"><citetitle>
-	the open callback</citetitle></link>.
-	Note that the runtime instance holds the copy of the
-	descriptor, not the pointer to the existing descriptor.  That
-	is, in the open callback, you can modify the copied descriptor
-	(<constant>runtime-&gt;hw</constant>) as you need.  For example, if the maximum
-	number of channels is 1 only on some chip models, you can
-	still use the same hardware descriptor and change the
-	channels_max later:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-          struct snd_pcm_runtime *runtime = substream->runtime;
-          ...
-          runtime->hw = snd_mychip_playback_hw; /* common definition */
-          if (chip->model == VERY_OLD_ONE)
-                  runtime->hw.channels_max = 1;
-]]>
-            </programlisting>
-          </informalexample>
-	</para>
-
-	<para>
-	  Typically, you'll have a hardware descriptor as below:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static struct snd_pcm_hardware snd_mychip_playback_hw = {
-          .info = (SNDRV_PCM_INFO_MMAP |
-                   SNDRV_PCM_INFO_INTERLEAVED |
-                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
-                   SNDRV_PCM_INFO_MMAP_VALID),
-          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
-          .rates =            SNDRV_PCM_RATE_8000_48000,
-          .rate_min =         8000,
-          .rate_max =         48000,
-          .channels_min =     2,
-          .channels_max =     2,
-          .buffer_bytes_max = 32768,
-          .period_bytes_min = 4096,
-          .period_bytes_max = 32768,
-          .periods_min =      1,
-          .periods_max =      1024,
-  };
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-	<itemizedlist>
-	<listitem><para>
-          The <structfield>info</structfield> field contains the type and
-        capabilities of this pcm. The bit flags are defined in
-        <filename>&lt;sound/asound.h&gt;</filename> as
-        <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you
-        have to specify whether the mmap is supported and which
-        interleaved format is supported.
-        When the hardware supports mmap, add the
-        <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the
-        hardware supports the interleaved or the non-interleaved
-        formats, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or
-        <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must
-        be set, respectively. If both are supported, you can set both,
-        too. 
-        </para>
-
-        <para>
-          In the above example, <constant>MMAP_VALID</constant> and
-        <constant>BLOCK_TRANSFER</constant> are specified for the OSS mmap
-        mode. Usually both are set. Of course,
-        <constant>MMAP_VALID</constant> is set only if the mmap is
-        really supported. 
-        </para>
-
-        <para>
-          The other possible flags are
-        <constant>SNDRV_PCM_INFO_PAUSE</constant> and
-        <constant>SNDRV_PCM_INFO_RESUME</constant>. The
-        <constant>PAUSE</constant> bit means that the pcm supports the
-        <quote>pause</quote> operation, while the
-        <constant>RESUME</constant> bit means that the pcm supports
-        the full <quote>suspend/resume</quote> operation.
-	If the <constant>PAUSE</constant> flag is set,
-	the <structfield>trigger</structfield> callback below
-        must handle the corresponding (pause push/release) commands.
-	The suspend/resume trigger commands can be defined even without
-	the <constant>RESUME</constant> flag.  See <link
-	linkend="power-management"><citetitle>
-	Power Management</citetitle></link> section for details.
-        </para>
-
-	<para>
-	  When the PCM substreams can be synchronized (typically,
-	synchronized start/stop of a playback and a capture streams),
-	you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>,
-	too.  In this case, you'll need to check the linked-list of
-	PCM substreams in the trigger callback.  This will be
-	described in the later section.
-	</para>
-	</listitem>
-
-	<listitem>
-        <para>
-          <structfield>formats</structfield> field contains the bit-flags
-        of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>).
-        If the hardware supports more than one format, give all or'ed
-        bits.  In the example above, the signed 16bit little-endian
-        format is specified.
-        </para>
-	</listitem>
-
-	<listitem>
-        <para>
-        <structfield>rates</structfield> field contains the bit-flags of
-        supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>).
-        When the chip supports continuous rates, pass
-        <constant>CONTINUOUS</constant> bit additionally.
-        The pre-defined rate bits are provided only for typical
-	rates. If your chip supports unconventional rates, you need to add
-        the <constant>KNOT</constant> bit and set up the hardware
-        constraint manually (explained later).
-        </para>
-	</listitem>
-
-	<listitem>
-	<para>
-	<structfield>rate_min</structfield> and
-	<structfield>rate_max</structfield> define the minimum and
-	maximum sample rate.  This should correspond somehow to
-	<structfield>rates</structfield> bits.
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	<structfield>channel_min</structfield> and
-	<structfield>channel_max</structfield> 
-	define, as you might already expected, the minimum and maximum
-	number of channels.
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	<structfield>buffer_bytes_max</structfield> defines the
-	maximum buffer size in bytes.  There is no
-	<structfield>buffer_bytes_min</structfield> field, since
-	it can be calculated from the minimum period size and the
-	minimum number of periods.
-	Meanwhile, <structfield>period_bytes_min</structfield> and
-	define the minimum and maximum size of the period in bytes.
-	<structfield>periods_max</structfield> and
-	<structfield>periods_min</structfield> define the maximum and
-	minimum number of periods in the buffer.
-        </para>
-
-	<para>
-	The <quote>period</quote> is a term that corresponds to
-	a fragment in the OSS world. The period defines the size at
-	which a PCM interrupt is generated. This size strongly
-	depends on the hardware. 
-	Generally, the smaller period size will give you more
-	interrupts, that is, more controls. 
-	In the case of capture, this size defines the input latency.
-	On the other hand, the whole buffer size defines the
-	output latency for the playback direction.
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	There is also a field <structfield>fifo_size</structfield>.
-	This specifies the size of the hardware FIFO, but currently it
-	is neither used in the driver nor in the alsa-lib.  So, you
-	can ignore this field.
-	</para>
-	</listitem>
-	</itemizedlist>
-	</para>
-	</section>
-
-	<section id="pcm-interface-runtime-config">
-	<title>PCM Configurations</title>
-	<para>
-	Ok, let's go back again to the PCM runtime records.
-	The most frequently referred records in the runtime instance are
-	the PCM configurations.
-	The PCM configurations are stored in the runtime instance
-	after the application sends <type>hw_params</type> data via
-	alsa-lib.  There are many fields copied from hw_params and
-	sw_params structs.  For example,
-	<structfield>format</structfield> holds the format type
-	chosen by the application.  This field contains the enum value
-	<constant>SNDRV_PCM_FORMAT_XXX</constant>.
-	</para>
-
-	<para>
-	One thing to be noted is that the configured buffer and period
-	sizes are stored in <quote>frames</quote> in the runtime.
-        In the ALSA world, 1 frame = channels * samples-size.
-	For conversion between frames and bytes, you can use the
-	<function>frames_to_bytes()</function> and
-          <function>bytes_to_frames()</function> helper functions. 
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  period_bytes = frames_to_bytes(runtime, runtime->period_size);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-	<para>
-	Also, many software parameters (sw_params) are
-	stored in frames, too.  Please check the type of the field.
-	<type>snd_pcm_uframes_t</type> is for the frames as unsigned
-	integer while <type>snd_pcm_sframes_t</type> is for the frames
-	as signed integer.
-	</para>
-	</section>
-
-	<section id="pcm-interface-runtime-dma">
-	<title>DMA Buffer Information</title>
-	<para>
-	The DMA buffer is defined by the following four fields,
-	<structfield>dma_area</structfield>,
-	<structfield>dma_addr</structfield>,
-	<structfield>dma_bytes</structfield> and
-	<structfield>dma_private</structfield>.
-	The <structfield>dma_area</structfield> holds the buffer
-	pointer (the logical address).  You can call
-	<function>memcpy</function> from/to 
-	this pointer.  Meanwhile, <structfield>dma_addr</structfield>
-	holds the physical address of the buffer.  This field is
-	specified only when the buffer is a linear buffer.
-	<structfield>dma_bytes</structfield> holds the size of buffer
-	in bytes.  <structfield>dma_private</structfield> is used for
-	the ALSA DMA allocator.
-	</para>
-
-	<para>
-	If you use a standard ALSA function,
-	<function>snd_pcm_lib_malloc_pages()</function>, for
-	allocating the buffer, these fields are set by the ALSA middle
-	layer, and you should <emphasis>not</emphasis> change them by
-	yourself.  You can read them but not write them.
-	On the other hand, if you want to allocate the buffer by
-	yourself, you'll need to manage it in hw_params callback.
-	At least, <structfield>dma_bytes</structfield> is mandatory.
-	<structfield>dma_area</structfield> is necessary when the
-	buffer is mmapped.  If your driver doesn't support mmap, this
-	field is not necessary.  <structfield>dma_addr</structfield>
-	is also optional.  You can use
-	<structfield>dma_private</structfield> as you like, too.
-	</para>
-	</section>
-
-	<section id="pcm-interface-runtime-status">
-	<title>Running Status</title>
-	<para>
-	The running status can be referred via <constant>runtime-&gt;status</constant>.
-	This is the pointer to the struct <structname>snd_pcm_mmap_status</structname>
-	record.  For example, you can get the current DMA hardware
-	pointer via <constant>runtime-&gt;status-&gt;hw_ptr</constant>.
-	</para>
-
-	<para>
-	The DMA application pointer can be referred via
-	<constant>runtime-&gt;control</constant>, which points to the
-	struct <structname>snd_pcm_mmap_control</structname> record.
-	However, accessing directly to this value is not recommended.
-	</para>
-	</section>
-
-	<section id="pcm-interface-runtime-private">
-	<title>Private Data</title> 
-	<para>
-	You can allocate a record for the substream and store it in
-	<constant>runtime-&gt;private_data</constant>.  Usually, this
-	is done in
-	<link linkend="pcm-interface-operators-open-callback"><citetitle>
-	the open callback</citetitle></link>.
-	Don't mix this with <constant>pcm-&gt;private_data</constant>.
-	The <constant>pcm-&gt;private_data</constant> usually points to the
-	chip instance assigned statically at the creation of PCM, while the 
-	<constant>runtime-&gt;private_data</constant> points to a dynamic
-	data structure created at the PCM open callback.
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_open(struct snd_pcm_substream *substream)
-  {
-          struct my_pcm_data *data;
-          ....
-          data = kmalloc(sizeof(*data), GFP_KERNEL);
-          substream->runtime->private_data = data;
-          ....
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          The allocated object must be released in
-	<link linkend="pcm-interface-operators-open-callback"><citetitle>
-	the close callback</citetitle></link>.
-        </para>
-	</section>
-
-    </section>
-
-    <section id="pcm-interface-operators">
-      <title>Operators</title>
-      <para>
-        OK, now let me give details about each pcm callback
-      (<parameter>ops</parameter>). In general, every callback must
-      return 0 if successful, or a negative error number
-      such as <constant>-EINVAL</constant>. To choose an appropriate
-      error number, it is advised to check what value other parts of
-      the kernel return when the same kind of request fails.
-      </para>
-
-      <para>
-        The callback function takes at least the argument with
-        <structname>snd_pcm_substream</structname> pointer. To retrieve
-        the chip record from the given substream instance, you can use the
-        following macro. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  int xxx() {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          ....
-  }
-]]>
-          </programlisting>
-        </informalexample>
-
-	The macro reads <constant>substream-&gt;private_data</constant>,
-	which is a copy of <constant>pcm-&gt;private_data</constant>.
-	You can override the former if you need to assign different data
-	records per PCM substream.  For example, the cmi8330 driver assigns
-	different private_data for playback and capture directions,
-	because it uses two different codecs (SB- and AD-compatible) for
-	different directions.
-      </para>
-
-      <section id="pcm-interface-operators-open-callback">
-        <title>open callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_open(struct snd_pcm_substream *substream);
-]]>
-            </programlisting>
-          </informalexample>
-
-          This is called when a pcm substream is opened.
-        </para>
-
-        <para>
-          At least, here you have to initialize the runtime-&gt;hw
-          record. Typically, this is done by like this: 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_open(struct snd_pcm_substream *substream)
-  {
-          struct mychip *chip = snd_pcm_substream_chip(substream);
-          struct snd_pcm_runtime *runtime = substream->runtime;
-
-          runtime->hw = snd_mychip_playback_hw;
-          return 0;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-
-          where <parameter>snd_mychip_playback_hw</parameter> is the
-          pre-defined hardware description.
-	</para>
-
-	<para>
-	You can allocate a private data in this callback, as described
-	in <link linkend="pcm-interface-runtime-private"><citetitle>
-	Private Data</citetitle></link> section.
-	</para>
-
-	<para>
-	If the hardware configuration needs more constraints, set the
-	hardware constraints here, too.
-	See <link linkend="pcm-interface-constraints"><citetitle>
-	Constraints</citetitle></link> for more details.
-	</para>
-      </section>
-
-      <section id="pcm-interface-operators-close-callback">
-        <title>close callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_close(struct snd_pcm_substream *substream);
-]]>
-            </programlisting>
-          </informalexample>
-
-          Obviously, this is called when a pcm substream is closed.
-        </para>
-
-        <para>
-          Any private instance for a pcm substream allocated in the
-          open callback will be released here. 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_close(struct snd_pcm_substream *substream)
-  {
-          ....
-          kfree(substream->runtime->private_data);
-          ....
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-ioctl-callback">
-        <title>ioctl callback</title>
-        <para>
-          This is used for any special call to pcm ioctls. But
-        usually you can pass a generic ioctl callback, 
-        <function>snd_pcm_lib_ioctl</function>.
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-hw-params-callback">
-        <title>hw_params callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
-                               struct snd_pcm_hw_params *hw_params);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          This is called when the hardware parameter
-        (<structfield>hw_params</structfield>) is set
-        up by the application, 
-        that is, once when the buffer size, the period size, the
-        format, etc. are defined for the pcm substream. 
-        </para>
-
-        <para>
-          Many hardware setups should be done in this callback,
-        including the allocation of buffers. 
-        </para>
-
-        <para>
-          Parameters to be initialized are retrieved by
-          <function>params_xxx()</function> macros. To allocate
-          buffer, you can call a helper function, 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
-]]>
-            </programlisting>
-          </informalexample>
-
-          <function>snd_pcm_lib_malloc_pages()</function> is available
-	  only when the DMA buffers have been pre-allocated.
-	  See the section <link
-	  linkend="buffer-and-memory-buffer-types"><citetitle>
-	  Buffer Types</citetitle></link> for more details.
-        </para>
-
-        <para>
-          Note that this and <structfield>prepare</structfield> callbacks
-        may be called multiple times per initialization.
-        For example, the OSS emulation may
-        call these callbacks at each change via its ioctl. 
-        </para>
-
-        <para>
-          Thus, you need to be careful not to allocate the same buffers
-        many times, which will lead to memory leaks!  Calling the
-        helper function above many times is OK. It will release the
-        previous buffer automatically when it was already allocated. 
-        </para>
-
-        <para>
-          Another note is that this callback is non-atomic
-        (schedulable) as default, i.e. when no
-	<structfield>nonatomic</structfield> flag set.
-	This is important, because the
-        <structfield>trigger</structfield> callback 
-        is atomic (non-schedulable). That is, mutexes or any
-        schedule-related functions are not available in
-        <structfield>trigger</structfield> callback.
-	Please see the subsection
-	<link linkend="pcm-interface-atomicity"><citetitle>
-	Atomicity</citetitle></link> for details.
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-hw-free-callback">
-        <title>hw_free callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          This is called to release the resources allocated via
-          <structfield>hw_params</structfield>. For example, releasing the
-          buffer via 
-          <function>snd_pcm_lib_malloc_pages()</function> is done by
-          calling the following: 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  snd_pcm_lib_free_pages(substream);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          This function is always called before the close callback is called.
-          Also, the callback may be called multiple times, too.
-          Keep track whether the resource was already released. 
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-prepare-callback">
-       <title>prepare callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_prepare(struct snd_pcm_substream *substream);
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          This callback is called when the pcm is
-        <quote>prepared</quote>. You can set the format type, sample
-        rate, etc. here. The difference from
-        <structfield>hw_params</structfield> is that the 
-        <structfield>prepare</structfield> callback will be called each
-        time 
-        <function>snd_pcm_prepare()</function> is called, i.e. when
-        recovering after underruns, etc. 
-        </para>
-
-        <para>
-	Note that this callback is now non-atomic.
-	You can use schedule-related functions safely in this callback.
-        </para>
-
-        <para>
-          In this and the following callbacks, you can refer to the
-        values via the runtime record,
-        substream-&gt;runtime.
-        For example, to get the current
-        rate, format or channels, access to
-        runtime-&gt;rate,
-        runtime-&gt;format or
-        runtime-&gt;channels, respectively. 
-        The physical address of the allocated buffer is set to
-	runtime-&gt;dma_area.  The buffer and period sizes are
-	in runtime-&gt;buffer_size and runtime-&gt;period_size,
-	respectively.
-        </para>
-
-        <para>
-          Be careful that this callback will be called many times at
-        each setup, too. 
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-trigger-callback">
-        <title>trigger callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
-]]>
-            </programlisting>
-          </informalexample>
-
-          This is called when the pcm is started, stopped or paused.
-        </para>
-
-        <para>
-          Which action is specified in the second argument,
-          <constant>SNDRV_PCM_TRIGGER_XXX</constant> in
-          <filename>&lt;sound/pcm.h&gt;</filename>. At least,
-          the <constant>START</constant> and <constant>STOP</constant>
-          commands must be defined in this callback. 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  switch (cmd) {
-  case SNDRV_PCM_TRIGGER_START:
-          /* do something to start the PCM engine */
-          break;
-  case SNDRV_PCM_TRIGGER_STOP:
-          /* do something to stop the PCM engine */
-          break;
-  default:
-          return -EINVAL;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-          When the pcm supports the pause operation (given in the info
-        field of the hardware table), the <constant>PAUSE_PUSH</constant>
-        and <constant>PAUSE_RELEASE</constant> commands must be
-        handled here, too. The former is the command to pause the pcm,
-        and the latter to restart the pcm again. 
-        </para>
-
-        <para>
-          When the pcm supports the suspend/resume operation,
-	regardless of full or partial suspend/resume support,
-        the <constant>SUSPEND</constant> and <constant>RESUME</constant>
-        commands must be handled, too.
-        These commands are issued when the power-management status is
-        changed.  Obviously, the <constant>SUSPEND</constant> and
-        <constant>RESUME</constant> commands
-        suspend and resume the pcm substream, and usually, they
-        are identical to the <constant>STOP</constant> and
-        <constant>START</constant> commands, respectively.
-	  See the <link linkend="power-management"><citetitle>
-	Power Management</citetitle></link> section for details.
-        </para>
-
-        <para>
-          As mentioned, this callback is atomic as default unless
-	  <structfield>nonatomic</structfield> flag set, and
-	  you cannot call functions which may sleep.
-	  The trigger callback should be as minimal as possible,
-	  just really triggering the DMA.  The other stuff should be
-	  initialized hw_params and prepare callbacks properly
-	  beforehand.
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-pointer-callback">
-        <title>pointer callback</title>
-        <para>
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
-]]>
-            </programlisting>
-          </informalexample>
-
-          This callback is called when the PCM middle layer inquires
-        the current hardware position on the buffer. The position must
-        be returned in frames,
-        ranging from 0 to buffer_size - 1.
-        </para>
-
-        <para>
-          This is called usually from the buffer-update routine in the
-        pcm middle layer, which is invoked when
-        <function>snd_pcm_period_elapsed()</function> is called in the
-        interrupt routine. Then the pcm middle layer updates the
-        position and calculates the available space, and wakes up the
-        sleeping poll threads, etc. 
-        </para>
-
-        <para>
-          This callback is also atomic as default.
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-copy-silence">
-        <title>copy and silence callbacks</title>
-        <para>
-          These callbacks are not mandatory, and can be omitted in
-        most cases. These callbacks are used when the hardware buffer
-        cannot be in the normal memory space. Some chips have their
-        own buffer on the hardware which is not mappable. In such a
-        case, you have to transfer the data manually from the memory
-        buffer to the hardware buffer. Or, if the buffer is
-        non-contiguous on both physical and virtual memory spaces,
-        these callbacks must be defined, too. 
-        </para>
-
-        <para>
-          If these two callbacks are defined, copy and set-silence
-        operations are done by them. The detailed will be described in
-        the later section <link
-        linkend="buffer-and-memory"><citetitle>Buffer and Memory
-        Management</citetitle></link>. 
-        </para>
-      </section>
-
-      <section id="pcm-interface-operators-ack">
-        <title>ack callback</title>
-        <para>
-          This callback is also not mandatory. This callback is called
-        when the appl_ptr is updated in read or write operations.
-        Some drivers like emu10k1-fx and cs46xx need to track the
-	current appl_ptr for the internal buffer, and this callback
-	is useful only for such a purpose.
-	</para>
-	<para>
-	  This callback is atomic as default.
-	</para>
-      </section>
-
-      <section id="pcm-interface-operators-page-callback">
-        <title>page callback</title>
-
-        <para>
-          This callback is optional too. This callback is used
-        mainly for non-contiguous buffers. The mmap calls this
-        callback to get the page address. Some examples will be
-        explained in the later section <link
-        linkend="buffer-and-memory"><citetitle>Buffer and Memory
-        Management</citetitle></link>, too. 
-        </para>
-      </section>
-    </section>
-
-    <section id="pcm-interface-interrupt-handler">
-      <title>Interrupt Handler</title>
-      <para>
-        The rest of pcm stuff is the PCM interrupt handler. The
-      role of PCM interrupt handler in the sound driver is to update
-      the buffer position and to tell the PCM middle layer when the
-      buffer position goes across the prescribed period size. To
-      inform this, call the <function>snd_pcm_period_elapsed()</function>
-      function. 
-      </para>
-
-      <para>
-        There are several types of sound chips to generate the interrupts.
-      </para>
-
-      <section id="pcm-interface-interrupt-handler-boundary">
-        <title>Interrupts at the period (fragment) boundary</title>
-        <para>
-          This is the most frequently found type:  the hardware
-        generates an interrupt at each period boundary.
-	In this case, you can call
-        <function>snd_pcm_period_elapsed()</function> at each 
-        interrupt. 
-        </para>
-
-        <para>
-          <function>snd_pcm_period_elapsed()</function> takes the
-        substream pointer as its argument. Thus, you need to keep the
-        substream pointer accessible from the chip instance. For
-        example, define substream field in the chip record to hold the
-        current running substream pointer, and set the pointer value
-        at open callback (and reset at close callback). 
-        </para>
-
-        <para>
-          If you acquire a spinlock in the interrupt handler, and the
-        lock is used in other pcm callbacks, too, then you have to
-        release the lock before calling
-        <function>snd_pcm_period_elapsed()</function>, because
-        <function>snd_pcm_period_elapsed()</function> calls other pcm
-        callbacks inside. 
-        </para>
-
-        <para>
-          Typical code would be like:
-
-          <example>
-	    <title>Interrupt Handler Case #1</title>
-            <programlisting>
-<![CDATA[
-  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
-  {
-          struct mychip *chip = dev_id;
-          spin_lock(&chip->lock);
-          ....
-          if (pcm_irq_invoked(chip)) {
-                  /* call updater, unlock before it */
-                  spin_unlock(&chip->lock);
-                  snd_pcm_period_elapsed(chip->substream);
-                  spin_lock(&chip->lock);
-                  /* acknowledge the interrupt if necessary */
-          }
-          ....
-          spin_unlock(&chip->lock);
-          return IRQ_HANDLED;
-  }
-]]>
-            </programlisting>
-          </example>
-        </para>
-      </section>
-
-      <section id="pcm-interface-interrupt-handler-timer">
-        <title>High frequency timer interrupts</title>
-        <para>
-	This happens when the hardware doesn't generate interrupts
-        at the period boundary but issues timer interrupts at a fixed
-        timer rate (e.g. es1968 or ymfpci drivers). 
-        In this case, you need to check the current hardware
-        position and accumulate the processed sample length at each
-        interrupt.  When the accumulated size exceeds the period
-        size, call 
-        <function>snd_pcm_period_elapsed()</function> and reset the
-        accumulator. 
-        </para>
-
-        <para>
-          Typical code would be like the following.
-
-          <example>
-	    <title>Interrupt Handler Case #2</title>
-            <programlisting>
-<![CDATA[
-  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
-  {
-          struct mychip *chip = dev_id;
-          spin_lock(&chip->lock);
-          ....
-          if (pcm_irq_invoked(chip)) {
-                  unsigned int last_ptr, size;
-                  /* get the current hardware pointer (in frames) */
-                  last_ptr = get_hw_ptr(chip);
-                  /* calculate the processed frames since the
-                   * last update
-                   */
-                  if (last_ptr < chip->last_ptr)
-                          size = runtime->buffer_size + last_ptr 
-                                   - chip->last_ptr; 
-                  else
-                          size = last_ptr - chip->last_ptr;
-                  /* remember the last updated point */
-                  chip->last_ptr = last_ptr;
-                  /* accumulate the size */
-                  chip->size += size;
-                  /* over the period boundary? */
-                  if (chip->size >= runtime->period_size) {
-                          /* reset the accumulator */
-                          chip->size %= runtime->period_size;
-                          /* call updater */
-                          spin_unlock(&chip->lock);
-                          snd_pcm_period_elapsed(substream);
-                          spin_lock(&chip->lock);
-                  }
-                  /* acknowledge the interrupt if necessary */
-          }
-          ....
-          spin_unlock(&chip->lock);
-          return IRQ_HANDLED;
-  }
-]]>
-            </programlisting>
-          </example>
-        </para>
-      </section>
-
-      <section id="pcm-interface-interrupt-handler-both">
-        <title>On calling <function>snd_pcm_period_elapsed()</function></title>
-        <para>
-          In both cases, even if more than one period are elapsed, you
-        don't have to call
-        <function>snd_pcm_period_elapsed()</function> many times. Call
-        only once. And the pcm layer will check the current hardware
-        pointer and update to the latest status. 
-        </para>
-      </section>
-    </section>
-
-    <section id="pcm-interface-atomicity">
-      <title>Atomicity</title>
-      <para>
-      One of the most important (and thus difficult to debug) problems
-      in kernel programming are race conditions.
-      In the Linux kernel, they are usually avoided via spin-locks, mutexes
-      or semaphores.  In general, if a race condition can happen
-      in an interrupt handler, it has to be managed atomically, and you
-      have to use a spinlock to protect the critical session. If the
-      critical section is not in interrupt handler code and
-      if taking a relatively long time to execute is acceptable, you
-      should use mutexes or semaphores instead.
-      </para>
-
-      <para>
-      As already seen, some pcm callbacks are atomic and some are
-      not.  For example, the <parameter>hw_params</parameter> callback is
-      non-atomic, while <parameter>trigger</parameter> callback is
-      atomic.  This means, the latter is called already in a spinlock
-      held by the PCM middle layer. Please take this atomicity into
-      account when you choose a locking scheme in the callbacks.
-      </para>
-
-      <para>
-      In the atomic callbacks, you cannot use functions which may call
-      <function>schedule</function> or go to
-      <function>sleep</function>.  Semaphores and mutexes can sleep,
-      and hence they cannot be used inside the atomic callbacks
-      (e.g. <parameter>trigger</parameter> callback).
-      To implement some delay in such a callback, please use
-      <function>udelay()</function> or <function>mdelay()</function>.
-      </para>
-
-      <para>
-      All three atomic callbacks (trigger, pointer, and ack) are
-      called with local interrupts disabled.
-      </para>
-
-      <para>
-      The recent changes in PCM core code, however, allow all PCM
-      operations to be non-atomic.  This assumes that the all caller
-      sides are in non-atomic contexts.  For example, the function
-      <function>snd_pcm_period_elapsed()</function> is called
-      typically from the interrupt handler.  But, if you set up the
-      driver to use a threaded interrupt handler, this call can be in
-      non-atomic context, too.  In such a case, you can set
-      <structfield>nonatomic</structfield> filed of
-      <structname>snd_pcm</structname> object after creating it.
-      When this flag is set, mutex and rwsem are used internally in
-      the PCM core instead of spin and rwlocks, so that you can call
-      all PCM functions safely in a non-atomic context.
-      </para>
-
-    </section>
-    <section id="pcm-interface-constraints">
-      <title>Constraints</title>
-      <para>
-        If your chip supports unconventional sample rates, or only the
-      limited samples, you need to set a constraint for the
-      condition. 
-      </para>
-
-      <para>
-        For example, in order to restrict the sample rates in the some
-        supported values, use
-	<function>snd_pcm_hw_constraint_list()</function>.
-	You need to call this function in the open callback.
-
-        <example>
-	  <title>Example of Hardware Constraints</title>
-          <programlisting>
-<![CDATA[
-  static unsigned int rates[] =
-          {4000, 10000, 22050, 44100};
-  static struct snd_pcm_hw_constraint_list constraints_rates = {
-          .count = ARRAY_SIZE(rates),
-          .list = rates,
-          .mask = 0,
-  };
-
-  static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
-  {
-          int err;
-          ....
-          err = snd_pcm_hw_constraint_list(substream->runtime, 0,
-                                           SNDRV_PCM_HW_PARAM_RATE,
-                                           &constraints_rates);
-          if (err < 0)
-                  return err;
-          ....
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
-
-      <para>
-        There are many different constraints.
-        Look at <filename>sound/pcm.h</filename> for a complete list.
-        You can even define your own constraint rules.
-        For example, let's suppose my_chip can manage a substream of 1 channel
-        if and only if the format is S16_LE, otherwise it supports any format
-        specified in the <structname>snd_pcm_hardware</structname> structure (or in any
-        other constraint_list). You can build a rule like this:
-
-        <example>
-	  <title>Example of Hardware Constraints for Channels</title>
-	  <programlisting>
-<![CDATA[
-  static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
-                                        struct snd_pcm_hw_rule *rule)
-  {
-          struct snd_interval *c = hw_param_interval(params,
-                        SNDRV_PCM_HW_PARAM_CHANNELS);
-          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
-          struct snd_interval ch;
-
-          snd_interval_any(&ch);
-          if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
-                  ch.min = ch.max = 1;
-                  ch.integer = 1;
-                  return snd_interval_refine(c, &ch);
-          }
-          return 0;
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
- 
-      <para>
-        Then you need to call this function to add your rule:
-
-       <informalexample>
-	 <programlisting>
-<![CDATA[
-  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
-                      hw_rule_channels_by_format, NULL,
-                      SNDRV_PCM_HW_PARAM_FORMAT, -1);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The rule function is called when an application sets the PCM
-	format, and it refines the number of channels accordingly.
-        But an application may set the number of channels before
-	setting the format. Thus you also need to define the inverse rule:
-
-       <example>
-	 <title>Example of Hardware Constraints for Formats</title>
-	 <programlisting>
-<![CDATA[
-  static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
-                                        struct snd_pcm_hw_rule *rule)
-  {
-          struct snd_interval *c = hw_param_interval(params,
-                SNDRV_PCM_HW_PARAM_CHANNELS);
-          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
-          struct snd_mask fmt;
-
-          snd_mask_any(&fmt);    /* Init the struct */
-          if (c->min < 2) {
-                  fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
-                  return snd_mask_refine(f, &fmt);
-          }
-          return 0;
-  }
-]]>
-          </programlisting>
-        </example>
-      </para>
-
-      <para>
-      ...and in the open callback:
-       <informalexample>
-	 <programlisting>
-<![CDATA[
-  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
-                      hw_rule_format_by_channels, NULL,
-                      SNDRV_PCM_HW_PARAM_CHANNELS, -1);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        I won't give more details here, rather I
-        would like to say, <quote>Luke, use the source.</quote>
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Control Interface  -->
-<!-- ****************************************************** -->
-  <chapter id="control-interface">
-    <title>Control Interface</title>
-
-    <section id="control-interface-general">
-      <title>General</title>
-      <para>
-        The control interface is used widely for many switches,
-      sliders, etc. which are accessed from user-space. Its most
-      important use is the mixer interface. In other words, since ALSA
-      0.9.x, all the mixer stuff is implemented on the control kernel API.
-      </para>
-
-      <para>
-        ALSA has a well-defined AC97 control module. If your chip
-      supports only the AC97 and nothing else, you can skip this
-      section. 
-      </para>
-
-      <para>
-        The control API is defined in
-      <filename>&lt;sound/control.h&gt;</filename>.
-      Include this file if you want to add your own controls.
-      </para>
-    </section>
-
-    <section id="control-interface-definition">
-      <title>Definition of Controls</title>
-      <para>
-        To create a new control, you need to define the
-	following three
-      callbacks: <structfield>info</structfield>,
-      <structfield>get</structfield> and
-      <structfield>put</structfield>. Then, define a
-      struct <structname>snd_kcontrol_new</structname> record, such as: 
-
-        <example>
-	  <title>Definition of a Control</title>
-          <programlisting>
-<![CDATA[
-  static struct snd_kcontrol_new my_control = {
-          .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
-          .name = "PCM Playback Switch",
-          .index = 0,
-          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
-          .private_value = 0xffff,
-          .info = my_control_info,
-          .get = my_control_get,
-          .put = my_control_put
-  };
-]]>
-          </programlisting>
-        </example>
-      </para>
-
-      <para>
-        The <structfield>iface</structfield> field specifies the control
-      type, <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>, which
-      is usually <constant>MIXER</constant>.
-      Use <constant>CARD</constant> for global controls that are not
-      logically part of the mixer.
-      If the control is closely associated with some specific device on
-      the sound card, use <constant>HWDEP</constant>,
-      <constant>PCM</constant>, <constant>RAWMIDI</constant>,
-      <constant>TIMER</constant>, or <constant>SEQUENCER</constant>, and
-      specify the device number with the
-      <structfield>device</structfield> and
-      <structfield>subdevice</structfield> fields.
-      </para>
-
-      <para>
-        The <structfield>name</structfield> is the name identifier
-      string. Since ALSA 0.9.x, the control name is very important,
-      because its role is classified from its name. There are
-      pre-defined standard control names. The details are described in
-      the <link linkend="control-interface-control-names"><citetitle>
-      Control Names</citetitle></link> subsection.
-      </para>
-
-      <para>
-        The <structfield>index</structfield> field holds the index number
-      of this control. If there are several different controls with
-      the same name, they can be distinguished by the index
-      number. This is the case when 
-      several codecs exist on the card. If the index is zero, you can
-      omit the definition above. 
-      </para>
-
-      <para>
-        The <structfield>access</structfield> field contains the access
-      type of this control. Give the combination of bit masks,
-      <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there.
-      The details will be explained in
-      the <link linkend="control-interface-access-flags"><citetitle>
-      Access Flags</citetitle></link> subsection.
-      </para>
-
-      <para>
-        The <structfield>private_value</structfield> field contains
-      an arbitrary long integer value for this record. When using
-      the generic <structfield>info</structfield>,
-      <structfield>get</structfield> and
-      <structfield>put</structfield> callbacks, you can pass a value 
-      through this field. If several small numbers are necessary, you can
-      combine them in bitwise. Or, it's possible to give a pointer
-      (casted to unsigned long) of some record to this field, too. 
-      </para>
-
-      <para>
-      The <structfield>tlv</structfield> field can be used to provide
-      metadata about the control; see the
-      <link linkend="control-interface-tlv">
-      <citetitle>Metadata</citetitle></link> subsection.
-      </para>
-
-      <para>
-        The other three are
-	<link linkend="control-interface-callbacks"><citetitle>
-	callback functions</citetitle></link>.
-      </para>
-    </section>
-
-    <section id="control-interface-control-names">
-      <title>Control Names</title>
-      <para>
-        There are some standards to define the control names. A
-      control is usually defined from the three parts as
-      <quote>SOURCE DIRECTION FUNCTION</quote>. 
-      </para>
-
-      <para>
-        The first, <constant>SOURCE</constant>, specifies the source
-      of the control, and is a string such as <quote>Master</quote>,
-      <quote>PCM</quote>, <quote>CD</quote> and
-      <quote>Line</quote>. There are many pre-defined sources. 
-      </para>
-
-      <para>
-        The second, <constant>DIRECTION</constant>, is one of the
-      following strings according to the direction of the control:
-      <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass
-      Playback</quote> and <quote>Bypass Capture</quote>. Or, it can
-      be omitted, meaning both playback and capture directions. 
-      </para>
-
-      <para>
-        The third, <constant>FUNCTION</constant>, is one of the
-      following strings according to the function of the control:
-      <quote>Switch</quote>, <quote>Volume</quote> and
-      <quote>Route</quote>. 
-      </para>
-
-      <para>
-        The example of control names are, thus, <quote>Master Capture
-      Switch</quote> or <quote>PCM Playback Volume</quote>. 
-      </para>
-
-      <para>
-        There are some exceptions:
-      </para>
-
-      <section id="control-interface-control-names-global">
-        <title>Global capture and playback</title>
-        <para>
-          <quote>Capture Source</quote>, <quote>Capture Switch</quote>
-        and <quote>Capture Volume</quote> are used for the global
-        capture (input) source, switch and volume. Similarly,
-        <quote>Playback Switch</quote> and <quote>Playback
-        Volume</quote> are used for the global output gain switch and
-        volume. 
-        </para>
-      </section>
-
-      <section id="control-interface-control-names-tone">
-        <title>Tone-controls</title>
-        <para>
-          tone-control switch and volumes are specified like
-        <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control -
-        Switch</quote>, <quote>Tone Control - Bass</quote>,
-        <quote>Tone Control - Center</quote>.  
-        </para>
-      </section>
-
-      <section id="control-interface-control-names-3d">
-        <title>3D controls</title>
-        <para>
-          3D-control switches and volumes are specified like <quote>3D
-        Control - XXX</quote>, e.g. <quote>3D Control -
-        Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D
-        Control - Space</quote>. 
-        </para>
-      </section>
-
-      <section id="control-interface-control-names-mic">
-        <title>Mic boost</title>
-        <para>
-          Mic-boost switch is set as <quote>Mic Boost</quote> or
-        <quote>Mic Boost (6dB)</quote>. 
-        </para>
-
-        <para>
-          More precise information can be found in
-        <filename>Documentation/sound/alsa/ControlNames.txt</filename>.
-        </para>
-      </section>
-    </section>
-
-    <section id="control-interface-access-flags">
-      <title>Access Flags</title>
-
-      <para>
-      The access flag is the bitmask which specifies the access type
-      of the given control.  The default access type is
-      <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>, 
-      which means both read and write are allowed to this control.
-      When the access flag is omitted (i.e. = 0), it is
-      considered as <constant>READWRITE</constant> access as default. 
-      </para>
-
-      <para>
-      When the control is read-only, pass
-      <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead.
-      In this case, you don't have to define
-      the <structfield>put</structfield> callback.
-      Similarly, when the control is write-only (although it's a rare
-      case), you can use the <constant>WRITE</constant> flag instead, and
-      you don't need the <structfield>get</structfield> callback.
-      </para>
-
-      <para>
-      If the control value changes frequently (e.g. the VU meter),
-      <constant>VOLATILE</constant> flag should be given.  This means
-      that the control may be changed without
-      <link linkend="control-interface-change-notification"><citetitle>
-      notification</citetitle></link>. Applications should poll such
-      a control constantly.
-      </para>
-
-      <para>
-      When the control is inactive, set
-      the <constant>INACTIVE</constant> flag, too.
-      There are <constant>LOCK</constant> and
-      <constant>OWNER</constant> flags to change the write
-      permissions.
-      </para>
-
-    </section>
-
-    <section id="control-interface-callbacks">
-      <title>Callbacks</title>
-
-      <section id="control-interface-callbacks-info">
-        <title>info callback</title>
-        <para>
-          The <structfield>info</structfield> callback is used to get
-        detailed information on this control. This must store the
-        values of the given struct <structname>snd_ctl_elem_info</structname>
-        object. For example, for a boolean control with a single
-        element: 
-
-          <example>
-	    <title>Example of info callback</title>
-            <programlisting>
-<![CDATA[
-  static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
-                          struct snd_ctl_elem_info *uinfo)
-  {
-          uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
-          uinfo->count = 1;
-          uinfo->value.integer.min = 0;
-          uinfo->value.integer.max = 1;
-          return 0;
-  }
-]]>
-            </programlisting>
-          </example>
-        </para>
-
-        <para>
-          The <structfield>type</structfield> field specifies the type
-        of the control. There are <constant>BOOLEAN</constant>,
-        <constant>INTEGER</constant>, <constant>ENUMERATED</constant>,
-        <constant>BYTES</constant>, <constant>IEC958</constant> and
-        <constant>INTEGER64</constant>. The
-        <structfield>count</structfield> field specifies the 
-        number of elements in this control. For example, a stereo
-        volume would have count = 2. The
-        <structfield>value</structfield> field is a union, and 
-        the values stored are depending on the type. The boolean and
-        integer types are identical. 
-        </para>
-
-        <para>
-          The enumerated type is a bit different from others.  You'll
-          need to set the string for the currently given item index. 
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
-                          struct snd_ctl_elem_info *uinfo)
-  {
-          static char *texts[4] = {
-                  "First", "Second", "Third", "Fourth"
-          };
-          uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
-          uinfo->count = 1;
-          uinfo->value.enumerated.items = 4;
-          if (uinfo->value.enumerated.item > 3)
-                  uinfo->value.enumerated.item = 3;
-          strcpy(uinfo->value.enumerated.name,
-                 texts[uinfo->value.enumerated.item]);
-          return 0;
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-	  The above callback can be simplified with a helper function,
-	  <function>snd_ctl_enum_info</function>.  The final code
-	  looks like below.
-	  (You can pass ARRAY_SIZE(texts) instead of 4 in the third
-	   argument; it's a matter of taste.)
-
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
-                          struct snd_ctl_elem_info *uinfo)
-  {
-          static char *texts[4] = {
-                  "First", "Second", "Third", "Fourth"
-          };
-          return snd_ctl_enum_info(uinfo, 1, 4, texts);
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-	  Some common info callbacks are available for your convenience:
-	<function>snd_ctl_boolean_mono_info()</function> and
-	<function>snd_ctl_boolean_stereo_info()</function>.
-	Obviously, the former is an info callback for a mono channel
-	boolean item, just like <function>snd_myctl_mono_info</function>
-	above, and the latter is for a stereo channel boolean item.
-	</para>
-
-      </section>
-
-      <section id="control-interface-callbacks-get">
-        <title>get callback</title>
-
-        <para>
-          This callback is used to read the current value of the
-        control and to return to user-space. 
-        </para>
-
-        <para>
-          For example,
-
-          <example>
-	    <title>Example of get callback</title>
-            <programlisting>
-<![CDATA[
-  static int snd_myctl_get(struct snd_kcontrol *kcontrol,
-                           struct snd_ctl_elem_value *ucontrol)
-  {
-          struct mychip *chip = snd_kcontrol_chip(kcontrol);
-          ucontrol->value.integer.value[0] = get_some_value(chip);
-          return 0;
-  }
-]]>
-            </programlisting>
-          </example>
-        </para>
-
-        <para>
-	The <structfield>value</structfield> field depends on 
-        the type of control as well as on the info callback.  For example,
-	the sb driver uses this field to store the register offset,
-        the bit-shift and the bit-mask.  The
-        <structfield>private_value</structfield> field is set as follows:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  .private_value = reg | (shift << 16) | (mask << 24)
-]]>
-            </programlisting>
-          </informalexample>
-	and is retrieved in callbacks like
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
-                                    struct snd_ctl_elem_value *ucontrol)
-  {
-          int reg = kcontrol->private_value & 0xff;
-          int shift = (kcontrol->private_value >> 16) & 0xff;
-          int mask = (kcontrol->private_value >> 24) & 0xff;
-          ....
-  }
-]]>
-            </programlisting>
-          </informalexample>
-	</para>
-
-	<para>
-	In the <structfield>get</structfield> callback,
-	you have to fill all the elements if the
-        control has more than one elements,
-        i.e. <structfield>count</structfield> &gt; 1.
-	In the example above, we filled only one element
-        (<structfield>value.integer.value[0]</structfield>) since it's
-        assumed as <structfield>count</structfield> = 1.
-        </para>
-      </section>
-
-      <section id="control-interface-callbacks-put">
-        <title>put callback</title>
-
-        <para>
-          This callback is used to write a value from user-space.
-        </para>
-
-        <para>
-          For example,
-
-          <example>
-	    <title>Example of put callback</title>
-            <programlisting>
-<![CDATA[
-  static int snd_myctl_put(struct snd_kcontrol *kcontrol,
-                           struct snd_ctl_elem_value *ucontrol)
-  {
-          struct mychip *chip = snd_kcontrol_chip(kcontrol);
-          int changed = 0;
-          if (chip->current_value !=
-               ucontrol->value.integer.value[0]) {
-                  change_current_value(chip,
-                              ucontrol->value.integer.value[0]);
-                  changed = 1;
-          }
-          return changed;
-  }
-]]>
-            </programlisting>
-          </example>
-
-          As seen above, you have to return 1 if the value is
-        changed. If the value is not changed, return 0 instead. 
-	If any fatal error happens, return a negative error code as
-        usual.
-        </para>
-
-        <para>
-	As in the <structfield>get</structfield> callback,
-	when the control has more than one elements,
-	all elements must be evaluated in this callback, too.
-        </para>
-      </section>
-
-      <section id="control-interface-callbacks-all">
-        <title>Callbacks are not atomic</title>
-        <para>
-          All these three callbacks are basically not atomic.
-        </para>
-      </section>
-    </section>
-
-    <section id="control-interface-constructor">
-      <title>Constructor</title>
-      <para>
-        When everything is ready, finally we can create a new
-      control. To create a control, there are two functions to be
-      called, <function>snd_ctl_new1()</function> and
-      <function>snd_ctl_add()</function>. 
-      </para>
-
-      <para>
-        In the simplest way, you can do like this:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
-  if (err < 0)
-          return err;
-]]>
-          </programlisting>
-        </informalexample>
-
-        where <parameter>my_control</parameter> is the
-      struct <structname>snd_kcontrol_new</structname> object defined above, and chip
-      is the object pointer to be passed to
-      kcontrol-&gt;private_data 
-      which can be referred to in callbacks. 
-      </para>
-
-      <para>
-        <function>snd_ctl_new1()</function> allocates a new
-      <structname>snd_kcontrol</structname> instance,
-      and <function>snd_ctl_add</function> assigns the given
-      control component to the card. 
-      </para>
-    </section>
-
-    <section id="control-interface-change-notification">
-      <title>Change Notification</title>
-      <para>
-        If you need to change and update a control in the interrupt
-      routine, you can call <function>snd_ctl_notify()</function>. For
-      example, 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
-]]>
-          </programlisting>
-        </informalexample>
-
-        This function takes the card pointer, the event-mask, and the
-      control id pointer for the notification. The event-mask
-      specifies the types of notification, for example, in the above
-      example, the change of control values is notified.
-      The id pointer is the pointer of struct <structname>snd_ctl_elem_id</structname>
-      to be notified.
-      You can find some examples in <filename>es1938.c</filename> or
-      <filename>es1968.c</filename> for hardware volume interrupts. 
-      </para>
-    </section>
-
-    <section id="control-interface-tlv">
-      <title>Metadata</title>
-      <para>
-      To provide information about the dB values of a mixer control, use
-      on of the <constant>DECLARE_TLV_xxx</constant> macros from
-      <filename>&lt;sound/tlv.h&gt;</filename> to define a variable
-      containing this information, set the<structfield>tlv.p
-      </structfield> field to point to this variable, and include the
-      <constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the
-      <structfield>access</structfield> field; like this:
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
-
-  static struct snd_kcontrol_new my_control = {
-          ...
-          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
-                    SNDRV_CTL_ELEM_ACCESS_TLV_READ,
-          ...
-          .tlv.p = db_scale_my_control,
-  };
-]]>
-        </programlisting>
-      </informalexample>
-      </para>
-
-      <para>
-      The <function>DECLARE_TLV_DB_SCALE</function> macro defines
-      information about a mixer control where each step in the control's
-      value changes the dB value by a constant dB amount.
-      The first parameter is the name of the variable to be defined.
-      The second parameter is the minimum value, in units of 0.01 dB.
-      The third parameter is the step size, in units of 0.01 dB.
-      Set the fourth parameter to 1 if the minimum value actually mutes
-      the control.
-      </para>
-
-      <para>
-      The <function>DECLARE_TLV_DB_LINEAR</function> macro defines
-      information about a mixer control where the control's value affects
-      the output linearly.
-      The first parameter is the name of the variable to be defined.
-      The second parameter is the minimum value, in units of 0.01 dB.
-      The third parameter is the maximum value, in units of 0.01 dB.
-      If the minimum value mutes the control, set the second parameter to
-      <constant>TLV_DB_GAIN_MUTE</constant>.
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- API for AC97 Codec  -->
-<!-- ****************************************************** -->
-  <chapter id="api-ac97">
-    <title>API for AC97 Codec</title>
-
-    <section>
-      <title>General</title>
-      <para>
-        The ALSA AC97 codec layer is a well-defined one, and you don't
-      have to write much code to control it. Only low-level control
-      routines are necessary. The AC97 codec API is defined in
-      <filename>&lt;sound/ac97_codec.h&gt;</filename>. 
-      </para>
-    </section>
-
-    <section id="api-ac97-example">
-      <title>Full Code Example</title>
-      <para>
-          <example>
-	    <title>Example of AC97 Interface</title>
-            <programlisting>
-<![CDATA[
-  struct mychip {
-          ....
-          struct snd_ac97 *ac97;
-          ....
-  };
-
-  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
-                                             unsigned short reg)
-  {
-          struct mychip *chip = ac97->private_data;
-          ....
-          /* read a register value here from the codec */
-          return the_register_value;
-  }
-
-  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
-                                   unsigned short reg, unsigned short val)
-  {
-          struct mychip *chip = ac97->private_data;
-          ....
-          /* write the given register value to the codec */
-  }
-
-  static int snd_mychip_ac97(struct mychip *chip)
-  {
-          struct snd_ac97_bus *bus;
-          struct snd_ac97_template ac97;
-          int err;
-          static struct snd_ac97_bus_ops ops = {
-                  .write = snd_mychip_ac97_write,
-                  .read = snd_mychip_ac97_read,
-          };
-
-          err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
-          if (err < 0)
-                  return err;
-          memset(&ac97, 0, sizeof(ac97));
-          ac97.private_data = chip;
-          return snd_ac97_mixer(bus, &ac97, &chip->ac97);
-  }
-
-]]>
-          </programlisting>
-        </example>
-      </para>
-    </section>
-
-    <section id="api-ac97-constructor">
-      <title>Constructor</title>
-      <para>
-        To create an ac97 instance, first call <function>snd_ac97_bus</function>
-      with an <type>ac97_bus_ops_t</type> record with callback functions.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_ac97_bus *bus;
-  static struct snd_ac97_bus_ops ops = {
-        .write = snd_mychip_ac97_write,
-        .read = snd_mychip_ac97_read,
-  };
-
-  snd_ac97_bus(card, 0, &ops, NULL, &pbus);
-]]>
-          </programlisting>
-        </informalexample>
-
-      The bus record is shared among all belonging ac97 instances.
-      </para>
-
-      <para>
-      And then call <function>snd_ac97_mixer()</function> with an
-      struct <structname>snd_ac97_template</structname>
-      record together with the bus pointer created above.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_ac97_template ac97;
-  int err;
-
-  memset(&ac97, 0, sizeof(ac97));
-  ac97.private_data = chip;
-  snd_ac97_mixer(bus, &ac97, &chip->ac97);
-]]>
-          </programlisting>
-        </informalexample>
-
-        where chip-&gt;ac97 is a pointer to a newly created
-        <type>ac97_t</type> instance.
-        In this case, the chip pointer is set as the private data, so that
-        the read/write callback functions can refer to this chip instance.
-        This instance is not necessarily stored in the chip
-	record.  If you need to change the register values from the
-        driver, or need the suspend/resume of ac97 codecs, keep this
-        pointer to pass to the corresponding functions.
-      </para>
-    </section>
-
-    <section id="api-ac97-callbacks">
-      <title>Callbacks</title>
-      <para>
-        The standard callbacks are <structfield>read</structfield> and
-      <structfield>write</structfield>. Obviously they 
-      correspond to the functions for read and write accesses to the
-      hardware low-level codes. 
-      </para>
-
-      <para>
-        The <structfield>read</structfield> callback returns the
-        register value specified in the argument. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
-                                             unsigned short reg)
-  {
-          struct mychip *chip = ac97->private_data;
-          ....
-          return the_register_value;
-  }
-]]>
-          </programlisting>
-        </informalexample>
-
-        Here, the chip can be cast from ac97-&gt;private_data.
-      </para>
-
-      <para>
-        Meanwhile, the <structfield>write</structfield> callback is
-        used to set the register value. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
-                       unsigned short reg, unsigned short val)
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-      These callbacks are non-atomic like the control API callbacks.
-      </para>
-
-      <para>
-        There are also other callbacks:
-      <structfield>reset</structfield>,
-      <structfield>wait</structfield> and
-      <structfield>init</structfield>. 
-      </para>
-
-      <para>
-        The <structfield>reset</structfield> callback is used to reset
-      the codec. If the chip requires a special kind of reset, you can
-      define this callback. 
-      </para>
-
-      <para>
-        The <structfield>wait</structfield> callback is used to
-      add some waiting time in the standard initialization of the codec. If the
-      chip requires the extra waiting time, define this callback. 
-      </para>
-
-      <para>
-        The <structfield>init</structfield> callback is used for
-      additional initialization of the codec.
-      </para>
-    </section>
-
-    <section id="api-ac97-updating-registers">
-      <title>Updating Registers in The Driver</title>
-      <para>
-        If you need to access to the codec from the driver, you can
-      call the following functions:
-      <function>snd_ac97_write()</function>,
-      <function>snd_ac97_read()</function>,
-      <function>snd_ac97_update()</function> and
-      <function>snd_ac97_update_bits()</function>. 
-      </para>
-
-      <para>
-        Both <function>snd_ac97_write()</function> and
-        <function>snd_ac97_update()</function> functions are used to
-        set a value to the given register
-        (<constant>AC97_XXX</constant>). The difference between them is
-        that <function>snd_ac97_update()</function> doesn't write a
-        value if the given value has been already set, while
-        <function>snd_ac97_write()</function> always rewrites the
-        value. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_ac97_write(ac97, AC97_MASTER, 0x8080);
-  snd_ac97_update(ac97, AC97_MASTER, 0x8080);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        <function>snd_ac97_read()</function> is used to read the value
-        of the given register. For example, 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  value = snd_ac97_read(ac97, AC97_MASTER);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        <function>snd_ac97_update_bits()</function> is used to update
-        some bits in the given register.  
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_ac97_update_bits(ac97, reg, mask, value);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        Also, there is a function to change the sample rate (of a
-        given register such as
-        <constant>AC97_PCM_FRONT_DAC_RATE</constant>) when VRA or
-        DRA is supported by the codec:
-        <function>snd_ac97_set_rate()</function>. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The following registers are available to set the rate:
-      <constant>AC97_PCM_MIC_ADC_RATE</constant>,
-      <constant>AC97_PCM_FRONT_DAC_RATE</constant>,
-      <constant>AC97_PCM_LR_ADC_RATE</constant>,
-      <constant>AC97_SPDIF</constant>. When
-      <constant>AC97_SPDIF</constant> is specified, the register is
-      not really changed but the corresponding IEC958 status bits will
-      be updated. 
-      </para>
-    </section>
-
-    <section id="api-ac97-clock-adjustment">
-      <title>Clock Adjustment</title>
-      <para>
-        In some chips, the clock of the codec isn't 48000 but using a
-      PCI clock (to save a quartz!). In this case, change the field
-      bus-&gt;clock to the corresponding
-      value. For example, intel8x0 
-      and es1968 drivers have their own function to read from the clock.
-      </para>
-    </section>
-
-    <section id="api-ac97-proc-files">
-      <title>Proc Files</title>
-      <para>
-        The ALSA AC97 interface will create a proc file such as
-      <filename>/proc/asound/card0/codec97#0/ac97#0-0</filename> and
-      <filename>ac97#0-0+regs</filename>. You can refer to these files to
-      see the current status and registers of the codec. 
-      </para>
-    </section>
-
-    <section id="api-ac97-multiple-codecs">
-      <title>Multiple Codecs</title>
-      <para>
-        When there are several codecs on the same card, you need to
-      call <function>snd_ac97_mixer()</function> multiple times with
-      ac97.num=1 or greater. The <structfield>num</structfield> field
-      specifies the codec number. 
-      </para>
-
-      <para>
-        If you set up multiple codecs, you either need to write
-      different callbacks for each codec or check
-      ac97-&gt;num in the callback routines. 
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- MIDI (MPU401-UART) Interface  -->
-<!-- ****************************************************** -->
-  <chapter id="midi-interface">
-    <title>MIDI (MPU401-UART) Interface</title>
-
-    <section id="midi-interface-general">
-      <title>General</title>
-      <para>
-        Many soundcards have built-in MIDI (MPU401-UART)
-      interfaces. When the soundcard supports the standard MPU401-UART
-      interface, most likely you can use the ALSA MPU401-UART API. The
-      MPU401-UART API is defined in
-      <filename>&lt;sound/mpu401.h&gt;</filename>. 
-      </para>
-
-      <para>
-        Some soundchips have a similar but slightly different
-      implementation of mpu401 stuff. For example, emu10k1 has its own
-      mpu401 routines. 
-      </para>
-    </section>
-
-    <section id="midi-interface-constructor">
-      <title>Constructor</title>
-      <para>
-        To create a rawmidi object, call
-      <function>snd_mpu401_uart_new()</function>. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_rawmidi *rmidi;
-  snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
-                      irq, &rmidi);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The first argument is the card pointer, and the second is the
-      index of this component. You can create up to 8 rawmidi
-      devices. 
-      </para>
-
-      <para>
-        The third argument is the type of the hardware,
-      <constant>MPU401_HW_XXX</constant>. If it's not a special one,
-      you can use <constant>MPU401_HW_MPU401</constant>. 
-      </para>
-
-      <para>
-        The 4th argument is the I/O port address. Many
-      backward-compatible MPU401 have an I/O port such as 0x330. Or, it
-      might be a part of its own PCI I/O region. It depends on the
-      chip design. 
-      </para>
-
-      <para>
-	The 5th argument is a bitflag for additional information.
-        When the I/O port address above is part of the PCI I/O
-      region, the MPU401 I/O port might have been already allocated
-      (reserved) by the driver itself. In such a case, pass a bit flag
-      <constant>MPU401_INFO_INTEGRATED</constant>,
-      and the mpu401-uart layer will allocate the I/O ports by itself. 
-      </para>
-
-	<para>
-	When the controller supports only the input or output MIDI stream,
-	pass the <constant>MPU401_INFO_INPUT</constant> or
-	<constant>MPU401_INFO_OUTPUT</constant> bitflag, respectively.
-	Then the rawmidi instance is created as a single stream.
-	</para>
-
-	<para>
-	<constant>MPU401_INFO_MMIO</constant> bitflag is used to change
-	the access method to MMIO (via readb and writeb) instead of
-	iob and outb. In this case, you have to pass the iomapped address
-	to <function>snd_mpu401_uart_new()</function>.
-	</para>
-
-	<para>
-	When <constant>MPU401_INFO_TX_IRQ</constant> is set, the output
-	stream isn't checked in the default interrupt handler.  The driver
-	needs to call <function>snd_mpu401_uart_interrupt_tx()</function>
-	by itself to start processing the output stream in the irq handler.
-	</para>
-
-	<para>
-	If the MPU-401 interface shares its interrupt with the other logical
-	devices on the card, set <constant>MPU401_INFO_IRQ_HOOK</constant>
-	(see <link linkend="midi-interface-interrupt-handler"><citetitle>
-	below</citetitle></link>).
-	</para>
-
-      <para>
-        Usually, the port address corresponds to the command port and
-        port + 1 corresponds to the data port. If not, you may change
-        the <structfield>cport</structfield> field of
-        struct <structname>snd_mpu401</structname> manually 
-        afterward. However, <structname>snd_mpu401</structname> pointer is not
-        returned explicitly by
-        <function>snd_mpu401_uart_new()</function>. You need to cast
-        rmidi-&gt;private_data to
-        <structname>snd_mpu401</structname> explicitly, 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_mpu401 *mpu;
-  mpu = rmidi->private_data;
-]]>
-          </programlisting>
-        </informalexample>
-
-        and reset the cport as you like:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  mpu->cport = my_own_control_port;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-	The 6th argument specifies the ISA irq number that will be
-	allocated.  If no interrupt is to be allocated (because your
-	code is already allocating a shared interrupt, or because the
-	device does not use interrupts), pass -1 instead.
-	For a MPU-401 device without an interrupt, a polling timer
-	will be used instead.
-      </para>
-    </section>
-
-    <section id="midi-interface-interrupt-handler">
-      <title>Interrupt Handler</title>
-      <para>
-        When the interrupt is allocated in
-      <function>snd_mpu401_uart_new()</function>, an exclusive ISA
-      interrupt handler is automatically used, hence you don't have
-      anything else to do than creating the mpu401 stuff.  Otherwise, you
-      have to set <constant>MPU401_INFO_IRQ_HOOK</constant>, and call
-      <function>snd_mpu401_uart_interrupt()</function> explicitly from your
-      own interrupt handler when it has determined that a UART interrupt
-      has occurred.
-      </para>
-
-      <para>
-        In this case, you need to pass the private_data of the
-        returned rawmidi object from
-        <function>snd_mpu401_uart_new()</function> as the second
-        argument of <function>snd_mpu401_uart_interrupt()</function>. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- RawMIDI Interface  -->
-<!-- ****************************************************** -->
-  <chapter id="rawmidi-interface">
-    <title>RawMIDI Interface</title>
-
-    <section id="rawmidi-interface-overview">
-      <title>Overview</title>
-
-      <para>
-      The raw MIDI interface is used for hardware MIDI ports that can
-      be accessed as a byte stream.  It is not used for synthesizer
-      chips that do not directly understand MIDI.
-      </para>
-
-      <para>
-      ALSA handles file and buffer management.  All you have to do is
-      to write some code to move data between the buffer and the
-      hardware.
-      </para>
-
-      <para>
-      The rawmidi API is defined in
-      <filename>&lt;sound/rawmidi.h&gt;</filename>.
-      </para>
-    </section>
-
-    <section id="rawmidi-interface-constructor">
-      <title>Constructor</title>
-
-      <para>
-      To create a rawmidi device, call the
-      <function>snd_rawmidi_new</function> function:
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_rawmidi *rmidi;
-  err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
-  if (err < 0)
-          return err;
-  rmidi->private_data = chip;
-  strcpy(rmidi->name, "My MIDI");
-  rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
-                      SNDRV_RAWMIDI_INFO_INPUT |
-                      SNDRV_RAWMIDI_INFO_DUPLEX;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-      The first argument is the card pointer, the second argument is
-      the ID string.
-      </para>
-
-      <para>
-      The third argument is the index of this component.  You can
-      create up to 8 rawmidi devices.
-      </para>
-
-      <para>
-      The fourth and fifth arguments are the number of output and
-      input substreams, respectively, of this device (a substream is
-      the equivalent of a MIDI port).
-      </para>
-
-      <para>
-      Set the <structfield>info_flags</structfield> field to specify
-      the capabilities of the device.
-      Set <constant>SNDRV_RAWMIDI_INFO_OUTPUT</constant> if there is
-      at least one output port,
-      <constant>SNDRV_RAWMIDI_INFO_INPUT</constant> if there is at
-      least one input port,
-      and <constant>SNDRV_RAWMIDI_INFO_DUPLEX</constant> if the device
-      can handle output and input at the same time.
-      </para>
-
-      <para>
-      After the rawmidi device is created, you need to set the
-      operators (callbacks) for each substream.  There are helper
-      functions to set the operators for all the substreams of a device:
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
-  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-      The operators are usually defined like this:
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static struct snd_rawmidi_ops snd_mymidi_output_ops = {
-          .open =    snd_mymidi_output_open,
-          .close =   snd_mymidi_output_close,
-          .trigger = snd_mymidi_output_trigger,
-  };
-]]>
-          </programlisting>
-        </informalexample>
-      These callbacks are explained in the <link
-      linkend="rawmidi-interface-callbacks"><citetitle>Callbacks</citetitle></link>
-      section.
-      </para>
-
-      <para>
-      If there are more than one substream, you should give a
-      unique name to each of them:
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_rawmidi_substream *substream;
-  list_for_each_entry(substream,
-                      &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
-                      list {
-          sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
-  }
-  /* same for SNDRV_RAWMIDI_STREAM_INPUT */
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-    </section>
-
-    <section id="rawmidi-interface-callbacks">
-      <title>Callbacks</title>
-
-      <para>
-      In all the callbacks, the private data that you've set for the
-      rawmidi device can be accessed as
-      substream-&gt;rmidi-&gt;private_data.
-      <!-- <code> isn't available before DocBook 4.3 -->
-      </para>
-
-      <para>
-      If there is more than one port, your callbacks can determine the
-      port index from the struct snd_rawmidi_substream data passed to each
-      callback:
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_rawmidi_substream *substream;
-  int index = substream->number;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <section id="rawmidi-interface-op-open">
-      <title><function>open</function> callback</title>
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int snd_xxx_open(struct snd_rawmidi_substream *substream);
-]]>
-          </programlisting>
-        </informalexample>
-
-        <para>
-        This is called when a substream is opened.
-        You can initialize the hardware here, but you shouldn't
-        start transmitting/receiving data yet.
-        </para>
-      </section>
-
-      <section id="rawmidi-interface-op-close">
-      <title><function>close</function> callback</title>
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int snd_xxx_close(struct snd_rawmidi_substream *substream);
-]]>
-          </programlisting>
-        </informalexample>
-
-        <para>
-        Guess what.
-        </para>
-
-        <para>
-        The <function>open</function> and <function>close</function>
-        callbacks of a rawmidi device are serialized with a mutex,
-        and can sleep.
-        </para>
-      </section>
-
-      <section id="rawmidi-interface-op-trigger-out">
-      <title><function>trigger</function> callback for output
-      substreams</title>
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
-]]>
-          </programlisting>
-        </informalexample>
-
-        <para>
-        This is called with a nonzero <parameter>up</parameter>
-        parameter when there is some data in the substream buffer that
-        must be transmitted.
-        </para>
-
-        <para>
-        To read data from the buffer, call
-        <function>snd_rawmidi_transmit_peek</function>.  It will
-        return the number of bytes that have been read; this will be
-        less than the number of bytes requested when there are no more
-        data in the buffer.
-        After the data have been transmitted successfully, call
-        <function>snd_rawmidi_transmit_ack</function> to remove the
-        data from the substream buffer:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  unsigned char data;
-  while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
-          if (snd_mychip_try_to_transmit(data))
-                  snd_rawmidi_transmit_ack(substream, 1);
-          else
-                  break; /* hardware FIFO full */
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-        If you know beforehand that the hardware will accept data, you
-        can use the <function>snd_rawmidi_transmit</function> function
-        which reads some data and removes them from the buffer at once:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  while (snd_mychip_transmit_possible()) {
-          unsigned char data;
-          if (snd_rawmidi_transmit(substream, &data, 1) != 1)
-                  break; /* no more data */
-          snd_mychip_transmit(data);
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-
-        <para>
-        If you know beforehand how many bytes you can accept, you can
-        use a buffer size greater than one with the
-        <function>snd_rawmidi_transmit*</function> functions.
-        </para>
-
-        <para>
-        The <function>trigger</function> callback must not sleep.  If
-        the hardware FIFO is full before the substream buffer has been
-        emptied, you have to continue transmitting data later, either
-        in an interrupt handler, or with a timer if the hardware
-        doesn't have a MIDI transmit interrupt.
-        </para>
-
-        <para>
-        The <function>trigger</function> callback is called with a
-        zero <parameter>up</parameter> parameter when the transmission
-        of data should be aborted.
-        </para>
-      </section>
-
-      <section id="rawmidi-interface-op-trigger-in">
-      <title><function>trigger</function> callback for input
-      substreams</title>
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
-]]>
-          </programlisting>
-        </informalexample>
-
-        <para>
-        This is called with a nonzero <parameter>up</parameter>
-        parameter to enable receiving data, or with a zero
-        <parameter>up</parameter> parameter do disable receiving data.
-        </para>
-
-        <para>
-        The <function>trigger</function> callback must not sleep; the
-        actual reading of data from the device is usually done in an
-        interrupt handler.
-        </para>
-
-        <para>
-        When data reception is enabled, your interrupt handler should
-        call <function>snd_rawmidi_receive</function> for all received
-        data:
-          <informalexample>
-            <programlisting>
-<![CDATA[
-  void snd_mychip_midi_interrupt(...)
-  {
-          while (mychip_midi_available()) {
-                  unsigned char data;
-                  data = mychip_midi_read();
-                  snd_rawmidi_receive(substream, &data, 1);
-          }
-  }
-]]>
-            </programlisting>
-          </informalexample>
-        </para>
-      </section>
-
-      <section id="rawmidi-interface-op-drain">
-      <title><function>drain</function> callback</title>
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
-]]>
-          </programlisting>
-        </informalexample>
-
-        <para>
-        This is only used with output substreams.  This function should wait
-        until all data read from the substream buffer have been transmitted.
-        This ensures that the device can be closed and the driver unloaded
-        without losing data.
-        </para>
-
-        <para>
-        This callback is optional. If you do not set
-        <structfield>drain</structfield> in the struct snd_rawmidi_ops
-        structure, ALSA will simply wait for 50&nbsp;milliseconds
-        instead.
-        </para>
-      </section>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Miscellaneous Devices  -->
-<!-- ****************************************************** -->
-  <chapter id="misc-devices">
-    <title>Miscellaneous Devices</title>
-
-    <section id="misc-devices-opl3">
-      <title>FM OPL3</title>
-      <para>
-        The FM OPL3 is still used in many chips (mainly for backward
-      compatibility). ALSA has a nice OPL3 FM control layer, too. The
-      OPL3 API is defined in
-      <filename>&lt;sound/opl3.h&gt;</filename>. 
-      </para>
-
-      <para>
-        FM registers can be directly accessed through the direct-FM API,
-      defined in <filename>&lt;sound/asound_fm.h&gt;</filename>. In
-      ALSA native mode, FM registers are accessed through
-      the Hardware-Dependent Device direct-FM extension API, whereas in
-      OSS compatible mode, FM registers can be accessed with the OSS
-      direct-FM compatible API in <filename>/dev/dmfmX</filename> device. 
-      </para>
-
-      <para>
-        To create the OPL3 component, you have two functions to
-        call. The first one is a constructor for the <type>opl3_t</type>
-        instance. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_opl3 *opl3;
-  snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
-                  integrated, &opl3);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The first argument is the card pointer, the second one is the
-      left port address, and the third is the right port address. In
-      most cases, the right port is placed at the left port + 2. 
-      </para>
-
-      <para>
-        The fourth argument is the hardware type.
-      </para>
-
-      <para>
-        When the left and right ports have been already allocated by
-      the card driver, pass non-zero to the fifth argument
-      (<parameter>integrated</parameter>). Otherwise, the opl3 module will
-      allocate the specified ports by itself. 
-      </para>
-
-      <para>
-        When the accessing the hardware requires special method
-        instead of the standard I/O access, you can create opl3 instance
-        separately with <function>snd_opl3_new()</function>.
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_opl3 *opl3;
-  snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-	Then set <structfield>command</structfield>,
-	<structfield>private_data</structfield> and
-	<structfield>private_free</structfield> for the private
-	access function, the private data and the destructor.
-	The l_port and r_port are not necessarily set.  Only the
-	command must be set properly.  You can retrieve the data
-	from the opl3-&gt;private_data field.
-      </para>
-
-      <para>
-	After creating the opl3 instance via <function>snd_opl3_new()</function>,
-	call <function>snd_opl3_init()</function> to initialize the chip to the
-	proper state. Note that <function>snd_opl3_create()</function> always
-	calls it internally.
-      </para>
-
-      <para>
-        If the opl3 instance is created successfully, then create a
-        hwdep device for this opl3. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_hwdep *opl3hwdep;
-  snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The first argument is the <type>opl3_t</type> instance you
-      created, and the second is the index number, usually 0. 
-      </para>
-
-      <para>
-        The third argument is the index-offset for the sequencer
-      client assigned to the OPL3 port. When there is an MPU401-UART,
-      give 1 for here (UART always takes 0). 
-      </para>
-    </section>
-
-    <section id="misc-devices-hardware-dependent">
-      <title>Hardware-Dependent Devices</title>
-      <para>
-        Some chips need user-space access for special
-      controls or for loading the micro code. In such a case, you can
-      create a hwdep (hardware-dependent) device. The hwdep API is
-      defined in <filename>&lt;sound/hwdep.h&gt;</filename>. You can
-      find examples in opl3 driver or
-      <filename>isa/sb/sb16_csp.c</filename>. 
-      </para>
-
-      <para>
-        The creation of the <type>hwdep</type> instance is done via
-        <function>snd_hwdep_new()</function>. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_hwdep *hw;
-  snd_hwdep_new(card, "My HWDEP", 0, &hw);
-]]>
-          </programlisting>
-        </informalexample>
-
-        where the third argument is the index number.
-      </para>
-
-      <para>
-        You can then pass any pointer value to the
-        <parameter>private_data</parameter>.
-        If you assign a private data, you should define the
-        destructor, too. The destructor function is set in
-        the <structfield>private_free</structfield> field.  
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
-  hw->private_data = p;
-  hw->private_free = mydata_free;
-]]>
-          </programlisting>
-        </informalexample>
-
-        and the implementation of the destructor would be:
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static void mydata_free(struct snd_hwdep *hw)
-  {
-          struct mydata *p = hw->private_data;
-          kfree(p);
-  }
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The arbitrary file operations can be defined for this
-        instance. The file operators are defined in
-        the <parameter>ops</parameter> table. For example, assume that
-        this chip needs an ioctl. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  hw->ops.open = mydata_open;
-  hw->ops.ioctl = mydata_ioctl;
-  hw->ops.release = mydata_release;
-]]>
-          </programlisting>
-        </informalexample>
-
-        And implement the callback functions as you like.
-      </para>
-    </section>
-
-    <section id="misc-devices-IEC958">
-      <title>IEC958 (S/PDIF)</title>
-      <para>
-        Usually the controls for IEC958 devices are implemented via
-      the control interface. There is a macro to compose a name string for
-      IEC958 controls, <function>SNDRV_CTL_NAME_IEC958()</function>
-      defined in <filename>&lt;include/asound.h&gt;</filename>.  
-      </para>
-
-      <para>
-        There are some standard controls for IEC958 status bits. These
-      controls use the type <type>SNDRV_CTL_ELEM_TYPE_IEC958</type>,
-      and the size of element is fixed as 4 bytes array
-      (value.iec958.status[x]). For the <structfield>info</structfield>
-      callback, you don't specify 
-      the value field for this type (the count field must be set,
-      though). 
-      </para>
-
-      <para>
-        <quote>IEC958 Playback Con Mask</quote> is used to return the
-      bit-mask for the IEC958 status bits of consumer mode. Similarly,
-      <quote>IEC958 Playback Pro Mask</quote> returns the bitmask for
-      professional mode. They are read-only controls, and are defined
-      as MIXER controls (iface =
-      <constant>SNDRV_CTL_ELEM_IFACE_MIXER</constant>).  
-      </para>
-
-      <para>
-        Meanwhile, <quote>IEC958 Playback Default</quote> control is
-      defined for getting and setting the current default IEC958
-      bits. Note that this one is usually defined as a PCM control
-      (iface = <constant>SNDRV_CTL_ELEM_IFACE_PCM</constant>),
-      although in some places it's defined as a MIXER control. 
-      </para>
-
-      <para>
-        In addition, you can define the control switches to
-      enable/disable or to set the raw bit mode. The implementation
-      will depend on the chip, but the control should be named as
-      <quote>IEC958 xxx</quote>, preferably using
-      the <function>SNDRV_CTL_NAME_IEC958()</function> macro. 
-      </para>
-
-      <para>
-        You can find several cases, for example,
-      <filename>pci/emu10k1</filename>,
-      <filename>pci/ice1712</filename>, or
-      <filename>pci/cmipci.c</filename>.  
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Buffer and Memory Management  -->
-<!-- ****************************************************** -->
-  <chapter id="buffer-and-memory">
-    <title>Buffer and Memory Management</title>
-
-    <section id="buffer-and-memory-buffer-types">
-      <title>Buffer Types</title>
-      <para>
-        ALSA provides several different buffer allocation functions
-      depending on the bus and the architecture. All these have a
-      consistent API. The allocation of physically-contiguous pages is
-      done via 
-      <function>snd_malloc_xxx_pages()</function> function, where xxx
-      is the bus type. 
-      </para>
-
-      <para>
-        The allocation of pages with fallback is
-      <function>snd_malloc_xxx_pages_fallback()</function>. This
-      function tries to allocate the specified pages but if the pages
-      are not available, it tries to reduce the page sizes until
-      enough space is found.
-      </para>
-
-      <para>
-      The release the pages, call
-      <function>snd_free_xxx_pages()</function> function. 
-      </para>
-
-      <para>
-      Usually, ALSA drivers try to allocate and reserve
-       a large contiguous physical space
-       at the time the module is loaded for the later use.
-       This is called <quote>pre-allocation</quote>.
-       As already written, you can call the following function at 
-       pcm instance construction time (in the case of PCI bus). 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
-                                        snd_dma_pci_data(pci), size, max);
-]]>
-          </programlisting>
-        </informalexample>
-
-        where <parameter>size</parameter> is the byte size to be
-      pre-allocated and the <parameter>max</parameter> is the maximum
-      size to be changed via the <filename>prealloc</filename> proc file.
-      The allocator will try to get an area as large as possible
-      within the given size. 
-      </para>
-
-      <para>
-      The second argument (type) and the third argument (device pointer)
-      are dependent on the bus.
-      In the case of the ISA bus, pass <function>snd_dma_isa_data()</function>
-      as the third argument with <constant>SNDRV_DMA_TYPE_DEV</constant> type.
-      For the continuous buffer unrelated to the bus can be pre-allocated
-      with <constant>SNDRV_DMA_TYPE_CONTINUOUS</constant> type and the
-      <function>snd_dma_continuous_data(GFP_KERNEL)</function> device pointer,
-      where <constant>GFP_KERNEL</constant> is the kernel allocation flag to
-      use.
-      For the PCI scatter-gather buffers, use
-      <constant>SNDRV_DMA_TYPE_DEV_SG</constant> with
-      <function>snd_dma_pci_data(pci)</function>
-      (see the 
-          <link linkend="buffer-and-memory-non-contiguous"><citetitle>Non-Contiguous Buffers
-          </citetitle></link> section).
-      </para>
-
-      <para>
-        Once the buffer is pre-allocated, you can use the
-        allocator in the <structfield>hw_params</structfield> callback: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_pcm_lib_malloc_pages(substream, size);
-]]>
-          </programlisting>
-        </informalexample>
-
-        Note that you have to pre-allocate to use this function.
-      </para>
-    </section>
-
-    <section id="buffer-and-memory-external-hardware">
-      <title>External Hardware Buffers</title>
-      <para>
-        Some chips have their own hardware buffers and the DMA
-      transfer from the host memory is not available. In such a case,
-      you need to either 1) copy/set the audio data directly to the
-      external hardware buffer, or 2) make an intermediate buffer and
-      copy/set the data from it to the external hardware buffer in
-      interrupts (or in tasklets, preferably).
-      </para>
-
-      <para>
-        The first case works fine if the external hardware buffer is large
-      enough.  This method doesn't need any extra buffers and thus is
-      more effective. You need to define the
-      <structfield>copy</structfield> and
-      <structfield>silence</structfield> callbacks for 
-      the data transfer. However, there is a drawback: it cannot
-      be mmapped. The examples are GUS's GF1 PCM or emu8000's
-      wavetable PCM. 
-      </para>
-
-      <para>
-        The second case allows for mmap on the buffer, although you have
-      to handle an interrupt or a tasklet to transfer the data
-      from the intermediate buffer to the hardware buffer. You can find an
-      example in the vxpocket driver. 
-      </para>
-
-      <para>
-        Another case is when the chip uses a PCI memory-map
-      region for the buffer instead of the host memory. In this case,
-      mmap is available only on certain architectures like the Intel one.
-      In non-mmap mode, the data cannot be transferred as in the normal
-      way. Thus you need to define the <structfield>copy</structfield> and
-      <structfield>silence</structfield> callbacks as well, 
-      as in the cases above. The examples are found in
-      <filename>rme32.c</filename> and <filename>rme96.c</filename>. 
-      </para>
-
-      <para>
-        The implementation of the <structfield>copy</structfield> and
-        <structfield>silence</structfield> callbacks depends upon 
-        whether the hardware supports interleaved or non-interleaved
-        samples. The <structfield>copy</structfield> callback is
-        defined like below, a bit 
-        differently depending whether the direction is playback or
-        capture: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int playback_copy(struct snd_pcm_substream *substream, int channel,
-               snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
-  static int capture_copy(struct snd_pcm_substream *substream, int channel,
-               snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        In the case of interleaved samples, the second argument
-      (<parameter>channel</parameter>) is not used. The third argument
-      (<parameter>pos</parameter>) points the 
-      current position offset in frames. 
-      </para>
-
-      <para>
-        The meaning of the fourth argument is different between
-      playback and capture. For playback, it holds the source data
-      pointer, and for capture, it's the destination data pointer. 
-      </para>
-
-      <para>
-        The last argument is the number of frames to be copied.
-      </para>
-
-      <para>
-        What you have to do in this callback is again different
-        between playback and capture directions. In the
-        playback case, you copy the given amount of data
-        (<parameter>count</parameter>) at the specified pointer
-        (<parameter>src</parameter>) to the specified offset
-        (<parameter>pos</parameter>) on the hardware buffer. When
-        coded like memcpy-like way, the copy would be like: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
-            frames_to_bytes(runtime, count));
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        For the capture direction, you copy the given amount of
-        data (<parameter>count</parameter>) at the specified offset
-        (<parameter>pos</parameter>) on the hardware buffer to the
-        specified pointer (<parameter>dst</parameter>). 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
-            frames_to_bytes(runtime, count));
-]]>
-          </programlisting>
-        </informalexample>
-
-        Note that both the position and the amount of data are given
-      in frames. 
-      </para>
-
-      <para>
-        In the case of non-interleaved samples, the implementation
-      will be a bit more complicated. 
-      </para>
-
-      <para>
-        You need to check the channel argument, and if it's -1, copy
-      the whole channels. Otherwise, you have to copy only the
-      specified channel. Please check
-      <filename>isa/gus/gus_pcm.c</filename> as an example. 
-      </para>
-
-      <para>
-        The <structfield>silence</structfield> callback is also
-        implemented in a similar way. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  static int silence(struct snd_pcm_substream *substream, int channel,
-                     snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        The meanings of arguments are the same as in the
-      <structfield>copy</structfield> 
-      callback, although there is no <parameter>src/dst</parameter>
-      argument. In the case of interleaved samples, the channel
-      argument has no meaning, as well as on
-      <structfield>copy</structfield> callback.  
-      </para>
-
-      <para>
-        The role of <structfield>silence</structfield> callback is to
-        set the given amount 
-        (<parameter>count</parameter>) of silence data at the
-        specified offset (<parameter>pos</parameter>) on the hardware
-        buffer. Suppose that the data format is signed (that is, the
-        silent-data is 0), and the implementation using a memset-like
-        function would be like: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
-            frames_to_bytes(runtime, count));
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        In the case of non-interleaved samples, again, the
-      implementation becomes a bit more complicated. See, for example,
-      <filename>isa/gus/gus_pcm.c</filename>. 
-      </para>
-    </section>
-
-    <section id="buffer-and-memory-non-contiguous">
-      <title>Non-Contiguous Buffers</title>
-      <para>
-        If your hardware supports the page table as in emu10k1 or the
-      buffer descriptors as in via82xx, you can use the scatter-gather
-      (SG) DMA. ALSA provides an interface for handling SG-buffers.
-      The API is provided in <filename>&lt;sound/pcm.h&gt;</filename>. 
-      </para>
-
-      <para>
-        For creating the SG-buffer handler, call
-        <function>snd_pcm_lib_preallocate_pages()</function> or
-        <function>snd_pcm_lib_preallocate_pages_for_all()</function>
-        with <constant>SNDRV_DMA_TYPE_DEV_SG</constant>
-	in the PCM constructor like other PCI pre-allocator.
-        You need to pass <function>snd_dma_pci_data(pci)</function>,
-        where pci is the struct <structname>pci_dev</structname> pointer
-        of the chip as well.
-        The <type>struct snd_sg_buf</type> instance is created as
-        substream-&gt;dma_private. You can cast
-        the pointer like: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        Then call <function>snd_pcm_lib_malloc_pages()</function>
-      in the <structfield>hw_params</structfield> callback
-      as well as in the case of normal PCI buffer.
-      The SG-buffer handler will allocate the non-contiguous kernel
-      pages of the given size and map them onto the virtually contiguous
-      memory.  The virtual pointer is addressed in runtime-&gt;dma_area.
-      The physical address (runtime-&gt;dma_addr) is set to zero,
-      because the buffer is physically non-contiguous.
-      The physical address table is set up in sgbuf-&gt;table.
-      You can get the physical address at a certain offset via
-      <function>snd_pcm_sgbuf_get_addr()</function>. 
-      </para>
-
-      <para>
-        When a SG-handler is used, you need to set
-      <function>snd_pcm_sgbuf_ops_page</function> as
-      the <structfield>page</structfield> callback.
-      (See <link linkend="pcm-interface-operators-page-callback">
-      <citetitle>page callback section</citetitle></link>.)
-      </para>
-
-      <para>
-        To release the data, call
-      <function>snd_pcm_lib_free_pages()</function> in the
-      <structfield>hw_free</structfield> callback as usual.
-      </para>
-    </section>
-
-    <section id="buffer-and-memory-vmalloced">
-      <title>Vmalloc'ed Buffers</title>
-      <para>
-        It's possible to use a buffer allocated via
-      <function>vmalloc</function>, for example, for an intermediate
-      buffer. Since the allocated pages are not contiguous, you need
-      to set the <structfield>page</structfield> callback to obtain
-      the physical address at every offset. 
-      </para>
-
-      <para>
-        The implementation of <structfield>page</structfield> callback
-        would be like this: 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  #include <linux/vmalloc.h>
-
-  /* get the physical page pointer on the given offset */
-  static struct page *mychip_page(struct snd_pcm_substream *substream,
-                                  unsigned long offset)
-  {
-          void *pageptr = substream->runtime->dma_area + offset;
-          return vmalloc_to_page(pageptr);
-  }
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Proc Interface  -->
-<!-- ****************************************************** -->
-  <chapter id="proc-interface">
-    <title>Proc Interface</title>
-    <para>
-      ALSA provides an easy interface for procfs. The proc files are
-      very useful for debugging. I recommend you set up proc files if
-      you write a driver and want to get a running status or register
-      dumps. The API is found in
-      <filename>&lt;sound/info.h&gt;</filename>. 
-    </para>
-
-    <para>
-      To create a proc file, call
-      <function>snd_card_proc_new()</function>. 
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  struct snd_info_entry *entry;
-  int err = snd_card_proc_new(card, "my-file", &entry);
-]]>
-        </programlisting>
-      </informalexample>
-
-      where the second argument specifies the name of the proc file to be
-    created. The above example will create a file
-    <filename>my-file</filename> under the card directory,
-    e.g. <filename>/proc/asound/card0/my-file</filename>. 
-    </para>
-
-    <para>
-    Like other components, the proc entry created via
-    <function>snd_card_proc_new()</function> will be registered and
-    released automatically in the card registration and release
-    functions.
-    </para>
-
-    <para>
-      When the creation is successful, the function stores a new
-    instance in the pointer given in the third argument.
-    It is initialized as a text proc file for read only.  To use
-    this proc file as a read-only text file as it is, set the read
-    callback with a private data via 
-     <function>snd_info_set_text_ops()</function>.
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  snd_info_set_text_ops(entry, chip, my_proc_read);
-]]>
-        </programlisting>
-      </informalexample>
-    
-    where the second argument (<parameter>chip</parameter>) is the
-    private data to be used in the callbacks. The third parameter
-    specifies the read buffer size and the fourth
-    (<parameter>my_proc_read</parameter>) is the callback function, which
-    is defined like
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static void my_proc_read(struct snd_info_entry *entry,
-                           struct snd_info_buffer *buffer);
-]]>
-        </programlisting>
-      </informalexample>
-    
-    </para>
-
-    <para>
-    In the read callback, use <function>snd_iprintf()</function> for
-    output strings, which works just like normal
-    <function>printf()</function>.  For example,
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static void my_proc_read(struct snd_info_entry *entry,
-                           struct snd_info_buffer *buffer)
-  {
-          struct my_chip *chip = entry->private_data;
-
-          snd_iprintf(buffer, "This is my chip!\n");
-          snd_iprintf(buffer, "Port = %ld\n", chip->port);
-  }
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-    The file permissions can be changed afterwards.  As default, it's
-    set as read only for all users.  If you want to add write
-    permission for the user (root as default), do as follows:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
- entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
-]]>
-        </programlisting>
-      </informalexample>
-
-    and set the write buffer size and the callback
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  entry->c.text.write = my_proc_write;
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-      For the write callback, you can use
-    <function>snd_info_get_line()</function> to get a text line, and
-    <function>snd_info_get_str()</function> to retrieve a string from
-    the line. Some examples are found in
-    <filename>core/oss/mixer_oss.c</filename>, core/oss/and
-    <filename>pcm_oss.c</filename>. 
-    </para>
-
-    <para>
-      For a raw-data proc-file, set the attributes as follows:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static struct snd_info_entry_ops my_file_io_ops = {
-          .read = my_file_io_read,
-  };
-
-  entry->content = SNDRV_INFO_CONTENT_DATA;
-  entry->private_data = chip;
-  entry->c.ops = &my_file_io_ops;
-  entry->size = 4096;
-  entry->mode = S_IFREG | S_IRUGO;
-]]>
-        </programlisting>
-      </informalexample>
-
-      For the raw data, <structfield>size</structfield> field must be
-      set properly.  This specifies the maximum size of the proc file access.
-    </para>
-
-    <para>
-      The read/write callbacks of raw mode are more direct than the text mode.
-      You need to use a low-level I/O functions such as
-      <function>copy_from/to_user()</function> to transfer the
-      data.
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static ssize_t my_file_io_read(struct snd_info_entry *entry,
-                              void *file_private_data,
-                              struct file *file,
-                              char *buf,
-                              size_t count,
-                              loff_t pos)
-  {
-          if (copy_to_user(buf, local_data + pos, count))
-                  return -EFAULT;
-          return count;
-  }
-]]>
-        </programlisting>
-      </informalexample>
-
-      If the size of the info entry has been set up properly,
-      <structfield>count</structfield> and <structfield>pos</structfield> are
-      guaranteed to fit within 0 and the given size.
-      You don't have to check the range in the callbacks unless any
-      other condition is required.
-
-    </para>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Power Management  -->
-<!-- ****************************************************** -->
-  <chapter id="power-management">
-    <title>Power Management</title>
-    <para>
-      If the chip is supposed to work with suspend/resume
-      functions, you need to add power-management code to the
-      driver. The additional code for power-management should be
-      <function>ifdef</function>'ed with
-      <constant>CONFIG_PM</constant>. 
-    </para>
-
-	<para>
-	If the driver <emphasis>fully</emphasis> supports suspend/resume
-	that is, the device can be
-	properly resumed to its state when suspend was called,
-	you can set the <constant>SNDRV_PCM_INFO_RESUME</constant> flag
-	in the pcm info field.  Usually, this is possible when the
-	registers of the chip can be safely saved and restored to
-	RAM. If this is set, the trigger callback is called with
-	<constant>SNDRV_PCM_TRIGGER_RESUME</constant> after the resume
-	callback completes. 
-	</para>
-
-	<para>
-	Even if the driver doesn't support PM fully but 
-	partial suspend/resume is still possible, it's still worthy to
-	implement suspend/resume callbacks. In such a case, applications
-	would reset the status by calling
-	<function>snd_pcm_prepare()</function> and restart the stream
-	appropriately.  Hence, you can define suspend/resume callbacks
-	below but don't set <constant>SNDRV_PCM_INFO_RESUME</constant>
-	info flag to the PCM.
-	</para>
-	
-	<para>
-	Note that the trigger with SUSPEND can always be called when
-	<function>snd_pcm_suspend_all</function> is called,
-	regardless of the <constant>SNDRV_PCM_INFO_RESUME</constant> flag.
-	The <constant>RESUME</constant> flag affects only the behavior
-	of <function>snd_pcm_resume()</function>.
-	(Thus, in theory,
-	<constant>SNDRV_PCM_TRIGGER_RESUME</constant> isn't needed
-	to be handled in the trigger callback when no
-	<constant>SNDRV_PCM_INFO_RESUME</constant> flag is set.  But,
-	it's better to keep it for compatibility reasons.)
-	</para>
-    <para>
-      In the earlier version of ALSA drivers, a common
-      power-management layer was provided, but it has been removed.
-      The driver needs to define the suspend/resume hooks according to
-      the bus the device is connected to.  In the case of PCI drivers, the
-      callbacks look like below:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  #ifdef CONFIG_PM
-  static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
-  {
-          .... /* do things for suspend */
-          return 0;
-  }
-  static int snd_my_resume(struct pci_dev *pci)
-  {
-          .... /* do things for suspend */
-          return 0;
-  }
-  #endif
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-      The scheme of the real suspend job is as follows.
-
-      <orderedlist>
-        <listitem><para>Retrieve the card and the chip data.</para></listitem>
-        <listitem><para>Call <function>snd_power_change_state()</function> with
-	  <constant>SNDRV_CTL_POWER_D3hot</constant> to change the
-	  power status.</para></listitem>
-        <listitem><para>Call <function>snd_pcm_suspend_all()</function> to suspend the running PCM streams.</para></listitem>
-	<listitem><para>If AC97 codecs are used, call
-	<function>snd_ac97_suspend()</function> for each codec.</para></listitem>
-        <listitem><para>Save the register values if necessary.</para></listitem>
-        <listitem><para>Stop the hardware if necessary.</para></listitem>
-        <listitem><para>Disable the PCI device by calling
-	  <function>pci_disable_device()</function>.  Then, call
-          <function>pci_save_state()</function> at last.</para></listitem>
-      </orderedlist>
-    </para>
-
-    <para>
-      A typical code would be like:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
-  {
-          /* (1) */
-          struct snd_card *card = pci_get_drvdata(pci);
-          struct mychip *chip = card->private_data;
-          /* (2) */
-          snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
-          /* (3) */
-          snd_pcm_suspend_all(chip->pcm);
-          /* (4) */
-          snd_ac97_suspend(chip->ac97);
-          /* (5) */
-          snd_mychip_save_registers(chip);
-          /* (6) */
-          snd_mychip_stop_hardware(chip);
-          /* (7) */
-          pci_disable_device(pci);
-          pci_save_state(pci);
-          return 0;
-  }
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-    The scheme of the real resume job is as follows.
-
-    <orderedlist>
-    <listitem><para>Retrieve the card and the chip data.</para></listitem>
-    <listitem><para>Set up PCI. First, call <function>pci_restore_state()</function>.
-    	Then enable the pci device again by calling <function>pci_enable_device()</function>.
-	Call <function>pci_set_master()</function> if necessary, too.</para></listitem>
-    <listitem><para>Re-initialize the chip.</para></listitem>
-    <listitem><para>Restore the saved registers if necessary.</para></listitem>
-    <listitem><para>Resume the mixer, e.g. calling
-    <function>snd_ac97_resume()</function>.</para></listitem>
-    <listitem><para>Restart the hardware (if any).</para></listitem>
-    <listitem><para>Call <function>snd_power_change_state()</function> with
-	<constant>SNDRV_CTL_POWER_D0</constant> to notify the processes.</para></listitem>
-    </orderedlist>
-    </para>
-
-    <para>
-    A typical code would be like:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static int mychip_resume(struct pci_dev *pci)
-  {
-          /* (1) */
-          struct snd_card *card = pci_get_drvdata(pci);
-          struct mychip *chip = card->private_data;
-          /* (2) */
-          pci_restore_state(pci);
-          pci_enable_device(pci);
-          pci_set_master(pci);
-          /* (3) */
-          snd_mychip_reinit_chip(chip);
-          /* (4) */
-          snd_mychip_restore_registers(chip);
-          /* (5) */
-          snd_ac97_resume(chip->ac97);
-          /* (6) */
-          snd_mychip_restart_chip(chip);
-          /* (7) */
-          snd_power_change_state(card, SNDRV_CTL_POWER_D0);
-          return 0;
-  }
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-	As shown in the above, it's better to save registers after
-	suspending the PCM operations via
-	<function>snd_pcm_suspend_all()</function> or
-	<function>snd_pcm_suspend()</function>.  It means that the PCM
-	streams are already stopped when the register snapshot is
-	taken.  But, remember that you don't have to restart the PCM
-	stream in the resume callback. It'll be restarted via 
-	trigger call with <constant>SNDRV_PCM_TRIGGER_RESUME</constant>
-	when necessary.
-    </para>
-
-    <para>
-      OK, we have all callbacks now. Let's set them up. In the
-      initialization of the card, make sure that you can get the chip
-      data from the card instance, typically via
-      <structfield>private_data</structfield> field, in case you
-      created the chip data individually.
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static int snd_mychip_probe(struct pci_dev *pci,
-                              const struct pci_device_id *pci_id)
-  {
-          ....
-          struct snd_card *card;
-          struct mychip *chip;
-          int err;
-          ....
-          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                             0, &card);
-          ....
-          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
-          ....
-          card->private_data = chip;
-          ....
-  }
-]]>
-        </programlisting>
-      </informalexample>
-
-	When you created the chip data with
-	<function>snd_card_new()</function>, it's anyway accessible
-	via <structfield>private_data</structfield> field.
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static int snd_mychip_probe(struct pci_dev *pci,
-                              const struct pci_device_id *pci_id)
-  {
-          ....
-          struct snd_card *card;
-          struct mychip *chip;
-          int err;
-          ....
-          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
-                             sizeof(struct mychip), &card);
-          ....
-          chip = card->private_data;
-          ....
-  }
-]]>
-        </programlisting>
-      </informalexample>
-
-    </para>
-
-    <para>
-      If you need a space to save the registers, allocate the
-	buffer for it here, too, since it would be fatal
-    if you cannot allocate a memory in the suspend phase.
-    The allocated buffer should be released in the corresponding
-    destructor.
-    </para>
-
-    <para>
-      And next, set suspend/resume callbacks to the pci_driver.
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static struct pci_driver driver = {
-          .name = KBUILD_MODNAME,
-          .id_table = snd_my_ids,
-          .probe = snd_my_probe,
-          .remove = snd_my_remove,
-  #ifdef CONFIG_PM
-          .suspend = snd_my_suspend,
-          .resume = snd_my_resume,
-  #endif
-  };
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Module Parameters  -->
-<!-- ****************************************************** -->
-  <chapter id="module-parameters">
-    <title>Module Parameters</title>
-    <para>
-      There are standard module options for ALSA. At least, each
-      module should have the <parameter>index</parameter>,
-      <parameter>id</parameter> and <parameter>enable</parameter>
-      options. 
-    </para>
-
-    <para>
-      If the module supports multiple cards (usually up to
-      8 = <constant>SNDRV_CARDS</constant> cards), they should be
-      arrays. The default initial values are defined already as
-      constants for easier programming:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
-  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
-  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-      If the module supports only a single card, they could be single
-    variables, instead.  <parameter>enable</parameter> option is not
-    always necessary in this case, but it would be better to have a
-    dummy option for compatibility.
-    </para>
-
-    <para>
-      The module parameters must be declared with the standard
-    <function>module_param()()</function>,
-    <function>module_param_array()()</function> and
-    <function>MODULE_PARM_DESC()</function> macros.
-    </para>
-
-    <para>
-      The typical coding would be like below:
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  #define CARD_NAME "My Chip"
-
-  module_param_array(index, int, NULL, 0444);
-  MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
-  module_param_array(id, charp, NULL, 0444);
-  MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
-  module_param_array(enable, bool, NULL, 0444);
-  MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-    <para>
-      Also, don't forget to define the module description, classes,
-      license and devices. Especially, the recent modprobe requires to
-      define the module license as GPL, etc., otherwise the system is
-      shown as <quote>tainted</quote>. 
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  MODULE_DESCRIPTION("My Chip");
-  MODULE_LICENSE("GPL");
-  MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
-]]>
-        </programlisting>
-      </informalexample>
-    </para>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- How To Put Your Driver  -->
-<!-- ****************************************************** -->
-  <chapter id="how-to-put-your-driver">
-    <title>How To Put Your Driver Into ALSA Tree</title>
-	<section>
-	<title>General</title>
-	<para>
-	So far, you've learned how to write the driver codes.
-	And you might have a question now: how to put my own
-	driver into the ALSA driver tree?
-	Here (finally :) the standard procedure is described briefly.
-	</para>
-
-	<para>
-	Suppose that you create a new PCI driver for the card
-	<quote>xyz</quote>.  The card module name would be
-	snd-xyz.  The new driver is usually put into the alsa-driver
-	tree, <filename>alsa-driver/pci</filename> directory in
-	the case of PCI cards.
-	Then the driver is evaluated, audited and tested
-	by developers and users.  After a certain time, the driver
-	will go to the alsa-kernel tree (to the corresponding directory,
-	such as <filename>alsa-kernel/pci</filename>) and eventually
- 	will be integrated into the Linux 2.6 tree (the directory would be
-	<filename>linux/sound/pci</filename>).
-	</para>
-
-	<para>
-	In the following sections, the driver code is supposed
-	to be put into alsa-driver tree. The two cases are covered:
-	a driver consisting of a single source file and one consisting
-	of several source files.
-	</para>
-	</section>
-
-	<section>
-	<title>Driver with A Single Source File</title>
-	<para>
-	<orderedlist>
-	<listitem>
-	<para>
-	Modify alsa-driver/pci/Makefile
-	</para>
-
-	<para>
-	Suppose you have a file xyz.c.  Add the following
-	two lines
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  snd-xyz-objs := xyz.o
-  obj-$(CONFIG_SND_XYZ) += snd-xyz.o
-]]>
-        </programlisting>
-      </informalexample>
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	Create the Kconfig entry
-	</para>
-
-	<para>
-	Add the new entry of Kconfig for your xyz driver.
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  config SND_XYZ
-          tristate "Foobar XYZ"
-          depends on SND
-          select SND_PCM
-          help
-            Say Y here to include support for Foobar XYZ soundcard.
-
-            To compile this driver as a module, choose M here: the module
-            will be called snd-xyz.
-]]>
-        </programlisting>
-      </informalexample>
-
-	the line, select SND_PCM, specifies that the driver xyz supports
-	PCM.  In addition to SND_PCM, the following components are
-	supported for select command:
-	SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
-	SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
-	Add the select command for each supported component.
-	</para>
-
-	<para>
-	Note that some selections imply the lowlevel selections.
-	For example, PCM includes TIMER, MPU401_UART includes RAWMIDI,
-	AC97_CODEC includes PCM, and OPL3_LIB includes HWDEP.
-	You don't need to give the lowlevel selections again.
-	</para>
-
-	<para>
-	For the details of Kconfig script, refer to the kbuild
-	documentation.
-	</para>
-
-	</listitem>
-
-	<listitem>
-	<para>
-	Run cvscompile script to re-generate the configure script and
-	build the whole stuff again.
-	</para>
-	</listitem>
-	</orderedlist>
-	</para>
-	</section>
-
-	<section>
-	<title>Drivers with Several Source Files</title>
-	<para>
-	Suppose that the driver snd-xyz have several source files.
-	They are located in the new subdirectory,
-	pci/xyz.
-
-	<orderedlist>
-	<listitem>
-	<para>
-	Add a new directory (<filename>xyz</filename>) in
-	<filename>alsa-driver/pci/Makefile</filename> as below
-
-      <informalexample>
-        <programlisting>
-<![CDATA[
-  obj-$(CONFIG_SND) += xyz/
-]]>
-        </programlisting>
-      </informalexample>
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	Under the directory <filename>xyz</filename>, create a Makefile
-
-      <example>
-	<title>Sample Makefile for a driver xyz</title>
-        <programlisting>
-<![CDATA[
-  ifndef SND_TOPDIR
-  SND_TOPDIR=../..
-  endif
-
-  include $(SND_TOPDIR)/toplevel.config
-  include $(SND_TOPDIR)/Makefile.conf
-
-  snd-xyz-objs := xyz.o abc.o def.o
-
-  obj-$(CONFIG_SND_XYZ) += snd-xyz.o
-
-  include $(SND_TOPDIR)/Rules.make
-]]>
-        </programlisting>
-      </example>
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	Create the Kconfig entry
-	</para>
-
-	<para>
-	This procedure is as same as in the last section.
-	</para>
-	</listitem>
-
-	<listitem>
-	<para>
-	Run cvscompile script to re-generate the configure script and
-	build the whole stuff again.
-	</para>
-	</listitem>
-	</orderedlist>
-	</para>
-	</section>
-
-  </chapter>
-
-<!-- ****************************************************** -->
-<!-- Useful Functions  -->
-<!-- ****************************************************** -->
-  <chapter id="useful-functions">
-    <title>Useful Functions</title>
-
-    <section id="useful-functions-snd-printk">
-      <title><function>snd_printk()</function> and friends</title>
-      <para>
-        ALSA provides a verbose version of the
-      <function>printk()</function> function. If a kernel config
-      <constant>CONFIG_SND_VERBOSE_PRINTK</constant> is set, this
-      function prints the given message together with the file name
-      and the line of the caller. The <constant>KERN_XXX</constant>
-      prefix is processed as 
-      well as the original <function>printk()</function> does, so it's
-      recommended to add this prefix, e.g. 
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\n");
-]]>
-          </programlisting>
-        </informalexample>
-      </para>
-
-      <para>
-        There are also <function>printk()</function>'s for
-      debugging. <function>snd_printd()</function> can be used for
-      general debugging purposes. If
-      <constant>CONFIG_SND_DEBUG</constant> is set, this function is
-      compiled, and works just like
-      <function>snd_printk()</function>. If the ALSA is compiled
-      without the debugging flag, it's ignored. 
-      </para>
-
-      <para>
-        <function>snd_printdd()</function> is compiled in only when
-      <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is set. Please note
-      that <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is not set as default
-      even if you configure the alsa-driver with
-      <option>--with-debug=full</option> option. You need to give
-      explicitly <option>--with-debug=detect</option> option instead. 
-      </para>
-    </section>
-
-    <section id="useful-functions-snd-bug">
-      <title><function>snd_BUG()</function></title>
-      <para>
-        It shows the <computeroutput>BUG?</computeroutput> message and
-      stack trace as well as <function>snd_BUG_ON</function> at the point.
-      It's useful to show that a fatal error happens there. 
-      </para>
-      <para>
-	 When no debug flag is set, this macro is ignored. 
-      </para>
-    </section>
-
-    <section id="useful-functions-snd-bug-on">
-      <title><function>snd_BUG_ON()</function></title>
-      <para>
-        <function>snd_BUG_ON()</function> macro is similar with
-	<function>WARN_ON()</function> macro. For example,  
-
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  snd_BUG_ON(!pointer);
-]]>
-          </programlisting>
-        </informalexample>
-
-	or it can be used as the condition,
-        <informalexample>
-          <programlisting>
-<![CDATA[
-  if (snd_BUG_ON(non_zero_is_bug))
-          return -EINVAL;
-]]>
-          </programlisting>
-        </informalexample>
-
-      </para>
-
-      <para>
-        The macro takes an conditional expression to evaluate.
-	When <constant>CONFIG_SND_DEBUG</constant>, is set, if the
-	expression is non-zero, it shows the warning message such as
-	<computeroutput>BUG? (xxx)</computeroutput>
-	normally followed by stack trace.
-
-	In both cases it returns the evaluated value.
-      </para>
-
-    </section>
-
-  </chapter>
-
-
-<!-- ****************************************************** -->
-<!-- Acknowledgments  -->
-<!-- ****************************************************** -->
-  <chapter id="acknowledgments">
-    <title>Acknowledgments</title>
-    <para>
-      I would like to thank Phil Kerr for his help for improvement and
-      corrections of this document. 
-    </para>
-    <para>
-    Kevin Conder reformatted the original plain-text to the
-    DocBook format.
-    </para>
-    <para>
-    Giuliano Pochini corrected typos and contributed the example codes
-    in the hardware constraints section.
-    </para>
-  </chapter>
-</book>

Documentation/sound/kernel-api/index.rst

···
    :maxdepth: 2
 
    alsa-driver-api
+   writing-an-alsa-driver

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

···
+======================
+Writing an ALSA Driver
+======================
+
+:Author: Takashi Iwai <[e-mail]>
+:Date:   Oct 15, 2007
+:Edition: 0.3.7
+
+Preface
+=======
+
+This document describes how to write an `ALSA (Advanced Linux Sound
+Architecture) <http://www.alsa-project.org/>`__ driver. The document
+focuses mainly on PCI soundcards. In the case of other device types, the
+API might be different, too. However, at least the ALSA kernel API is
+consistent, and therefore it would be still a bit help for writing them.
+
+This document targets people who already have enough C language skills
+and have basic linux kernel programming knowledge. This document doesn't
+explain the general topic of linux kernel coding and doesn't cover
+low-level driver implementation details. It only describes the standard
+way to write a PCI sound driver on ALSA.
+
+If you are already familiar with the older ALSA ver.0.5.x API, you can
+check the drivers such as ``sound/pci/es1938.c`` or
+``sound/pci/maestro3.c`` which have also almost the same code-base in
+the ALSA 0.5.x tree, so you can compare the differences.
+
+This document is still a draft version. Any feedback and corrections,
+please!!
+
+File Tree Structure
+===================
+
+General
+-------
+
+The ALSA drivers are provided in two ways.
+
+One is the trees provided as a tarball or via cvs from the ALSA's ftp
+site, and another is the 2.6 (or later) Linux kernel tree. To
+synchronize both, the ALSA driver tree is split into two different
+trees: alsa-kernel and alsa-driver. The former contains purely the
+source code for the Linux 2.6 (or later) tree. This tree is designed
+only for compilation on 2.6 or later environment. The latter,
+alsa-driver, contains many subtle files for compiling ALSA drivers
+outside of the Linux kernel tree, wrapper functions for older 2.2 and
+2.4 kernels, to adapt the latest kernel API, and additional drivers
+which are still in development or in tests. The drivers in alsa-driver
+tree will be moved to alsa-kernel (and eventually to the 2.6 kernel
+tree) when they are finished and confirmed to work fine.
+
+The file tree structure of ALSA driver is depicted below. Both
+alsa-kernel and alsa-driver have almost the same file structure, except
+for “core” directory. It's named as “acore” in alsa-driver tree.
+
+::
+
+            sound
+                    /core
+                            /oss
+                            /seq
+                                    /oss
+                                    /instr
+                    /ioctl32
+                    /include
+                    /drivers
+                            /mpu401
+                            /opl3
+                    /i2c
+                            /l3
+                    /synth
+                            /emux
+                    /pci
+                            /(cards)
+                    /isa
+                            /(cards)
+                    /arm
+                    /ppc
+                    /sparc
+                    /usb
+                    /pcmcia /(cards)
+                    /oss
+
+
+core directory
+--------------
+
+This directory contains the middle layer which is the heart of ALSA
+drivers. In this directory, the native ALSA modules are stored. The
+sub-directories contain different modules and are dependent upon the
+kernel config.
+
+core/oss
+~~~~~~~~
+
+The codes for PCM and mixer OSS emulation modules are stored in this
+directory. The rawmidi OSS emulation is included in the ALSA rawmidi
+code since it's quite small. The sequencer code is stored in
+``core/seq/oss`` directory (see `below <#core-seq-oss>`__).
+
+core/ioctl32
+~~~~~~~~~~~~
+
+This directory contains the 32bit-ioctl wrappers for 64bit architectures
+such like x86-64, ppc64 and sparc64. For 32bit and alpha architectures,
+these are not compiled.
+
+core/seq
+~~~~~~~~
+
+This directory and its sub-directories are for the ALSA sequencer. This
+directory contains the sequencer core and primary sequencer modules such
+like snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
+``CONFIG_SND_SEQUENCER`` is set in the kernel config.
+
+core/seq/oss
+~~~~~~~~~~~~
+
+This contains the OSS sequencer emulation codes.
+
+core/seq/instr
+~~~~~~~~~~~~~~
+
+This directory contains the modules for the sequencer instrument layer.
+
+include directory
+-----------------
+
+This is the place for the public header files of ALSA drivers, which are
+to be exported to user-space, or included by several files at different
+directories. Basically, the private header files should not be placed in
+this directory, but you may still find files there, due to historical
+reasons :)
+
+drivers directory
+-----------------
+
+This directory contains code shared among different drivers on different
+architectures. They are hence supposed not to be architecture-specific.
+For example, the dummy pcm driver and the serial MIDI driver are found
+in this directory. In the sub-directories, there is code for components
+which are independent from bus and cpu architectures.
+
+drivers/mpu401
+~~~~~~~~~~~~~~
+
+The MPU401 and MPU401-UART modules are stored here.
+
+drivers/opl3 and opl4
+~~~~~~~~~~~~~~~~~~~~~
+
+The OPL3 and OPL4 FM-synth stuff is found here.
+
+i2c directory
+-------------
+
+This contains the ALSA i2c components.
+
+Although there is a standard i2c layer on Linux, ALSA has its own i2c
+code for some cards, because the soundcard needs only a simple operation
+and the standard i2c API is too complicated for such a purpose.
+
+i2c/l3
+~~~~~~
+
+This is a sub-directory for ARM L3 i2c.
+
+synth directory
+---------------
+
+This contains the synth middle-level modules.
+
+So far, there is only Emu8000/Emu10k1 synth driver under the
+``synth/emux`` sub-directory.
+
+pci directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for PCI soundcards and the code specific to the PCI BUS.
+
+The drivers compiled from a single file are stored directly in the pci
+directory, while the drivers with several source files are stored on
+their own sub-directory (e.g. emu10k1, ice1712).
+
+isa directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for ISA soundcards.
+
+arm, ppc, and sparc directories
+-------------------------------
+
+They are used for top-level card modules which are specific to one of
+these architectures.
+
+usb directory
+-------------
+
+This directory contains the USB-audio driver. In the latest version, the
+USB MIDI driver is integrated in the usb-audio driver.
+
+pcmcia directory
+----------------
+
+The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
+be in the pci directory, because their API is identical to that of
+standard PCI cards.
+
+oss directory
+-------------
+
+The OSS/Lite source files are stored here in Linux 2.6 (or later) tree.
+In the ALSA driver tarball, this directory is empty, of course :)
+
+Basic Flow for PCI Drivers
+==========================
+
+Outline
+-------
+
+The minimum flow for PCI soundcards is as follows:
+
+-  define the PCI ID table (see the section `PCI Entries`_).
+
+-  create ``probe`` callback.
+
+-  create ``remove`` callback.
+
+-  create a :c:type:`struct pci_driver <pci_driver>` structure
+   containing the three pointers above.
+
+-  create an ``init`` function just calling the
+   :c:func:`pci_register_driver()` to register the pci_driver
+   table defined above.
+
+-  create an ``exit`` function to call the
+   :c:func:`pci_unregister_driver()` function.
+
+Full Code Example
+-----------------
+
+The code example is shown below. Some parts are kept unimplemented at
+this moment but will be filled in the next sections. The numbers in the
+comment lines of the :c:func:`snd_mychip_probe()` function refer
+to details explained in the following section.
+
+::
+
+      #include <linux/init.h>
+      #include <linux/pci.h>
+      #include <linux/slab.h>
+      #include <sound/core.h>
+      #include <sound/initval.h>
+
+      /* module parameters (see "Module Parameters") */
+      /* SNDRV_CARDS: maximum number of cards supported by this module */
+      static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+      static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+      static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+      /* definition of the chip-specific record */
+      struct mychip {
+              struct snd_card *card;
+              /* the rest of the implementation will be in section
+               * "PCI Resource Management"
+               */
+      };
+
+      /* chip-specific destructor
+       * (see "PCI Resource Management")
+       */
+      static int snd_mychip_free(struct mychip *chip)
+      {
+              .... /* will be implemented later... */
+      }
+
+      /* component-destructor
+       * (see "Management of Cards and Components")
+       */
+      static int snd_mychip_dev_free(struct snd_device *device)
+      {
+              return snd_mychip_free(device->device_data);
+      }
+
+      /* chip-specific constructor
+       * (see "Management of Cards and Components")
+       */
+      static int snd_mychip_create(struct snd_card *card,
+                                   struct pci_dev *pci,
+                                   struct mychip **rchip)
+      {
+              struct mychip *chip;
+              int err;
+              static struct snd_device_ops ops = {
+                     .dev_free = snd_mychip_dev_free,
+              };
+
+              *rchip = NULL;
+
+              /* check PCI availability here
+               * (see "PCI Resource Management")
+               */
+              ....
+
+              /* allocate a chip-specific data with zero filled */
+              chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+              if (chip == NULL)
+                      return -ENOMEM;
+
+              chip->card = card;
+
+              /* rest of initialization here; will be implemented
+               * later, see "PCI Resource Management"
+               */
+              ....
+
+              err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+              if (err < 0) {
+                      snd_mychip_free(chip);
+                      return err;
+              }
+
+              *rchip = chip;
+              return 0;
+      }
+
+      /* constructor -- see "Driver Constructor" sub-section */
+      static int snd_mychip_probe(struct pci_dev *pci,
+                                  const struct pci_device_id *pci_id)
+      {
+              static int dev;
+              struct snd_card *card;
+              struct mychip *chip;
+              int err;
+
+              /* (1) */
+              if (dev >= SNDRV_CARDS)
+                      return -ENODEV;
+              if (!enable[dev]) {
+                      dev++;
+                      return -ENOENT;
+              }
+
+              /* (2) */
+              err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                                 0, &card);
+              if (err < 0)
+                      return err;
+
+              /* (3) */
+              err = snd_mychip_create(card, pci, &chip);
+              if (err < 0) {
+                      snd_card_free(card);
+                      return err;
+              }
+
+              /* (4) */
+              strcpy(card->driver, "My Chip");
+              strcpy(card->shortname, "My Own Chip 123");
+              sprintf(card->longname, "%s at 0x%lx irq %i",
+                      card->shortname, chip->ioport, chip->irq);
+
+              /* (5) */
+              .... /* implemented later */
+
+              /* (6) */
+              err = snd_card_register(card);
+              if (err < 0) {
+                      snd_card_free(card);
+                      return err;
+              }
+
+              /* (7) */
+              pci_set_drvdata(pci, card);
+              dev++;
+              return 0;
+      }
+
+      /* destructor -- see the "Destructor" sub-section */
+      static void snd_mychip_remove(struct pci_dev *pci)
+      {
+              snd_card_free(pci_get_drvdata(pci));
+              pci_set_drvdata(pci, NULL);
+      }
+
+
+
+Driver Constructor
+------------------
+
+The real constructor of PCI drivers is the ``probe`` callback. The
+``probe`` callback and other component-constructors which are called
+from the ``probe`` callback cannot be used with the ``__init`` prefix
+because any PCI device could be a hotplug device.
+
+In the ``probe`` callback, the following scheme is often used.
+
+1) Check and increment the device index.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  static int dev;
+  ....
+  if (dev >= SNDRV_CARDS)
+          return -ENODEV;
+  if (!enable[dev]) {
+          dev++;
+          return -ENOENT;
+  }
+
+
+where ``enable[dev]`` is the module option.
+
+Each time the ``probe`` callback is called, check the availability of
+the device. If not available, simply increment the device index and
+returns. dev will be incremented also later (`step 7
+<#set-the-pci-driver-data-and-return-zero>`__).
+
+2) Create a card instance
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  struct snd_card *card;
+  int err;
+  ....
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     0, &card);
+
+
+The details will be explained in the section `Management of Cards and
+Components`_.
+
+3) Create a main component
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In this part, the PCI resources are allocated.
+
+::
+
+  struct mychip *chip;
+  ....
+  err = snd_mychip_create(card, pci, &chip);
+  if (err < 0) {
+          snd_card_free(card);
+          return err;
+  }
+
+The details will be explained in the section `PCI Resource
+Management`_.
+
+4) Set the driver ID and name strings.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  strcpy(card->driver, "My Chip");
+  strcpy(card->shortname, "My Own Chip 123");
+  sprintf(card->longname, "%s at 0x%lx irq %i",
+          card->shortname, chip->ioport, chip->irq);
+
+The driver field holds the minimal ID string of the chip. This is used
+by alsa-lib's configurator, so keep it simple but unique. Even the
+same driver can have different driver IDs to distinguish the
+functionality of each chip type.
+
+The shortname field is a string shown as more verbose name. The longname
+field contains the information shown in ``/proc/asound/cards``.
+
+5) Create other components, such as mixer, MIDI, etc.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Here you define the basic components such as `PCM <#PCM-Interface>`__,
+mixer (e.g. `AC97 <#API-for-AC97-Codec>`__), MIDI (e.g.
+`MPU-401 <#MIDI-MPU401-UART-Interface>`__), and other interfaces.
+Also, if you want a `proc file <#Proc-Interface>`__, define it here,
+too.
+
+6) Register the card instance.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  err = snd_card_register(card);
+  if (err < 0) {
+          snd_card_free(card);
+          return err;
+  }
+
+Will be explained in the section `Management of Cards and
+Components`_, too.
+
+7) Set the PCI driver data and return zero.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  pci_set_drvdata(pci, card);
+  dev++;
+  return 0;
+
+In the above, the card record is stored. This pointer is used in the
+remove callback and power-management callbacks, too.
+
+Destructor
+----------
+
+The destructor, remove callback, simply releases the card instance. Then
+the ALSA middle layer will release all the attached components
+automatically.
+
+It would be typically like the following:
+
+::
+
+  static void snd_mychip_remove(struct pci_dev *pci)
+  {
+          snd_card_free(pci_get_drvdata(pci));
+          pci_set_drvdata(pci, NULL);
+  }
+
+
+The above code assumes that the card pointer is set to the PCI driver
+data.
+
+Header Files
+------------
+
+For the above example, at least the following include files are
+necessary.
+
+::
+
+  #include <linux/init.h>
+  #include <linux/pci.h>
+  #include <linux/slab.h>
+  #include <sound/core.h>
+  #include <sound/initval.h>
+
+where the last one is necessary only when module options are defined
+in the source file. If the code is split into several files, the files
+without module options don't need them.
+
+In addition to these headers, you'll need ``<linux/interrupt.h>`` for
+interrupt handling, and ``<asm/io.h>`` for I/O access. If you use the
+:c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
+to include ``<linux/delay.h>`` too.
+
+The ALSA interfaces like the PCM and control APIs are defined in other
+``<sound/xxx.h>`` header files. They have to be included after
+``<sound/core.h>``.
+
+Management of Cards and Components
+==================================
+
+Card Instance
+-------------
+
+For each soundcard, a “card” record must be allocated.
+
+A card record is the headquarters of the soundcard. It manages the whole
+list of devices (components) on the soundcard, such as PCM, mixers,
+MIDI, synthesizer, and so on. Also, the card record holds the ID and the
+name strings of the card, manages the root of proc files, and controls
+the power-management states and hotplug disconnections. The component
+list on the card record is used to manage the correct release of
+resources at destruction.
+
+As mentioned above, to create a card instance, call
+:c:func:`snd_card_new()`.
+
+::
+
+  struct snd_card *card;
+  int err;
+  err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
+
+
+The function takes six arguments: the parent device pointer, the
+card-index number, the id string, the module pointer (usually
+``THIS_MODULE``), the size of extra-data space, and the pointer to
+return the card instance. The extra_size argument is used to allocate
+card->private_data for the chip-specific data. Note that these data are
+allocated by :c:func:`snd_card_new()`.
+
+The first argument, the pointer of struct :c:type:`struct device
+<device>`, specifies the parent device. For PCI devices, typically
+``&pci->`` is passed there.
+
+Components
+----------
+
+After the card is created, you can attach the components (devices) to
+the card instance. In an ALSA driver, a component is represented as a
+:c:type:`struct snd_device <snd_device>` object. A component
+can be a PCM instance, a control interface, a raw MIDI interface, etc.
+Each such instance has one component entry.
+
+A component can be created via :c:func:`snd_device_new()`
+function.
+
+::
+
+  snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
+
+This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the
+data pointer, and the callback pointers (``&ops``). The device-level
+defines the type of components and the order of registration and
+de-registration. For most components, the device-level is already
+defined. For a user-defined component, you can use
+``SNDRV_DEV_LOWLEVEL``.
+
+This function itself doesn't allocate the data space. The data must be
+allocated manually beforehand, and its pointer is passed as the
+argument. This pointer (``chip`` in the above example) is used as the
+identifier for the instance.
+
+Each pre-defined ALSA component such as ac97 and pcm calls
+:c:func:`snd_device_new()` inside its constructor. The destructor
+for each component is defined in the callback pointers. Hence, you don't
+need to take care of calling a destructor for such a component.
+
+If you wish to create your own component, you need to set the destructor
+function to the dev_free callback in the ``ops``, so that it can be
+released automatically via :c:func:`snd_card_free()`. The next
+example will show an implementation of chip-specific data.
+
+Chip-Specific Data
+------------------
+
+Chip-specific information, e.g. the I/O port address, its resource
+pointer, or the irq number, is stored in the chip-specific record.
+
+::
+
+  struct mychip {
+          ....
+  };
+
+
+In general, there are two ways of allocating the chip record.
+
+1. Allocating via :c:func:`snd_card_new()`.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As mentioned above, you can pass the extra-data-length to the 5th
+argument of :c:func:`snd_card_new()`, i.e.
+
+::
+
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     sizeof(struct mychip), &card);
+
+:c:type:`struct mychip <mychip>` is the type of the chip record.
+
+In return, the allocated record can be accessed as
+
+::
+
+  struct mychip *chip = card->private_data;
+
+With this method, you don't have to allocate twice. The record is
+released together with the card instance.
+
+2. Allocating an extra device.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+After allocating a card instance via :c:func:`snd_card_new()`
+(with ``0`` on the 4th arg), call :c:func:`kzalloc()`.
+
+::
+
+  struct snd_card *card;
+  struct mychip *chip;
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     0, &card);
+  .....
+  chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+
+The chip record should have the field to hold the card pointer at least,
+
+::
+
+  struct mychip {
+          struct snd_card *card;
+          ....
+  };
+
+
+Then, set the card pointer in the returned chip instance.
+
+::
+
+  chip->card = card;
+
+Next, initialize the fields, and register this chip record as a
+low-level device with a specified ``ops``,
+
+::
+
+  static struct snd_device_ops ops = {
+          .dev_free =        snd_mychip_dev_free,
+  };
+  ....
+  snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+
+:c:func:`snd_mychip_dev_free()` is the device-destructor
+function, which will call the real destructor.
+
+::
+
+  static int snd_mychip_dev_free(struct snd_device *device)
+  {
+          return snd_mychip_free(device->device_data);
+  }
+
+where :c:func:`snd_mychip_free()` is the real destructor.
+
+Registration and Release
+------------------------
+
+After all components are assigned, register the card instance by calling
+:c:func:`snd_card_register()`. Access to the device files is
+enabled at this point. That is, before
+:c:func:`snd_card_register()` is called, the components are safely
+inaccessible from external side. If this call fails, exit the probe
+function after releasing the card via :c:func:`snd_card_free()`.
+
+For releasing the card instance, you can call simply
+:c:func:`snd_card_free()`. As mentioned earlier, all components
+are released automatically by this call.
+
+For a device which allows hotplugging, you can use
+:c:func:`snd_card_free_when_closed()`. This one will postpone
+the destruction until all devices are closed.
+
+PCI Resource Management
+=======================
+
+Full Code Example
+-----------------
+
+In this section, we'll complete the chip-specific constructor,
+destructor and PCI entries. Example code is shown first, below.
+
+::
+
+      struct mychip {
+              struct snd_card *card;
+              struct pci_dev *pci;
+
+              unsigned long port;
+              int irq;
+      };
+
+      static int snd_mychip_free(struct mychip *chip)
+      {
+              /* disable hardware here if any */
+              .... /* (not implemented in this document) */
+
+              /* release the irq */
+              if (chip->irq >= 0)
+                      free_irq(chip->irq, chip);
+              /* release the I/O ports & memory */
+              pci_release_regions(chip->pci);
+              /* disable the PCI entry */
+              pci_disable_device(chip->pci);
+              /* release the data */
+              kfree(chip);
+              return 0;
+      }
+
+      /* chip-specific constructor */
+      static int snd_mychip_create(struct snd_card *card,
+                                   struct pci_dev *pci,
+                                   struct mychip **rchip)
+      {
+              struct mychip *chip;
+              int err;
+              static struct snd_device_ops ops = {
+                     .dev_free = snd_mychip_dev_free,
+              };
+
+              *rchip = NULL;
+
+              /* initialize the PCI entry */
+              err = pci_enable_device(pci);
+              if (err < 0)
+                      return err;
+              /* check PCI availability (28bit DMA) */
+              if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+                  pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+                      printk(KERN_ERR "error to set 28bit mask DMA\n");
+                      pci_disable_device(pci);
+                      return -ENXIO;
+              }
+
+              chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+              if (chip == NULL) {
+                      pci_disable_device(pci);
+                      return -ENOMEM;
+              }
+
+              /* initialize the stuff */
+              chip->card = card;
+              chip->pci = pci;
+              chip->irq = -1;
+
+              /* (1) PCI resource allocation */
+              err = pci_request_regions(pci, "My Chip");
+              if (err < 0) {
+                      kfree(chip);
+                      pci_disable_device(pci);
+                      return err;
+              }
+              chip->port = pci_resource_start(pci, 0);
+              if (request_irq(pci->irq, snd_mychip_interrupt,
+                              IRQF_SHARED, KBUILD_MODNAME, chip)) {
+                      printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+                      snd_mychip_free(chip);
+                      return -EBUSY;
+              }
+              chip->irq = pci->irq;
+
+              /* (2) initialization of the chip hardware */
+              .... /*   (not implemented in this document) */
+
+              err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+              if (err < 0) {
+                      snd_mychip_free(chip);
+                      return err;
+              }
+
+              *rchip = chip;
+              return 0;
+      }
+
+      /* PCI IDs */
+      static struct pci_device_id snd_mychip_ids[] = {
+              { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+                PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+              ....
+              { 0, }
+      };
+      MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+      /* pci_driver definition */
+      static struct pci_driver driver = {
+              .name = KBUILD_MODNAME,
+              .id_table = snd_mychip_ids,
+              .probe = snd_mychip_probe,
+              .remove = snd_mychip_remove,
+      };
+
+      /* module initialization */
+      static int __init alsa_card_mychip_init(void)
+      {
+              return pci_register_driver(&driver);
+      }
+
+      /* module clean up */
+      static void __exit alsa_card_mychip_exit(void)
+      {
+              pci_unregister_driver(&driver);
+      }
+
+      module_init(alsa_card_mychip_init)
+      module_exit(alsa_card_mychip_exit)
+
+      EXPORT_NO_SYMBOLS; /* for old kernels only */
+
+Some Hafta's
+------------
+
+The allocation of PCI resources is done in the ``probe`` function, and
+usually an extra :c:func:`xxx_create()` function is written for this
+purpose.
+
+In the case of PCI devices, you first have to call the
+:c:func:`pci_enable_device()` function before allocating
+resources. Also, you need to set the proper PCI DMA mask to limit the
+accessed I/O range. In some cases, you might need to call
+:c:func:`pci_set_master()` function, too.
+
+Suppose the 28bit mask, and the code to be added would be like:
+
+::
+
+  err = pci_enable_device(pci);
+  if (err < 0)
+          return err;
+  if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+      pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+          printk(KERN_ERR "error to set 28bit mask DMA\n");
+          pci_disable_device(pci);
+          return -ENXIO;
+  }
+  
+
+Resource Allocation
+-------------------
+
+The allocation of I/O ports and irqs is done via standard kernel
+functions. Unlike ALSA ver.0.5.x., there are no helpers for that. And
+these resources must be released in the destructor function (see below).
+Also, on ALSA 0.9.x, you don't need to allocate (pseudo-)DMA for PCI
+like in ALSA 0.5.x.
+
+Now assume that the PCI device has an I/O port with 8 bytes and an
+interrupt. Then :c:type:`struct mychip <mychip>` will have the
+following fields:
+
+::
+
+  struct mychip {
+          struct snd_card *card;
+
+          unsigned long port;
+          int irq;
+  };
+
+
+For an I/O port (and also a memory region), you need to have the
+resource pointer for the standard resource management. For an irq, you
+have to keep only the irq number (integer). But you need to initialize
+this number as -1 before actual allocation, since irq 0 is valid. The
+port address and its resource pointer can be initialized as null by
+:c:func:`kzalloc()` automatically, so you don't have to take care of
+resetting them.
+
+The allocation of an I/O port is done like this:
+
+::
+
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) { 
+          kfree(chip);
+          pci_disable_device(pci);
+          return err;
+  }
+  chip->port = pci_resource_start(pci, 0);
+
+It will reserve the I/O port region of 8 bytes of the given PCI device.
+The returned value, ``chip->res_port``, is allocated via
+:c:func:`kmalloc()` by :c:func:`request_region()`. The pointer
+must be released via :c:func:`kfree()`, but there is a problem with
+this. This issue will be explained later.
+
+The allocation of an interrupt source is done like this:
+
+::
+
+  if (request_irq(pci->irq, snd_mychip_interrupt,
+                  IRQF_SHARED, KBUILD_MODNAME, chip)) {
+          printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+          snd_mychip_free(chip);
+          return -EBUSY;
+  }
+  chip->irq = pci->irq;
+
+where :c:func:`snd_mychip_interrupt()` is the interrupt handler
+defined `later <#pcm-interface-interrupt-handler>`__. Note that
+``chip->irq`` should be defined only when :c:func:`request_irq()`
+succeeded.
+
+On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used
+as the interrupt flag of :c:func:`request_irq()`.
+
+The last argument of :c:func:`request_irq()` is the data pointer
+passed to the interrupt handler. Usually, the chip-specific record is
+used for that, but you can use what you like, too.
+
+I won't give details about the interrupt handler at this point, but at
+least its appearance can be explained now. The interrupt handler looks
+usually like the following:
+
+::
+
+  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+  {
+          struct mychip *chip = dev_id;
+          ....
+          return IRQ_HANDLED;
+  }
+
+
+Now let's write the corresponding destructor for the resources above.
+The role of destructor is simple: disable the hardware (if already
+activated) and release the resources. So far, we have no hardware part,
+so the disabling code is not written here.
+
+To release the resources, the “check-and-release” method is a safer way.
+For the interrupt, do like this:
+
+::
+
+  if (chip->irq >= 0)
+          free_irq(chip->irq, chip);
+
+Since the irq number can start from 0, you should initialize
+``chip->irq`` with a negative value (e.g. -1), so that you can check
+the validity of the irq number as above.
+
+When you requested I/O ports or memory regions via
+:c:func:`pci_request_region()` or
+:c:func:`pci_request_regions()` like in this example, release the
+resource(s) using the corresponding function,
+:c:func:`pci_release_region()` or
+:c:func:`pci_release_regions()`.
+
+::
+
+  pci_release_regions(chip->pci);
+
+When you requested manually via :c:func:`request_region()` or
+:c:func:`request_mem_region()`, you can release it via
+:c:func:`release_resource()`. Suppose that you keep the resource
+pointer returned from :c:func:`request_region()` in
+chip->res_port, the release procedure looks like:
+
+::
+
+  release_and_free_resource(chip->res_port);
+
+Don't forget to call :c:func:`pci_disable_device()` before the
+end.
+
+And finally, release the chip-specific record.
+
+::
+
+  kfree(chip);
+
+We didn't implement the hardware disabling part in the above. If you
+need to do this, please note that the destructor may be called even
+before the initialization of the chip is completed. It would be better
+to have a flag to skip hardware disabling if the hardware was not
+initialized yet.
+
+When the chip-data is assigned to the card using
+:c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL`` , its
+destructor is called at the last. That is, it is assured that all other
+components like PCMs and controls have already been released. You don't
+have to stop PCMs, etc. explicitly, but just call low-level hardware
+stopping.
+
+The management of a memory-mapped region is almost as same as the
+management of an I/O port. You'll need three fields like the
+following:
+
+::
+
+  struct mychip {
+          ....
+          unsigned long iobase_phys;
+          void __iomem *iobase_virt;
+  };
+
+and the allocation would be like below:
+
+::
+
+  if ((err = pci_request_regions(pci, "My Chip")) < 0) {
+          kfree(chip);
+          return err;
+  }
+  chip->iobase_phys = pci_resource_start(pci, 0);
+  chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
+                                      pci_resource_len(pci, 0));
+
+and the corresponding destructor would be:
+
+::
+
+  static int snd_mychip_free(struct mychip *chip)
+  {
+          ....
+          if (chip->iobase_virt)
+                  iounmap(chip->iobase_virt);
+          ....
+          pci_release_regions(chip->pci);
+          ....
+  }
+
+PCI Entries
+-----------
+
+So far, so good. Let's finish the missing PCI stuff. At first, we need a
+:c:type:`struct pci_device_id <pci_device_id>` table for
+this chipset. It's a table of PCI vendor/device ID number, and some
+masks.
+
+For example,
+
+::
+
+  static struct pci_device_id snd_mychip_ids[] = {
+          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+          ....
+          { 0, }
+  };
+  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+The first and second fields of the :c:type:`struct pci_device_id
+<pci_device_id>` structure are the vendor and device IDs. If you
+have no reason to filter the matching devices, you can leave the
+remaining fields as above. The last field of the :c:type:`struct
+pci_device_id <pci_device_id>` struct contains private data
+for this entry. You can specify any value here, for example, to define
+specific operations for supported device IDs. Such an example is found
+in the intel8x0 driver.
+
+The last entry of this list is the terminator. You must specify this
+all-zero entry.
+
+Then, prepare the :c:type:`struct pci_driver <pci_driver>`
+record:
+
+::
+
+  static struct pci_driver driver = {
+          .name = KBUILD_MODNAME,
+          .id_table = snd_mychip_ids,
+          .probe = snd_mychip_probe,
+          .remove = snd_mychip_remove,
+  };
+
+The ``probe`` and ``remove`` functions have already been defined in
+the previous sections. The ``name`` field is the name string of this
+device. Note that you must not use a slash “/” in this string.
+
+And at last, the module entries:
+
+::
+
+  static int __init alsa_card_mychip_init(void)
+  {
+          return pci_register_driver(&driver);
+  }
+
+  static void __exit alsa_card_mychip_exit(void)
+  {
+          pci_unregister_driver(&driver);
+  }
+
+  module_init(alsa_card_mychip_init)
+  module_exit(alsa_card_mychip_exit)
+
+Note that these module entries are tagged with ``__init`` and ``__exit``
+prefixes.
+
+Oh, one thing was forgotten. If you have no exported symbols, you need
+to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).
+
+::
+
+  EXPORT_NO_SYMBOLS;
+
+That's all!
+
+PCM Interface
+=============
+
+General
+-------
+
+The PCM middle layer of ALSA is quite powerful and it is only necessary
+for each driver to implement the low-level functions to access its
+hardware.
+
+For accessing to the PCM layer, you need to include ``<sound/pcm.h>``
+first. In addition, ``<sound/pcm_params.h>`` might be needed if you
+access to some functions related with hw_param.
+
+Each card device can have up to four pcm instances. A pcm instance
+corresponds to a pcm device file. The limitation of number of instances
+comes only from the available bit size of the Linux's device numbers.
+Once when 64bit device number is used, we'll have more pcm instances
+available.
+
+A pcm instance consists of pcm playback and capture streams, and each
+pcm stream consists of one or more pcm substreams. Some soundcards
+support multiple playback functions. For example, emu10k1 has a PCM
+playback of 32 stereo substreams. In this case, at each open, a free
+substream is (usually) automatically chosen and opened. Meanwhile, when
+only one substream exists and it was already opened, the successful open
+will either block or error with ``EAGAIN`` according to the file open
+mode. But you don't have to care about such details in your driver. The
+PCM middle layer will take care of such work.
+
+Full Code Example
+-----------------
+
+The example code below does not include any hardware access routines but
+shows only the skeleton, how to build up the PCM interfaces.
+
+::
+
+      #include <sound/pcm.h>
+      ....
+
+      /* hardware definition */
+      static struct snd_pcm_hardware snd_mychip_playback_hw = {
+              .info = (SNDRV_PCM_INFO_MMAP |
+                       SNDRV_PCM_INFO_INTERLEAVED |
+                       SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                       SNDRV_PCM_INFO_MMAP_VALID),
+              .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+              .rates =            SNDRV_PCM_RATE_8000_48000,
+              .rate_min =         8000,
+              .rate_max =         48000,
+              .channels_min =     2,
+              .channels_max =     2,
+              .buffer_bytes_max = 32768,
+              .period_bytes_min = 4096,
+              .period_bytes_max = 32768,
+              .periods_min =      1,
+              .periods_max =      1024,
+      };
+
+      /* hardware definition */
+      static struct snd_pcm_hardware snd_mychip_capture_hw = {
+              .info = (SNDRV_PCM_INFO_MMAP |
+                       SNDRV_PCM_INFO_INTERLEAVED |
+                       SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                       SNDRV_PCM_INFO_MMAP_VALID),
+              .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+              .rates =            SNDRV_PCM_RATE_8000_48000,
+              .rate_min =         8000,
+              .rate_max =         48000,
+              .channels_min =     2,
+              .channels_max =     2,
+              .buffer_bytes_max = 32768,
+              .period_bytes_min = 4096,
+              .period_bytes_max = 32768,
+              .periods_min =      1,
+              .periods_max =      1024,
+      };
+
+      /* open callback */
+      static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              runtime->hw = snd_mychip_playback_hw;
+              /* more hardware-initialization will be done here */
+              ....
+              return 0;
+      }
+
+      /* close callback */
+      static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+
+      }
+
+      /* open callback */
+      static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              runtime->hw = snd_mychip_capture_hw;
+              /* more hardware-initialization will be done here */
+              ....
+              return 0;
+      }
+
+      /* close callback */
+      static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+
+      }
+
+      /* hw_params callback */
+      static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
+                                   struct snd_pcm_hw_params *hw_params)
+      {
+              return snd_pcm_lib_malloc_pages(substream,
+                                         params_buffer_bytes(hw_params));
+      }
+
+      /* hw_free callback */
+      static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
+      {
+              return snd_pcm_lib_free_pages(substream);
+      }
+
+      /* prepare callback */
+      static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              /* set up the hardware with the current configuration
+               * for example...
+               */
+              mychip_set_sample_format(chip, runtime->format);
+              mychip_set_sample_rate(chip, runtime->rate);
+              mychip_set_channels(chip, runtime->channels);
+              mychip_set_dma_setup(chip, runtime->dma_addr,
+                                   chip->buffer_size,
+                                   chip->period_size);
+              return 0;
+      }
+
+      /* trigger callback */
+      static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
+                                        int cmd)
+      {
+              switch (cmd) {
+              case SNDRV_PCM_TRIGGER_START:
+                      /* do something to start the PCM engine */
+                      ....
+                      break;
+              case SNDRV_PCM_TRIGGER_STOP:
+                      /* do something to stop the PCM engine */
+                      ....
+                      break;
+              default:
+                      return -EINVAL;
+              }
+      }
+
+      /* pointer callback */
+      static snd_pcm_uframes_t
+      snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              unsigned int current_ptr;
+
+              /* get the current hardware pointer */
+              current_ptr = mychip_get_hw_pointer(chip);
+              return current_ptr;
+      }
+
+      /* operators */
+      static struct snd_pcm_ops snd_mychip_playback_ops = {
+              .open =        snd_mychip_playback_open,
+              .close =       snd_mychip_playback_close,
+              .ioctl =       snd_pcm_lib_ioctl,
+              .hw_params =   snd_mychip_pcm_hw_params,
+              .hw_free =     snd_mychip_pcm_hw_free,
+              .prepare =     snd_mychip_pcm_prepare,
+              .trigger =     snd_mychip_pcm_trigger,
+              .pointer =     snd_mychip_pcm_pointer,
+      };
+
+      /* operators */
+      static struct snd_pcm_ops snd_mychip_capture_ops = {
+              .open =        snd_mychip_capture_open,
+              .close =       snd_mychip_capture_close,
+              .ioctl =       snd_pcm_lib_ioctl,
+              .hw_params =   snd_mychip_pcm_hw_params,
+              .hw_free =     snd_mychip_pcm_hw_free,
+              .prepare =     snd_mychip_pcm_prepare,
+              .trigger =     snd_mychip_pcm_trigger,
+              .pointer =     snd_mychip_pcm_pointer,
+      };
+
+      /*
+       *  definitions of capture are omitted here...
+       */
+
+      /* create a pcm device */
+      static int snd_mychip_new_pcm(struct mychip *chip)
+      {
+              struct snd_pcm *pcm;
+              int err;
+
+              err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+              if (err < 0)
+                      return err;
+              pcm->private_data = chip;
+              strcpy(pcm->name, "My Chip");
+              chip->pcm = pcm;
+              /* set operators */
+              snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+                              &snd_mychip_playback_ops);
+              snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+                              &snd_mychip_capture_ops);
+              /* pre-allocation of buffers */
+              /* NOTE: this may fail */
+              snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                                    snd_dma_pci_data(chip->pci),
+                                                    64*1024, 64*1024);
+              return 0;
+      }
+
+
+PCM Constructor
+---------------
+
+A pcm instance is allocated by the :c:func:`snd_pcm_new()`
+function. It would be better to create a constructor for pcm, namely,
+
+::
+
+  static int snd_mychip_new_pcm(struct mychip *chip)
+  {
+          struct snd_pcm *pcm;
+          int err;
+
+          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+          if (err < 0) 
+                  return err;
+          pcm->private_data = chip;
+          strcpy(pcm->name, "My Chip");
+          chip->pcm = pcm;
+	  ....
+          return 0;
+  }
+
+The :c:func:`snd_pcm_new()` function takes four arguments. The
+first argument is the card pointer to which this pcm is assigned, and
+the second is the ID string.
+
+The third argument (``index``, 0 in the above) is the index of this new
+pcm. It begins from zero. If you create more than one pcm instances,
+specify the different numbers in this argument. For example, ``index =
+1`` for the second PCM device.
+
+The fourth and fifth arguments are the number of substreams for playback
+and capture, respectively. Here 1 is used for both arguments. When no
+playback or capture substreams are available, pass 0 to the
+corresponding argument.
+
+If a chip supports multiple playbacks or captures, you can specify more
+numbers, but they must be handled properly in open/close, etc.
+callbacks. When you need to know which substream you are referring to,
+then it can be obtained from :c:type:`struct snd_pcm_substream
+<snd_pcm_substream>` data passed to each callback as follows:
+
+::
+
+  struct snd_pcm_substream *substream;
+  int index = substream->number;
+
+
+After the pcm is created, you need to set operators for each pcm stream.
+
+::
+
+  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+                  &snd_mychip_playback_ops);
+  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+                  &snd_mychip_capture_ops);
+
+The operators are defined typically like this:
+
+::
+
+  static struct snd_pcm_ops snd_mychip_playback_ops = {
+          .open =        snd_mychip_pcm_open,
+          .close =       snd_mychip_pcm_close,
+          .ioctl =       snd_pcm_lib_ioctl,
+          .hw_params =   snd_mychip_pcm_hw_params,
+          .hw_free =     snd_mychip_pcm_hw_free,
+          .prepare =     snd_mychip_pcm_prepare,
+          .trigger =     snd_mychip_pcm_trigger,
+          .pointer =     snd_mychip_pcm_pointer,
+  };
+
+All the callbacks are described in the Operators_ subsection.
+
+After setting the operators, you probably will want to pre-allocate the
+buffer. For the pre-allocation, simply call the following:
+
+::
+
+  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                        snd_dma_pci_data(chip->pci),
+                                        64*1024, 64*1024);
+
+It will allocate a buffer up to 64kB as default. Buffer management
+details will be described in the later section `Buffer and Memory
+Management`_.
+
+Additionally, you can set some extra information for this pcm in
+``pcm->info_flags``. The available values are defined as
+``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the
+hardware definition (described later). When your soundchip supports only
+half-duplex, specify like this:
+
+::
+
+  pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
+
+
+... And the Destructor?
+-----------------------
+
+The destructor for a pcm instance is not always necessary. Since the pcm
+device will be released by the middle layer code automatically, you
+don't have to call the destructor explicitly.
+
+The destructor would be necessary if you created special records
+internally and needed to release them. In such a case, set the
+destructor function to ``pcm->private_free``:
+
+::
+
+      static void mychip_pcm_free(struct snd_pcm *pcm)
+      {
+              struct mychip *chip = snd_pcm_chip(pcm);
+              /* free your own data */
+              kfree(chip->my_private_pcm_data);
+              /* do what you like else */
+              ....
+      }
+
+      static int snd_mychip_new_pcm(struct mychip *chip)
+      {
+              struct snd_pcm *pcm;
+              ....
+              /* allocate your own data */
+              chip->my_private_pcm_data = kmalloc(...);
+              /* set the destructor */
+              pcm->private_data = chip;
+              pcm->private_free = mychip_pcm_free;
+              ....
+      }
+
+
+
+Runtime Pointer - The Chest of PCM Information
+----------------------------------------------
+
+When the PCM substream is opened, a PCM runtime instance is allocated
+and assigned to the substream. This pointer is accessible via
+``substream->runtime``. This runtime pointer holds most information you
+need to control the PCM: the copy of hw_params and sw_params
+configurations, the buffer pointers, mmap records, spinlocks, etc.
+
+The definition of runtime instance is found in ``<sound/pcm.h>``. Here
+are the contents of this file:
+
+::
+
+  struct _snd_pcm_runtime {
+          /* -- Status -- */
+          struct snd_pcm_substream *trigger_master;
+          snd_timestamp_t trigger_tstamp;	/* trigger timestamp */
+          int overrange;
+          snd_pcm_uframes_t avail_max;
+          snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
+          snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
+  
+          /* -- HW params -- */
+          snd_pcm_access_t access;	/* access mode */
+          snd_pcm_format_t format;	/* SNDRV_PCM_FORMAT_* */
+          snd_pcm_subformat_t subformat;	/* subformat */
+          unsigned int rate;		/* rate in Hz */
+          unsigned int channels;		/* channels */
+          snd_pcm_uframes_t period_size;	/* period size */
+          unsigned int periods;		/* periods */
+          snd_pcm_uframes_t buffer_size;	/* buffer size */
+          unsigned int tick_time;		/* tick time */
+          snd_pcm_uframes_t min_align;	/* Min alignment for the format */
+          size_t byte_align;
+          unsigned int frame_bits;
+          unsigned int sample_bits;
+          unsigned int info;
+          unsigned int rate_num;
+          unsigned int rate_den;
+  
+          /* -- SW params -- */
+          struct timespec tstamp_mode;	/* mmap timestamp is updated */
+          unsigned int period_step;
+          unsigned int sleep_min;		/* min ticks to sleep */
+          snd_pcm_uframes_t start_threshold;
+          snd_pcm_uframes_t stop_threshold;
+          snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
+                                                  noise is nearest than this */
+          snd_pcm_uframes_t silence_size;	/* Silence filling size */
+          snd_pcm_uframes_t boundary;	/* pointers wrap point */
+  
+          snd_pcm_uframes_t silenced_start;
+          snd_pcm_uframes_t silenced_size;
+  
+          snd_pcm_sync_id_t sync;		/* hardware synchronization ID */
+  
+          /* -- mmap -- */
+          volatile struct snd_pcm_mmap_status *status;
+          volatile struct snd_pcm_mmap_control *control;
+          atomic_t mmap_count;
+  
+          /* -- locking / scheduling -- */
+          spinlock_t lock;
+          wait_queue_head_t sleep;
+          struct timer_list tick_timer;
+          struct fasync_struct *fasync;
+
+          /* -- private section -- */
+          void *private_data;
+          void (*private_free)(struct snd_pcm_runtime *runtime);
+  
+          /* -- hardware description -- */
+          struct snd_pcm_hardware hw;
+          struct snd_pcm_hw_constraints hw_constraints;
+  
+          /* -- timer -- */
+          unsigned int timer_resolution;	/* timer resolution */
+  
+          /* -- DMA -- */           
+          unsigned char *dma_area;	/* DMA area */
+          dma_addr_t dma_addr;		/* physical bus address (not accessible from main CPU) */
+          size_t dma_bytes;		/* size of DMA area */
+  
+          struct snd_dma_buffer *dma_buffer_p;	/* allocated buffer */
+  
+  #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
+          /* -- OSS things -- */
+          struct snd_pcm_oss_runtime oss;
+  #endif
+  };
+
+
+For the operators (callbacks) of each sound driver, most of these
+records are supposed to be read-only. Only the PCM middle-layer changes
+/ updates them. The exceptions are the hardware description (hw) DMA
+buffer information and the private data. Besides, if you use the
+standard buffer allocation method via
+:c:func:`snd_pcm_lib_malloc_pages()`, you don't need to set the
+DMA buffer information by yourself.
+
+In the sections below, important records are explained.
+
+Hardware Description
+~~~~~~~~~~~~~~~~~~~~
+
+The hardware descriptor (:c:type:`struct snd_pcm_hardware
+<snd_pcm_hardware>`) contains the definitions of the fundamental
+hardware configuration. Above all, you'll need to define this in the
+`PCM open callback`_. Note that the runtime instance holds the copy of
+the descriptor, not the pointer to the existing descriptor. That is,
+in the open callback, you can modify the copied descriptor
+(``runtime->hw``) as you need. For example, if the maximum number of
+channels is 1 only on some chip models, you can still use the same
+hardware descriptor and change the channels_max later:
+
+::
+
+          struct snd_pcm_runtime *runtime = substream->runtime;
+          ...
+          runtime->hw = snd_mychip_playback_hw; /* common definition */
+          if (chip->model == VERY_OLD_ONE)
+                  runtime->hw.channels_max = 1;
+
+Typically, you'll have a hardware descriptor as below:
+
+::
+
+  static struct snd_pcm_hardware snd_mychip_playback_hw = {
+          .info = (SNDRV_PCM_INFO_MMAP |
+                   SNDRV_PCM_INFO_INTERLEAVED |
+                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                   SNDRV_PCM_INFO_MMAP_VALID),
+          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+          .rates =            SNDRV_PCM_RATE_8000_48000,
+          .rate_min =         8000,
+          .rate_max =         48000,
+          .channels_min =     2,
+          .channels_max =     2,
+          .buffer_bytes_max = 32768,
+          .period_bytes_min = 4096,
+          .period_bytes_max = 32768,
+          .periods_min =      1,
+          .periods_max =      1024,
+  };
+
+-  The ``info`` field contains the type and capabilities of this
+   pcm. The bit flags are defined in ``<sound/asound.h>`` as
+   ``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether
+   the mmap is supported and which interleaved format is
+   supported. When the hardware supports mmap, add the
+   ``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the
+   interleaved or the non-interleaved formats,
+   ``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED``
+   flag must be set, respectively. If both are supported, you can set
+   both, too.
+
+   In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are
+   specified for the OSS mmap mode. Usually both are set. Of course,
+   ``MMAP_VALID`` is set only if the mmap is really supported.
+
+   The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and
+   ``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the pcm
+   supports the “pause” operation, while the ``RESUME`` bit means that
+   the pcm supports the full “suspend/resume” operation. If the
+   ``PAUSE`` flag is set, the ``trigger`` callback below must handle
+   the corresponding (pause push/release) commands. The suspend/resume
+   trigger commands can be defined even without the ``RESUME``
+   flag. See `Power Management`_ section for details.
+
+   When the PCM substreams can be synchronized (typically,
+   synchronized start/stop of a playback and a capture streams), you
+   can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll
+   need to check the linked-list of PCM substreams in the trigger
+   callback. This will be described in the later section.
+
+-  ``formats`` field contains the bit-flags of supported formats
+   (``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one
+   format, give all or'ed bits. In the example above, the signed 16bit
+   little-endian format is specified.
+
+-  ``rates`` field contains the bit-flags of supported rates
+   (``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates,
+   pass ``CONTINUOUS`` bit additionally. The pre-defined rate bits are
+   provided only for typical rates. If your chip supports
+   unconventional rates, you need to add the ``KNOT`` bit and set up
+   the hardware constraint manually (explained later).
+
+-  ``rate_min`` and ``rate_max`` define the minimum and maximum sample
+   rate. This should correspond somehow to ``rates`` bits.
+
+-  ``channel_min`` and ``channel_max`` define, as you might already
+   expected, the minimum and maximum number of channels.
+
+-  ``buffer_bytes_max`` defines the maximum buffer size in
+   bytes. There is no ``buffer_bytes_min`` field, since it can be
+   calculated from the minimum period size and the minimum number of
+   periods. Meanwhile, ``period_bytes_min`` and define the minimum and
+   maximum size of the period in bytes. ``periods_max`` and
+   ``periods_min`` define the maximum and minimum number of periods in
+   the buffer.
+
+   The “period” is a term that corresponds to a fragment in the OSS
+   world. The period defines the size at which a PCM interrupt is
+   generated. This size strongly depends on the hardware. Generally,
+   the smaller period size will give you more interrupts, that is,
+   more controls. In the case of capture, this size defines the input
+   latency. On the other hand, the whole buffer size defines the
+   output latency for the playback direction.
+
+-  There is also a field ``fifo_size``. This specifies the size of the
+   hardware FIFO, but currently it is neither used in the driver nor
+   in the alsa-lib. So, you can ignore this field.
+
+PCM Configurations
+~~~~~~~~~~~~~~~~~~
+
+Ok, let's go back again to the PCM runtime records. The most
+frequently referred records in the runtime instance are the PCM
+configurations. The PCM configurations are stored in the runtime
+instance after the application sends ``hw_params`` data via
+alsa-lib. There are many fields copied from hw_params and sw_params
+structs. For example, ``format`` holds the format type chosen by the
+application. This field contains the enum value
+``SNDRV_PCM_FORMAT_XXX``.
+
+One thing to be noted is that the configured buffer and period sizes
+are stored in “frames” in the runtime. In the ALSA world, ``1 frame =
+channels \* samples-size``. For conversion between frames and bytes,
+you can use the :c:func:`frames_to_bytes()` and
+:c:func:`bytes_to_frames()` helper functions.
+
+::
+
+  period_bytes = frames_to_bytes(runtime, runtime->period_size);
+
+Also, many software parameters (sw_params) are stored in frames, too.
+Please check the type of the field. ``snd_pcm_uframes_t`` is for the
+frames as unsigned integer while ``snd_pcm_sframes_t`` is for the
+frames as signed integer.
+
+DMA Buffer Information
+~~~~~~~~~~~~~~~~~~~~~~
+
+The DMA buffer is defined by the following four fields, ``dma_area``,
+``dma_addr``, ``dma_bytes`` and ``dma_private``. The ``dma_area``
+holds the buffer pointer (the logical address). You can call
+:c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds
+the physical address of the buffer. This field is specified only when
+the buffer is a linear buffer. ``dma_bytes`` holds the size of buffer
+in bytes. ``dma_private`` is used for the ALSA DMA allocator.
+
+If you use a standard ALSA function,
+:c:func:`snd_pcm_lib_malloc_pages()`, for allocating the buffer,
+these fields are set by the ALSA middle layer, and you should *not*
+change them by yourself. You can read them but not write them. On the
+other hand, if you want to allocate the buffer by yourself, you'll
+need to manage it in hw_params callback. At least, ``dma_bytes`` is
+mandatory. ``dma_area`` is necessary when the buffer is mmapped. If
+your driver doesn't support mmap, this field is not
+necessary. ``dma_addr`` is also optional. You can use dma_private as
+you like, too.
+
+Running Status
+~~~~~~~~~~~~~~
+
+The running status can be referred via ``runtime->status``. This is
+the pointer to the :c:type:`struct snd_pcm_mmap_status
+<snd_pcm_mmap_status>` record. For example, you can get the current
+DMA hardware pointer via ``runtime->status->hw_ptr``.
+
+The DMA application pointer can be referred via ``runtime->control``,
+which points to the :c:type:`struct snd_pcm_mmap_control
+<snd_pcm_mmap_control>` record. However, accessing directly to
+this value is not recommended.
+
+Private Data
+~~~~~~~~~~~~
+
+You can allocate a record for the substream and store it in
+``runtime->private_data``. Usually, this is done in the `PCM open
+callback`_. Don't mix this with ``pcm->private_data``. The
+``pcm->private_data`` usually points to the chip instance assigned
+statically at the creation of PCM, while the ``runtime->private_data``
+points to a dynamic data structure created at the PCM open
+callback.
+
+::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream)
+  {
+          struct my_pcm_data *data;
+          ....
+          data = kmalloc(sizeof(*data), GFP_KERNEL);
+          substream->runtime->private_data = data;
+          ....
+  }
+
+
+The allocated object must be released in the `close callback`_.
+
+Operators
+---------
+
+OK, now let me give details about each pcm callback (``ops``). In
+general, every callback must return 0 if successful, or a negative
+error number such as ``-EINVAL``. To choose an appropriate error
+number, it is advised to check what value other parts of the kernel
+return when the same kind of request fails.
+
+The callback function takes at least the argument with :c:type:`struct
+snd_pcm_substream <snd_pcm_substream>` pointer. To retrieve the chip
+record from the given substream instance, you can use the following
+macro.
+
+::
+
+  int xxx() {
+          struct mychip *chip = snd_pcm_substream_chip(substream);
+          ....
+  }
+
+The macro reads ``substream->private_data``, which is a copy of
+``pcm->private_data``. You can override the former if you need to
+assign different data records per PCM substream. For example, the
+cmi8330 driver assigns different ``private_data`` for playback and
+capture directions, because it uses two different codecs (SB- and
+AD-compatible) for different directions.
+
+PCM open callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream);
+
+This is called when a pcm substream is opened.
+
+At least, here you have to initialize the ``runtime->hw``
+record. Typically, this is done by like this:
+
+::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream)
+  {
+          struct mychip *chip = snd_pcm_substream_chip(substream);
+          struct snd_pcm_runtime *runtime = substream->runtime;
+
+          runtime->hw = snd_mychip_playback_hw;
+          return 0;
+  }
+
+where ``snd_mychip_playback_hw`` is the pre-defined hardware
+description.
+
+You can allocate a private data in this callback, as described in
+`Private Data`_ section.
+
+If the hardware configuration needs more constraints, set the hardware
+constraints here, too. See Constraints_ for more details.
+
+close callback
+~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_close(struct snd_pcm_substream *substream);
+
+
+Obviously, this is called when a pcm substream is closed.
+
+Any private instance for a pcm substream allocated in the ``open``
+callback will be released here.
+
+::
+
+  static int snd_xxx_close(struct snd_pcm_substream *substream)
+  {
+          ....
+          kfree(substream->runtime->private_data);
+          ....
+  }
+
+ioctl callback
+~~~~~~~~~~~~~~
+
+This is used for any special call to pcm ioctls. But usually you can
+pass a generic ioctl callback, :c:func:`snd_pcm_lib_ioctl()`.
+
+hw_params callback
+~~~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
+                               struct snd_pcm_hw_params *hw_params);
+
+This is called when the hardware parameter (``hw_params``) is set up
+by the application, that is, once when the buffer size, the period
+size, the format, etc. are defined for the pcm substream.
+
+Many hardware setups should be done in this callback, including the
+allocation of buffers.
+
+Parameters to be initialized are retrieved by
+:c:func:`params_xxx()` macros. To allocate buffer, you can call a
+helper function,
+
+::
+
+  snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
+
+:c:func:`snd_pcm_lib_malloc_pages()` is available only when the
+DMA buffers have been pre-allocated. See the section `Buffer Types`_
+for more details.
+
+Note that this and ``prepare`` callbacks may be called multiple times
+per initialization. For example, the OSS emulation may call these
+callbacks at each change via its ioctl.
+
+Thus, you need to be careful not to allocate the same buffers many
+times, which will lead to memory leaks! Calling the helper function
+above many times is OK. It will release the previous buffer
+automatically when it was already allocated.
+
+Another note is that this callback is non-atomic (schedulable) as
+default, i.e. when no ``nonatomic`` flag set. This is important,
+because the ``trigger`` callback is atomic (non-schedulable). That is,
+mutexes or any schedule-related functions are not available in
+``trigger`` callback. Please see the subsection Atomicity_ for
+details.
+
+hw_free callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
+
+This is called to release the resources allocated via
+``hw_params``. For example, releasing the buffer via
+:c:func:`snd_pcm_lib_malloc_pages()` is done by calling the
+following:
+
+::
+
+  snd_pcm_lib_free_pages(substream);
+
+This function is always called before the close callback is called.
+Also, the callback may be called multiple times, too. Keep track
+whether the resource was already released.
+
+prepare callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_prepare(struct snd_pcm_substream *substream);
+
+This callback is called when the pcm is “prepared”. You can set the
+format type, sample rate, etc. here. The difference from ``hw_params``
+is that the ``prepare`` callback will be called each time
+:c:func:`snd_pcm_prepare()` is called, i.e. when recovering after
+underruns, etc.
+
+Note that this callback is now non-atomic. You can use
+schedule-related functions safely in this callback.
+
+In this and the following callbacks, you can refer to the values via
+the runtime record, ``substream->runtime``. For example, to get the
+current rate, format or channels, access to ``runtime->rate``,
+``runtime->format`` or ``runtime->channels``, respectively. The
+physical address of the allocated buffer is set to
+``runtime->dma_area``. The buffer and period sizes are in
+``runtime->buffer_size`` and ``runtime->period_size``, respectively.
+
+Be careful that this callback will be called many times at each setup,
+too.
+
+trigger callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
+
+This is called when the pcm is started, stopped or paused.
+
+Which action is specified in the second argument,
+``SNDRV_PCM_TRIGGER_XXX`` in ``<sound/pcm.h>``. At least, the ``START``
+and ``STOP`` commands must be defined in this callback.
+
+::
+
+  switch (cmd) {
+  case SNDRV_PCM_TRIGGER_START:
+          /* do something to start the PCM engine */
+          break;
+  case SNDRV_PCM_TRIGGER_STOP:
+          /* do something to stop the PCM engine */
+          break;
+  default:
+          return -EINVAL;
+  }
+
+When the pcm supports the pause operation (given in the info field of
+the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands
+must be handled here, too. The former is the command to pause the pcm,
+and the latter to restart the pcm again.
+
+When the pcm supports the suspend/resume operation, regardless of full
+or partial suspend/resume support, the ``SUSPEND`` and ``RESUME``
+commands must be handled, too. These commands are issued when the
+power-management status is changed. Obviously, the ``SUSPEND`` and
+``RESUME`` commands suspend and resume the pcm substream, and usually,
+they are identical to the ``STOP`` and ``START`` commands, respectively.
+See the `Power Management`_ section for details.
+
+As mentioned, this callback is atomic as default unless ``nonatomic``
+flag set, and you cannot call functions which may sleep. The
+``trigger`` callback should be as minimal as possible, just really
+triggering the DMA. The other stuff should be initialized
+``hw_params`` and ``prepare`` callbacks properly beforehand.
+
+pointer callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
+
+This callback is called when the PCM middle layer inquires the current
+hardware position on the buffer. The position must be returned in
+frames, ranging from 0 to ``buffer_size - 1``. 
+
+This is called usually from the buffer-update routine in the pcm
+middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()`
+is called in the interrupt routine. Then the pcm middle layer updates
+the position and calculates the available space, and wakes up the
+sleeping poll threads, etc.
+
+This callback is also atomic as default.
+
+copy and silence callbacks
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These callbacks are not mandatory, and can be omitted in most cases.
+These callbacks are used when the hardware buffer cannot be in the
+normal memory space. Some chips have their own buffer on the hardware
+which is not mappable. In such a case, you have to transfer the data
+manually from the memory buffer to the hardware buffer. Or, if the
+buffer is non-contiguous on both physical and virtual memory spaces,
+these callbacks must be defined, too.
+
+If these two callbacks are defined, copy and set-silence operations
+are done by them. The detailed will be described in the later section
+`Buffer and Memory Management`_.
+
+ack callback
+~~~~~~~~~~~~
+
+This callback is also not mandatory. This callback is called when the
+``appl_ptr`` is updated in read or write operations. Some drivers like
+emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
+internal buffer, and this callback is useful only for such a purpose.
+
+This callback is atomic as default.
+
+page callback
+~~~~~~~~~~~~~
+
+This callback is optional too. This callback is used mainly for
+non-contiguous buffers. The mmap calls this callback to get the page
+address. Some examples will be explained in the later section `Buffer
+and Memory Management`_, too.
+
+PCM Interrupt Handler
+---------------------
+
+The rest of pcm stuff is the PCM interrupt handler. The role of PCM
+interrupt handler in the sound driver is to update the buffer position
+and to tell the PCM middle layer when the buffer position goes across
+the prescribed period size. To inform this, call the
+:c:func:`snd_pcm_period_elapsed()` function.
+
+There are several types of sound chips to generate the interrupts.
+
+Interrupts at the period (fragment) boundary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This is the most frequently found type: the hardware generates an
+interrupt at each period boundary. In this case, you can call
+:c:func:`snd_pcm_period_elapsed()` at each interrupt.
+
+:c:func:`snd_pcm_period_elapsed()` takes the substream pointer as
+its argument. Thus, you need to keep the substream pointer accessible
+from the chip instance. For example, define ``substream`` field in the
+chip record to hold the current running substream pointer, and set the
+pointer value at ``open`` callback (and reset at ``close`` callback).
+
+If you acquire a spinlock in the interrupt handler, and the lock is used
+in other pcm callbacks, too, then you have to release the lock before
+calling :c:func:`snd_pcm_period_elapsed()`, because
+:c:func:`snd_pcm_period_elapsed()` calls other pcm callbacks
+inside.
+
+Typical code would be like:
+
+::
+
+
+      static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+      {
+              struct mychip *chip = dev_id;
+              spin_lock(&chip->lock);
+              ....
+              if (pcm_irq_invoked(chip)) {
+                      /* call updater, unlock before it */
+                      spin_unlock(&chip->lock);
+                      snd_pcm_period_elapsed(chip->substream);
+                      spin_lock(&chip->lock);
+                      /* acknowledge the interrupt if necessary */
+              }
+              ....
+              spin_unlock(&chip->lock);
+              return IRQ_HANDLED;
+      }
+
+
+
+High frequency timer interrupts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This happens when the hardware doesn't generate interrupts at the period
+boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
+or ymfpci drivers). In this case, you need to check the current hardware
+position and accumulate the processed sample length at each interrupt.
+When the accumulated size exceeds the period size, call
+:c:func:`snd_pcm_period_elapsed()` and reset the accumulator.
+
+Typical code would be like the following.
+
+::
+
+
+      static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+      {
+              struct mychip *chip = dev_id;
+              spin_lock(&chip->lock);
+              ....
+              if (pcm_irq_invoked(chip)) {
+                      unsigned int last_ptr, size;
+                      /* get the current hardware pointer (in frames) */
+                      last_ptr = get_hw_ptr(chip);
+                      /* calculate the processed frames since the
+                       * last update
+                       */
+                      if (last_ptr < chip->last_ptr)
+                              size = runtime->buffer_size + last_ptr
+                                       - chip->last_ptr;
+                      else
+                              size = last_ptr - chip->last_ptr;
+                      /* remember the last updated point */
+                      chip->last_ptr = last_ptr;
+                      /* accumulate the size */
+                      chip->size += size;
+                      /* over the period boundary? */
+                      if (chip->size >= runtime->period_size) {
+                              /* reset the accumulator */
+                              chip->size %= runtime->period_size;
+                              /* call updater */
+                              spin_unlock(&chip->lock);
+                              snd_pcm_period_elapsed(substream);
+                              spin_lock(&chip->lock);
+                      }
+                      /* acknowledge the interrupt if necessary */
+              }
+              ....
+              spin_unlock(&chip->lock);
+              return IRQ_HANDLED;
+      }
+
+
+
+On calling :c:func:`snd_pcm_period_elapsed()`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In both cases, even if more than one period are elapsed, you don't have
+to call :c:func:`snd_pcm_period_elapsed()` many times. Call only
+once. And the pcm layer will check the current hardware pointer and
+update to the latest status.
+
+Atomicity
+---------
+
+One of the most important (and thus difficult to debug) problems in
+kernel programming are race conditions. In the Linux kernel, they are
+usually avoided via spin-locks, mutexes or semaphores. In general, if a
+race condition can happen in an interrupt handler, it has to be managed
+atomically, and you have to use a spinlock to protect the critical
+session. If the critical section is not in interrupt handler code and if
+taking a relatively long time to execute is acceptable, you should use
+mutexes or semaphores instead.
+
+As already seen, some pcm callbacks are atomic and some are not. For
+example, the ``hw_params`` callback is non-atomic, while ``trigger``
+callback is atomic. This means, the latter is called already in a
+spinlock held by the PCM middle layer. Please take this atomicity into
+account when you choose a locking scheme in the callbacks.
+
+In the atomic callbacks, you cannot use functions which may call
+:c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
+mutexes can sleep, and hence they cannot be used inside the atomic
+callbacks (e.g. ``trigger`` callback). To implement some delay in such a
+callback, please use :c:func:`udelay()` or :c:func:`mdelay()`.
+
+All three atomic callbacks (trigger, pointer, and ack) are called with
+local interrupts disabled.
+
+The recent changes in PCM core code, however, allow all PCM operations
+to be non-atomic. This assumes that the all caller sides are in
+non-atomic contexts. For example, the function
+:c:func:`snd_pcm_period_elapsed()` is called typically from the
+interrupt handler. But, if you set up the driver to use a threaded
+interrupt handler, this call can be in non-atomic context, too. In such
+a case, you can set ``nonatomic`` filed of :c:type:`struct snd_pcm
+<snd_pcm>` object after creating it. When this flag is set, mutex
+and rwsem are used internally in the PCM core instead of spin and
+rwlocks, so that you can call all PCM functions safely in a non-atomic
+context.
+
+Constraints
+-----------
+
+If your chip supports unconventional sample rates, or only the limited
+samples, you need to set a constraint for the condition.
+
+For example, in order to restrict the sample rates in the some supported
+values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to
+call this function in the open callback.
+
+::
+
+      static unsigned int rates[] =
+              {4000, 10000, 22050, 44100};
+      static struct snd_pcm_hw_constraint_list constraints_rates = {
+              .count = ARRAY_SIZE(rates),
+              .list = rates,
+              .mask = 0,
+      };
+
+      static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
+      {
+              int err;
+              ....
+              err = snd_pcm_hw_constraint_list(substream->runtime, 0,
+                                               SNDRV_PCM_HW_PARAM_RATE,
+                                               &constraints_rates);
+              if (err < 0)
+                      return err;
+              ....
+      }
+
+
+
+There are many different constraints. Look at ``sound/pcm.h`` for a
+complete list. You can even define your own constraint rules. For
+example, let's suppose my_chip can manage a substream of 1 channel if
+and only if the format is ``S16_LE``, otherwise it supports any format
+specified in the :c:type:`struct snd_pcm_hardware
+<snd_pcm_hardware>` structure (or in any other
+constraint_list). You can build a rule like this:
+
+::
+
+      static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
+                                            struct snd_pcm_hw_rule *rule)
+      {
+              struct snd_interval *c = hw_param_interval(params,
+                            SNDRV_PCM_HW_PARAM_CHANNELS);
+              struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+              struct snd_interval ch;
+
+              snd_interval_any(&ch);
+              if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
+                      ch.min = ch.max = 1;
+                      ch.integer = 1;
+                      return snd_interval_refine(c, &ch);
+              }
+              return 0;
+      }
+
+
+Then you need to call this function to add your rule:
+
+::
+
+  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
+                      hw_rule_channels_by_format, NULL,
+                      SNDRV_PCM_HW_PARAM_FORMAT, -1);
+
+The rule function is called when an application sets the PCM format, and
+it refines the number of channels accordingly. But an application may
+set the number of channels before setting the format. Thus you also need
+to define the inverse rule:
+
+::
+
+      static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
+                                            struct snd_pcm_hw_rule *rule)
+      {
+              struct snd_interval *c = hw_param_interval(params,
+                    SNDRV_PCM_HW_PARAM_CHANNELS);
+              struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+              struct snd_mask fmt;
+
+              snd_mask_any(&fmt);    /* Init the struct */
+              if (c->min < 2) {
+                      fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
+                      return snd_mask_refine(f, &fmt);
+              }
+              return 0;
+      }
+
+
+... and in the open callback:
+
+::
+
+  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
+                      hw_rule_format_by_channels, NULL,
+                      SNDRV_PCM_HW_PARAM_CHANNELS, -1);
+
+I won't give more details here, rather I would like to say, “Luke, use
+the source.”
+
+Control Interface
+=================
+
+General
+-------
+
+The control interface is used widely for many switches, sliders, etc.
+which are accessed from user-space. Its most important use is the mixer
+interface. In other words, since ALSA 0.9.x, all the mixer stuff is
+implemented on the control kernel API.
+
+ALSA has a well-defined AC97 control module. If your chip supports only
+the AC97 and nothing else, you can skip this section.
+
+The control API is defined in ``<sound/control.h>``. Include this file
+if you want to add your own controls.
+
+Definition of Controls
+----------------------
+
+To create a new control, you need to define the following three
+callbacks: ``info``, ``get`` and ``put``. Then, define a
+:c:type:`struct snd_kcontrol_new <snd_kcontrol_new>` record, such as:
+
+::
+
+
+      static struct snd_kcontrol_new my_control = {
+              .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+              .name = "PCM Playback Switch",
+              .index = 0,
+              .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
+              .private_value = 0xffff,
+              .info = my_control_info,
+              .get = my_control_get,
+              .put = my_control_put
+      };
+
+
+The ``iface`` field specifies the control type,
+``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD``
+for global controls that are not logically part of the mixer. If the
+control is closely associated with some specific device on the sound
+card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``,
+and specify the device number with the ``device`` and ``subdevice``
+fields.
+
+The ``name`` is the name identifier string. Since ALSA 0.9.x, the
+control name is very important, because its role is classified from
+its name. There are pre-defined standard control names. The details
+are described in the `Control Names`_ subsection.
+
+The ``index`` field holds the index number of this control. If there
+are several different controls with the same name, they can be
+distinguished by the index number. This is the case when several
+codecs exist on the card. If the index is zero, you can omit the
+definition above. 
+
+The ``access`` field contains the access type of this control. Give
+the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``,
+there. The details will be explained in the `Access Flags`_
+subsection.
+
+The ``private_value`` field contains an arbitrary long integer value
+for this record. When using the generic ``info``, ``get`` and ``put``
+callbacks, you can pass a value through this field. If several small
+numbers are necessary, you can combine them in bitwise. Or, it's
+possible to give a pointer (casted to unsigned long) of some record to
+this field, too. 
+
+The ``tlv`` field can be used to provide metadata about the control;
+see the `Metadata`_ subsection.
+
+The other three are `Control Callbacks`_.
+
+Control Names
+-------------
+
+There are some standards to define the control names. A control is
+usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
+
+The first, ``SOURCE``, specifies the source of the control, and is a
+string such as “Master”, “PCM”, “CD” and “Line”. There are many
+pre-defined sources.
+
+The second, ``DIRECTION``, is one of the following strings according to
+the direction of the control: “Playback”, “Capture”, “Bypass Playback”
+and “Bypass Capture”. Or, it can be omitted, meaning both playback and
+capture directions.
+
+The third, ``FUNCTION``, is one of the following strings according to
+the function of the control: “Switch”, “Volume” and “Route”.
+
+The example of control names are, thus, “Master Capture Switch” or “PCM
+Playback Volume”.
+
+There are some exceptions:
+
+Global capture and playback
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+“Capture Source”, “Capture Switch” and “Capture Volume” are used for the
+global capture (input) source, switch and volume. Similarly, “Playback
+Switch” and “Playback Volume” are used for the global output gain switch
+and volume.
+
+Tone-controls
+~~~~~~~~~~~~~
+
+tone-control switch and volumes are specified like “Tone Control - XXX”,
+e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control -
+Center”.
+
+3D controls
+~~~~~~~~~~~
+
+3D-control switches and volumes are specified like “3D Control - XXX”,
+e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
+
+Mic boost
+~~~~~~~~~
+
+Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
+
+More precise information can be found in
+``Documentation/sound/alsa/ControlNames.txt``.
+
+Access Flags
+------------
+
+The access flag is the bitmask which specifies the access type of the
+given control. The default access type is
+``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are
+allowed to this control. When the access flag is omitted (i.e. = 0), it
+is considered as ``READWRITE`` access as default.
+
+When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ``
+instead. In this case, you don't have to define the ``put`` callback.
+Similarly, when the control is write-only (although it's a rare case),
+you can use the ``WRITE`` flag instead, and you don't need the ``get``
+callback.
+
+If the control value changes frequently (e.g. the VU meter),
+``VOLATILE`` flag should be given. This means that the control may be
+changed without `Change notification`_. Applications should poll such
+a control constantly.
+
+When the control is inactive, set the ``INACTIVE`` flag, too. There are
+``LOCK`` and ``OWNER`` flags to change the write permissions.
+
+Control Callbacks
+-----------------
+
+info callback
+~~~~~~~~~~~~~
+
+The ``info`` callback is used to get detailed information on this
+control. This must store the values of the given :c:type:`struct
+snd_ctl_elem_info <snd_ctl_elem_info>` object. For example,
+for a boolean control with a single element:
+
+::
+
+
+      static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
+                              struct snd_ctl_elem_info *uinfo)
+      {
+              uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
+              uinfo->count = 1;
+              uinfo->value.integer.min = 0;
+              uinfo->value.integer.max = 1;
+              return 0;
+      }
+
+
+
+The ``type`` field specifies the type of the control. There are
+``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and
+``INTEGER64``. The ``count`` field specifies the number of elements in
+this control. For example, a stereo volume would have count = 2. The
+``value`` field is a union, and the values stored are depending on the
+type. The boolean and integer types are identical.
+
+The enumerated type is a bit different from others. You'll need to set
+the string for the currently given item index.
+
+::
+
+  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+                          struct snd_ctl_elem_info *uinfo)
+  {
+          static char *texts[4] = {
+                  "First", "Second", "Third", "Fourth"
+          };
+          uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
+          uinfo->count = 1;
+          uinfo->value.enumerated.items = 4;
+          if (uinfo->value.enumerated.item > 3)
+                  uinfo->value.enumerated.item = 3;
+          strcpy(uinfo->value.enumerated.name,
+                 texts[uinfo->value.enumerated.item]);
+          return 0;
+  }
+
+The above callback can be simplified with a helper function,
+:c:func:`snd_ctl_enum_info()`. The final code looks like below.
+(You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument;
+it's a matter of taste.)
+
+::
+
+  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+                          struct snd_ctl_elem_info *uinfo)
+  {
+          static char *texts[4] = {
+                  "First", "Second", "Third", "Fourth"
+          };
+          return snd_ctl_enum_info(uinfo, 1, 4, texts);
+  }
+
+
+Some common info callbacks are available for your convenience:
+:c:func:`snd_ctl_boolean_mono_info()` and
+:c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former
+is an info callback for a mono channel boolean item, just like
+:c:func:`snd_myctl_mono_info()` above, and the latter is for a
+stereo channel boolean item.
+
+get callback
+~~~~~~~~~~~~
+
+This callback is used to read the current value of the control and to
+return to user-space.
+
+For example,
+
+::
+
+
+      static int snd_myctl_get(struct snd_kcontrol *kcontrol,
+                               struct snd_ctl_elem_value *ucontrol)
+      {
+              struct mychip *chip = snd_kcontrol_chip(kcontrol);
+              ucontrol->value.integer.value[0] = get_some_value(chip);
+              return 0;
+      }
+
+
+
+The ``value`` field depends on the type of control as well as on the
+info callback. For example, the sb driver uses this field to store the
+register offset, the bit-shift and the bit-mask. The ``private_value``
+field is set as follows:
+
+::
+
+  .private_value = reg | (shift << 16) | (mask << 24)
+
+and is retrieved in callbacks like
+
+::
+
+  static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
+                                    struct snd_ctl_elem_value *ucontrol)
+  {
+          int reg = kcontrol->private_value & 0xff;
+          int shift = (kcontrol->private_value >> 16) & 0xff;
+          int mask = (kcontrol->private_value >> 24) & 0xff;
+          ....
+  }
+
+In the ``get`` callback, you have to fill all the elements if the
+control has more than one elements, i.e. ``count > 1``. In the example
+above, we filled only one element (``value.integer.value[0]``) since
+it's assumed as ``count = 1``.
+
+put callback
+~~~~~~~~~~~~
+
+This callback is used to write a value from user-space.
+
+For example,
+
+::
+
+
+      static int snd_myctl_put(struct snd_kcontrol *kcontrol,
+                               struct snd_ctl_elem_value *ucontrol)
+      {
+              struct mychip *chip = snd_kcontrol_chip(kcontrol);
+              int changed = 0;
+              if (chip->current_value !=
+                   ucontrol->value.integer.value[0]) {
+                      change_current_value(chip,
+                                  ucontrol->value.integer.value[0]);
+                      changed = 1;
+              }
+              return changed;
+      }
+
+
+
+As seen above, you have to return 1 if the value is changed. If the
+value is not changed, return 0 instead. If any fatal error happens,
+return a negative error code as usual.
+
+As in the ``get`` callback, when the control has more than one
+elements, all elements must be evaluated in this callback, too.
+
+Callbacks are not atomic
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+All these three callbacks are basically not atomic.
+
+Control Constructor
+-------------------
+
+When everything is ready, finally we can create a new control. To create
+a control, there are two functions to be called,
+:c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`.
+
+In the simplest way, you can do like this:
+
+::
+
+  err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
+  if (err < 0)
+          return err;
+
+where ``my_control`` is the :c:type:`struct snd_kcontrol_new
+<snd_kcontrol_new>` object defined above, and chip is the object
+pointer to be passed to kcontrol->private_data which can be referred
+to in callbacks.
+
+:c:func:`snd_ctl_new1()` allocates a new :c:type:`struct
+snd_kcontrol <snd_kcontrol>` instance, and
+:c:func:`snd_ctl_add()` assigns the given control component to the
+card.
+
+Change Notification
+-------------------
+
+If you need to change and update a control in the interrupt routine, you
+can call :c:func:`snd_ctl_notify()`. For example,
+
+::
+
+  snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
+
+This function takes the card pointer, the event-mask, and the control id
+pointer for the notification. The event-mask specifies the types of
+notification, for example, in the above example, the change of control
+values is notified. The id pointer is the pointer of :c:type:`struct
+snd_ctl_elem_id <snd_ctl_elem_id>` to be notified. You can
+find some examples in ``es1938.c`` or ``es1968.c`` for hardware volume
+interrupts.
+
+Metadata
+--------
+
+To provide information about the dB values of a mixer control, use on of
+the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a
+variable containing this information, set the ``tlv.p`` field to point to
+this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag
+in the ``access`` field; like this:
+
+::
+
+  static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
+
+  static struct snd_kcontrol_new my_control = {
+          ...
+          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
+                    SNDRV_CTL_ELEM_ACCESS_TLV_READ,
+          ...
+          .tlv.p = db_scale_my_control,
+  };
+
+
+The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information
+about a mixer control where each step in the control's value changes the
+dB value by a constant dB amount. The first parameter is the name of the
+variable to be defined. The second parameter is the minimum value, in
+units of 0.01 dB. The third parameter is the step size, in units of 0.01
+dB. Set the fourth parameter to 1 if the minimum value actually mutes
+the control.
+
+The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information
+about a mixer control where the control's value affects the output
+linearly. The first parameter is the name of the variable to be defined.
+The second parameter is the minimum value, in units of 0.01 dB. The
+third parameter is the maximum value, in units of 0.01 dB. If the
+minimum value mutes the control, set the second parameter to
+``TLV_DB_GAIN_MUTE``.
+
+API for AC97 Codec
+==================
+
+General
+-------
+
+The ALSA AC97 codec layer is a well-defined one, and you don't have to
+write much code to control it. Only low-level control routines are
+necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``.
+
+Full Code Example
+-----------------
+
+::
+
+      struct mychip {
+              ....
+              struct snd_ac97 *ac97;
+              ....
+      };
+
+      static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+                                                 unsigned short reg)
+      {
+              struct mychip *chip = ac97->private_data;
+              ....
+              /* read a register value here from the codec */
+              return the_register_value;
+      }
+
+      static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+                                       unsigned short reg, unsigned short val)
+      {
+              struct mychip *chip = ac97->private_data;
+              ....
+              /* write the given register value to the codec */
+      }
+
+      static int snd_mychip_ac97(struct mychip *chip)
+      {
+              struct snd_ac97_bus *bus;
+              struct snd_ac97_template ac97;
+              int err;
+              static struct snd_ac97_bus_ops ops = {
+                      .write = snd_mychip_ac97_write,
+                      .read = snd_mychip_ac97_read,
+              };
+
+              err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
+              if (err < 0)
+                      return err;
+              memset(&ac97, 0, sizeof(ac97));
+              ac97.private_data = chip;
+              return snd_ac97_mixer(bus, &ac97, &chip->ac97);
+      }
+
+
+AC97 Constructor
+----------------
+
+To create an ac97 instance, first call :c:func:`snd_ac97_bus()`
+with an ``ac97_bus_ops_t`` record with callback functions.
+
+::
+
+  struct snd_ac97_bus *bus;
+  static struct snd_ac97_bus_ops ops = {
+        .write = snd_mychip_ac97_write,
+        .read = snd_mychip_ac97_read,
+  };
+
+  snd_ac97_bus(card, 0, &ops, NULL, &pbus);
+
+The bus record is shared among all belonging ac97 instances.
+
+And then call :c:func:`snd_ac97_mixer()` with an :c:type:`struct
+snd_ac97_template <snd_ac97_template>` record together with
+the bus pointer created above.
+
+::
+
+  struct snd_ac97_template ac97;
+  int err;
+
+  memset(&ac97, 0, sizeof(ac97));
+  ac97.private_data = chip;
+  snd_ac97_mixer(bus, &ac97, &chip->ac97);
+
+where chip->ac97 is a pointer to a newly created ``ac97_t``
+instance. In this case, the chip pointer is set as the private data,
+so that the read/write callback functions can refer to this chip
+instance. This instance is not necessarily stored in the chip
+record. If you need to change the register values from the driver, or
+need the suspend/resume of ac97 codecs, keep this pointer to pass to
+the corresponding functions.
+
+AC97 Callbacks
+--------------
+
+The standard callbacks are ``read`` and ``write``. Obviously they
+correspond to the functions for read and write accesses to the
+hardware low-level codes.
+
+The ``read`` callback returns the register value specified in the
+argument.
+
+::
+
+  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+                                             unsigned short reg)
+  {
+          struct mychip *chip = ac97->private_data;
+          ....
+          return the_register_value;
+  }
+
+Here, the chip can be cast from ``ac97->private_data``.
+
+Meanwhile, the ``write`` callback is used to set the register
+value
+
+::
+
+  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+                       unsigned short reg, unsigned short val)
+
+
+These callbacks are non-atomic like the control API callbacks.
+
+There are also other callbacks: ``reset``, ``wait`` and ``init``.
+
+The ``reset`` callback is used to reset the codec. If the chip
+requires a special kind of reset, you can define this callback.
+
+The ``wait`` callback is used to add some waiting time in the standard
+initialization of the codec. If the chip requires the extra waiting
+time, define this callback.
+
+The ``init`` callback is used for additional initialization of the
+codec.
+
+Updating Registers in The Driver
+--------------------------------
+
+If you need to access to the codec from the driver, you can call the
+following functions: :c:func:`snd_ac97_write()`,
+:c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and
+:c:func:`snd_ac97_update_bits()`.
+
+Both :c:func:`snd_ac97_write()` and
+:c:func:`snd_ac97_update()` functions are used to set a value to
+the given register (``AC97_XXX``). The difference between them is that
+:c:func:`snd_ac97_update()` doesn't write a value if the given
+value has been already set, while :c:func:`snd_ac97_write()`
+always rewrites the value.
+
+::
+
+  snd_ac97_write(ac97, AC97_MASTER, 0x8080);
+  snd_ac97_update(ac97, AC97_MASTER, 0x8080);
+
+:c:func:`snd_ac97_read()` is used to read the value of the given
+register. For example,
+
+::
+
+  value = snd_ac97_read(ac97, AC97_MASTER);
+
+:c:func:`snd_ac97_update_bits()` is used to update some bits in
+the given register.
+
+::
+
+  snd_ac97_update_bits(ac97, reg, mask, value);
+
+Also, there is a function to change the sample rate (of a given register
+such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the
+codec: :c:func:`snd_ac97_set_rate()`.
+
+::
+
+  snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
+
+
+The following registers are available to set the rate:
+``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``,
+``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is
+specified, the register is not really changed but the corresponding
+IEC958 status bits will be updated.
+
+Clock Adjustment
+----------------
+
+In some chips, the clock of the codec isn't 48000 but using a PCI clock
+(to save a quartz!). In this case, change the field ``bus->clock`` to
+the corresponding value. For example, intel8x0 and es1968 drivers have
+their own function to read from the clock.
+
+Proc Files
+----------
+
+The ALSA AC97 interface will create a proc file such as
+``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You
+can refer to these files to see the current status and registers of
+the codec.
+
+Multiple Codecs
+---------------
+
+When there are several codecs on the same card, you need to call
+:c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or
+greater. The ``num`` field specifies the codec number.
+
+If you set up multiple codecs, you either need to write different
+callbacks for each codec or check ``ac97->num`` in the callback
+routines.
+
+MIDI (MPU401-UART) Interface
+============================
+
+General
+-------
+
+Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
+soundcard supports the standard MPU401-UART interface, most likely you
+can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
+``<sound/mpu401.h>``.
+
+Some soundchips have a similar but slightly different implementation of
+mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
+
+MIDI Constructor
+----------------
+
+To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`.
+
+::
+
+  struct snd_rawmidi *rmidi;
+  snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
+                      irq, &rmidi);
+
+
+The first argument is the card pointer, and the second is the index of
+this component. You can create up to 8 rawmidi devices.
+
+The third argument is the type of the hardware, ``MPU401_HW_XXX``. If
+it's not a special one, you can use ``MPU401_HW_MPU401``.
+
+The 4th argument is the I/O port address. Many backward-compatible
+MPU401 have an I/O port such as 0x330. Or, it might be a part of its own
+PCI I/O region. It depends on the chip design.
+
+The 5th argument is a bitflag for additional information. When the I/O
+port address above is part of the PCI I/O region, the MPU401 I/O port
+might have been already allocated (reserved) by the driver itself. In
+such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the
+mpu401-uart layer will allocate the I/O ports by itself.
+
+When the controller supports only the input or output MIDI stream, pass
+the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag,
+respectively. Then the rawmidi instance is created as a single stream.
+
+``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO
+(via readb and writeb) instead of iob and outb. In this case, you have
+to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`.
+
+When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in
+the default interrupt handler. The driver needs to call
+:c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start
+processing the output stream in the irq handler.
+
+If the MPU-401 interface shares its interrupt with the other logical
+devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see
+`below <#MIDI-Interrupt-Handler>`__).
+
+Usually, the port address corresponds to the command port and port + 1
+corresponds to the data port. If not, you may change the ``cport``
+field of :c:type:`struct snd_mpu401 <snd_mpu401>` manually afterward.
+However, :c:type:`struct snd_mpu401 <snd_mpu401>` pointer is
+not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You
+need to cast ``rmidi->private_data`` to :c:type:`struct snd_mpu401
+<snd_mpu401>` explicitly,
+
+::
+
+  struct snd_mpu401 *mpu;
+  mpu = rmidi->private_data;
+
+and reset the ``cport`` as you like:
+
+::
+
+  mpu->cport = my_own_control_port;
+
+The 6th argument specifies the ISA irq number that will be allocated. If
+no interrupt is to be allocated (because your code is already allocating
+a shared interrupt, or because the device does not use interrupts), pass
+-1 instead. For a MPU-401 device without an interrupt, a polling timer
+will be used instead.
+
+MIDI Interrupt Handler
+----------------------
+
+When the interrupt is allocated in
+:c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt
+handler is automatically used, hence you don't have anything else to do
+than creating the mpu401 stuff. Otherwise, you have to set
+``MPU401_INFO_IRQ_HOOK``, and call
+:c:func:`snd_mpu401_uart_interrupt()` explicitly from your own
+interrupt handler when it has determined that a UART interrupt has
+occurred.
+
+In this case, you need to pass the private_data of the returned rawmidi
+object from :c:func:`snd_mpu401_uart_new()` as the second
+argument of :c:func:`snd_mpu401_uart_interrupt()`.
+
+::
+
+  snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
+
+
+RawMIDI Interface
+=================
+
+Overview
+--------
+
+The raw MIDI interface is used for hardware MIDI ports that can be
+accessed as a byte stream. It is not used for synthesizer chips that do
+not directly understand MIDI.
+
+ALSA handles file and buffer management. All you have to do is to write
+some code to move data between the buffer and the hardware.
+
+The rawmidi API is defined in ``<sound/rawmidi.h>``.
+
+RawMIDI Constructor
+-------------------
+
+To create a rawmidi device, call the :c:func:`snd_rawmidi_new()`
+function:
+
+::
+
+  struct snd_rawmidi *rmidi;
+  err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
+  if (err < 0)
+          return err;
+  rmidi->private_data = chip;
+  strcpy(rmidi->name, "My MIDI");
+  rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
+                      SNDRV_RAWMIDI_INFO_INPUT |
+                      SNDRV_RAWMIDI_INFO_DUPLEX;
+
+The first argument is the card pointer, the second argument is the ID
+string.
+
+The third argument is the index of this component. You can create up to
+8 rawmidi devices.
+
+The fourth and fifth arguments are the number of output and input
+substreams, respectively, of this device (a substream is the equivalent
+of a MIDI port).
+
+Set the ``info_flags`` field to specify the capabilities of the
+device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one
+output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one
+input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle
+output and input at the same time.
+
+After the rawmidi device is created, you need to set the operators
+(callbacks) for each substream. There are helper functions to set the
+operators for all the substreams of a device:
+
+::
+
+  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
+  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
+
+The operators are usually defined like this:
+
+::
+
+  static struct snd_rawmidi_ops snd_mymidi_output_ops = {
+          .open =    snd_mymidi_output_open,
+          .close =   snd_mymidi_output_close,
+          .trigger = snd_mymidi_output_trigger,
+  };
+
+These callbacks are explained in the `RawMIDI Callbacks`_ section.
+
+If there are more than one substream, you should give a unique name to
+each of them:
+
+::
+
+  struct snd_rawmidi_substream *substream;
+  list_for_each_entry(substream,
+                      &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
+                      list {
+          sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
+  }
+  /* same for SNDRV_RAWMIDI_STREAM_INPUT */
+
+RawMIDI Callbacks
+-----------------
+
+In all the callbacks, the private data that you've set for the rawmidi
+device can be accessed as ``substream->rmidi->private_data``.
+
+If there is more than one port, your callbacks can determine the port
+index from the struct snd_rawmidi_substream data passed to each
+callback:
+
+::
+
+  struct snd_rawmidi_substream *substream;
+  int index = substream->number;
+
+RawMIDI open callback
+~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static int snd_xxx_open(struct snd_rawmidi_substream *substream);
+
+
+This is called when a substream is opened. You can initialize the
+hardware here, but you shouldn't start transmitting/receiving data yet.
+
+RawMIDI close callback
+~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static int snd_xxx_close(struct snd_rawmidi_substream *substream);
+
+Guess what.
+
+The ``open`` and ``close`` callbacks of a rawmidi device are
+serialized with a mutex, and can sleep.
+
+Rawmidi trigger callback for output substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter when there is some data
+in the substream buffer that must be transmitted.
+
+To read data from the buffer, call
+:c:func:`snd_rawmidi_transmit_peek()`. It will return the number
+of bytes that have been read; this will be less than the number of bytes
+requested when there are no more data in the buffer. After the data have
+been transmitted successfully, call
+:c:func:`snd_rawmidi_transmit_ack()` to remove the data from the
+substream buffer:
+
+::
+
+  unsigned char data;
+  while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
+          if (snd_mychip_try_to_transmit(data))
+                  snd_rawmidi_transmit_ack(substream, 1);
+          else
+                  break; /* hardware FIFO full */
+  }
+
+If you know beforehand that the hardware will accept data, you can use
+the :c:func:`snd_rawmidi_transmit()` function which reads some
+data and removes them from the buffer at once:
+
+::
+
+  while (snd_mychip_transmit_possible()) {
+          unsigned char data;
+          if (snd_rawmidi_transmit(substream, &data, 1) != 1)
+                  break; /* no more data */
+          snd_mychip_transmit(data);
+  }
+
+If you know beforehand how many bytes you can accept, you can use a
+buffer size greater than one with the
+:c:func:`snd_rawmidi_transmit\*()` functions.
+
+The ``trigger`` callback must not sleep. If the hardware FIFO is full
+before the substream buffer has been emptied, you have to continue
+transmitting data later, either in an interrupt handler, or with a
+timer if the hardware doesn't have a MIDI transmit interrupt.
+
+The ``trigger`` callback is called with a zero ``up`` parameter when
+the transmission of data should be aborted.
+
+RawMIDI trigger callback for input substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter to enable receiving data,
+or with a zero ``up`` parameter do disable receiving data.
+
+The ``trigger`` callback must not sleep; the actual reading of data
+from the device is usually done in an interrupt handler.
+
+When data reception is enabled, your interrupt handler should call
+:c:func:`snd_rawmidi_receive()` for all received data:
+
+::
+
+  void snd_mychip_midi_interrupt(...)
+  {
+          while (mychip_midi_available()) {
+                  unsigned char data;
+                  data = mychip_midi_read();
+                  snd_rawmidi_receive(substream, &data, 1);
+          }
+  }
+
+
+drain callback
+~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
+
+
+This is only used with output substreams. This function should wait
+until all data read from the substream buffer have been transmitted.
+This ensures that the device can be closed and the driver unloaded
+without losing data.
+
+This callback is optional. If you do not set ``drain`` in the struct
+snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
+instead.
+
+Miscellaneous Devices
+=====================
+
+FM OPL3
+-------
+
+The FM OPL3 is still used in many chips (mainly for backward
+compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
+is defined in ``<sound/opl3.h>``.
+
+FM registers can be directly accessed through the direct-FM API, defined
+in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are
+accessed through the Hardware-Dependent Device direct-FM extension API,
+whereas in OSS compatible mode, FM registers can be accessed with the
+OSS direct-FM compatible API in ``/dev/dmfmX`` device.
+
+To create the OPL3 component, you have two functions to call. The first
+one is a constructor for the ``opl3_t`` instance.
+
+::
+
+  struct snd_opl3 *opl3;
+  snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
+                  integrated, &opl3);
+
+The first argument is the card pointer, the second one is the left port
+address, and the third is the right port address. In most cases, the
+right port is placed at the left port + 2.
+
+The fourth argument is the hardware type.
+
+When the left and right ports have been already allocated by the card
+driver, pass non-zero to the fifth argument (``integrated``). Otherwise,
+the opl3 module will allocate the specified ports by itself.
+
+When the accessing the hardware requires special method instead of the
+standard I/O access, you can create opl3 instance separately with
+:c:func:`snd_opl3_new()`.
+
+::
+
+  struct snd_opl3 *opl3;
+  snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
+
+Then set ``command``, ``private_data`` and ``private_free`` for the
+private access function, the private data and the destructor. The
+``l_port`` and ``r_port`` are not necessarily set. Only the command
+must be set properly. You can retrieve the data from the
+``opl3->private_data`` field. 
+
+After creating the opl3 instance via :c:func:`snd_opl3_new()`,
+call :c:func:`snd_opl3_init()` to initialize the chip to the
+proper state. Note that :c:func:`snd_opl3_create()` always calls
+it internally.
+
+If the opl3 instance is created successfully, then create a hwdep device
+for this opl3.
+
+::
+
+  struct snd_hwdep *opl3hwdep;
+  snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
+
+The first argument is the ``opl3_t`` instance you created, and the
+second is the index number, usually 0.
+
+The third argument is the index-offset for the sequencer client assigned
+to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART
+always takes 0).
+
+Hardware-Dependent Devices
+--------------------------
+
+Some chips need user-space access for special controls or for loading
+the micro code. In such a case, you can create a hwdep
+(hardware-dependent) device. The hwdep API is defined in
+``<sound/hwdep.h>``. You can find examples in opl3 driver or
+``isa/sb/sb16_csp.c``.
+
+The creation of the ``hwdep`` instance is done via
+:c:func:`snd_hwdep_new()`.
+
+::
+
+  struct snd_hwdep *hw;
+  snd_hwdep_new(card, "My HWDEP", 0, &hw);
+
+where the third argument is the index number.
+
+You can then pass any pointer value to the ``private_data``. If you
+assign a private data, you should define the destructor, too. The
+destructor function is set in the ``private_free`` field.
+
+::
+
+  struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
+  hw->private_data = p;
+  hw->private_free = mydata_free;
+
+and the implementation of the destructor would be:
+
+::
+
+  static void mydata_free(struct snd_hwdep *hw)
+  {
+          struct mydata *p = hw->private_data;
+          kfree(p);
+  }
+
+The arbitrary file operations can be defined for this instance. The file
+operators are defined in the ``ops`` table. For example, assume that
+this chip needs an ioctl.
+
+::
+
+  hw->ops.open = mydata_open;
+  hw->ops.ioctl = mydata_ioctl;
+  hw->ops.release = mydata_release;
+
+And implement the callback functions as you like.
+
+IEC958 (S/PDIF)
+---------------
+
+Usually the controls for IEC958 devices are implemented via the control
+interface. There is a macro to compose a name string for IEC958
+controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in
+``<include/asound.h>``.
+
+There are some standard controls for IEC958 status bits. These controls
+use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is
+fixed as 4 bytes array (value.iec958.status[x]). For the ``info``
+callback, you don't specify the value field for this type (the count
+field must be set, though).
+
+“IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958
+status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask”
+returns the bitmask for professional mode. They are read-only controls,
+and are defined as MIXER controls (iface =
+``SNDRV_CTL_ELEM_IFACE_MIXER``).
+
+Meanwhile, “IEC958 Playback Default” control is defined for getting and
+setting the current default IEC958 bits. Note that this one is usually
+defined as a PCM control (iface = ``SNDRV_CTL_ELEM_IFACE_PCM``),
+although in some places it's defined as a MIXER control.
+
+In addition, you can define the control switches to enable/disable or to
+set the raw bit mode. The implementation will depend on the chip, but
+the control should be named as “IEC958 xxx”, preferably using the
+:c:func:`SNDRV_CTL_NAME_IEC958()` macro.
+
+You can find several cases, for example, ``pci/emu10k1``,
+``pci/ice1712``, or ``pci/cmipci.c``.
+
+Buffer and Memory Management
+============================
+
+Buffer Types
+------------
+
+ALSA provides several different buffer allocation functions depending on
+the bus and the architecture. All these have a consistent API. The
+allocation of physically-contiguous pages is done via
+:c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus
+type.
+
+The allocation of pages with fallback is
+:c:func:`snd_malloc_xxx_pages_fallback()`. This function tries
+to allocate the specified pages but if the pages are not available, it
+tries to reduce the page sizes until enough space is found.
+
+The release the pages, call :c:func:`snd_free_xxx_pages()`
+function.
+
+Usually, ALSA drivers try to allocate and reserve a large contiguous
+physical space at the time the module is loaded for the later use. This
+is called “pre-allocation”. As already written, you can call the
+following function at pcm instance construction time (in the case of PCI
+bus).
+
+::
+
+  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                        snd_dma_pci_data(pci), size, max);
+
+where ``size`` is the byte size to be pre-allocated and the ``max`` is
+the maximum size to be changed via the ``prealloc`` proc file. The
+allocator will try to get an area as large as possible within the
+given size.
+
+The second argument (type) and the third argument (device pointer) are
+dependent on the bus. In the case of the ISA bus, pass
+:c:func:`snd_dma_isa_data()` as the third argument with
+``SNDRV_DMA_TYPE_DEV`` type. For the continuous buffer unrelated to the
+bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type and the
+``snd_dma_continuous_data(GFP_KERNEL)`` device pointer, where
+``GFP_KERNEL`` is the kernel allocation flag to use. For the PCI
+scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with
+``snd_dma_pci_data(pci)`` (see the `Non-Contiguous Buffers`_
+section).
+
+Once the buffer is pre-allocated, you can use the allocator in the
+``hw_params`` callback:
+
+::
+
+  snd_pcm_lib_malloc_pages(substream, size);
+
+Note that you have to pre-allocate to use this function.
+
+External Hardware Buffers
+-------------------------
+
+Some chips have their own hardware buffers and the DMA transfer from the
+host memory is not available. In such a case, you need to either 1)
+copy/set the audio data directly to the external hardware buffer, or 2)
+make an intermediate buffer and copy/set the data from it to the
+external hardware buffer in interrupts (or in tasklets, preferably).
+
+The first case works fine if the external hardware buffer is large
+enough. This method doesn't need any extra buffers and thus is more
+effective. You need to define the ``copy`` and ``silence`` callbacks
+for the data transfer. However, there is a drawback: it cannot be
+mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM.
+
+The second case allows for mmap on the buffer, although you have to
+handle an interrupt or a tasklet to transfer the data from the
+intermediate buffer to the hardware buffer. You can find an example in
+the vxpocket driver.
+
+Another case is when the chip uses a PCI memory-map region for the
+buffer instead of the host memory. In this case, mmap is available only
+on certain architectures like the Intel one. In non-mmap mode, the data
+cannot be transferred as in the normal way. Thus you need to define the
+``copy`` and ``silence`` callbacks as well, as in the cases above. The
+examples are found in ``rme32.c`` and ``rme96.c``.
+
+The implementation of the ``copy`` and ``silence`` callbacks depends
+upon whether the hardware supports interleaved or non-interleaved
+samples. The ``copy`` callback is defined like below, a bit
+differently depending whether the direction is playback or capture:
+
+::
+
+  static int playback_copy(struct snd_pcm_substream *substream, int channel,
+               snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
+  static int capture_copy(struct snd_pcm_substream *substream, int channel,
+               snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
+
+In the case of interleaved samples, the second argument (``channel``) is
+not used. The third argument (``pos``) points the current position
+offset in frames.
+
+The meaning of the fourth argument is different between playback and
+capture. For playback, it holds the source data pointer, and for
+capture, it's the destination data pointer.
+
+The last argument is the number of frames to be copied.
+
+What you have to do in this callback is again different between playback
+and capture directions. In the playback case, you copy the given amount
+of data (``count``) at the specified pointer (``src``) to the specified
+offset (``pos``) on the hardware buffer. When coded like memcpy-like
+way, the copy would be like:
+
+::
+
+  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
+            frames_to_bytes(runtime, count));
+
+For the capture direction, you copy the given amount of data (``count``)
+at the specified offset (``pos``) on the hardware buffer to the
+specified pointer (``dst``).
+
+::
+
+  my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
+            frames_to_bytes(runtime, count));
+
+Note that both the position and the amount of data are given in frames.
+
+In the case of non-interleaved samples, the implementation will be a bit
+more complicated.
+
+You need to check the channel argument, and if it's -1, copy the whole
+channels. Otherwise, you have to copy only the specified channel. Please
+check ``isa/gus/gus_pcm.c`` as an example.
+
+The ``silence`` callback is also implemented in a similar way
+
+::
+
+  static int silence(struct snd_pcm_substream *substream, int channel,
+                     snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
+
+The meanings of arguments are the same as in the ``copy`` callback,
+although there is no ``src/dst`` argument. In the case of interleaved
+samples, the channel argument has no meaning, as well as on ``copy``
+callback.
+
+The role of ``silence`` callback is to set the given amount
+(``count``) of silence data at the specified offset (``pos``) on the
+hardware buffer. Suppose that the data format is signed (that is, the
+silent-data is 0), and the implementation using a memset-like function
+would be like: 
+
+::
+
+  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
+            frames_to_bytes(runtime, count));
+
+In the case of non-interleaved samples, again, the implementation
+becomes a bit more complicated. See, for example, ``isa/gus/gus_pcm.c``.
+
+Non-Contiguous Buffers
+----------------------
+
+If your hardware supports the page table as in emu10k1 or the buffer
+descriptors as in via82xx, you can use the scatter-gather (SG) DMA. ALSA
+provides an interface for handling SG-buffers. The API is provided in
+``<sound/pcm.h>``.
+
+For creating the SG-buffer handler, call
+:c:func:`snd_pcm_lib_preallocate_pages()` or
+:c:func:`snd_pcm_lib_preallocate_pages_for_all()` with
+``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like other PCI
+pre-allocator. You need to pass ``snd_dma_pci_data(pci)``, where pci is
+the :c:type:`struct pci_dev <pci_dev>` pointer of the chip as
+well. The ``struct snd_sg_buf`` instance is created as
+``substream->dma_private``. You can cast the pointer like:
+
+::
+
+  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
+
+Then call :c:func:`snd_pcm_lib_malloc_pages()` in the ``hw_params``
+callback as well as in the case of normal PCI buffer. The SG-buffer
+handler will allocate the non-contiguous kernel pages of the given size
+and map them onto the virtually contiguous memory. The virtual pointer
+is addressed in runtime->dma_area. The physical address
+(``runtime->dma_addr``) is set to zero, because the buffer is
+physically non-contiguous. The physical address table is set up in
+``sgbuf->table``. You can get the physical address at a certain offset
+via :c:func:`snd_pcm_sgbuf_get_addr()`.
+
+When a SG-handler is used, you need to set
+:c:func:`snd_pcm_sgbuf_ops_page()` as the ``page`` callback. (See
+`page callback`_ section.)
+
+To release the data, call :c:func:`snd_pcm_lib_free_pages()` in
+the ``hw_free`` callback as usual.
+
+Vmalloc'ed Buffers
+------------------
+
+It's possible to use a buffer allocated via :c:func:`vmalloc()`, for
+example, for an intermediate buffer. Since the allocated pages are not
+contiguous, you need to set the ``page`` callback to obtain the physical
+address at every offset.
+
+The implementation of ``page`` callback would be like this:
+
+::
+
+  #include <linux/vmalloc.h>
+
+  /* get the physical page pointer on the given offset */
+  static struct page *mychip_page(struct snd_pcm_substream *substream,
+                                  unsigned long offset)
+  {
+          void *pageptr = substream->runtime->dma_area + offset;
+          return vmalloc_to_page(pageptr);
+  }
+
+Proc Interface
+==============
+
+ALSA provides an easy interface for procfs. The proc files are very
+useful for debugging. I recommend you set up proc files if you write a
+driver and want to get a running status or register dumps. The API is
+found in ``<sound/info.h>``.
+
+To create a proc file, call :c:func:`snd_card_proc_new()`.
+
+::
+
+  struct snd_info_entry *entry;
+  int err = snd_card_proc_new(card, "my-file", &entry);
+
+where the second argument specifies the name of the proc file to be
+created. The above example will create a file ``my-file`` under the
+card directory, e.g. ``/proc/asound/card0/my-file``.
+
+Like other components, the proc entry created via
+:c:func:`snd_card_proc_new()` will be registered and released
+automatically in the card registration and release functions.
+
+When the creation is successful, the function stores a new instance in
+the pointer given in the third argument. It is initialized as a text
+proc file for read only. To use this proc file as a read-only text file
+as it is, set the read callback with a private data via
+:c:func:`snd_info_set_text_ops()`.
+
+::
+
+  snd_info_set_text_ops(entry, chip, my_proc_read);
+
+where the second argument (``chip``) is the private data to be used in
+the callbacks. The third parameter specifies the read buffer size and
+the fourth (``my_proc_read``) is the callback function, which is
+defined like
+
+::
+
+  static void my_proc_read(struct snd_info_entry *entry,
+                           struct snd_info_buffer *buffer);
+
+In the read callback, use :c:func:`snd_iprintf()` for output
+strings, which works just like normal :c:func:`printf()`. For
+example,
+
+::
+
+  static void my_proc_read(struct snd_info_entry *entry,
+                           struct snd_info_buffer *buffer)
+  {
+          struct my_chip *chip = entry->private_data;
+
+          snd_iprintf(buffer, "This is my chip!\n");
+          snd_iprintf(buffer, "Port = %ld\n", chip->port);
+  }
+
+The file permissions can be changed afterwards. As default, it's set as
+read only for all users. If you want to add write permission for the
+user (root as default), do as follows:
+
+::
+
+ entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
+
+and set the write buffer size and the callback
+
+::
+
+  entry->c.text.write = my_proc_write;
+
+For the write callback, you can use :c:func:`snd_info_get_line()`
+to get a text line, and :c:func:`snd_info_get_str()` to retrieve
+a string from the line. Some examples are found in
+``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``.
+
+For a raw-data proc-file, set the attributes as follows:
+
+::
+
+  static struct snd_info_entry_ops my_file_io_ops = {
+          .read = my_file_io_read,
+  };
+
+  entry->content = SNDRV_INFO_CONTENT_DATA;
+  entry->private_data = chip;
+  entry->c.ops = &my_file_io_ops;
+  entry->size = 4096;
+  entry->mode = S_IFREG | S_IRUGO;
+
+For the raw data, ``size`` field must be set properly. This specifies
+the maximum size of the proc file access.
+
+The read/write callbacks of raw mode are more direct than the text mode.
+You need to use a low-level I/O functions such as
+:c:func:`copy_from/to_user()` to transfer the data.
+
+::
+
+  static ssize_t my_file_io_read(struct snd_info_entry *entry,
+                              void *file_private_data,
+                              struct file *file,
+                              char *buf,
+                              size_t count,
+                              loff_t pos)
+  {
+          if (copy_to_user(buf, local_data + pos, count))
+                  return -EFAULT;
+          return count;
+  }
+
+If the size of the info entry has been set up properly, ``count`` and
+``pos`` are guaranteed to fit within 0 and the given size. You don't
+have to check the range in the callbacks unless any other condition is
+required.
+
+Power Management
+================
+
+If the chip is supposed to work with suspend/resume functions, you need
+to add power-management code to the driver. The additional code for
+power-management should be ifdef-ed with ``CONFIG_PM``.
+
+If the driver *fully* supports suspend/resume that is, the device can be
+properly resumed to its state when suspend was called, you can set the
+``SNDRV_PCM_INFO_RESUME`` flag in the pcm info field. Usually, this is
+possible when the registers of the chip can be safely saved and restored
+to RAM. If this is set, the trigger callback is called with
+``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes.
+
+Even if the driver doesn't support PM fully but partial suspend/resume
+is still possible, it's still worthy to implement suspend/resume
+callbacks. In such a case, applications would reset the status by
+calling :c:func:`snd_pcm_prepare()` and restart the stream
+appropriately. Hence, you can define suspend/resume callbacks below but
+don't set ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM.
+
+Note that the trigger with SUSPEND can always be called when
+:c:func:`snd_pcm_suspend_all()` is called, regardless of the
+``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the
+behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory,
+``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger
+callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better
+to keep it for compatibility reasons.)
+
+In the earlier version of ALSA drivers, a common power-management layer
+was provided, but it has been removed. The driver needs to define the
+suspend/resume hooks according to the bus the device is connected to. In
+the case of PCI drivers, the callbacks look like below:
+
+::
+
+  #ifdef CONFIG_PM
+  static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
+  {
+          .... /* do things for suspend */
+          return 0;
+  }
+  static int snd_my_resume(struct pci_dev *pci)
+  {
+          .... /* do things for suspend */
+          return 0;
+  }
+  #endif
+
+The scheme of the real suspend job is as follows.
+
+1. Retrieve the card and the chip data.
+
+2. Call :c:func:`snd_power_change_state()` with
+   ``SNDRV_CTL_POWER_D3hot`` to change the power status.
+
+3. Call :c:func:`snd_pcm_suspend_all()` to suspend the running
+   PCM streams.
+
+4. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for
+   each codec.
+
+5. Save the register values if necessary.
+
+6. Stop the hardware if necessary.
+
+7. Disable the PCI device by calling
+   :c:func:`pci_disable_device()`. Then, call
+   :c:func:`pci_save_state()` at last.
+
+A typical code would be like:
+
+::
+
+  static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
+  {
+          /* (1) */
+          struct snd_card *card = pci_get_drvdata(pci);
+          struct mychip *chip = card->private_data;
+          /* (2) */
+          snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
+          /* (3) */
+          snd_pcm_suspend_all(chip->pcm);
+          /* (4) */
+          snd_ac97_suspend(chip->ac97);
+          /* (5) */
+          snd_mychip_save_registers(chip);
+          /* (6) */
+          snd_mychip_stop_hardware(chip);
+          /* (7) */
+          pci_disable_device(pci);
+          pci_save_state(pci);
+          return 0;
+  }
+
+
+The scheme of the real resume job is as follows.
+
+1. Retrieve the card and the chip data.
+
+2. Set up PCI. First, call :c:func:`pci_restore_state()`. Then
+   enable the pci device again by calling
+   :c:func:`pci_enable_device()`. Call
+   :c:func:`pci_set_master()` if necessary, too.
+
+3. Re-initialize the chip.
+
+4. Restore the saved registers if necessary.
+
+5. Resume the mixer, e.g. calling :c:func:`snd_ac97_resume()`.
+
+6. Restart the hardware (if any).
+
+7. Call :c:func:`snd_power_change_state()` with
+   ``SNDRV_CTL_POWER_D0`` to notify the processes.
+
+A typical code would be like:
+
+::
+
+  static int mychip_resume(struct pci_dev *pci)
+  {
+          /* (1) */
+          struct snd_card *card = pci_get_drvdata(pci);
+          struct mychip *chip = card->private_data;
+          /* (2) */
+          pci_restore_state(pci);
+          pci_enable_device(pci);
+          pci_set_master(pci);
+          /* (3) */
+          snd_mychip_reinit_chip(chip);
+          /* (4) */
+          snd_mychip_restore_registers(chip);
+          /* (5) */
+          snd_ac97_resume(chip->ac97);
+          /* (6) */
+          snd_mychip_restart_chip(chip);
+          /* (7) */
+          snd_power_change_state(card, SNDRV_CTL_POWER_D0);
+          return 0;
+  }
+
+As shown in the above, it's better to save registers after suspending
+the PCM operations via :c:func:`snd_pcm_suspend_all()` or
+:c:func:`snd_pcm_suspend()`. It means that the PCM streams are
+already stopped when the register snapshot is taken. But, remember that
+you don't have to restart the PCM stream in the resume callback. It'll
+be restarted via trigger call with ``SNDRV_PCM_TRIGGER_RESUME`` when
+necessary.
+
+OK, we have all callbacks now. Let's set them up. In the initialization
+of the card, make sure that you can get the chip data from the card
+instance, typically via ``private_data`` field, in case you created the
+chip data individually.
+
+::
+
+  static int snd_mychip_probe(struct pci_dev *pci,
+                              const struct pci_device_id *pci_id)
+  {
+          ....
+          struct snd_card *card;
+          struct mychip *chip;
+          int err;
+          ....
+          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                             0, &card);
+          ....
+          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+          ....
+          card->private_data = chip;
+          ....
+  }
+
+When you created the chip data with :c:func:`snd_card_new()`, it's
+anyway accessible via ``private_data`` field.
+
+::
+
+  static int snd_mychip_probe(struct pci_dev *pci,
+                              const struct pci_device_id *pci_id)
+  {
+          ....
+          struct snd_card *card;
+          struct mychip *chip;
+          int err;
+          ....
+          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                             sizeof(struct mychip), &card);
+          ....
+          chip = card->private_data;
+          ....
+  }
+
+If you need a space to save the registers, allocate the buffer for it
+here, too, since it would be fatal if you cannot allocate a memory in
+the suspend phase. The allocated buffer should be released in the
+corresponding destructor.
+
+And next, set suspend/resume callbacks to the pci_driver.
+
+::
+
+  static struct pci_driver driver = {
+          .name = KBUILD_MODNAME,
+          .id_table = snd_my_ids,
+          .probe = snd_my_probe,
+          .remove = snd_my_remove,
+  #ifdef CONFIG_PM
+          .suspend = snd_my_suspend,
+          .resume = snd_my_resume,
+  #endif
+  };
+
+Module Parameters
+=================
+
+There are standard module options for ALSA. At least, each module should
+have the ``index``, ``id`` and ``enable`` options.
+
+If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS``
+cards), they should be arrays. The default initial values are defined
+already as constants for easier programming:
+
+::
+
+  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+If the module supports only a single card, they could be single
+variables, instead. ``enable`` option is not always necessary in this
+case, but it would be better to have a dummy option for compatibility.
+
+The module parameters must be declared with the standard
+``module_param()()``, ``module_param_array()()`` and
+:c:func:`MODULE_PARM_DESC()` macros.
+
+The typical coding would be like below:
+
+::
+
+  #define CARD_NAME "My Chip"
+
+  module_param_array(index, int, NULL, 0444);
+  MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
+  module_param_array(id, charp, NULL, 0444);
+  MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
+  module_param_array(enable, bool, NULL, 0444);
+  MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
+
+Also, don't forget to define the module description, classes, license
+and devices. Especially, the recent modprobe requires to define the
+module license as GPL, etc., otherwise the system is shown as “tainted”.
+
+::
+
+  MODULE_DESCRIPTION("My Chip");
+  MODULE_LICENSE("GPL");
+  MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
+
+
+How To Put Your Driver Into ALSA Tree
+=====================================
+
+General
+-------
+
+So far, you've learned how to write the driver codes. And you might have
+a question now: how to put my own driver into the ALSA driver tree? Here
+(finally :) the standard procedure is described briefly.
+
+Suppose that you create a new PCI driver for the card “xyz”. The card
+module name would be snd-xyz. The new driver is usually put into the
+alsa-driver tree, ``alsa-driver/pci`` directory in the case of PCI
+cards. Then the driver is evaluated, audited and tested by developers
+and users. After a certain time, the driver will go to the alsa-kernel
+tree (to the corresponding directory, such as ``alsa-kernel/pci``) and
+eventually will be integrated into the Linux 2.6 tree (the directory
+would be ``linux/sound/pci``).
+
+In the following sections, the driver code is supposed to be put into
+alsa-driver tree. The two cases are covered: a driver consisting of a
+single source file and one consisting of several source files.
+
+Driver with A Single Source File
+--------------------------------
+
+1. Modify alsa-driver/pci/Makefile
+
+   Suppose you have a file xyz.c. Add the following two lines
+
+::
+
+  snd-xyz-objs := xyz.o
+  obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+2. Create the Kconfig entry
+
+   Add the new entry of Kconfig for your xyz driver. config SND_XYZ
+   tristate "Foobar XYZ" depends on SND select SND_PCM help Say Y here
+   to include support for Foobar XYZ soundcard. To compile this driver
+   as a module, choose M here: the module will be called snd-xyz. the
+   line, select SND_PCM, specifies that the driver xyz supports PCM. In
+   addition to SND_PCM, the following components are supported for
+   select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP,
+   SND_MPU401_UART, SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB,
+   SND_AC97_CODEC. Add the select command for each supported
+   component.
+
+   Note that some selections imply the lowlevel selections. For example,
+   PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC
+   includes PCM, and OPL3_LIB includes HWDEP. You don't need to give
+   the lowlevel selections again.
+
+   For the details of Kconfig script, refer to the kbuild documentation.
+
+3. Run cvscompile script to re-generate the configure script and build
+   the whole stuff again.
+
+Drivers with Several Source Files
+---------------------------------
+
+Suppose that the driver snd-xyz have several source files. They are
+located in the new subdirectory, pci/xyz.
+
+1. Add a new directory (``xyz``) in ``alsa-driver/pci/Makefile`` as
+   below
+
+::
+
+  obj-$(CONFIG_SND) += xyz/
+
+
+2. Under the directory ``xyz``, create a Makefile
+
+::
+
+         ifndef SND_TOPDIR
+         SND_TOPDIR=../..
+         endif
+
+         include $(SND_TOPDIR)/toplevel.config
+         include $(SND_TOPDIR)/Makefile.conf
+
+         snd-xyz-objs := xyz.o abc.o def.o
+
+         obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+         include $(SND_TOPDIR)/Rules.make
+
+3. Create the Kconfig entry
+
+   This procedure is as same as in the last section.
+
+4. Run cvscompile script to re-generate the configure script and build
+   the whole stuff again.
+
+Useful Functions
+================
+
+:c:func:`snd_printk()` and friends
+---------------------------------------
+
+ALSA provides a verbose version of the :c:func:`printk()` function.
+If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
+prints the given message together with the file name and the line of the
+caller. The ``KERN_XXX`` prefix is processed as well as the original
+:c:func:`printk()` does, so it's recommended to add this prefix,
+e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n");
+
+There are also :c:func:`printk()`'s for debugging.
+:c:func:`snd_printd()` can be used for general debugging purposes.
+If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works
+just like :c:func:`snd_printk()`. If the ALSA is compiled without
+the debugging flag, it's ignored.
+
+:c:func:`snd_printdd()` is compiled in only when
+``CONFIG_SND_DEBUG_VERBOSE`` is set. Please note that
+``CONFIG_SND_DEBUG_VERBOSE`` is not set as default even if you configure
+the alsa-driver with ``--with-debug=full`` option. You need to give
+explicitly ``--with-debug=detect`` option instead.
+
+:c:func:`snd_BUG()`
+------------------------
+
+It shows the ``BUG?`` message and stack trace as well as
+:c:func:`snd_BUG_ON()` at the point. It's useful to show that a
+fatal error happens there.
+
+When no debug flag is set, this macro is ignored.
+
+:c:func:`snd_BUG_ON()`
+----------------------------
+
+:c:func:`snd_BUG_ON()` macro is similar with
+:c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
+it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
+return -EINVAL;
+
+The macro takes an conditional expression to evaluate. When
+``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows
+the warning message such as ``BUG? (xxx)`` normally followed by stack
+trace. In both cases it returns the evaluated value.
+
+Acknowledgments
+===============
+
+I would like to thank Phil Kerr for his help for improvement and
+corrections of this document.
+
+Kevin Conder reformatted the original plain-text to the DocBook format.
+
+Giuliano Pochini corrected typos and contributed the example codes in
+the hardware constraints section.