Docs/sound: Add documentation for userspace-driven ALSA timers

Add the documentation which describes the new userspace-driven timers
API introduced in this patch series. The documentation contains:

- Description of userspace-driven ALSA timers, what they are for
- Description of the timers API
- Example of how the timers can be created and triggered
- How the timers can be used as a timer sources for snd-aloop module

Suggested-by: Axel Holzinger <[e-mail]>
Signed-off-by: Ivan Orlov <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

Documentation/sound/index.rst

···
    alsa-configuration
    hd-audio/index
    cards/index
+   utimers
 
 .. only::  subproject and html
 

Documentation/sound/utimers.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+Userspace-driven timers
+=======================
+
+:Author: Ivan Orlov <[e-mail]>
+
+Preface
+=======
+
+This document describes the userspace-driven timers: virtual ALSA timers
+which could be created and controlled by userspace applications using
+IOCTL calls. Such timers could be useful when synchronizing audio
+stream with timer sources which we don't have ALSA timers exported for
+(e.g. PTP clocks), and when synchronizing the audio stream going through
+two virtual sound devices using ``snd-aloop`` (for instance, when
+we have a network application sending frames to one snd-aloop device,
+and another sound application listening on the other end of snd-aloop).
+
+Enabling userspace-driven timers
+================================
+
+The userspace-driven timers could be enabled in the kernel using the
+``CONFIG_SND_UTIMER`` configuration option. It depends on the
+``CONFIG_SND_TIMER`` option, so it also should be enabled.
+
+Userspace-driven timers API
+===========================
+
+Userspace application can create a userspace-driven ALSA timer by
+executing the ``SNDRV_TIMER_IOCTL_CREATE`` ioctl call on the
+``/dev/snd/timer`` device file descriptor. The ``snd_timer_uinfo``
+structure should be passed as an ioctl argument:
+
+::
+
+    struct snd_timer_uinfo {
+        __u64 resolution;
+        int fd;
+        unsigned int id;
+        unsigned char reserved[16];
+    }
+
+The ``resolution`` field sets the desired resolution in nanoseconds for
+the virtual timer. ``resolution`` field simply provides an information
+about the virtual timer, but does not affect the timing itself. ``id``
+field gets overwritten by the ioctl, and the identifier you get in this
+field after the call can be used as a timer subdevice number when
+passing the timer to ``snd-aloop`` kernel module or other userspace
+applications. There could be up to 128 userspace-driven timers in the
+system at one moment of time, thus the id value ranges from 0 to 127.
+
+Besides from overwriting the ``snd_timer_uinfo`` struct, ioctl stores
+a timer file descriptor, which can be used to trigger the timer, in the
+``fd`` field of the ``snd_timer_uinfo`` struct. Allocation of a file
+descriptor for the timer guarantees that the timer can only be triggered
+by the process which created it. The timer then can be triggered with
+``SNDRV_TIMER_IOCTL_TRIGGER`` ioctl call on the timer file descriptor.
+
+So, the example code for creating and triggering the timer would be:
+
+::
+
+    static struct snd_timer_uinfo utimer_info = {
+        /* Timer is going to tick (presumably) every 1000000 ns */
+        .resolution = 1000000ULL,
+        .id = -1,
+    };
+
+    int timer_device_fd = open("/dev/snd/timer",  O_RDWR | O_CLOEXEC);
+
+    if (ioctl(timer_device_fd, SNDRV_TIMER_IOCTL_CREATE, &utimer_info)) {
+        perror("Failed to create the timer");
+        return -1;
+    }
+
+    ...
+
+    /*
+     * Now we want to trigger the timer. Callbacks of all of the
+     * timer instances binded to this timer will be executed after
+     * this call.
+     */
+    ioctl(utimer_info.fd, SNDRV_TIMER_IOCTL_TRIGGER, NULL);
+
+    ...
+
+    /* Now, destroy the timer */
+    close(timer_info.fd);
+
+
+More detailed example of creating and ticking the timer could be found
+in the utimer ALSA selftest.
+
+Userspace-driven timers and snd-aloop
+-------------------------------------
+
+Userspace-driven timers could be easily used with ``snd-aloop`` module
+when synchronizing two sound applications on both ends of the virtual
+sound loopback. For instance, if one of the applications receives sound
+frames from network and sends them to snd-aloop pcm device, and another
+application listens for frames on the other snd-aloop pcm device, it
+makes sense that the ALSA middle layer should initiate a data
+transaction when the new period of data is received through network, but
+not when the certain amount of jiffies elapses. Userspace-driven ALSA
+timers could be used to achieve this.
+
+To use userspace-driven ALSA timer as a timer source of snd-aloop, pass
+the following string as the snd-aloop ``timer_source`` parameter:
+
+::
+
+  # modprobe snd-aloop timer_source="-1.4.<utimer_id>"
+
+Where ``utimer_id`` is the id of the timer you created with
+``SNDRV_TIMER_IOCTL_CREATE``, and ``4`` is the number of
+userspace-driven timers device (``SNDRV_TIMER_GLOBAL_UDRIVEN``).
+
+``resolution`` for the userspace-driven ALSA timer used with snd-aloop
+should be calculated as ``1000000000ULL / frame_rate * period_size`` as
+the timer is going to tick every time a new period of frames is ready.
+
+After that, each time you trigger the timer with
+``SNDRV_TIMER_IOCTL_TRIGGER`` the new period of data will be transferred
+from one snd-aloop device to another.