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 <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

3 files changed, 2772 insertions, 1 deletions

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 3141dd3b6e53..51a57009aec8 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -276,11 +276,18 @@ config N_HDLC
 	  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"
 	depends on SERIAL_NONSTANDARD
diff --git a/drivers/char/Makefile b/drivers/char/Makefile
index f957edf7e45d..d39be4cf1f5d 100644
--- a/drivers/char/Makefile
+++ b/drivers/char/Makefile
@@ -40,6 +40,7 @@ obj-$(CONFIG_SYNCLINK)		+= synclink.o
 obj-$(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
diff --git a/drivers/char/n_gsm.c b/drivers/char/n_gsm.c
new file mode 100644
index 000000000000..c4161d5e053d
--- /dev/null
+++ b/drivers/char/n_gsm.c
@@ -0,0 +1,2763 @@
+/*
+ * 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