tty: n_gsm line discipline

Add an implementation of GSM 0710 MUX. The implementation currently supports

- Basic and advanced framing (as either end of the link)
- UI or UIH data frames
- Adaption layer 1-4 (1 and 2 via tty, 3 and 4 as skbuff lists)
- Modem and control messages including the correct retry process
- Flow control

and exposes the MUX channels as a set of virtual tty devices including modem
signals. This is an experimental driver.

Signed-off-by: Alan Cox <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/Kconfig

···
 	  Allows synchronous HDLC communications with tty device drivers that
 	  support synchronous HDLC such as the Microgate SyncLink adapter.
 
-	  This driver can only be built as a module ( = code which can be
+	  This driver can be built as a module ( = code which can be
 	  inserted in and removed from the running kernel whenever you want).
 	  The module will be called n_hdlc. If you want to do that, say M
 	  here.
+
+config N_GSM
+	tristate "GSM MUX line discipline support (EXPERIMENTAL)"
+	depends on EXPERIMENTAL
+	help
+	  This line discipline provides support for the GSM MUX protocol and
+	  presents the mux as a set of 61 individual tty devices.
 
 config RISCOM8
 	tristate "SDL RISCom/8 card support"

drivers/char/Makefile

···
 obj-$(CONFIG_SYNCLINKMP)	+= synclinkmp.o
 obj-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_N_HDLC)		+= n_hdlc.o
+obj-$(CONFIG_N_GSM)		+= n_gsm.o
 obj-$(CONFIG_AMIGA_BUILTIN_SERIAL) += amiserial.o
 obj-$(CONFIG_SX)		+= sx.o generic_serial.o
 obj-$(CONFIG_RIO)		+= rio/ generic_serial.o

drivers/char/n_gsm.c

···
+/*
+ * n_gsm.c GSM 0710 tty multiplexor
+ * Copyright (c) 2009/10 Intel Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * 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., 675 Mass Ave, Cambridge, MA 02139, USA.
+ *
+ *	* THIS IS A DEVELOPMENT SNAPSHOT IT IS NOT A FINAL RELEASE *
+ *
+ * TO DO:
+ *	Mostly done:	ioctls for setting modes/timing
+ *	Partly done: 	hooks so you can pull off frames to non tty devs
+ *	Restart DLCI 0 when it closes ?
+ *	Test basic encoding
+ *	Improve the tx engine
+ *	Resolve tx side locking by adding a queue_head and routing
+ *		all control traffic via it
+ *	General tidy/document
+ *	Review the locking/move to refcounts more (mux now moved to an
+ *		alloc/free model ready)
+ *	Use newest tty open/close port helpers and install hooks
+ *	What to do about power functions ?
+ *	Termios setting and negotiation
+ *	Do we need a 'which mux are you' ioctl to correlate mux and tty sets
+ *
+ */
+
+#include <linux/types.h>
+#include <linux/major.h>
+#include <linux/errno.h>
+#include <linux/signal.h>
+#include <linux/fcntl.h>
+#include <linux/sched.h>
+#include <linux/interrupt.h>
+#include <linux/tty.h>
+#include <linux/timer.h>
+#include <linux/ctype.h>
+#include <linux/mm.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/poll.h>
+#include <linux/bitops.h>
+#include <linux/file.h>
+#include <linux/uaccess.h>
+#include <linux/module.h>
+#include <linux/timer.h>
+#include <linux/tty_flip.h>
+#include <linux/tty_driver.h>
+#include <linux/serial.h>
+#include <linux/kfifo.h>
+#include <linux/skbuff.h>
+#include <linux/gsmmux.h>
+
+static int debug;
+module_param(debug, int, 0600);
+
+#define T1	(HZ/10)
+#define T2	(HZ/3)
+#define N2	3
+
+/* Use long timers for testing at low speed with debug on */
+#ifdef DEBUG_TIMING
+#define T1	HZ
+#define T2	(2 * HZ)
+#endif
+
+/* Semi-arbitary buffer size limits. 0710 is normally run with 32-64 byte
+   limits so this is plenty */
+#define MAX_MRU 512
+#define MAX_MTU 512
+
+/*
+ *	Each block of data we have queued to go out is in the form of
+ *	a gsm_msg which holds everything we need in a link layer independant
+ *	format
+ */
+
+struct gsm_msg {
+	struct gsm_msg *next;
+	u8 addr;		/* DLCI address + flags */
+	u8 ctrl;		/* Control byte + flags */
+	unsigned int len;	/* Length of data block (can be zero) */
+	unsigned char *data;	/* Points into buffer but not at the start */
+	unsigned char buffer[0];
+};
+
+/*
+ *	Each active data link has a gsm_dlci structure associated which ties
+ *	the link layer to an optional tty (if the tty side is open). To avoid
+ *	complexity right now these are only ever freed up when the mux is
+ *	shut down.
+ *
+ *	At the moment we don't free DLCI objects until the mux is torn down
+ *	this avoid object life time issues but might be worth review later.
+ */
+
+struct gsm_dlci {
+	struct gsm_mux *gsm;
+	int addr;
+	int state;
+#define DLCI_CLOSED		0
+#define DLCI_OPENING		1	/* Sending SABM not seen UA */
+#define DLCI_OPEN		2	/* SABM/UA complete */
+#define DLCI_CLOSING		3	/* Sending DISC not seen UA/DM */
+
+	/* Link layer */
+	spinlock_t lock;	/* Protects the internal state */
+	struct timer_list t1;	/* Retransmit timer for SABM and UA */
+	int retries;
+	/* Uplink tty if active */
+	struct tty_port port;	/* The tty bound to this DLCI if there is one */
+	struct kfifo *fifo;	/* Queue fifo for the DLCI */
+	struct kfifo _fifo;	/* For new fifo API porting only */
+	int adaption;		/* Adaption layer in use */
+	u32 modem_rx;		/* Our incoming virtual modem lines */
+	u32 modem_tx;		/* Our outgoing modem lines */
+	int dead;		/* Refuse re-open */
+	/* Flow control */
+	int throttled;		/* Private copy of throttle state */
+	int constipated;	/* Throttle status for outgoing */
+	/* Packetised I/O */
+	struct sk_buff *skb;	/* Frame being sent */
+	struct sk_buff_head skb_list;	/* Queued frames */
+	/* Data handling callback */
+	void (*data)(struct gsm_dlci *dlci, u8 *data, int len);
+};
+
+/* DLCI 0, 62/63 are special or reseved see gsmtty_open */
+
+#define NUM_DLCI		64
+
+/*
+ *	DLCI 0 is used to pass control blocks out of band of the data
+ *	flow (and with a higher link priority). One command can be outstanding
+ *	at a time and we use this structure to manage them. They are created
+ *	and destroyed by the user context, and updated by the receive paths
+ *	and timers
+ */
+
+struct gsm_control {
+	u8 cmd;		/* Command we are issuing */
+	u8 *data;	/* Data for the command in case we retransmit */
+	int len;	/* Length of block for retransmission */
+	int done;	/* Done flag */
+	int error;	/* Error if any */
+};
+
+/*
+ *	Each GSM mux we have is represented by this structure. If we are
+ *	operating as an ldisc then we use this structure as our ldisc
+ *	state. We need to sort out lifetimes and locking with respect
+ *	to the gsm mux array. For now we don't free DLCI objects that
+ *	have been instantiated until the mux itself is terminated.
+ *
+ *	To consider further: tty open versus mux shutdown.
+ */
+
+struct gsm_mux {
+	struct tty_struct *tty;		/* The tty our ldisc is bound to */
+	spinlock_t lock;
+
+	/* Events on the GSM channel */
+	wait_queue_head_t event;
+
+	/* Bits for GSM mode decoding */
+
+	/* Framing Layer */
+	unsigned char *buf;
+	int state;
+#define GSM_SEARCH		0
+#define GSM_START		1
+#define GSM_ADDRESS		2
+#define GSM_CONTROL		3
+#define GSM_LEN			4
+#define GSM_DATA		5
+#define GSM_FCS			6
+#define GSM_OVERRUN		7
+	unsigned int len;
+	unsigned int address;
+	unsigned int count;
+	int escape;
+	int encoding;
+	u8 control;
+	u8 fcs;
+	u8 *txframe;			/* TX framing buffer */
+
+	/* Methods for the receiver side */
+	void (*receive)(struct gsm_mux *gsm, u8 ch);
+	void (*error)(struct gsm_mux *gsm, u8 ch, u8 flag);
+	/* And transmit side */
+	int (*output)(struct gsm_mux *mux, u8 *data, int len);
+
+	/* Link Layer */
+	unsigned int mru;
+	unsigned int mtu;
+	int initiator;			/* Did we initiate connection */
+	int dead;			/* Has the mux been shut down */
+	struct gsm_dlci *dlci[NUM_DLCI];
+	int constipated;		/* Asked by remote to shut up */
+
+	spinlock_t tx_lock;
+	unsigned int tx_bytes;		/* TX data outstanding */
+#define TX_THRESH_HI		8192
+#define TX_THRESH_LO		2048
+	struct gsm_msg *tx_head;	/* Pending data packets */
+	struct gsm_msg *tx_tail;
+
+	/* Control messages */
+	struct timer_list t2_timer;	/* Retransmit timer for commands */
+	int cretries;			/* Command retry counter */
+	struct gsm_control *pending_cmd;/* Our current pending command */
+	spinlock_t control_lock;	/* Protects the pending command */
+
+	/* Configuration */
+	int adaption;		/* 1 or 2 supported */
+	u8 ftype;		/* UI or UIH */
+	int t1, t2;		/* Timers in 1/100th of a sec */
+	int n2;			/* Retry count */
+
+	/* Statistics (not currently exposed) */
+	unsigned long bad_fcs;
+	unsigned long malformed;
+	unsigned long io_error;
+	unsigned long bad_size;
+	unsigned long unsupported;
+};
+
+
+/*
+ *	Mux objects - needed so that we can translate a tty index into the
+ *	relevant mux and DLCI.
+ */
+
+#define MAX_MUX		4			/* 256 minors */
+static struct gsm_mux *gsm_mux[MAX_MUX];	/* GSM muxes */
+static spinlock_t gsm_mux_lock;
+
+/*
+ *	This section of the driver logic implements the GSM encodings
+ *	both the basic and the 'advanced'. Reliable transport is not
+ *	supported.
+ */
+
+#define CR			0x02
+#define EA			0x01
+#define	PF			0x10
+
+/* I is special: the rest are ..*/
+#define RR			0x01
+#define UI			0x03
+#define RNR			0x05
+#define REJ			0x09
+#define DM			0x0F
+#define SABM			0x2F
+#define DISC			0x43
+#define UA			0x63
+#define	UIH			0xEF
+
+/* Channel commands */
+#define CMD_NSC			0x09
+#define CMD_TEST		0x11
+#define CMD_PSC			0x21
+#define CMD_RLS			0x29
+#define CMD_FCOFF		0x31
+#define CMD_PN			0x41
+#define CMD_RPN			0x49
+#define CMD_FCON		0x51
+#define CMD_CLD			0x61
+#define CMD_SNC			0x69
+#define CMD_MSC			0x71
+
+/* Virtual modem bits */
+#define MDM_FC			0x01
+#define MDM_RTC			0x02
+#define MDM_RTR			0x04
+#define MDM_IC			0x20
+#define MDM_DV			0x40
+
+#define GSM0_SOF		0xF9
+#define GSM1_SOF 		0x7E
+#define GSM1_ESCAPE		0x7D
+#define GSM1_ESCAPE_BITS	0x20
+#define XON			0x11
+#define XOFF			0x13
+
+static const struct tty_port_operations gsm_port_ops;
+
+/*
+ *	CRC table for GSM 0710
+ */
+
+static const u8 gsm_fcs8[256] = {
+	0x00, 0x91, 0xE3, 0x72, 0x07, 0x96, 0xE4, 0x75,
+	0x0E, 0x9F, 0xED, 0x7C, 0x09, 0x98, 0xEA, 0x7B,
+	0x1C, 0x8D, 0xFF, 0x6E, 0x1B, 0x8A, 0xF8, 0x69,
+	0x12, 0x83, 0xF1, 0x60, 0x15, 0x84, 0xF6, 0x67,
+	0x38, 0xA9, 0xDB, 0x4A, 0x3F, 0xAE, 0xDC, 0x4D,
+	0x36, 0xA7, 0xD5, 0x44, 0x31, 0xA0, 0xD2, 0x43,
+	0x24, 0xB5, 0xC7, 0x56, 0x23, 0xB2, 0xC0, 0x51,
+	0x2A, 0xBB, 0xC9, 0x58, 0x2D, 0xBC, 0xCE, 0x5F,
+	0x70, 0xE1, 0x93, 0x02, 0x77, 0xE6, 0x94, 0x05,
+	0x7E, 0xEF, 0x9D, 0x0C, 0x79, 0xE8, 0x9A, 0x0B,
+	0x6C, 0xFD, 0x8F, 0x1E, 0x6B, 0xFA, 0x88, 0x19,
+	0x62, 0xF3, 0x81, 0x10, 0x65, 0xF4, 0x86, 0x17,
+	0x48, 0xD9, 0xAB, 0x3A, 0x4F, 0xDE, 0xAC, 0x3D,
+	0x46, 0xD7, 0xA5, 0x34, 0x41, 0xD0, 0xA2, 0x33,
+	0x54, 0xC5, 0xB7, 0x26, 0x53, 0xC2, 0xB0, 0x21,
+	0x5A, 0xCB, 0xB9, 0x28, 0x5D, 0xCC, 0xBE, 0x2F,
+	0xE0, 0x71, 0x03, 0x92, 0xE7, 0x76, 0x04, 0x95,
+	0xEE, 0x7F, 0x0D, 0x9C, 0xE9, 0x78, 0x0A, 0x9B,
+	0xFC, 0x6D, 0x1F, 0x8E, 0xFB, 0x6A, 0x18, 0x89,
+	0xF2, 0x63, 0x11, 0x80, 0xF5, 0x64, 0x16, 0x87,
+	0xD8, 0x49, 0x3B, 0xAA, 0xDF, 0x4E, 0x3C, 0xAD,
+	0xD6, 0x47, 0x35, 0xA4, 0xD1, 0x40, 0x32, 0xA3,
+	0xC4, 0x55, 0x27, 0xB6, 0xC3, 0x52, 0x20, 0xB1,
+	0xCA, 0x5B, 0x29, 0xB8, 0xCD, 0x5C, 0x2E, 0xBF,
+	0x90, 0x01, 0x73, 0xE2, 0x97, 0x06, 0x74, 0xE5,
+	0x9E, 0x0F, 0x7D, 0xEC, 0x99, 0x08, 0x7A, 0xEB,
+	0x8C, 0x1D, 0x6F, 0xFE, 0x8B, 0x1A, 0x68, 0xF9,
+	0x82, 0x13, 0x61, 0xF0, 0x85, 0x14, 0x66, 0xF7,
+	0xA8, 0x39, 0x4B, 0xDA, 0xAF, 0x3E, 0x4C, 0xDD,
+	0xA6, 0x37, 0x45, 0xD4, 0xA1, 0x30, 0x42, 0xD3,
+	0xB4, 0x25, 0x57, 0xC6, 0xB3, 0x22, 0x50, 0xC1,
+	0xBA, 0x2B, 0x59, 0xC8, 0xBD, 0x2C, 0x5E, 0xCF
+};
+
+#define INIT_FCS	0xFF
+#define GOOD_FCS	0xCF
+
+/**
+ *	gsm_fcs_add	-	update FCS
+ *	@fcs: Current FCS
+ *	@c: Next data
+ *
+ *	Update the FCS to include c. Uses the algorithm in the specification
+ *	notes.
+ */
+
+static inline u8 gsm_fcs_add(u8 fcs, u8 c)
+{
+	return gsm_fcs8[fcs ^ c];
+}
+
+/**
+ *	gsm_fcs_add_block	-	update FCS for a block
+ *	@fcs: Current FCS
+ *	@c: buffer of data
+ *	@len: length of buffer
+ *
+ *	Update the FCS to include c. Uses the algorithm in the specification
+ *	notes.
+ */
+
+static inline u8 gsm_fcs_add_block(u8 fcs, u8 *c, int len)
+{
+	while (len--)
+		fcs = gsm_fcs8[fcs ^ *c++];
+	return fcs;
+}
+
+/**
+ *	gsm_read_ea		-	read a byte into an EA
+ *	@val: variable holding value
+ *	c: byte going into the EA
+ *
+ *	Processes one byte of an EA. Updates the passed variable
+ *	and returns 1 if the EA is now completely read
+ */
+
+static int gsm_read_ea(unsigned int *val, u8 c)
+{
+	/* Add the next 7 bits into the value */
+	*val <<= 7;
+	*val |= c >> 1;
+	/* Was this the last byte of the EA 1 = yes*/
+	return c & EA;
+}
+
+/**
+ *	gsm_encode_modem	-	encode modem data bits
+ *	@dlci: DLCI to encode from
+ *
+ *	Returns the correct GSM encoded modem status bits (6 bit field) for
+ *	the current status of the DLCI and attached tty object
+ */
+
+static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
+{
+	u8 modembits = 0;
+	/* FC is true flow control not modem bits */
+	if (dlci->throttled)
+		modembits |= MDM_FC;
+	if (dlci->modem_tx & TIOCM_DTR)
+		modembits |= MDM_RTC;
+	if (dlci->modem_tx & TIOCM_RTS)
+		modembits |= MDM_RTR;
+	if (dlci->modem_tx & TIOCM_RI)
+		modembits |= MDM_IC;
+	if (dlci->modem_tx & TIOCM_CD)
+		modembits |= MDM_DV;
+	return modembits;
+}
+
+/**
+ *	gsm_print_packet	-	display a frame for debug
+ *	@hdr: header to print before decode
+ *	@addr: address EA from the frame
+ *	@cr: C/R bit from the frame
+ *	@control: control including PF bit
+ *	@data: following data bytes
+ *	@dlen: length of data
+ *
+ *	Displays a packet in human readable format for debugging purposes. The
+ *	style is based on amateur radio LAP-B dump display.
+ */
+
+static void gsm_print_packet(const char *hdr, int addr, int cr,
+					u8 control, const u8 *data, int dlen)
+{
+	if (!(debug & 1))
+		return;
+
+	printk(KERN_INFO "%s %d) %c: ", hdr, addr, "RC"[cr]);
+
+	switch (control & ~PF) {
+	case SABM:
+		printk(KERN_CONT "SABM");
+		break;
+	case UA:
+		printk(KERN_CONT "UA");
+		break;
+	case DISC:
+		printk(KERN_CONT "DISC");
+		break;
+	case DM:
+		printk(KERN_CONT "DM");
+		break;
+	case UI:
+		printk(KERN_CONT "UI");
+		break;
+	case UIH:
+		printk(KERN_CONT "UIH");
+		break;
+	default:
+		if (!(control & 0x01)) {
+			printk(KERN_CONT "I N(S)%d N(R)%d",
+				(control & 0x0E) >> 1, (control & 0xE)>> 5);
+		} else switch (control & 0x0F) {
+		case RR:
+			printk("RR(%d)", (control & 0xE0) >> 5);
+			break;
+		case RNR:
+			printk("RNR(%d)", (control & 0xE0) >> 5);
+			break;
+		case REJ:
+			printk("REJ(%d)", (control & 0xE0) >> 5);
+			break;
+		default:
+			printk(KERN_CONT "[%02X]", control);
+		}
+	}
+
+	if (control & PF)
+		printk(KERN_CONT "(P)");
+	else
+		printk(KERN_CONT "(F)");
+
+	if (dlen) {
+		int ct = 0;
+		while (dlen--) {
+			if (ct % 8 == 0)
+				printk(KERN_CONT "\n    ");
+			printk(KERN_CONT "%02X ", *data++);
+			ct++;
+		}
+	}
+	printk(KERN_CONT "\n");
+}
+
+
+/*
+ *	Link level transmission side
+ */
+
+/**
+ *	gsm_stuff_packet	-	bytestuff a packet
+ *	@ibuf: input
+ *	@obuf: output
+ *	@len: length of input
+ *
+ *	Expand a buffer by bytestuffing it. The worst case size change
+ *	is doubling and the caller is responsible for handing out
+ *	suitable sized buffers.
+ */
+
+static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
+{
+	int olen = 0;
+	while (len--) {
+		if (*input == GSM1_SOF || *input == GSM1_ESCAPE
+		    || *input == XON || *input == XOFF) {
+			*output++ = GSM1_ESCAPE;
+			*output++ = *input++ ^ GSM1_ESCAPE_BITS;
+			olen++;
+		} else
+			*output++ = *input++;
+		olen++;
+	}
+	return olen;
+}
+
+static void hex_packet(const unsigned char *p, int len)
+{
+	int i;
+	for (i = 0; i < len; i++) {
+		if (i && (i % 16) == 0)
+			printk("\n");
+		printk("%02X ", *p++);
+	}
+	printk("\n");
+}
+
+/**
+ *	gsm_send	-	send a control frame
+ *	@gsm: our GSM mux
+ *	@addr: address for control frame
+ *	@cr: command/response bit
+ *	@control:  control byte including PF bit
+ *
+ *	Format up and transmit a control frame. These do not go via the
+ *	queueing logic as they should be transmitted ahead of data when
+ *	they are needed.
+ *
+ *	FIXME: Lock versus data TX path
+ */
+
+static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
+{
+	int len;
+	u8 cbuf[10];
+	u8 ibuf[3];
+
+	switch (gsm->encoding) {
+	case 0:
+		cbuf[0] = GSM0_SOF;
+		cbuf[1] = (addr << 2) | (cr << 1) | EA;
+		cbuf[2] = control;
+		cbuf[3] = EA;	/* Length of data = 0 */
+		cbuf[4] = 0xFF - gsm_fcs_add_block(INIT_FCS, cbuf + 1, 3);
+		cbuf[5] = GSM0_SOF;
+		len = 6;
+		break;
+	case 1:
+	case 2:
+		/* Control frame + packing (but not frame stuffing) in mode 1 */
+		ibuf[0] = (addr << 2) | (cr << 1) | EA;
+		ibuf[1] = control;
+		ibuf[2] = 0xFF - gsm_fcs_add_block(INIT_FCS, ibuf, 2);
+		/* Stuffing may double the size worst case */
+		len = gsm_stuff_frame(ibuf, cbuf + 1, 3);
+		/* Now add the SOF markers */
+		cbuf[0] = GSM1_SOF;
+		cbuf[len + 1] = GSM1_SOF;
+		/* FIXME: we can omit the lead one in many cases */
+		len += 2;
+		break;
+	default:
+		WARN_ON(1);
+		return;
+	}
+	gsm->output(gsm, cbuf, len);
+	gsm_print_packet("-->", addr, cr, control, NULL, 0);
+}
+
+/**
+ *	gsm_response	-	send a control response
+ *	@gsm: our GSM mux
+ *	@addr: address for control frame
+ *	@control:  control byte including PF bit
+ *
+ *	Format up and transmit a link level response frame.
+ */
+
+static inline void gsm_response(struct gsm_mux *gsm, int addr, int control)
+{
+	gsm_send(gsm, addr, 0, control);
+}
+
+/**
+ *	gsm_command	-	send a control command
+ *	@gsm: our GSM mux
+ *	@addr: address for control frame
+ *	@control:  control byte including PF bit
+ *
+ *	Format up and transmit a link level command frame.
+ */
+
+static inline void gsm_command(struct gsm_mux *gsm, int addr, int control)
+{
+	gsm_send(gsm, addr, 1, control);
+}
+
+/* Data transmission */
+
+#define HDR_LEN		6	/* ADDR CTRL [LEN.2] DATA FCS */
+
+/**
+ *	gsm_data_alloc		-	allocate data frame
+ *	@gsm: GSM mux
+ *	@addr: DLCI address
+ *	@len: length excluding header and FCS
+ *	@ctrl: control byte
+ *
+ *	Allocate a new data buffer for sending frames with data. Space is left
+ *	at the front for header bytes but that is treated as an implementation
+ *	detail and not for the high level code to use
+ */
+
+static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
+								u8 ctrl)
+{
+	struct gsm_msg *m = kmalloc(sizeof(struct gsm_msg) + len + HDR_LEN,
+								GFP_ATOMIC);
+	if (m == NULL)
+		return NULL;
+	m->data = m->buffer + HDR_LEN - 1;	/* Allow for FCS */
+	m->len = len;
+	m->addr = addr;
+	m->ctrl = ctrl;
+	m->next = NULL;
+	return m;
+}
+
+/**
+ *	gsm_data_kick		-	poke the queue
+ *	@gsm: GSM Mux
+ *
+ *	The tty device has called us to indicate that room has appeared in
+ *	the transmit queue. Ram more data into the pipe if we have any
+ *
+ *	FIXME: lock against link layer control transmissions
+ */
+
+static void gsm_data_kick(struct gsm_mux *gsm)
+{
+	struct gsm_msg *msg = gsm->tx_head;
+	int len;
+	int skip_sof = 0;
+
+	/* FIXME: We need to apply this solely to data messages */
+	if (gsm->constipated)
+		return;
+
+	while (gsm->tx_head != NULL) {
+		msg = gsm->tx_head;
+		if (gsm->encoding != 0) {
+			gsm->txframe[0] = GSM1_SOF;
+			len = gsm_stuff_frame(msg->data,
+						gsm->txframe + 1, msg->len);
+			gsm->txframe[len + 1] = GSM1_SOF;
+			len += 2;
+		} else {
+			gsm->txframe[0] = GSM0_SOF;
+			memcpy(gsm->txframe + 1 , msg->data, msg->len);
+			gsm->txframe[msg->len + 1] = GSM0_SOF;
+			len = msg->len + 2;
+		}
+
+		if (debug & 4) {
+			printk("gsm_data_kick: \n");
+			hex_packet(gsm->txframe, len);
+		}
+
+		if (gsm->output(gsm, gsm->txframe + skip_sof,
+						len - skip_sof) < 0)
+			break;
+		/* FIXME: Can eliminate one SOF in many more cases */
+		gsm->tx_head = msg->next;
+		if (gsm->tx_head == NULL)
+			gsm->tx_tail = NULL;
+		gsm->tx_bytes -= msg->len;
+		kfree(msg);
+		/* For a burst of frames skip the extra SOF within the
+		   burst */
+		skip_sof = 1;
+	}
+}
+
+/**
+ *	__gsm_data_queue		-	queue a UI or UIH frame
+ *	@dlci: DLCI sending the data
+ *	@msg: message queued
+ *
+ *	Add data to the transmit queue and try and get stuff moving
+ *	out of the mux tty if not already doing so. The Caller must hold
+ *	the gsm tx lock.
+ */
+
+static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
+{
+	struct gsm_mux *gsm = dlci->gsm;
+	u8 *dp = msg->data;
+	u8 *fcs = dp + msg->len;
+
+	/* Fill in the header */
+	if (gsm->encoding == 0) {
+		if (msg->len < 128)
+			*--dp = (msg->len << 1) | EA;
+		else {
+			*--dp = (msg->len >> 6) | EA;
+			*--dp = (msg->len & 127) << 1;
+		}
+	}
+
+	*--dp = msg->ctrl;
+	if (gsm->initiator)
+		*--dp = (msg->addr << 2) | 2 | EA;
+	else
+		*--dp = (msg->addr << 2) | EA;
+	*fcs = gsm_fcs_add_block(INIT_FCS, dp , msg->data - dp);
+	/* Ugly protocol layering violation */
+	if (msg->ctrl == UI || msg->ctrl == (UI|PF))
+		*fcs = gsm_fcs_add_block(*fcs, msg->data, msg->len);
+	*fcs = 0xFF - *fcs;
+
+	gsm_print_packet("Q> ", msg->addr, gsm->initiator, msg->ctrl,
+							msg->data, msg->len);
+
+	/* Move the header back and adjust the length, also allow for the FCS
+	   now tacked on the end */
+	msg->len += (msg->data - dp) + 1;
+	msg->data = dp;
+
+	/* Add to the actual output queue */
+	if (gsm->tx_tail)
+		gsm->tx_tail->next = msg;
+	else
+		gsm->tx_head = msg;
+	gsm->tx_tail = msg;
+	gsm->tx_bytes += msg->len;
+	gsm_data_kick(gsm);
+}
+
+/**
+ *	gsm_data_queue		-	queue a UI or UIH frame
+ *	@dlci: DLCI sending the data
+ *	@msg: message queued
+ *
+ *	Add data to the transmit queue and try and get stuff moving
+ *	out of the mux tty if not already doing so. Take the
+ *	the gsm tx lock and dlci lock.
+ */
+
+static void gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&dlci->gsm->tx_lock, flags);
+	__gsm_data_queue(dlci, msg);
+	spin_unlock_irqrestore(&dlci->gsm->tx_lock, flags);
+}
+
+/**
+ *	gsm_dlci_data_output	-	try and push data out of a DLCI
+ *	@gsm: mux
+ *	@dlci: the DLCI to pull data from
+ *
+ *	Pull data from a DLCI and send it into the transmit queue if there
+ *	is data. Keep to the MRU of the mux. This path handles the usual tty
+ *	interface which is a byte stream with optional modem data.
+ *
+ *	Caller must hold the tx_lock of the mux.
+ */
+
+static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
+{
+	struct gsm_msg *msg;
+	u8 *dp;
+	int len, size;
+	int h = dlci->adaption - 1;
+
+	len = kfifo_len(dlci->fifo);
+	if (len == 0)
+		return 0;
+
+	/* MTU/MRU count only the data bits */
+	if (len > gsm->mtu)
+		len = gsm->mtu;
+
+	size = len + h;
+
+	msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
+	/* FIXME: need a timer or something to kick this so it can't
+	   get stuck with no work outstanding and no buffer free */
+	if (msg == NULL)
+		return -ENOMEM;
+	dp = msg->data;
+	switch (dlci->adaption) {
+	case 1:	/* Unstructured */
+		break;
+	case 2:	/* Unstructed with modem bits. Always one byte as we never
+		   send inline break data */
+		*dp += gsm_encode_modem(dlci);
+		len--;
+		break;
+	}
+	WARN_ON(kfifo_out_locked(dlci->fifo, dp , len, &dlci->lock) != len);
+	__gsm_data_queue(dlci, msg);
+	/* Bytes of data we used up */
+	return size;
+}
+
+/**
+ *	gsm_dlci_data_output_framed  -	try and push data out of a DLCI
+ *	@gsm: mux
+ *	@dlci: the DLCI to pull data from
+ *
+ *	Pull data from a DLCI and send it into the transmit queue if there
+ *	is data. Keep to the MRU of the mux. This path handles framed data
+ *	queued as skbuffs to the DLCI.
+ *
+ *	Caller must hold the tx_lock of the mux.
+ */
+
+static int gsm_dlci_data_output_framed(struct gsm_mux *gsm,
+						struct gsm_dlci *dlci)
+{
+	struct gsm_msg *msg;
+	u8 *dp;
+	int len, size;
+	int last = 0, first = 0;
+	int overhead = 0;
+
+	/* One byte per frame is used for B/F flags */
+	if (dlci->adaption == 4)
+		overhead = 1;
+
+	/* dlci->skb is locked by tx_lock */
+	if (dlci->skb == NULL) {
+		dlci->skb = skb_dequeue(&dlci->skb_list);
+		if (dlci->skb == NULL)
+			return 0;
+		first = 1;
+	}
+	len = dlci->skb->len + overhead;
+
+	/* MTU/MRU count only the data bits */
+	if (len > gsm->mtu) {
+		if (dlci->adaption == 3) {
+			/* Over long frame, bin it */
+			kfree_skb(dlci->skb);
+			dlci->skb = NULL;
+			return 0;
+		}
+		len = gsm->mtu;
+	} else
+		last = 1;
+
+	size = len + overhead;
+	msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
+
+	/* FIXME: need a timer or something to kick this so it can't
+	   get stuck with no work outstanding and no buffer free */
+	if (msg == NULL)
+		return -ENOMEM;
+	dp = msg->data;
+
+	if (dlci->adaption == 4) { /* Interruptible framed (Packetised Data) */
+		/* Flag byte to carry the start/end info */
+		*dp++ = last << 7 | first << 6 | 1;	/* EA */
+		len--;
+	}
+	memcpy(dp, skb_pull(dlci->skb, len), len);
+	__gsm_data_queue(dlci, msg);
+	if (last)
+		dlci->skb = NULL;
+	return size;
+}
+
+/**
+ *	gsm_dlci_data_sweep		-	look for data to send
+ *	@gsm: the GSM mux
+ *
+ *	Sweep the GSM mux channels in priority order looking for ones with
+ *	data to send. We could do with optimising this scan a bit. We aim
+ *	to fill the queue totally or up to TX_THRESH_HI bytes. Once we hit
+ *	TX_THRESH_LO we get called again
+ *
+ *	FIXME: We should round robin between groups and in theory you can
+ *	renegotiate DLCI priorities with optional stuff. Needs optimising.
+ */
+
+static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
+{
+	int len;
+	/* Priority ordering: We should do priority with RR of the groups */
+	int i = 1;
+	unsigned long flags;
+
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	while (i < NUM_DLCI) {
+		struct gsm_dlci *dlci;
+
+		if (gsm->tx_bytes > TX_THRESH_HI)
+			break;
+		dlci = gsm->dlci[i];
+		if (dlci == NULL || dlci->constipated) {
+			i++;
+			continue;
+		}
+		if (dlci->adaption < 3)
+			len = gsm_dlci_data_output(gsm, dlci);
+		else
+			len = gsm_dlci_data_output_framed(gsm, dlci);
+		if (len < 0)
+			return;
+		/* DLCI empty - try the next */
+		if (len == 0)
+			i++;
+	}
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+}
+
+/**
+ *	gsm_dlci_data_kick	-	transmit if possible
+ *	@dlci: DLCI to kick
+ *
+ *	Transmit data from this DLCI if the queue is empty. We can't rely on
+ *	a tty wakeup except when we filled the pipe so we need to fire off
+ *	new data ourselves in other cases.
+ */
+
+static void gsm_dlci_data_kick(struct gsm_dlci *dlci)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&dlci->gsm->tx_lock, flags);
+	/* If we have nothing running then we need to fire up */
+	if (dlci->gsm->tx_bytes == 0)
+		gsm_dlci_data_output(dlci->gsm, dlci);
+	else if (dlci->gsm->tx_bytes < TX_THRESH_LO)
+		gsm_dlci_data_sweep(dlci->gsm);
+	spin_unlock_irqrestore(&dlci->gsm->tx_lock, flags);
+}
+
+/*
+ *	Control message processing
+ */
+
+
+/**
+ *	gsm_control_reply	-	send a response frame to a control
+ *	@gsm: gsm channel
+ *	@cmd: the command to use
+ *	@data: data to follow encoded info
+ *	@dlen: length of data
+ *
+ *	Encode up and queue a UI/UIH frame containing our response.
+ */
+
+static void gsm_control_reply(struct gsm_mux *gsm, int cmd, u8 *data,
+					int dlen)
+{
+	struct gsm_msg *msg;
+	msg = gsm_data_alloc(gsm, 0, dlen + 2, gsm->ftype);
+	msg->data[0] = (cmd & 0xFE) << 1 | EA;	/* Clear C/R */
+	msg->data[1] = (dlen << 1) | EA;
+	memcpy(msg->data + 2, data, dlen);
+	gsm_data_queue(gsm->dlci[0], msg);
+}
+
+/**
+ *	gsm_process_modem	-	process received modem status
+ *	@tty: virtual tty bound to the DLCI
+ *	@dlci: DLCI to affect
+ *	@modem: modem bits (full EA)
+ *
+ *	Used when a modem control message or line state inline in adaption
+ *	layer 2 is processed. Sort out the local modem state and throttles
+ */
+
+static void gsm_process_modem(struct tty_struct *tty, struct gsm_dlci *dlci,
+							u32 modem)
+{
+	int  mlines = 0;
+	u8 brk = modem >> 6;
+
+	/* Flow control/ready to communicate */
+	if (modem & MDM_FC) {
+		/* Need to throttle our output on this device */
+		dlci->constipated = 1;
+	}
+	if (modem & MDM_RTC) {
+		mlines |= TIOCM_DSR | TIOCM_DTR;
+		dlci->constipated = 0;
+		gsm_dlci_data_kick(dlci);
+	}
+	/* Map modem bits */
+	if (modem & MDM_RTR)
+		mlines |= TIOCM_RTS | TIOCM_CTS;
+	if (modem & MDM_IC)
+		mlines |= TIOCM_RI;
+	if (modem & MDM_DV)
+		mlines |= TIOCM_CD;
+
+	/* Carrier drop -> hangup */
+	if (tty) {
+		if ((mlines & TIOCM_CD) == 0 && (dlci->modem_rx & TIOCM_CD))
+			if (!(tty->termios->c_cflag & CLOCAL))
+				tty_hangup(tty);
+		if (brk & 0x01)
+			tty_insert_flip_char(tty, 0, TTY_BREAK);
+	}
+	dlci->modem_rx = mlines;
+}
+
+/**
+ *	gsm_control_modem	-	modem status received
+ *	@gsm: GSM channel
+ *	@data: data following command
+ *	@clen: command length
+ *
+ *	We have received a modem status control message. This is used by
+ *	the GSM mux protocol to pass virtual modem line status and optionally
+ *	to indicate break signals. Unpack it, convert to Linux representation
+ *	and if need be stuff a break message down the tty.
+ */
+
+static void gsm_control_modem(struct gsm_mux *gsm, u8 *data, int clen)
+{
+	unsigned int addr = 0;
+	unsigned int modem = 0;
+	struct gsm_dlci *dlci;
+	int len = clen;
+	u8 *dp = data;
+	struct tty_struct *tty;
+
+	while (gsm_read_ea(&addr, *dp++) == 0) {
+		len--;
+		if (len == 0)
+			return;
+	}
+	/* Must be at least one byte following the EA */
+	len--;
+	if (len <= 0)
+		return;
+
+	addr >>= 1;
+	/* Closed port, or invalid ? */
+	if (addr == 0 || addr >= NUM_DLCI || gsm->dlci[addr] == NULL)
+		return;
+	dlci = gsm->dlci[addr];
+
+	while (gsm_read_ea(&modem, *dp++) == 0) {
+		len--;
+		if (len == 0)
+			return;
+	}
+	tty = tty_port_tty_get(&dlci->port);
+	gsm_process_modem(tty, dlci, modem);
+	if (tty) {
+		tty_wakeup(tty);
+		tty_kref_put(tty);
+	}
+	gsm_control_reply(gsm, CMD_MSC, data, clen);
+}
+
+/**
+ *	gsm_control_rls		-	remote line status
+ *	@gsm: GSM channel
+ *	@data: data bytes
+ *	@clen: data length
+ *
+ *	The modem sends us a two byte message on the control channel whenever
+ *	it wishes to send us an error state from the virtual link. Stuff
+ *	this into the uplink tty if present
+ */
+
+static void gsm_control_rls(struct gsm_mux *gsm, u8 *data, int clen)
+{
+	struct tty_struct *tty;
+	unsigned int addr = 0 ;
+	u8 bits;
+	int len = clen;
+	u8 *dp = data;
+
+	while (gsm_read_ea(&addr, *dp++) == 0) {
+		len--;
+		if (len == 0)
+			return;
+	}
+	/* Must be at least one byte following ea */
+	len--;
+	if (len <= 0)
+		return;
+	addr >>= 1;
+	/* Closed port, or invalid ? */
+	if (addr == 0 || addr >= NUM_DLCI || gsm->dlci[addr] == NULL)
+		return;
+	/* No error ? */
+	bits = *dp;
+	if ((bits & 1) == 0)
+		return;
+	/* See if we have an uplink tty */
+	tty = tty_port_tty_get(&gsm->dlci[addr]->port);
+
+	if (tty) {
+		if (bits & 2)
+			tty_insert_flip_char(tty, 0, TTY_OVERRUN);
+		if (bits & 4)
+			tty_insert_flip_char(tty, 0, TTY_PARITY);
+		if (bits & 8)
+			tty_insert_flip_char(tty, 0, TTY_FRAME);
+		tty_flip_buffer_push(tty);
+		tty_kref_put(tty);
+	}
+	gsm_control_reply(gsm, CMD_RLS, data, clen);
+}
+
+static void gsm_dlci_begin_close(struct gsm_dlci *dlci);
+
+/**
+ *	gsm_control_message	-	DLCI 0 control processing
+ *	@gsm: our GSM mux
+ *	@command:  the command EA
+ *	@data: data beyond the command/length EAs
+ *	@clen: length
+ *
+ *	Input processor for control messages from the other end of the link.
+ *	Processes the incoming request and queues a response frame or an
+ *	NSC response if not supported
+ */
+
+static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
+							u8 *data, int clen)
+{
+	u8 buf[1];
+	switch (command) {
+	case CMD_CLD: {
+		struct gsm_dlci *dlci = gsm->dlci[0];
+		/* Modem wishes to close down */
+		if (dlci) {
+			dlci->dead = 1;
+			gsm->dead = 1;
+			gsm_dlci_begin_close(dlci);
+		}
+		}
+		break;
+	case CMD_TEST:
+		/* Modem wishes to test, reply with the data */
+		gsm_control_reply(gsm, CMD_TEST, data, clen);
+		break;
+	case CMD_FCON:
+		/* Modem wants us to STFU */
+		gsm->constipated = 1;
+		gsm_control_reply(gsm, CMD_FCON, NULL, 0);
+		break;
+	case CMD_FCOFF:
+		/* Modem can accept data again */
+		gsm->constipated = 0;
+		gsm_control_reply(gsm, CMD_FCOFF, NULL, 0);
+		/* Kick the link in case it is idling */
+		gsm_data_kick(gsm);
+		break;
+	case CMD_MSC:
+		/* Out of band modem line change indicator for a DLCI */
+		gsm_control_modem(gsm, data, clen);
+		break;
+	case CMD_RLS:
+		/* Out of band error reception for a DLCI */
+		gsm_control_rls(gsm, data, clen);
+		break;
+	case CMD_PSC:
+		/* Modem wishes to enter power saving state */
+		gsm_control_reply(gsm, CMD_PSC, NULL, 0);
+		break;
+		/* Optional unsupported commands */
+	case CMD_PN:	/* Parameter negotiation */
+	case CMD_RPN:	/* Remote port negotation */
+	case CMD_SNC:	/* Service negotation command */
+	default:
+		/* Reply to bad commands with an NSC */
+		buf[0] = command;
+		gsm_control_reply(gsm, CMD_NSC, buf, 1);
+		break;
+	}
+}
+
+/**
+ *	gsm_control_response	-	process a response to our control
+ *	@gsm: our GSM mux
+ *	@command: the command (response) EA
+ *	@data: data beyond the command/length EA
+ *	@clen: length
+ *
+ *	Process a response to an outstanding command. We only allow a single
+ *	control message in flight so this is fairly easy. All the clean up
+ *	is done by the caller, we just update the fields, flag it as done
+ *	and return
+ */
+
+static void gsm_control_response(struct gsm_mux *gsm, unsigned int command,
+							u8 *data, int clen)
+{
+	struct gsm_control *ctrl;
+	unsigned long flags;
+
+	spin_lock_irqsave(&gsm->control_lock, flags);
+
+	ctrl = gsm->pending_cmd;
+	/* Does the reply match our command */
+	command |= 1;
+	if (ctrl != NULL && (command == ctrl->cmd || command == CMD_NSC)) {
+		/* Our command was replied to, kill the retry timer */
+		del_timer(&gsm->t2_timer);
+		gsm->pending_cmd = NULL;
+		/* Rejected by the other end */
+		if (command == CMD_NSC)
+			ctrl->error = -EOPNOTSUPP;
+		ctrl->done = 1;
+		wake_up(&gsm->event);
+	}
+	spin_unlock_irqrestore(&gsm->control_lock, flags);
+}
+
+/**
+ *	gsm_control_transmit 	-	send control packet
+ *	@gsm: gsm mux
+ *	@ctrl: frame to send
+ *
+ *	Send out a pending control command (called under control lock)
+ */
+
+static void gsm_control_transmit(struct gsm_mux *gsm, struct gsm_control *ctrl)
+{
+	struct gsm_msg *msg = gsm_data_alloc(gsm, 0, ctrl->len + 1,
+							gsm->ftype|PF);
+	if (msg == NULL)
+		return;
+	msg->data[0] = (ctrl->cmd << 1) | 2 | EA;	/* command */
+	memcpy(msg->data + 1, ctrl->data, ctrl->len);
+	gsm_data_queue(gsm->dlci[0], msg);
+}
+
+/**
+ *	gsm_control_retransmit	-	retransmit a control frame
+ *	@data: pointer to our gsm object
+ *
+ *	Called off the T2 timer expiry in order to retransmit control frames
+ *	that have been lost in the system somewhere. The control_lock protects
+ *	us from colliding with another sender or a receive completion event.
+ *	In that situation the timer may still occur in a small window but
+ *	gsm->pending_cmd will be NULL and we just let the timer expire.
+ */
+
+static void gsm_control_retransmit(unsigned long data)
+{
+	struct gsm_mux *gsm = (struct gsm_mux *)data;
+	struct gsm_control *ctrl;
+	unsigned long flags;
+	spin_lock_irqsave(&gsm->control_lock, flags);
+	ctrl = gsm->pending_cmd;
+	if (ctrl) {
+		gsm->cretries--;
+		if (gsm->cretries == 0) {
+			gsm->pending_cmd = NULL;
+			ctrl->error = -ETIMEDOUT;
+			ctrl->done = 1;
+			spin_unlock_irqrestore(&gsm->control_lock, flags);
+			wake_up(&gsm->event);
+			return;
+		}
+		gsm_control_transmit(gsm, ctrl);
+		mod_timer(&gsm->t2_timer, jiffies + gsm->t2 * HZ / 100);
+	}
+	spin_unlock_irqrestore(&gsm->control_lock, flags);
+}
+
+/**
+ *	gsm_control_send	-	send a control frame on DLCI 0
+ *	@gsm: the GSM channel
+ *	@command: command  to send including CR bit
+ *	@data: bytes of data (must be kmalloced)
+ *	@len: length of the block to send
+ *
+ *	Queue and dispatch a control command. Only one command can be
+ *	active at a time. In theory more can be outstanding but the matching
+ *	gets really complicated so for now stick to one outstanding.
+ */
+
+static struct gsm_control *gsm_control_send(struct gsm_mux *gsm,
+		unsigned int command, u8 *data, int clen)
+{
+	struct gsm_control *ctrl = kzalloc(sizeof(struct gsm_control),
+						GFP_KERNEL);
+	unsigned long flags;
+	if (ctrl == NULL)
+		return NULL;
+retry:
+	wait_event(gsm->event, gsm->pending_cmd == NULL);
+	spin_lock_irqsave(&gsm->control_lock, flags);
+	if (gsm->pending_cmd != NULL) {
+		spin_unlock_irqrestore(&gsm->control_lock, flags);
+		goto retry;
+	}
+	ctrl->cmd = command;
+	ctrl->data = data;
+	ctrl->len = clen;
+	gsm->pending_cmd = ctrl;
+	gsm->cretries = gsm->n2;
+	mod_timer(&gsm->t2_timer, jiffies + gsm->t2 * HZ / 100);
+	gsm_control_transmit(gsm, ctrl);
+	spin_unlock_irqrestore(&gsm->control_lock, flags);
+	return ctrl;
+}
+
+/**
+ *	gsm_control_wait	-	wait for a control to finish
+ *	@gsm: GSM mux
+ *	@control: control we are waiting on
+ *
+ *	Waits for the control to complete or time out. Frees any used
+ *	resources and returns 0 for success, or an error if the remote
+ *	rejected or ignored the request.
+ */
+
+static int gsm_control_wait(struct gsm_mux *gsm, struct gsm_control *control)
+{
+	int err;
+	wait_event(gsm->event, control->done == 1);
+	err = control->error;
+	kfree(control);
+	return err;
+}
+
+
+/*
+ *	DLCI level handling: Needs krefs
+ */
+
+/*
+ *	State transitions and timers
+ */
+
+/**
+ *	gsm_dlci_close		-	a DLCI has closed
+ *	@dlci: DLCI that closed
+ *
+ *	Perform processing when moving a DLCI into closed state. If there
+ *	is an attached tty this is hung up
+ */
+
+static void gsm_dlci_close(struct gsm_dlci *dlci)
+{
+	del_timer(&dlci->t1);
+	if (debug & 8)
+		printk("DLCI %d goes closed.\n", dlci->addr);
+	dlci->state = DLCI_CLOSED;
+	if (dlci->addr != 0) {
+		struct tty_struct  *tty = tty_port_tty_get(&dlci->port);
+		if (tty) {
+			tty_hangup(tty);
+			tty_kref_put(tty);
+		}
+		kfifo_reset(dlci->fifo);
+	} else
+		dlci->gsm->dead = 1;
+	wake_up(&dlci->gsm->event);
+	/* A DLCI 0 close is a MUX termination so we need to kick that
+	   back to userspace somehow */
+}
+
+/**
+ *	gsm_dlci_open		-	a DLCI has opened
+ *	@dlci: DLCI that opened
+ *
+ *	Perform processing when moving a DLCI into open state.
+ */
+
+static void gsm_dlci_open(struct gsm_dlci *dlci)
+{
+	/* Note that SABM UA .. SABM UA first UA lost can mean that we go
+	   open -> open */
+	del_timer(&dlci->t1);
+	/* This will let a tty open continue */
+	dlci->state = DLCI_OPEN;
+	if (debug & 8)
+		printk("DLCI %d goes open.\n", dlci->addr);
+	wake_up(&dlci->gsm->event);
+}
+
+/**
+ *	gsm_dlci_t1		-	T1 timer expiry
+ *	@dlci: DLCI that opened
+ *
+ *	The T1 timer handles retransmits of control frames (essentially of
+ *	SABM and DISC). We resend the command until the retry count runs out
+ *	in which case an opening port goes back to closed and a closing port
+ *	is simply put into closed state (any further frames from the other
+ *	end will get a DM response)
+ */
+
+static void gsm_dlci_t1(unsigned long data)
+{
+	struct gsm_dlci *dlci = (struct gsm_dlci *)data;
+	struct gsm_mux *gsm = dlci->gsm;
+
+	switch (dlci->state) {
+	case DLCI_OPENING:
+		dlci->retries--;
+		if (dlci->retries) {
+			gsm_command(dlci->gsm, dlci->addr, SABM|PF);
+			mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
+		} else
+			gsm_dlci_close(dlci);
+		break;
+	case DLCI_CLOSING:
+		dlci->retries--;
+		if (dlci->retries) {
+			gsm_command(dlci->gsm, dlci->addr, DISC|PF);
+			mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
+		} else
+			gsm_dlci_close(dlci);
+		break;
+	}
+}
+
+/**
+ *	gsm_dlci_begin_open	-	start channel open procedure
+ *	@dlci: DLCI to open
+ *
+ *	Commence opening a DLCI from the Linux side. We issue SABM messages
+ *	to the modem which should then reply with a UA, at which point we
+ *	will move into open state. Opening is done asynchronously with retry
+ *	running off timers and the responses.
+ */
+
+static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
+{
+	struct gsm_mux *gsm = dlci->gsm;
+	if (dlci->state == DLCI_OPEN || dlci->state == DLCI_OPENING)
+		return;
+	dlci->retries = gsm->n2;
+	dlci->state = DLCI_OPENING;
+	gsm_command(dlci->gsm, dlci->addr, SABM|PF);
+	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
+}
+
+/**
+ *	gsm_dlci_begin_close	-	start channel open procedure
+ *	@dlci: DLCI to open
+ *
+ *	Commence closing a DLCI from the Linux side. We issue DISC messages
+ *	to the modem which should then reply with a UA, at which point we
+ *	will move into closed state. Closing is done asynchronously with retry
+ *	off timers. We may also receive a DM reply from the other end which
+ *	indicates the channel was already closed.
+ */
+
+static void gsm_dlci_begin_close(struct gsm_dlci *dlci)
+{
+	struct gsm_mux *gsm = dlci->gsm;
+	if (dlci->state == DLCI_CLOSED || dlci->state == DLCI_CLOSING)
+		return;
+	dlci->retries = gsm->n2;
+	dlci->state = DLCI_CLOSING;
+	gsm_command(dlci->gsm, dlci->addr, DISC|PF);
+	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
+}
+
+/**
+ *	gsm_dlci_data		-	data arrived
+ *	@dlci: channel
+ *	@data: block of bytes received
+ *	@len: length of received block
+ *
+ *	A UI or UIH frame has arrived which contains data for a channel
+ *	other than the control channel. If the relevant virtual tty is
+ *	open we shovel the bits down it, if not we drop them.
+ */
+
+static void gsm_dlci_data(struct gsm_dlci *dlci, u8 *data, int len)
+{
+	/* krefs .. */
+	struct tty_port *port = &dlci->port;
+	struct tty_struct *tty = tty_port_tty_get(port);
+	unsigned int modem = 0;
+
+	if (debug & 16)
+		printk("%d bytes for tty %p\n", len, tty);
+	if (tty) {
+		switch (dlci->adaption)  {
+			/* Unsupported types */
+			/* Packetised interruptible data */
+			case 4:
+				break;
+			/* Packetised uininterruptible voice/data */
+			case 3:
+				break;
+			/* Asynchronous serial with line state in each frame */
+			case 2:
+				while (gsm_read_ea(&modem, *data++) == 0) {
+					len--;
+					if (len == 0)
+						return;
+				}
+				gsm_process_modem(tty, dlci, modem);
+			/* Line state will go via DLCI 0 controls only */
+			case 1:
+			default:
+				tty_insert_flip_string(tty, data, len);
+				tty_flip_buffer_push(tty);
+		}
+		tty_kref_put(tty);
+	}
+}
+
+/**
+ *	gsm_dlci_control	-	data arrived on control channel
+ *	@dlci: channel
+ *	@data: block of bytes received
+ *	@len: length of received block
+ *
+ *	A UI or UIH frame has arrived which contains data for DLCI 0 the
+ *	control channel. This should contain a command EA followed by
+ *	control data bytes. The command EA contains a command/response bit
+ *	and we divide up the work accordingly.
+ */
+
+static void gsm_dlci_command(struct gsm_dlci *dlci, u8 *data, int len)
+{
+	/* See what command is involved */
+	unsigned int command = 0;
+	while (len-- > 0) {
+		if (gsm_read_ea(&command, *data++) == 1) {
+			int clen = *data++;
+			len--;
+			/* FIXME: this is properly an EA */
+			clen >>= 1;
+			/* Malformed command ? */
+			if (clen > len)
+				return;
+			if (command & 1)
+				gsm_control_message(dlci->gsm, command,
+								data, clen);
+			else
+				gsm_control_response(dlci->gsm, command,
+								data, clen);
+			return;
+		}
+	}
+}
+
+/*
+ *	Allocate/Free DLCI channels
+ */
+
+/**
+ *	gsm_dlci_alloc		-	allocate a DLCI
+ *	@gsm: GSM mux
+ *	@addr: address of the DLCI
+ *
+ *	Allocate and install a new DLCI object into the GSM mux.
+ *
+ *	FIXME: review locking races
+ */
+
+static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
+{
+	struct gsm_dlci *dlci = kzalloc(sizeof(struct gsm_dlci), GFP_ATOMIC);
+	if (dlci == NULL)
+		return NULL;
+	spin_lock_init(&dlci->lock);
+	dlci->fifo = &dlci->_fifo;
+	if (kfifo_alloc(&dlci->_fifo, 4096, GFP_KERNEL) < 0) {
+		kfree(dlci);
+		return NULL;
+	}
+
+	skb_queue_head_init(&dlci->skb_list);
+	init_timer(&dlci->t1);
+	dlci->t1.function = gsm_dlci_t1;
+	dlci->t1.data = (unsigned long)dlci;
+	tty_port_init(&dlci->port);
+	dlci->port.ops = &gsm_port_ops;
+	dlci->gsm = gsm;
+	dlci->addr = addr;
+	dlci->adaption = gsm->adaption;
+	dlci->state = DLCI_CLOSED;
+	if (addr)
+		dlci->data = gsm_dlci_data;
+	else
+		dlci->data = gsm_dlci_command;
+	gsm->dlci[addr] = dlci;
+	return dlci;
+}
+
+/**
+ *	gsm_dlci_free		-	release DLCI
+ *	@dlci: DLCI to destroy
+ *
+ *	Free up a DLCI. Currently to keep the lifetime rules sane we only
+ *	clean up DLCI objects when the MUX closes rather than as the port
+ *	is closed down on both the tty and mux levels.
+ *
+ *	Can sleep.
+ */
+static void gsm_dlci_free(struct gsm_dlci *dlci)
+{
+	struct tty_struct *tty = tty_port_tty_get(&dlci->port);
+	if (tty) {
+		tty_vhangup(tty);
+		tty_kref_put(tty);
+	}
+	del_timer_sync(&dlci->t1);
+	dlci->gsm->dlci[dlci->addr] = NULL;
+	kfifo_free(dlci->fifo);
+	kfree(dlci);
+}
+
+
+/*
+ *	LAPBish link layer logic
+ */
+
+/**
+ *	gsm_queue		-	a GSM frame is ready to process
+ *	@gsm: pointer to our gsm mux
+ *
+ *	At this point in time a frame has arrived and been demangled from
+ *	the line encoding. All the differences between the encodings have
+ *	been handled below us and the frame is unpacked into the structures.
+ *	The fcs holds the header FCS but any data FCS must be added here.
+ */
+
+static void gsm_queue(struct gsm_mux *gsm)
+{
+	struct gsm_dlci *dlci;
+	u8 cr;
+	int address;
+	/* We have to sneak a look at the packet body to do the FCS.
+	   A somewhat layering violation in the spec */
+
+	if ((gsm->control & ~PF) == UI)
+		gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf, gsm->len);
+	if (gsm->fcs != GOOD_FCS) {
+		gsm->bad_fcs++;
+		if (debug & 4)
+			printk("BAD FCS %02x\n", gsm->fcs);
+		return;
+	}
+	address = gsm->address >> 1;
+	if (address >= NUM_DLCI)
+		goto invalid;
+
+	cr = gsm->address & 1;		/* C/R bit */
+
+	gsm_print_packet("<--", address, cr, gsm->control, gsm->buf, gsm->len);
+
+	cr ^= 1 - gsm->initiator;	/* Flip so 1 always means command */
+	dlci = gsm->dlci[address];
+
+	switch (gsm->control) {
+	case SABM|PF:
+		if (cr == 0)
+			goto invalid;
+		if (dlci == NULL)
+			dlci = gsm_dlci_alloc(gsm, address);
+		if (dlci == NULL)
+			return;
+		if (dlci->dead)
+			gsm_response(gsm, address, DM);
+		else {
+			gsm_response(gsm, address, UA);
+			gsm_dlci_open(dlci);
+		}
+		break;
+	case DISC|PF:
+		if (cr == 0)
+			goto invalid;
+		if (dlci == NULL || dlci->state == DLCI_CLOSED) {
+			gsm_response(gsm, address, DM);
+			return;
+		}
+		/* Real close complete */
+		gsm_response(gsm, address, UA);
+		gsm_dlci_close(dlci);
+		break;
+	case UA:
+	case UA|PF:
+		if (cr == 0 || dlci == NULL)
+			break;
+		switch (dlci->state) {
+		case DLCI_CLOSING:
+			gsm_dlci_close(dlci);
+			break;
+		case DLCI_OPENING:
+			gsm_dlci_open(dlci);
+			break;
+		}
+		break;
+	case DM:	/* DM can be valid unsolicited */
+	case DM|PF:
+		if (cr)
+			goto invalid;
+		if (dlci == NULL)
+			return;
+		gsm_dlci_close(dlci);
+		break;
+	case UI:
+	case UI|PF:
+	case UIH:
+	case UIH|PF:
+#if 0
+		if (cr)
+			goto invalid;
+#endif
+		if (dlci == NULL || dlci->state != DLCI_OPEN) {
+			gsm_command(gsm, address, DM|PF);
+			return;
+		}
+		dlci->data(dlci, gsm->buf, gsm->len);
+		break;
+	default:
+		goto invalid;
+	}
+	return;
+invalid:
+	gsm->malformed++;
+	return;
+}
+
+
+/**
+ *	gsm0_receive	-	perform processing for non-transparency
+ *	@gsm: gsm data for this ldisc instance
+ *	@c: character
+ *
+ *	Receive bytes in gsm mode 0
+ */
+
+static void gsm0_receive(struct gsm_mux *gsm, unsigned char c)
+{
+	switch (gsm->state) {
+	case GSM_SEARCH:	/* SOF marker */
+		if (c == GSM0_SOF) {
+			gsm->state = GSM_ADDRESS;
+			gsm->address = 0;
+			gsm->len = 0;
+			gsm->fcs = INIT_FCS;
+		}
+		break;		/* Address EA */
+	case GSM_ADDRESS:
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		if (gsm_read_ea(&gsm->address, c))
+			gsm->state = GSM_CONTROL;
+		break;
+	case GSM_CONTROL:	/* Control Byte */
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		gsm->control = c;
+		gsm->state = GSM_LEN;
+		break;
+	case GSM_LEN:		/* Length EA */
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		if (gsm_read_ea(&gsm->len, c)) {
+			if (gsm->len > gsm->mru) {
+				gsm->bad_size++;
+				gsm->state = GSM_SEARCH;
+				break;
+			}
+			gsm->count = 0;
+			gsm->state = GSM_DATA;
+		}
+		break;
+	case GSM_DATA:		/* Data */
+		gsm->buf[gsm->count++] = c;
+		if (gsm->count == gsm->len)
+			gsm->state = GSM_FCS;
+		break;
+	case GSM_FCS:		/* FCS follows the packet */
+		gsm->fcs = c;
+		gsm_queue(gsm);
+		/* And then back for the next frame */
+		gsm->state = GSM_SEARCH;
+		break;
+	}
+}
+
+/**
+ *	gsm0_receive	-	perform processing for non-transparency
+ *	@gsm: gsm data for this ldisc instance
+ *	@c: character
+ *
+ *	Receive bytes in mode 1 (Advanced option)
+ */
+
+static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
+{
+	if (c == GSM1_SOF) {
+		/* EOF is only valid in frame if we have got to the data state
+		   and received at least one byte (the FCS) */
+		if (gsm->state == GSM_DATA && gsm->count) {
+			/* Extract the FCS */
+			gsm->count--;
+			gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->buf[gsm->count]);
+			gsm->len = gsm->count;
+			gsm_queue(gsm);
+			gsm->state  = GSM_START;
+			return;
+		}
+		/* Any partial frame was a runt so go back to start */
+		if (gsm->state != GSM_START) {
+			gsm->malformed++;
+			gsm->state = GSM_START;
+		}
+		/* A SOF in GSM_START means we are still reading idling or
+		   framing bytes */
+		return;
+	}
+
+	if (c == GSM1_ESCAPE) {
+		gsm->escape = 1;
+		return;
+	}
+
+	/* Only an unescaped SOF gets us out of GSM search */
+	if (gsm->state == GSM_SEARCH)
+		return;
+
+	if (gsm->escape) {
+		c ^= GSM1_ESCAPE_BITS;
+		gsm->escape = 0;
+	}
+	switch (gsm->state) {
+	case GSM_START:		/* First byte after SOF */
+		gsm->address = 0;
+		gsm->state = GSM_ADDRESS;
+		gsm->fcs = INIT_FCS;
+		/* Drop through */
+	case GSM_ADDRESS:	/* Address continuation */
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		if (gsm_read_ea(&gsm->address, c))
+			gsm->state = GSM_CONTROL;
+		break;
+	case GSM_CONTROL:	/* Control Byte */
+		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
+		gsm->control = c;
+		gsm->count = 0;
+		gsm->state = GSM_DATA;
+		break;
+	case GSM_DATA:		/* Data */
+		if (gsm->count > gsm->mru ) {	/* Allow one for the FCS */
+			gsm->state = GSM_OVERRUN;
+			gsm->bad_size++;
+		} else
+			gsm->buf[gsm->count++] = c;
+		break;
+	case GSM_OVERRUN:	/* Over-long - eg a dropped SOF */
+		break;
+	}
+}
+
+/**
+ *	gsm_error		-	handle tty error
+ *	@gsm: ldisc data
+ *	@data: byte received (may be invalid)
+ *	@flag: error received
+ *
+ *	Handle an error in the receipt of data for a frame. Currently we just
+ *	go back to hunting for a SOF.
+ *
+ *	FIXME: better diagnostics ?
+ */
+
+static void gsm_error(struct gsm_mux *gsm,
+				unsigned char data, unsigned char flag)
+{
+	gsm->state = GSM_SEARCH;
+	gsm->io_error++;
+}
+
+/**
+ *	gsm_cleanup_mux		-	generic GSM protocol cleanup
+ *	@gsm: our mux
+ *
+ *	Clean up the bits of the mux which are the same for all framing
+ *	protocols. Remove the mux from the mux table, stop all the timers
+ *	and then shut down each device hanging up the channels as we go.
+ */
+
+void gsm_cleanup_mux(struct gsm_mux *gsm)
+{
+	int i;
+	struct gsm_dlci *dlci = gsm->dlci[0];
+	struct gsm_msg *txq;
+
+	gsm->dead = 1;
+
+	spin_lock(&gsm_mux_lock);
+	for (i = 0; i < MAX_MUX; i++) {
+		if (gsm_mux[i] == gsm) {
+			gsm_mux[i] = NULL;
+			break;
+		}
+	}
+	spin_unlock(&gsm_mux_lock);
+	WARN_ON(i == MAX_MUX);
+
+	del_timer_sync(&gsm->t2_timer);
+	/* Now we are sure T2 has stopped */
+	if (dlci) {
+		dlci->dead = 1;
+		gsm_dlci_begin_close(dlci);
+		wait_event_interruptible(gsm->event,
+					dlci->state == DLCI_CLOSED);
+	}
+	/* Free up any link layer users */
+	for (i = 0; i < NUM_DLCI; i++)
+		if (gsm->dlci[i])
+			gsm_dlci_free(gsm->dlci[i]);
+	/* Now wipe the queues */
+	for (txq = gsm->tx_head; txq != NULL; txq = gsm->tx_head) {
+		gsm->tx_head = txq->next;
+		kfree(txq);
+	}
+	gsm->tx_tail = NULL;
+}
+EXPORT_SYMBOL_GPL(gsm_cleanup_mux);
+
+/**
+ *	gsm_activate_mux	-	generic GSM setup
+ *	@gsm: our mux
+ *
+ *	Set up the bits of the mux which are the same for all framing
+ *	protocols. Add the mux to the mux table so it can be opened and
+ *	finally kick off connecting to DLCI 0 on the modem.
+ */
+
+int gsm_activate_mux(struct gsm_mux *gsm)
+{
+	struct gsm_dlci *dlci;
+	int i = 0;
+
+	init_timer(&gsm->t2_timer);
+	gsm->t2_timer.function = gsm_control_retransmit;
+	gsm->t2_timer.data = (unsigned long)gsm;
+	init_waitqueue_head(&gsm->event);
+	spin_lock_init(&gsm->control_lock);
+	spin_lock_init(&gsm->tx_lock);
+
+	if (gsm->encoding == 0)
+		gsm->receive = gsm0_receive;
+	else
+		gsm->receive = gsm1_receive;
+	gsm->error = gsm_error;
+
+	spin_lock(&gsm_mux_lock);
+	for (i = 0; i < MAX_MUX; i++) {
+		if (gsm_mux[i] == NULL) {
+			gsm_mux[i] = gsm;
+			break;
+		}
+	}
+	spin_unlock(&gsm_mux_lock);
+	if (i == MAX_MUX)
+		return -EBUSY;
+
+	dlci = gsm_dlci_alloc(gsm, 0);
+	if (dlci == NULL)
+		return -ENOMEM;
+	gsm->dead = 0;		/* Tty opens are now permissible */
+	return 0;
+}
+EXPORT_SYMBOL_GPL(gsm_activate_mux);
+
+/**
+ *	gsm_free_mux		-	free up a mux
+ *	@mux: mux to free
+ *
+ *	Dispose of allocated resources for a dead mux. No refcounting
+ *	at present so the mux must be truely dead.
+ */
+void gsm_free_mux(struct gsm_mux *gsm)
+{
+	kfree(gsm->txframe);
+	kfree(gsm->buf);
+	kfree(gsm);
+}
+EXPORT_SYMBOL_GPL(gsm_free_mux);
+
+/**
+ *	gsm_alloc_mux		-	allocate a mux
+ *
+ *	Creates a new mux ready for activation.
+ */
+
+struct gsm_mux *gsm_alloc_mux(void)
+{
+	struct gsm_mux *gsm = kzalloc(sizeof(struct gsm_mux), GFP_KERNEL);
+	if (gsm == NULL)
+		return NULL;
+	gsm->buf = kmalloc(MAX_MRU + 1, GFP_KERNEL);
+	if (gsm->buf == NULL) {
+		kfree(gsm);
+		return NULL;
+	}
+	gsm->txframe = kmalloc(2 * MAX_MRU + 2, GFP_KERNEL);
+	if (gsm->txframe == NULL) {
+		kfree(gsm->buf);
+		kfree(gsm);
+		return NULL;
+	}
+	spin_lock_init(&gsm->lock);
+
+	gsm->t1 = T1;
+	gsm->t2 = T2;
+	gsm->n2 = N2;
+	gsm->ftype = UIH;
+	gsm->initiator = 0;
+	gsm->adaption = 1;
+	gsm->encoding = 1;
+	gsm->mru = 64;	/* Default to encoding 1 so these should be 64 */
+	gsm->mtu = 64;
+	gsm->dead = 1;	/* Avoid early tty opens */
+
+	return gsm;
+}
+EXPORT_SYMBOL_GPL(gsm_alloc_mux);
+
+
+
+
+/**
+ *	gsmld_output		-	write to link
+ *	@gsm: our mux
+ *	@data: bytes to output
+ *	@len: size
+ *
+ *	Write a block of data from the GSM mux to the data channel. This
+ *	will eventually be serialized from above but at the moment isn't.
+ */
+
+static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
+{
+	if (tty_write_room(gsm->tty) < len) {
+		set_bit(TTY_DO_WRITE_WAKEUP, &gsm->tty->flags);
+		return -ENOSPC;
+	}
+	if (debug & 4) {
+		printk("-->%d bytes out\n", len);
+		hex_packet(data, len);
+	}
+	gsm->tty->ops->write(gsm->tty, data, len);
+	return len;
+}
+
+/**
+ *	gsmld_attach_gsm	-	mode set up
+ *	@tty: our tty structure
+ *	@gsm: our mux
+ *
+ *	Set up the MUX for basic mode and commence connecting to the
+ *	modem. Currently called from the line discipline set up but
+ *	will need moving to an ioctl path.
+ */
+
+static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
+{
+	int ret;
+
+	gsm->tty = tty_kref_get(tty);
+	gsm->output = gsmld_output;
+	ret =  gsm_activate_mux(gsm);
+	if (ret != 0)
+		tty_kref_put(gsm->tty);
+	return ret;
+}
+
+
+/**
+ *	gsmld_detach_gsm	-	stop doing 0710 mux
+ *	@tty: tty atttached to the mux
+ *	@gsm: mux
+ *
+ *	Shutdown and then clean up the resources used by the line discipline
+ */
+
+static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
+{
+	WARN_ON(tty != gsm->tty);
+	gsm_cleanup_mux(gsm);
+	tty_kref_put(gsm->tty);
+	gsm->tty = NULL;
+}
+
+static void gsmld_receive_buf(struct tty_struct *tty, const unsigned char *cp,
+			      char *fp, int count)
+{
+	struct gsm_mux *gsm = tty->disc_data;
+	const unsigned char *dp;
+	char *f;
+	int i;
+	char buf[64];
+	char flags;
+
+	if (debug & 4) {
+		printk("Inbytes %dd\n", count);
+		hex_packet(cp, count);
+	}
+
+	for (i = count, dp = cp, f = fp; i; i--, dp++) {
+		flags = *f++;
+		switch (flags) {
+		case TTY_NORMAL:
+			gsm->receive(gsm, *dp);
+			break;
+		case TTY_OVERRUN:
+		case TTY_BREAK:
+		case TTY_PARITY:
+		case TTY_FRAME:
+			gsm->error(gsm, *dp, flags);
+			break;
+		default:
+			printk(KERN_ERR "%s: unknown flag %d\n",
+			       tty_name(tty, buf), flags);
+			break;
+		}
+	}
+	/* FASYNC if needed ? */
+	/* If clogged call tty_throttle(tty); */
+}
+
+/**
+ *	gsmld_chars_in_buffer	-	report available bytes
+ *	@tty: tty device
+ *
+ *	Report the number of characters buffered to be delivered to user
+ *	at this instant in time.
+ *
+ *	Locking: gsm lock
+ */
+
+static ssize_t gsmld_chars_in_buffer(struct tty_struct *tty)
+{
+	return 0;
+}
+
+/**
+ *	gsmld_flush_buffer	-	clean input queue
+ *	@tty:	terminal device
+ *
+ *	Flush the input buffer. Called when the line discipline is
+ *	being closed, when the tty layer wants the buffer flushed (eg
+ *	at hangup).
+ */
+
+static void gsmld_flush_buffer(struct tty_struct *tty)
+{
+}
+
+/**
+ *	gsmld_close		-	close the ldisc for this tty
+ *	@tty: device
+ *
+ *	Called from the terminal layer when this line discipline is
+ *	being shut down, either because of a close or becsuse of a
+ *	discipline change. The function will not be called while other
+ *	ldisc methods are in progress.
+ */
+
+static void gsmld_close(struct tty_struct *tty)
+{
+	struct gsm_mux *gsm = tty->disc_data;
+
+	gsmld_detach_gsm(tty, gsm);
+
+	gsmld_flush_buffer(tty);
+	/* Do other clean up here */
+	gsm_free_mux(gsm);
+}
+
+/**
+ *	gsmld_open		-	open an ldisc
+ *	@tty: terminal to open
+ *
+ *	Called when this line discipline is being attached to the
+ *	terminal device. Can sleep. Called serialized so that no
+ *	other events will occur in parallel. No further open will occur
+ *	until a close.
+ */
+
+static int gsmld_open(struct tty_struct *tty)
+{
+	struct gsm_mux *gsm;
+
+	if (tty->ops->write == NULL)
+		return -EINVAL;
+
+	/* Attach our ldisc data */
+	gsm = gsm_alloc_mux();
+	if (gsm == NULL)
+		return -ENOMEM;
+
+	tty->disc_data = gsm;
+	tty->receive_room = 65536;
+
+	/* Attach the initial passive connection */
+	gsm->encoding = 1;
+	return gsmld_attach_gsm(tty, gsm);
+}
+
+/**
+ *	gsmld_write_wakeup	-	asynchronous I/O notifier
+ *	@tty: tty device
+ *
+ *	Required for the ptys, serial driver etc. since processes
+ *	that attach themselves to the master and rely on ASYNC
+ *	IO must be woken up
+ */
+
+static void gsmld_write_wakeup(struct tty_struct *tty)
+{
+	struct gsm_mux *gsm = tty->disc_data;
+
+	/* Queue poll */
+	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	gsm_data_kick(gsm);
+	if (gsm->tx_bytes < TX_THRESH_LO)
+		gsm_dlci_data_sweep(gsm);
+}
+
+/**
+ *	gsmld_read		-	read function for tty
+ *	@tty: tty device
+ *	@file: file object
+ *	@buf: userspace buffer pointer
+ *	@nr: size of I/O
+ *
+ *	Perform reads for the line discipline. We are guaranteed that the
+ *	line discipline will not be closed under us but we may get multiple
+ *	parallel readers and must handle this ourselves. We may also get
+ *	a hangup. Always called in user context, may sleep.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ */
+
+static ssize_t gsmld_read(struct tty_struct *tty, struct file *file,
+			 unsigned char __user *buf, size_t nr)
+{
+	return -EOPNOTSUPP;
+}
+
+/**
+ *	gsmld_write		-	write function for tty
+ *	@tty: tty device
+ *	@file: file object
+ *	@buf: userspace buffer pointer
+ *	@nr: size of I/O
+ *
+ *	Called when the owner of the device wants to send a frame
+ *	itself (or some other control data). The data is transferred
+ *	as-is and must be properly framed and checksummed as appropriate
+ *	by userspace. Frames are either sent whole or not at all as this
+ *	avoids pain user side.
+ */
+
+static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
+			   const unsigned char *buf, size_t nr)
+{
+	int space = tty_write_room(tty);
+	if (space >= nr)
+		return tty->ops->write(tty, buf, nr);
+	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	return -ENOBUFS;
+}
+
+/**
+ *	gsmld_poll		-	poll method for N_GSM0710
+ *	@tty: terminal device
+ *	@file: file accessing it
+ *	@wait: poll table
+ *
+ *	Called when the line discipline is asked to poll() for data or
+ *	for special events. This code is not serialized with respect to
+ *	other events save open/close.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ *	Called without the kernel lock held - fine
+ */
+
+static unsigned int gsmld_poll(struct tty_struct *tty, struct file *file,
+							poll_table *wait)
+{
+	unsigned int mask = 0;
+	struct gsm_mux *gsm = tty->disc_data;
+
+	poll_wait(file, &tty->read_wait, wait);
+	poll_wait(file, &tty->write_wait, wait);
+	if (tty_hung_up_p(file))
+		mask |= POLLHUP;
+	if (!tty_is_writelocked(tty) && tty_write_room(tty) > 0)
+		mask |= POLLOUT | POLLWRNORM;
+	if (gsm->dead)
+		mask |= POLLHUP;
+	return mask;
+}
+
+static int gsmld_config(struct tty_struct *tty, struct gsm_mux *gsm,
+							struct gsm_config *c)
+{
+	int need_close = 0;
+	int need_restart = 0;
+
+	/* Stuff we don't support yet - UI or I frame transport, windowing */
+	if ((c->adaption !=1 && c->adaption != 2) || c->k)
+		return -EOPNOTSUPP;
+	/* Check the MRU/MTU range looks sane */
+	if (c->mru > MAX_MRU || c->mtu > MAX_MTU || c->mru < 8 || c->mtu < 8)
+		return -EINVAL;
+	if (c->n2 < 3)
+		return -EINVAL;
+	if (c->encapsulation > 1)	/* Basic, advanced, no I */
+		return -EINVAL;
+	if (c->initiator > 1)
+		return -EINVAL;
+	if (c->i == 0 || c->i > 2)	/* UIH and UI only */
+		return -EINVAL;
+	/*
+	 *	See what is needed for reconfiguration
+	 */
+
+	/* Timing fields */
+	if (c->t1 != 0 && c->t1 != gsm->t1)
+		need_restart = 1;
+	if (c->t2 != 0 && c->t2 != gsm->t2)
+		need_restart = 1;
+	if (c->encapsulation != gsm->encoding)
+		need_restart = 1;
+	if (c->adaption != gsm->adaption)
+		need_restart = 1;
+	/* Requires care */
+	if (c->initiator != gsm->initiator)
+		need_close = 1;
+	if (c->mru != gsm->mru)
+		need_restart = 1;
+	if (c->mtu != gsm->mtu)
+		need_restart = 1;
+
+	/*
+	 *	Close down what is needed, restart and initiate the new
+	 *	configuration
+	 */
+
+	if (need_close || need_restart) {
+		gsm_dlci_begin_close(gsm->dlci[0]);
+		/* This will timeout if the link is down due to N2 expiring */
+		wait_event_interruptible(gsm->event,
+				gsm->dlci[0]->state == DLCI_CLOSED);
+		if (signal_pending(current))
+			return -EINTR;
+	}
+	if (need_restart)
+		gsm_cleanup_mux(gsm);
+
+	gsm->initiator = c->initiator;
+	gsm->mru = c->mru;
+	gsm->encoding = c->encapsulation;
+	gsm->adaption = c->adaption;
+
+	if (c->i == 1)
+		gsm->ftype = UIH;
+	else if (c->i == 2)
+		gsm->ftype = UI;
+
+	if (c->t1)
+		gsm->t1 = c->t1;
+	if (c->t2)
+		gsm->t2 = c->t2;
+
+	/* FIXME: We need to separate activation/deactivation from adding
+	   and removing from the mux array */
+	if (need_restart)
+		gsm_activate_mux(gsm);
+	if (gsm->initiator && need_close)
+		gsm_dlci_begin_open(gsm->dlci[0]);
+	return 0;
+}
+
+static int gsmld_ioctl(struct tty_struct *tty, struct file *file,
+		       unsigned int cmd, unsigned long arg)
+{
+	struct gsm_config c;
+	struct gsm_mux *gsm = tty->disc_data;
+
+	switch (cmd) {
+	case GSMIOC_GETCONF:
+		memset(&c, 0, sizeof(c));
+		c.adaption = gsm->adaption;
+		c.encapsulation = gsm->encoding;
+		c.initiator = gsm->initiator;
+		c.t1 = gsm->t1;
+		c.t2 = gsm->t2;
+		c.t3 = 0;	/* Not supported */
+		c.n2 = gsm->n2;
+		if (gsm->ftype == UIH)
+			c.i = 1;
+		else
+			c.i = 2;
+		printk("Ftype %d i %d\n", gsm->ftype, c.i);
+		c.mru = gsm->mru;
+		c.mtu = gsm->mtu;
+		c.k = 0;
+		if (copy_to_user((void *)arg, &c, sizeof(c)))
+			return -EFAULT;
+		return 0;
+	case GSMIOC_SETCONF:
+		if (copy_from_user(&c, (void *)arg, sizeof(c)))
+			return -EFAULT;
+		return gsmld_config(tty, gsm, &c);
+	default:
+		return n_tty_ioctl_helper(tty, file, cmd, arg);
+	}
+}
+
+
+/* Line discipline for real tty */
+struct tty_ldisc_ops tty_ldisc_packet = {
+	.owner		 = THIS_MODULE,
+	.magic           = TTY_LDISC_MAGIC,
+	.name            = "n_gsm",
+	.open            = gsmld_open,
+	.close           = gsmld_close,
+	.flush_buffer    = gsmld_flush_buffer,
+	.chars_in_buffer = gsmld_chars_in_buffer,
+	.read            = gsmld_read,
+	.write           = gsmld_write,
+	.ioctl           = gsmld_ioctl,
+	.poll            = gsmld_poll,
+	.receive_buf     = gsmld_receive_buf,
+	.write_wakeup    = gsmld_write_wakeup
+};
+
+/*
+ *	Virtual tty side
+ */
+
+#define TX_SIZE		512
+
+static int gsmtty_modem_update(struct gsm_dlci *dlci, u8 brk)
+{
+	u8 modembits[5];
+	struct gsm_control *ctrl;
+	int len = 2;
+
+	if (brk)
+		len++;
+
+	modembits[0] = len << 1 | EA;		/* Data bytes */
+	modembits[1] = dlci->addr << 2 | 3;	/* DLCI, EA, 1 */
+	modembits[2] = gsm_encode_modem(dlci) << 1 | EA;
+	if (brk)
+		modembits[3] = brk << 4 | 2 | EA;	/* Valid, EA */
+	ctrl = gsm_control_send(dlci->gsm, CMD_MSC, modembits, len + 1);
+	if (ctrl == NULL)
+		return -ENOMEM;
+	return gsm_control_wait(dlci->gsm, ctrl);
+}
+
+static int gsm_carrier_raised(struct tty_port *port)
+{
+	struct gsm_dlci *dlci = container_of(port, struct gsm_dlci, port);
+	/* Not yet open so no carrier info */
+	if (dlci->state != DLCI_OPEN)
+		return 0;
+	if (debug & 2)
+		return 1;
+	return dlci->modem_rx & TIOCM_CD;
+}
+
+static void gsm_dtr_rts(struct tty_port *port, int onoff)
+{
+	struct gsm_dlci *dlci = container_of(port, struct gsm_dlci, port);
+	unsigned int modem_tx = dlci->modem_tx;
+	if (onoff)
+		modem_tx |= TIOCM_DTR | TIOCM_RTS;
+	else
+		modem_tx &= ~(TIOCM_DTR | TIOCM_RTS);
+	if (modem_tx != dlci->modem_tx) {
+		dlci->modem_tx = modem_tx;
+		gsmtty_modem_update(dlci, 0);
+	}
+}
+
+static const struct tty_port_operations gsm_port_ops = {
+	.carrier_raised = gsm_carrier_raised,
+	.dtr_rts = gsm_dtr_rts,
+};
+
+
+static int gsmtty_open(struct tty_struct *tty, struct file *filp)
+{
+	struct gsm_mux *gsm;
+	struct gsm_dlci *dlci;
+	struct tty_port *port;
+	unsigned int line = tty->index;
+	unsigned int mux = line >> 6;
+
+	line = line & 0x3F;
+
+	if (mux >= MAX_MUX)
+		return -ENXIO;
+	/* FIXME: we need to lock gsm_mux for lifetimes of ttys eventually */
+	if (gsm_mux[mux] == NULL)
+		return -EUNATCH;
+	if (line == 0 || line > 61)	/* 62/63 reserved */
+		return -ECHRNG;
+	gsm = gsm_mux[mux];
+	if (gsm->dead)
+		return -EL2HLT;
+	dlci = gsm->dlci[line];
+	if (dlci == NULL)
+		dlci = gsm_dlci_alloc(gsm, line);
+	if (dlci == NULL)
+		return -ENOMEM;
+	port = &dlci->port;
+	port->count++;
+	tty->driver_data = dlci;
+	tty_port_tty_set(port, tty);
+
+	dlci->modem_rx = 0;
+	/* We could in theory open and close before we wait - eg if we get
+	   a DM straight back. This is ok as that will have caused a hangup */
+	set_bit(ASYNCB_INITIALIZED, &port->flags);
+	/* Start sending off SABM messages */
+	gsm_dlci_begin_open(dlci);
+	/* And wait for virtual carrier */
+	return tty_port_block_til_ready(port, tty, filp);
+}
+
+static void gsmtty_close(struct tty_struct *tty, struct file *filp)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	if (dlci == NULL)
+		return;
+	if (tty_port_close_start(&dlci->port, tty, filp) == 0)
+		return;
+	gsm_dlci_begin_close(dlci);
+	tty_port_close_end(&dlci->port, tty);
+	tty_port_tty_set(&dlci->port, NULL);
+}
+
+static void gsmtty_hangup(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	tty_port_hangup(&dlci->port);
+	gsm_dlci_begin_close(dlci);
+}
+
+static int gsmtty_write(struct tty_struct *tty, const unsigned char *buf,
+								    int len)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	/* Stuff the bytes into the fifo queue */
+	int sent = kfifo_in_locked(dlci->fifo, buf, len, &dlci->lock);
+	/* Need to kick the channel */
+	gsm_dlci_data_kick(dlci);
+	return sent;
+}
+
+static int gsmtty_write_room(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	return TX_SIZE - kfifo_len(dlci->fifo);
+}
+
+static int gsmtty_chars_in_buffer(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	return kfifo_len(dlci->fifo);
+}
+
+static void gsmtty_flush_buffer(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	/* Caution needed: If we implement reliable transport classes
+	   then the data being transmitted can't simply be junked once
+	   it has first hit the stack. Until then we can just blow it
+	   away */
+	kfifo_reset(dlci->fifo);
+	/* Need to unhook this DLCI from the transmit queue logic */
+}
+
+static void gsmtty_wait_until_sent(struct tty_struct *tty, int timeout)
+{
+	/* The FIFO handles the queue so the kernel will do the right
+	   thing waiting on chars_in_buffer before calling us. No work
+	   to do here */
+}
+
+static int gsmtty_tiocmget(struct tty_struct *tty, struct file *filp)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	return dlci->modem_rx;
+}
+
+static int gsmtty_tiocmset(struct tty_struct *tty, struct file *filp,
+	unsigned int set, unsigned int clear)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	unsigned int modem_tx = dlci->modem_tx;
+
+	modem_tx &= clear;
+	modem_tx |= set;
+
+	if (modem_tx != dlci->modem_tx) {
+		dlci->modem_tx = modem_tx;
+		return gsmtty_modem_update(dlci, 0);
+	}
+	return 0;
+}
+
+
+static int gsmtty_ioctl(struct tty_struct *tty, struct file *filp,
+			unsigned int cmd, unsigned long arg)
+{
+	return -ENOIOCTLCMD;
+}
+
+static void gsmtty_set_termios(struct tty_struct *tty, struct ktermios *old)
+{
+	/* For the moment its fixed. In actual fact the speed information
+	   for the virtual channel can be propogated in both directions by
+	   the RPN control message. This however rapidly gets nasty as we
+	   then have to remap modem signals each way according to whether
+	   our virtual cable is null modem etc .. */
+	tty_termios_copy_hw(tty->termios, old);
+}
+
+static void gsmtty_throttle(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	if (tty->termios->c_cflag & CRTSCTS)
+		dlci->modem_tx &= ~TIOCM_DTR;
+	dlci->throttled = 1;
+	/* Send an MSC with DTR cleared */
+	gsmtty_modem_update(dlci, 0);
+}
+
+static void gsmtty_unthrottle(struct tty_struct *tty)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	if (tty->termios->c_cflag & CRTSCTS)
+		dlci->modem_tx |= TIOCM_DTR;
+	dlci->throttled = 0;
+	/* Send an MSC with DTR set */
+	gsmtty_modem_update(dlci, 0);
+}
+
+static int gsmtty_break_ctl(struct tty_struct *tty, int state)
+{
+	struct gsm_dlci *dlci = tty->driver_data;
+	int encode = 0;	/* Off */
+
+	if (state == -1)	/* "On indefinitely" - we can't encode this
+				    properly */
+		encode = 0x0F;
+	else if (state > 0) {
+		encode = state / 200;	/* mS to encoding */
+		if (encode > 0x0F)
+			encode = 0x0F;	/* Best effort */
+	}
+	return gsmtty_modem_update(dlci, encode);
+}
+
+static struct tty_driver *gsm_tty_driver;
+
+/* Virtual ttys for the demux */
+static const struct tty_operations gsmtty_ops = {
+	.open			= gsmtty_open,
+	.close			= gsmtty_close,
+	.write			= gsmtty_write,
+	.write_room		= gsmtty_write_room,
+	.chars_in_buffer	= gsmtty_chars_in_buffer,
+	.flush_buffer		= gsmtty_flush_buffer,
+	.ioctl			= gsmtty_ioctl,
+	.throttle		= gsmtty_throttle,
+	.unthrottle		= gsmtty_unthrottle,
+	.set_termios		= gsmtty_set_termios,
+	.hangup			= gsmtty_hangup,
+	.wait_until_sent	= gsmtty_wait_until_sent,
+	.tiocmget		= gsmtty_tiocmget,
+	.tiocmset		= gsmtty_tiocmset,
+	.break_ctl		= gsmtty_break_ctl,
+};
+
+
+
+static int __init gsm_init(void)
+{
+	/* Fill in our line protocol discipline, and register it */
+	int status = tty_register_ldisc(N_GSM0710, &tty_ldisc_packet);
+	if (status != 0) {
+		printk(KERN_ERR "n_gsm: can't register line discipline (err = %d)\n", status);
+		return status;
+	}
+
+	gsm_tty_driver = alloc_tty_driver(256);
+	if (!gsm_tty_driver) {
+		tty_unregister_ldisc(N_GSM0710);
+		printk(KERN_ERR "gsm_init: tty allocation failed.\n");
+		return -EINVAL;
+	}
+	gsm_tty_driver->owner	= THIS_MODULE;
+	gsm_tty_driver->driver_name	= "gsmtty";
+	gsm_tty_driver->name		= "gsmtty";
+	gsm_tty_driver->major		= 0;	/* Dynamic */
+	gsm_tty_driver->minor_start	= 0;
+	gsm_tty_driver->type		= TTY_DRIVER_TYPE_SERIAL;
+	gsm_tty_driver->subtype	= SERIAL_TYPE_NORMAL;
+	gsm_tty_driver->flags	= TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV
+							| TTY_DRIVER_HARDWARE_BREAK;
+	gsm_tty_driver->init_termios	= tty_std_termios;
+	/* Fixme */
+	gsm_tty_driver->init_termios.c_lflag &= ~ECHO;
+	tty_set_operations(gsm_tty_driver, &gsmtty_ops);
+
+	spin_lock_init(&gsm_mux_lock);
+
+	if (tty_register_driver(gsm_tty_driver)) {
+		put_tty_driver(gsm_tty_driver);
+		tty_unregister_ldisc(N_GSM0710);
+		printk(KERN_ERR "gsm_init: tty registration failed.\n");
+		return -EBUSY;
+	}
+	printk(KERN_INFO "gsm_init: loaded as %d,%d.\n", gsm_tty_driver->major, gsm_tty_driver->minor_start);
+	return 0;
+}
+
+static void __exit gsm_exit(void)
+{
+	int status = tty_unregister_ldisc(N_GSM0710);
+	if (status != 0)
+		printk(KERN_ERR "n_gsm: can't unregister line discipline (err = %d)\n", status);
+	tty_unregister_driver(gsm_tty_driver);
+	put_tty_driver(gsm_tty_driver);
+	printk(KERN_INFO "gsm_init: unloaded.\n");
+}
+
+module_init(gsm_init);
+module_exit(gsm_exit);
+
+
+MODULE_LICENSE("GPL");
+MODULE_ALIAS_LDISC(N_GSM0710);

include/linux/gsmmux.h

···
+#ifndef _LINUX_GSMMUX_H
+#define _LINUX_GSMMUX_H
+
+struct gsm_config
+{
+	unsigned int adaption;
+	unsigned int encapsulation;
+	unsigned int initiator;
+	unsigned int t1;
+	unsigned int t2;
+	unsigned int t3;
+	unsigned int n2;
+	unsigned int mru;
+	unsigned int mtu;
+	unsigned int k;
+	unsigned int i;
+	unsigned int unused[8];		/* Padding for expansion without
+					   breaking stuff */
+};
+
+#define GSMIOC_GETCONF		_IOR('G', 0, struct gsm_config)
+#define GSMIOC_SETCONF		_IOW('G', 1, struct gsm_config)
+
+
+#endif

include/linux/tty.h

···
 #define N_PPS		18	/* Pulse per Second */
 #define N_V253		19	/* Codec control over voice modem */
 #define N_CAIF		20      /* CAIF protocol for talking to modems */
+#define N_GSM0710	21	/* GSM 0710 Mux */
 
 /*
  * This character is the same as _POSIX_VDISABLE: it cannot be used as