xref: /Linux-v5.15/drivers/tty/n_gsm.c

// SPDX-License-Identifier: GPL-2.0
/*
 * n_gsm.c GSM 0710 tty multiplexor
 * Copyright (c) 2009/10 Intel Corporation
 *
 *	* 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 ?
 *	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/signal.h>
#include <linux/interrupt.h>
#include <linux/tty.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 <net/arp.h>
#include <linux/ip.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/gsmmux.h>
#include "tty.h"

static int debug;
module_param(debug, int, 0600);

/* Defaults: these are from the specification */

#define T1	10		/* 100mS */
#define T2	34		/* 333mS */
#define N2	3		/* Retry 3 times */

/* Use long timers for testing at low speed with debug on */
#ifdef DEBUG_TIMING
#define T1	100
#define T2	200
#endif

/*
 * Semi-arbitrary buffer size limits. 0710 is normally run with 32-64 byte
 * limits so this is plenty
 */
#define MAX_MRU 1500
#define MAX_MTU 1500
#define	GSM_NET_TX_TIMEOUT (HZ*10)

/*
 *	struct gsm_mux_net	-	network interface
 *
 *	Created when net interface is initialized.
 */
struct gsm_mux_net {
	struct kref ref;
	struct gsm_dlci *dlci;
};

/*
 *	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 independent
 *	format
 */

struct gsm_msg {
	struct list_head list;
	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[];
};

enum gsm_dlci_state {
	DLCI_CLOSED,
	DLCI_OPENING,		/* Sending SABM not seen UA */
	DLCI_OPEN,		/* SABM/UA complete */
	DLCI_CLOSING,		/* Sending DISC not seen UA/DM */
};

enum gsm_dlci_mode {
	DLCI_MODE_ABM,		/* Normal Asynchronous Balanced Mode */
	DLCI_MODE_ADM,		/* Asynchronous Disconnected Mode */
};

/*
 *	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;
	enum gsm_dlci_state state;
	struct mutex mutex;

	/* Link layer */
	enum gsm_dlci_mode mode;
	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 */
	int adaption;		/* Adaption layer in use */
	int prev_adaption;
	u32 modem_rx;		/* Our incoming virtual modem lines */
	u32 modem_tx;		/* Our outgoing modem lines */
	bool dead;		/* Refuse re-open */
	/* Flow control */
	bool throttled;		/* Private copy of throttle state */
	bool 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, const u8 *data, int len);
	void (*prev_data)(struct gsm_dlci *dlci, const u8 *data, int len);
	struct net_device *net; /* network interface, if created */
};

/* DLCI 0, 62/63 are special or reserved 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 */
};

enum gsm_mux_state {
	GSM_SEARCH,
	GSM_START,
	GSM_ADDRESS,
	GSM_CONTROL,
	GSM_LEN,
	GSM_DATA,
	GSM_FCS,
	GSM_OVERRUN,
	GSM_LEN0,
	GSM_LEN1,
	GSM_SSOF,
};

/*
 *	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;
	struct mutex mutex;
	unsigned int num;
	struct kref ref;

	/* Events on the GSM channel */
	wait_queue_head_t event;

	/* Bits for GSM mode decoding */

	/* Framing Layer */
	unsigned char *buf;
	enum gsm_mux_state state;
	unsigned int len;
	unsigned int address;
	unsigned int count;
	bool escape;
	int encoding;
	u8 control;
	u8 fcs;
	u8 received_fcs;
	u8 *txframe;			/* TX framing buffer */

	/* Method for the receiver side */
	void (*receive)(struct gsm_mux *gsm, u8 ch);

	/* Link Layer */
	unsigned int mru;
	unsigned int mtu;
	int initiator;			/* Did we initiate connection */
	bool dead;			/* Has the mux been shut down */
	struct gsm_dlci *dlci[NUM_DLCI];
	bool 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 list_head tx_list;	/* Pending data packets */

	/* 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 DEFINE_SPINLOCK(gsm_mux_lock);

static struct tty_driver *gsm_tty_driver;

/*
 *	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

static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len);

/**
 *	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;

	pr_info("%s %d) %c: ", hdr, addr, "RC"[cr]);

	switch (control & ~PF) {
	case SABM:
		pr_cont("SABM");
		break;
	case UA:
		pr_cont("UA");
		break;
	case DISC:
		pr_cont("DISC");
		break;
	case DM:
		pr_cont("DM");
		break;
	case UI:
		pr_cont("UI");
		break;
	case UIH:
		pr_cont("UIH");
		break;
	default:
		if (!(control & 0x01)) {
			pr_cont("I N(S)%d N(R)%d",
				(control & 0x0E) >> 1, (control & 0xE0) >> 5);
		} else switch (control & 0x0F) {
			case RR:
				pr_cont("RR(%d)", (control & 0xE0) >> 5);
				break;
			case RNR:
				pr_cont("RNR(%d)", (control & 0xE0) >> 5);
				break;
			case REJ:
				pr_cont("REJ(%d)", (control & 0xE0) >> 5);
				break;
			default:
				pr_cont("[%02X]", control);
		}
	}

	if (control & PF)
		pr_cont("(P)");
	else
		pr_cont("(F)");

	print_hex_dump_bytes("", DUMP_PREFIX_NONE, data, dlen);
}


/*
 *	Link level transmission side
 */

/**
 *	gsm_stuff_frame	-	bytestuff a packet
 *	@input: input buffer
 *	@output: output buffer
 *	@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;
}

/**
 *	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;
	}
	gsmld_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;
	INIT_LIST_HEAD(&m->list);
	return m;
}

/**
 *	gsm_data_kick		-	poke the queue
 *	@gsm: GSM Mux
 *	@dlci: DLCI sending the data
 *
 *	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
 *	If we have been flow-stopped by a CMD_FCOFF, then we can only
 *	send messages on DLCI0 until CMD_FCON
 *
 *	FIXME: lock against link layer control transmissions
 */

static void gsm_data_kick(struct gsm_mux *gsm, struct gsm_dlci *dlci)
{
	struct gsm_msg *msg, *nmsg;
	int len;

	list_for_each_entry_safe(msg, nmsg, &gsm->tx_list, list) {
		if (gsm->constipated && msg->addr)
			continue;
		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)
			print_hex_dump_bytes("gsm_data_kick: ",
					     DUMP_PREFIX_OFFSET,
					     gsm->txframe, len);
		if (gsmld_output(gsm, gsm->txframe, len) < 0)
			break;
		/* FIXME: Can eliminate one SOF in many more cases */
		gsm->tx_bytes -= msg->len;

		list_del(&msg->list);
		kfree(msg);

		if (dlci) {
			tty_port_tty_wakeup(&dlci->port);
		} else {
			int i = 0;

			for (i = 0; i < NUM_DLCI; i++)
				if (gsm->dlci[i])
					tty_port_tty_wakeup(&gsm->dlci[i]->port);
		}
	}
}

/**
 *	__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 >> 7);	/* bits 7 - 15 */
			*--dp = (msg->len & 127) << 1;	/* bits 0 - 6 */
		}
	}

	*--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 */
	list_add_tail(&msg->list, &gsm->tx_list);
	gsm->tx_bytes += msg->len;
	gsm_data_kick(gsm, dlci);
}

/**
 *	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, total_size, size;
	int h = dlci->adaption - 1;

	total_size = 0;
	while (1) {
		len = kfifo_len(&dlci->fifo);
		if (len == 0)
			return total_size;

		/* 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);
			break;
		}
		WARN_ON(kfifo_out_locked(&dlci->fifo, dp , len, &dlci->lock) != len);
		__gsm_data_queue(dlci, msg);
		total_size += size;
	}
	/* Bytes of data we used up */
	return total_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_tail(&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 */
			dev_kfree_skb_any(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) {
		skb_queue_tail(&dlci->skb_list, dlci->skb);
		dlci->skb = 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, dlci->skb->data, len);
	skb_pull(dlci->skb, len);
	__gsm_data_queue(dlci, msg);
	if (last) {
		dev_kfree_skb_any(dlci->skb);
		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;

	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 && !dlci->net)
			len = gsm_dlci_data_output(gsm, dlci);
		else
			len = gsm_dlci_data_output_framed(gsm, dlci);
		if (len < 0)
			break;
		/* DLCI empty - try the next */
		if (len == 0)
			i++;
	}
}

/**
 *	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;
	int sweep;

	if (dlci->constipated)
		return;

	spin_lock_irqsave(&dlci->gsm->tx_lock, flags);
	/* If we have nothing running then we need to fire up */
	sweep = (dlci->gsm->tx_bytes < TX_THRESH_LO);
	if (dlci->gsm->tx_bytes == 0) {
		if (dlci->net)
			gsm_dlci_data_output_framed(dlci->gsm, dlci);
		else
			gsm_dlci_data_output(dlci->gsm, dlci);
	}
	if (sweep)
		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, const u8 *data,
					int dlen)
{
	struct gsm_msg *msg;
	msg = gsm_data_alloc(gsm, 0, dlen + 2, gsm->ftype);
	if (msg == NULL)
		return;
	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)
 *	@clen: command length
 *
 *	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 clen)
{
	int  mlines = 0;
	u8 brk = 0;
	int fc;

	/* The modem status command can either contain one octet (v.24 signals)
	   or two octets (v.24 signals + break signals). The length field will
	   either be 2 or 3 respectively. This is specified in section
	   5.4.6.3.7 of the  27.010 mux spec. */

	if (clen == 2)
		modem = modem & 0x7f;
	else {
		brk = modem & 0x7f;
		modem = (modem >> 7) & 0x7f;
	}

	/* Flow control/ready to communicate */
	fc = (modem & MDM_FC) || !(modem & MDM_RTR);
	if (fc && !dlci->constipated) {
		/* Need to throttle our output on this device */
		dlci->constipated = true;
	} else if (!fc && dlci->constipated) {
		dlci->constipated = false;
		gsm_dlci_data_kick(dlci);
	}

	/* Map modem bits */
	if (modem & MDM_RTC)
		mlines |= TIOCM_DSR | TIOCM_DTR;
	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 (!C_CLOCAL(tty))
				tty_hangup(tty);
	}
	if (brk & 0x01)
		tty_insert_flip_char(&dlci->port, 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, const u8 *data, int clen)
{
	unsigned int addr = 0;
	unsigned int modem = 0;
	unsigned int brk = 0;
	struct gsm_dlci *dlci;
	int len = clen;
	const 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;
	}
	len--;
	if (len > 0) {
		while (gsm_read_ea(&brk, *dp++) == 0) {
			len--;
			if (len == 0)
				return;
		}
		modem <<= 7;
		modem |= (brk & 0x7f);
	}
	tty = tty_port_tty_get(&dlci->port);
	gsm_process_modem(tty, dlci, modem, clen);
	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, const u8 *data, int clen)
{
	struct tty_port *port;
	unsigned int addr = 0;
	u8 bits;
	int len = clen;
	const 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;

	port = &gsm->dlci[addr]->port;

	if (bits & 2)
		tty_insert_flip_char(port, 0, TTY_OVERRUN);
	if (bits & 4)
		tty_insert_flip_char(port, 0, TTY_PARITY);
	if (bits & 8)
		tty_insert_flip_char(port, 0, TTY_FRAME);

	tty_flip_buffer_push(port);

	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,
						const u8 *data, int clen)
{
	u8 buf[1];
	unsigned long flags;

	switch (command) {
	case CMD_CLD: {
		struct gsm_dlci *dlci = gsm->dlci[0];
		/* Modem wishes to close down */
		if (dlci) {
			dlci->dead = true;
			gsm->dead = true;
			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 can accept data again */
		gsm->constipated = false;
		gsm_control_reply(gsm, CMD_FCON, NULL, 0);
		/* Kick the link in case it is idling */
		spin_lock_irqsave(&gsm->tx_lock, flags);
		gsm_data_kick(gsm, NULL);
		spin_unlock_irqrestore(&gsm->tx_lock, flags);
		break;
	case CMD_FCOFF:
		/* Modem wants us to STFU */
		gsm->constipated = true;
		gsm_control_reply(gsm, CMD_FCOFF, NULL, 0);
		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 negotiation */
	case CMD_SNC:	/* Service negotiation 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,
						const 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);
	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
 *	@t: timer contained in 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(struct timer_list *t)
{
	struct gsm_mux *gsm = from_timer(gsm, t, t2_timer);
	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)
 *	@clen: 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;

	/* If DLCI0 is in ADM mode skip retries, it won't respond */
	if (gsm->dlci[0]->mode == DLCI_MODE_ADM)
		gsm->cretries = 1;
	else
		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)
		pr_debug("DLCI %d goes closed.\n", dlci->addr);
	dlci->state = DLCI_CLOSED;
	if (dlci->addr != 0) {
		tty_port_tty_hangup(&dlci->port, false);
		kfifo_reset(&dlci->fifo);
	} else
		dlci->gsm->dead = true;
	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)
		pr_debug("DLCI %d goes open.\n", dlci->addr);
	wake_up(&dlci->gsm->event);
}

/**
 *	gsm_dlci_t1		-	T1 timer expiry
 *	@t: timer contained in the 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)
 *
 *	Some control dlci can stay in ADM mode with other dlci working just
 *	fine. In that case we can just keep the control dlci open after the
 *	DLCI_OPENING retries time out.
 */

static void gsm_dlci_t1(struct timer_list *t)
{
	struct gsm_dlci *dlci = from_timer(dlci, t, t1);
	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 if (!dlci->addr && gsm->control == (DM | PF)) {
			if (debug & 8)
				pr_info("DLCI %d opening in ADM mode.\n",
					dlci->addr);
			dlci->mode = DLCI_MODE_ADM;
			gsm_dlci_open(dlci);
		} 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;
	default:
		pr_debug("%s: unhandled state: %d\n", __func__, dlci->state);
		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 or ADM, 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
 *	@clen: 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, const u8 *data, int clen)
{
	/* krefs .. */
	struct tty_port *port = &dlci->port;
	struct tty_struct *tty;
	unsigned int modem = 0;
	int len = clen;

	if (debug & 16)
		pr_debug("%d bytes for tty\n", len);
	switch (dlci->adaption)  {
	/* Unsupported types */
	case 4:		/* Packetised interruptible data */
		break;
	case 3:		/* Packetised uininterruptible voice/data */
		break;
	case 2:		/* Asynchronous serial with line state in each frame */
		while (gsm_read_ea(&modem, *data++) == 0) {
			len--;
			if (len == 0)
				return;
		}
		tty = tty_port_tty_get(port);
		if (tty) {
			gsm_process_modem(tty, dlci, modem, clen);
			tty_kref_put(tty);
		}
		fallthrough;
	case 1:		/* Line state will go via DLCI 0 controls only */
	default:
		tty_insert_flip_string(port, data, len);
		tty_flip_buffer_push(port);
	}
}

/**
 *	gsm_dlci_command	-	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, const 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);
	mutex_init(&dlci->mutex);
	if (kfifo_alloc(&dlci->fifo, 4096, GFP_KERNEL) < 0) {
		kfree(dlci);
		return NULL;
	}

	skb_queue_head_init(&dlci->skb_list);
	timer_setup(&dlci->t1, gsm_dlci_t1, 0);
	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		-	free DLCI
 *	@port: tty port for DLCI to free
 *
 *	Free up a DLCI.
 *
 *	Can sleep.
 */
static void gsm_dlci_free(struct tty_port *port)
{
	struct gsm_dlci *dlci = container_of(port, struct gsm_dlci, port);

	del_timer_sync(&dlci->t1);
	dlci->gsm->dlci[dlci->addr] = NULL;
	kfifo_free(&dlci->fifo);
	while ((dlci->skb = skb_dequeue(&dlci->skb_list)))
		dev_kfree_skb(dlci->skb);
	kfree(dlci);
}

static inline void dlci_get(struct gsm_dlci *dlci)
{
	tty_port_get(&dlci->port);
}

static inline void dlci_put(struct gsm_dlci *dlci)
{
	tty_port_put(&dlci->port);
}

static void gsm_destroy_network(struct gsm_dlci *dlci);

/**
 *	gsm_dlci_release		-	release DLCI
 *	@dlci: DLCI to destroy
 *
 *	Release a DLCI. Actual free is deferred until either
 *	mux is closed or tty is closed - whichever is last.
 *
 *	Can sleep.
 */
static void gsm_dlci_release(struct gsm_dlci *dlci)
{
	struct tty_struct *tty = tty_port_tty_get(&dlci->port);
	if (tty) {
		mutex_lock(&dlci->mutex);
		gsm_destroy_network(dlci);
		mutex_unlock(&dlci->mutex);

		tty_hangup(tty);

		tty_port_tty_set(&dlci->port, NULL);
		tty_kref_put(tty);
	}
	dlci->state = DLCI_CLOSED;
	dlci_put(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->encoding == 0) {
		/* WARNING: gsm->received_fcs is used for
		gsm->encoding = 0 only.
		In this case it contain the last piece of data
		required to generate final CRC */
		gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
	}
	if (gsm->fcs != GOOD_FCS) {
		gsm->bad_fcs++;
		if (debug & 4)
			pr_debug("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;
		default:
			pr_debug("%s: unhandled state: %d\n", __func__,
					dlci->state);
			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)
{
	unsigned int len;

	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;
	case GSM_ADDRESS:	/* Address EA */
		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_LEN0;
		break;
	case GSM_LEN0:		/* 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;
			if (!gsm->len)
				gsm->state = GSM_FCS;
			else
				gsm->state = GSM_DATA;
			break;
		}
		gsm->state = GSM_LEN1;
		break;
	case GSM_LEN1:
		gsm->fcs = gsm_fcs_add(gsm->fcs, c);
		len = c;
		gsm->len |= len << 7;
		if (gsm->len > gsm->mru) {
			gsm->bad_size++;
			gsm->state = GSM_SEARCH;
			break;
		}
		gsm->count = 0;
		if (!gsm->len)
			gsm->state = GSM_FCS;
		else
			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->received_fcs = c;
		gsm_queue(gsm);
		gsm->state = GSM_SSOF;
		break;
	case GSM_SSOF:
		if (c == GSM0_SOF) {
			gsm->state = GSM_SEARCH;
			break;
		}
		break;
	default:
		pr_debug("%s: unhandled state: %d\n", __func__, gsm->state);
		break;
	}
}

/**
 *	gsm1_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 = true;
		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 = false;
	}
	switch (gsm->state) {
	case GSM_START:		/* First byte after SOF */
		gsm->address = 0;
		gsm->state = GSM_ADDRESS;
		gsm->fcs = INIT_FCS;
		fallthrough;
	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;
	default:
		pr_debug("%s: unhandled state: %d\n", __func__, gsm->state);
		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++;
}

static int gsm_disconnect(struct gsm_mux *gsm)
{
	struct gsm_dlci *dlci = gsm->dlci[0];
	struct gsm_control *gc;

	if (!dlci)
		return 0;

	/* In theory disconnecting DLCI 0 is sufficient but for some
	   modems this is apparently not the case. */
	gc = gsm_control_send(gsm, CMD_CLD, NULL, 0);
	if (gc)
		gsm_control_wait(gsm, gc);

	del_timer_sync(&gsm->t2_timer);
	/* Now we are sure T2 has stopped */

	gsm_dlci_begin_close(dlci);
	wait_event_interruptible(gsm->event,
				dlci->state == DLCI_CLOSED);

	if (signal_pending(current))
		return -EINTR;

	return 0;
}

/**
 *	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.
 */

static void gsm_cleanup_mux(struct gsm_mux *gsm)
{
	int i;
	struct gsm_dlci *dlci = gsm->dlci[0];
	struct gsm_msg *txq, *ntxq;

	gsm->dead = true;

	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);
	/* open failed before registering => nothing to do */
	if (i == MAX_MUX)
		return;

	del_timer_sync(&gsm->t2_timer);
	/* Now we are sure T2 has stopped */
	if (dlci)
		dlci->dead = true;

	/* Free up any link layer users */
	mutex_lock(&gsm->mutex);
	for (i = 0; i < NUM_DLCI; i++)
		if (gsm->dlci[i])
			gsm_dlci_release(gsm->dlci[i]);
	mutex_unlock(&gsm->mutex);
	/* Now wipe the queues */
	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
		kfree(txq);
	INIT_LIST_HEAD(&gsm->tx_list);
}

/**
 *	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.
 */

static int gsm_activate_mux(struct gsm_mux *gsm)
{
	struct gsm_dlci *dlci;
	int i = 0;

	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
	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;

	spin_lock(&gsm_mux_lock);
	for (i = 0; i < MAX_MUX; i++) {
		if (gsm_mux[i] == NULL) {
			gsm->num = i;
			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 = false;		/* Tty opens are now permissible */
	return 0;
}

/**
 *	gsm_free_mux		-	free up a mux
 *	@gsm: mux to free
 *
 *	Dispose of allocated resources for a dead mux
 */
static void gsm_free_mux(struct gsm_mux *gsm)
{
	kfree(gsm->txframe);
	kfree(gsm->buf);
	kfree(gsm);
}

/**
 *	gsm_free_muxr		-	free up a mux
 *	@ref: kreference to the mux to free
 *
 *	Dispose of allocated resources for a dead mux
 */
static void gsm_free_muxr(struct kref *ref)
{
	struct gsm_mux *gsm = container_of(ref, struct gsm_mux, ref);
	gsm_free_mux(gsm);
}

static inline void mux_get(struct gsm_mux *gsm)
{
	kref_get(&gsm->ref);
}

static inline void mux_put(struct gsm_mux *gsm)
{
	kref_put(&gsm->ref, gsm_free_muxr);
}

static inline unsigned int mux_num_to_base(struct gsm_mux *gsm)
{
	return gsm->num * NUM_DLCI;
}

static inline unsigned int mux_line_to_num(unsigned int line)
{
	return line / NUM_DLCI;
}

/**
 *	gsm_alloc_mux		-	allocate a mux
 *
 *	Creates a new mux ready for activation.
 */

static 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);
	mutex_init(&gsm->mutex);
	kref_init(&gsm->ref);
	INIT_LIST_HEAD(&gsm->tx_list);

	gsm->t1 = T1;
	gsm->t2 = T2;
	gsm->n2 = N2;
	gsm->ftype = UIH;
	gsm->adaption = 1;
	gsm->encoding = 1;
	gsm->mru = 64;	/* Default to encoding 1 so these should be 64 */
	gsm->mtu = 64;
	gsm->dead = true;	/* Avoid early tty opens */

	return gsm;
}

static void gsm_copy_config_values(struct gsm_mux *gsm,
				   struct gsm_config *c)
{
	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;
	pr_debug("Ftype %d i %d\n", gsm->ftype, c->i);
	c->mru = gsm->mru;
	c->mtu = gsm->mtu;
	c->k = 0;
}

static int gsm_config(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) {
		int ret;

		ret = gsm_disconnect(gsm);

		if (ret)
			return ret;
	}
	if (need_restart)
		gsm_cleanup_mux(gsm);

	gsm->initiator = c->initiator;
	gsm->mru = c->mru;
	gsm->mtu = c->mtu;
	gsm->encoding = c->encapsulation;
	gsm->adaption = c->adaption;
	gsm->n2 = c->n2;

	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;
}

/**
 *	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)
		print_hex_dump_bytes("gsmld_output: ", DUMP_PREFIX_OFFSET,
				     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)
{
	unsigned int base;
	int ret, i;

	gsm->tty = tty_kref_get(tty);
	ret =  gsm_activate_mux(gsm);
	if (ret != 0)
		tty_kref_put(gsm->tty);
	else {
		/* Don't register device 0 - this is the control channel and not
		   a usable tty interface */
		base = mux_num_to_base(gsm); /* Base for this MUX */
		for (i = 1; i < NUM_DLCI; i++) {
			struct device *dev;

			dev = tty_register_device(gsm_tty_driver,
							base + i, NULL);
			if (IS_ERR(dev)) {
				for (i--; i >= 1; i--)
					tty_unregister_device(gsm_tty_driver,
								base + i);
				return PTR_ERR(dev);
			}
		}
	}
	return ret;
}


/**
 *	gsmld_detach_gsm	-	stop doing 0710 mux
 *	@tty: tty attached 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)
{
	unsigned int base = mux_num_to_base(gsm); /* Base for this MUX */
	int i;

	WARN_ON(tty != gsm->tty);
	for (i = 1; i < NUM_DLCI; i++)
		tty_unregister_device(gsm_tty_driver, base + i);
	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,
			      const char *fp, int count)
{
	struct gsm_mux *gsm = tty->disc_data;
	char flags = TTY_NORMAL;

	if (debug & 4)
		print_hex_dump_bytes("gsmld_receive: ", DUMP_PREFIX_OFFSET,
				     cp, count);

	for (; count; count--, cp++) {
		if (fp)
			flags = *fp++;
		switch (flags) {
		case TTY_NORMAL:
			gsm->receive(gsm, *cp);
			break;
		case TTY_OVERRUN:
		case TTY_BREAK:
		case TTY_PARITY:
		case TTY_FRAME:
			gsm_error(gsm, *cp, flags);
			break;
		default:
			WARN_ONCE(1, "%s: unknown flag %d\n",
			       tty_name(tty), flags);
			break;
		}
	}
	/* FASYNC if needed ? */
	/* If clogged call tty_throttle(tty); */
}

/**
 *	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 */
	mux_put(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;
	int ret;

	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;

	ret = gsmld_attach_gsm(tty, gsm);
	if (ret != 0) {
		gsm_cleanup_mux(gsm);
		mux_put(gsm);
	}
	return ret;
}

/**
 *	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;
	unsigned long flags;

	/* Queue poll */
	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
	spin_lock_irqsave(&gsm->tx_lock, flags);
	gsm_data_kick(gsm, NULL);
	if (gsm->tx_bytes < TX_THRESH_LO) {
		gsm_dlci_data_sweep(gsm);
	}
	spin_unlock_irqrestore(&gsm->tx_lock, flags);
}

/**
 *	gsmld_read		-	read function for tty
 *	@tty: tty device
 *	@file: file object
 *	@buf: userspace buffer pointer
 *	@nr: size of I/O
 *	@cookie: unused
 *	@offset: unused
 *
 *	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 *buf, size_t nr,
			  void **cookie, unsigned long offset)
{
	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 __poll_t gsmld_poll(struct tty_struct *tty, struct file *file,
							poll_table *wait)
{
	__poll_t 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 |= EPOLLHUP;
	if (!tty_is_writelocked(tty) && tty_write_room(tty) > 0)
		mask |= EPOLLOUT | EPOLLWRNORM;
	if (gsm->dead)
		mask |= EPOLLHUP;
	return mask;
}

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;
	unsigned int base;

	switch (cmd) {
	case GSMIOC_GETCONF:
		gsm_copy_config_values(gsm, &c);
		if (copy_to_user((void __user *)arg, &c, sizeof(c)))
			return -EFAULT;
		return 0;
	case GSMIOC_SETCONF:
		if (copy_from_user(&c, (void __user *)arg, sizeof(c)))
			return -EFAULT;
		return gsm_config(gsm, &c);
	case GSMIOC_GETFIRST:
		base = mux_num_to_base(gsm);
		return put_user(base + 1, (__u32 __user *)arg);
	default:
		return n_tty_ioctl_helper(tty, file, cmd, arg);
	}
}

/*
 *	Network interface
 *
 */

static int gsm_mux_net_open(struct net_device *net)
{
	pr_debug("%s called\n", __func__);
	netif_start_queue(net);
	return 0;
}

static int gsm_mux_net_close(struct net_device *net)
{
	netif_stop_queue(net);
	return 0;
}

static void dlci_net_free(struct gsm_dlci *dlci)
{
	if (!dlci->net) {
		WARN_ON(1);
		return;
	}
	dlci->adaption = dlci->prev_adaption;
	dlci->data = dlci->prev_data;
	free_netdev(dlci->net);
	dlci->net = NULL;
}
static void net_free(struct kref *ref)
{
	struct gsm_mux_net *mux_net;
	struct gsm_dlci *dlci;

	mux_net = container_of(ref, struct gsm_mux_net, ref);
	dlci = mux_net->dlci;

	if (dlci->net) {
		unregister_netdev(dlci->net);
		dlci_net_free(dlci);
	}
}

static inline void muxnet_get(struct gsm_mux_net *mux_net)
{
	kref_get(&mux_net->ref);
}

static inline void muxnet_put(struct gsm_mux_net *mux_net)
{
	kref_put(&mux_net->ref, net_free);
}

static netdev_tx_t gsm_mux_net_start_xmit(struct sk_buff *skb,
				      struct net_device *net)
{
	struct gsm_mux_net *mux_net = netdev_priv(net);
	struct gsm_dlci *dlci = mux_net->dlci;
	muxnet_get(mux_net);

	skb_queue_head(&dlci->skb_list, skb);
	net->stats.tx_packets++;
	net->stats.tx_bytes += skb->len;
	gsm_dlci_data_kick(dlci);
	/* And tell the kernel when the last transmit started. */
	netif_trans_update(net);
	muxnet_put(mux_net);
	return NETDEV_TX_OK;
}

/* called when a packet did not ack after watchdogtimeout */
static void gsm_mux_net_tx_timeout(struct net_device *net, unsigned int txqueue)
{
	/* Tell syslog we are hosed. */
	dev_dbg(&net->dev, "Tx timed out.\n");

	/* Update statistics */
	net->stats.tx_errors++;
}

static void gsm_mux_rx_netchar(struct gsm_dlci *dlci,
				const unsigned char *in_buf, int size)
{
	struct net_device *net = dlci->net;
	struct sk_buff *skb;
	struct gsm_mux_net *mux_net = netdev_priv(net);
	muxnet_get(mux_net);

	/* Allocate an sk_buff */
	skb = dev_alloc_skb(size + NET_IP_ALIGN);
	if (!skb) {
		/* We got no receive buffer. */
		net->stats.rx_dropped++;
		muxnet_put(mux_net);
		return;
	}
	skb_reserve(skb, NET_IP_ALIGN);
	skb_put_data(skb, in_buf, size);

	skb->dev = net;
	skb->protocol = htons(ETH_P_IP);

	/* Ship it off to the kernel */
	netif_rx(skb);

	/* update out statistics */
	net->stats.rx_packets++;
	net->stats.rx_bytes += size;
	muxnet_put(mux_net);
	return;
}

static void gsm_mux_net_init(struct net_device *net)
{
	static const struct net_device_ops gsm_netdev_ops = {
		.ndo_open		= gsm_mux_net_open,
		.ndo_stop		= gsm_mux_net_close,
		.ndo_start_xmit		= gsm_mux_net_start_xmit,
		.ndo_tx_timeout		= gsm_mux_net_tx_timeout,
	};

	net->netdev_ops = &gsm_netdev_ops;

	/* fill in the other fields */
	net->watchdog_timeo = GSM_NET_TX_TIMEOUT;
	net->flags = IFF_POINTOPOINT | IFF_NOARP | IFF_MULTICAST;
	net->type = ARPHRD_NONE;
	net->tx_queue_len = 10;
}


/* caller holds the dlci mutex */
static void gsm_destroy_network(struct gsm_dlci *dlci)
{
	struct gsm_mux_net *mux_net;

	pr_debug("destroy network interface\n");
	if (!dlci->net)
		return;
	mux_net = netdev_priv(dlci->net);
	muxnet_put(mux_net);
}


/* caller holds the dlci mutex */
static int gsm_create_network(struct gsm_dlci *dlci, struct gsm_netconfig *nc)
{
	char *netname;
	int retval = 0;
	struct net_device *net;
	struct gsm_mux_net *mux_net;

	if (!capable(CAP_NET_ADMIN))
		return -EPERM;

	/* Already in a non tty mode */
	if (dlci->adaption > 2)
		return -EBUSY;

	if (nc->protocol != htons(ETH_P_IP))
		return -EPROTONOSUPPORT;

	if (nc->adaption != 3 && nc->adaption != 4)
		return -EPROTONOSUPPORT;

	pr_debug("create network interface\n");

	netname = "gsm%d";
	if (nc->if_name[0] != '\0')
		netname = nc->if_name;
	net = alloc_netdev(sizeof(struct gsm_mux_net), netname,
			   NET_NAME_UNKNOWN, gsm_mux_net_init);
	if (!net) {
		pr_err("alloc_netdev failed\n");
		return -ENOMEM;
	}
	net->mtu = dlci->gsm->mtu;
	net->min_mtu = 8;
	net->max_mtu = dlci->gsm->mtu;
	mux_net = netdev_priv(net);
	mux_net->dlci = dlci;
	kref_init(&mux_net->ref);
	strncpy(nc->if_name, net->name, IFNAMSIZ); /* return net name */

	/* reconfigure dlci for network */
	dlci->prev_adaption = dlci->adaption;
	dlci->prev_data = dlci->data;
	dlci->adaption = nc->adaption;
	dlci->data = gsm_mux_rx_netchar;
	dlci->net = net;

	pr_debug("register netdev\n");
	retval = register_netdev(net);
	if (retval) {
		pr_err("network register fail %d\n", retval);
		dlci_net_free(dlci);
		return retval;
	}
	return net->ifindex;	/* return network index */
}

/* Line discipline for real tty */
static struct tty_ldisc_ops tty_ldisc_packet = {
	.owner		 = THIS_MODULE,
	.num		 = N_GSM0710,
	.name            = "n_gsm",
	.open            = gsmld_open,
	.close           = gsmld_close,
	.flush_buffer    = gsmld_flush_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);
	struct gsm_mux *gsm = dlci->gsm;

	/* Not yet open so no carrier info */
	if (dlci->state != DLCI_OPEN)
		return 0;
	if (debug & 2)
		return 1;

	/*
	 * Basic mode with control channel in ADM mode may not respond
	 * to CMD_MSC at all and modem_rx is empty.
	 */
	if (gsm->encoding == 0 && gsm->dlci[0]->mode == DLCI_MODE_ADM &&
	    !dlci->modem_rx)
		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,
	.destruct = gsm_dlci_free,
};

static int gsmtty_install(struct tty_driver *driver, struct tty_struct *tty)
{
	struct gsm_mux *gsm;
	struct gsm_dlci *dlci;
	unsigned int line = tty->index;
	unsigned int mux = mux_line_to_num(line);
	bool alloc = false;
	int ret;

	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;
	/* If DLCI 0 is not yet fully open return an error.
	This is ok from a locking
	perspective as we don't have to worry about this
	if DLCI0 is lost */
	mutex_lock(&gsm->mutex);
	if (gsm->dlci[0] && gsm->dlci[0]->state != DLCI_OPEN) {
		mutex_unlock(&gsm->mutex);
		return -EL2NSYNC;
	}
	dlci = gsm->dlci[line];
	if (dlci == NULL) {
		alloc = true;
		dlci = gsm_dlci_alloc(gsm, line);
	}
	if (dlci == NULL) {
		mutex_unlock(&gsm->mutex);
		return -ENOMEM;
	}
	ret = tty_port_install(&dlci->port, driver, tty);
	if (ret) {
		if (alloc)
			dlci_put(dlci);
		mutex_unlock(&gsm->mutex);
		return ret;
	}

	dlci_get(dlci);
	dlci_get(gsm->dlci[0]);
	mux_get(gsm);
	tty->driver_data = dlci;
	mutex_unlock(&gsm->mutex);

	return 0;
}

static int gsmtty_open(struct tty_struct *tty, struct file *filp)
{
	struct gsm_dlci *dlci = tty->driver_data;
	struct tty_port *port = &dlci->port;

	port->count++;
	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 */
	tty_port_set_initialized(port, 1);
	/* 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 (dlci->state == DLCI_CLOSED)
		return;
	mutex_lock(&dlci->mutex);
	gsm_destroy_network(dlci);
	mutex_unlock(&dlci->mutex);
	if (tty_port_close_start(&dlci->port, tty, filp) == 0)
		return;
	gsm_dlci_begin_close(dlci);
	if (tty_port_initialized(&dlci->port) && C_HUPCL(tty))
		tty_port_lower_dtr_rts(&dlci->port);
	tty_port_close_end(&dlci->port, tty);
	tty_port_tty_set(&dlci->port, NULL);
	return;
}

static void gsmtty_hangup(struct tty_struct *tty)
{
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return;
	tty_port_hangup(&dlci->port);
	gsm_dlci_begin_close(dlci);
}

static int gsmtty_write(struct tty_struct *tty, const unsigned char *buf,
								    int len)
{
	int sent;
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return -EINVAL;
	/* Stuff the bytes into the fifo queue */
	sent = kfifo_in_locked(&dlci->fifo, buf, len, &dlci->lock);
	/* Need to kick the channel */
	gsm_dlci_data_kick(dlci);
	return sent;
}

static unsigned int gsmtty_write_room(struct tty_struct *tty)
{
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return 0;
	return TX_SIZE - kfifo_len(&dlci->fifo);
}

static unsigned int gsmtty_chars_in_buffer(struct tty_struct *tty)
{
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return 0;
	return kfifo_len(&dlci->fifo);
}

static void gsmtty_flush_buffer(struct tty_struct *tty)
{
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return;
	/* 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 gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return -EINVAL;
	return dlci->modem_rx;
}

static int gsmtty_tiocmset(struct tty_struct *tty,
	unsigned int set, unsigned int clear)
{
	struct gsm_dlci *dlci = tty->driver_data;
	unsigned int modem_tx = dlci->modem_tx;

	if (dlci->state == DLCI_CLOSED)
		return -EINVAL;
	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,
			unsigned int cmd, unsigned long arg)
{
	struct gsm_dlci *dlci = tty->driver_data;
	struct gsm_netconfig nc;
	int index;

	if (dlci->state == DLCI_CLOSED)
		return -EINVAL;
	switch (cmd) {
	case GSMIOC_ENABLE_NET:
		if (copy_from_user(&nc, (void __user *)arg, sizeof(nc)))
			return -EFAULT;
		nc.if_name[IFNAMSIZ-1] = '\0';
		/* return net interface index or error code */
		mutex_lock(&dlci->mutex);
		index = gsm_create_network(dlci, &nc);
		mutex_unlock(&dlci->mutex);
		if (copy_to_user((void __user *)arg, &nc, sizeof(nc)))
			return -EFAULT;
		return index;
	case GSMIOC_DISABLE_NET:
		if (!capable(CAP_NET_ADMIN))
			return -EPERM;
		mutex_lock(&dlci->mutex);
		gsm_destroy_network(dlci);
		mutex_unlock(&dlci->mutex);
		return 0;
	default:
		return -ENOIOCTLCMD;
	}
}

static void gsmtty_set_termios(struct tty_struct *tty, struct ktermios *old)
{
	struct gsm_dlci *dlci = tty->driver_data;
	if (dlci->state == DLCI_CLOSED)
		return;
	/* 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 (dlci->state == DLCI_CLOSED)
		return;
	if (C_CRTSCTS(tty))
		dlci->modem_tx &= ~TIOCM_DTR;
	dlci->throttled = true;
	/* 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 (dlci->state == DLCI_CLOSED)
		return;
	if (C_CRTSCTS(tty))
		dlci->modem_tx |= TIOCM_DTR;
	dlci->throttled = false;
	/* 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 (dlci->state == DLCI_CLOSED)
		return -EINVAL;

	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 void gsmtty_cleanup(struct tty_struct *tty)
{
	struct gsm_dlci *dlci = tty->driver_data;
	struct gsm_mux *gsm = dlci->gsm;

	dlci_put(dlci);
	dlci_put(gsm->dlci[0]);
	mux_put(gsm);
}

/* Virtual ttys for the demux */
static const struct tty_operations gsmtty_ops = {
	.install		= gsmtty_install,
	.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,
	.cleanup		= gsmtty_cleanup,
};



static int __init gsm_init(void)
{
	/* Fill in our line protocol discipline, and register it */
	int status = tty_register_ldisc(&tty_ldisc_packet);
	if (status != 0) {
		pr_err("n_gsm: can't register line discipline (err = %d)\n",
								status);
		return status;
	}

	gsm_tty_driver = tty_alloc_driver(256, TTY_DRIVER_REAL_RAW |
			TTY_DRIVER_DYNAMIC_DEV | TTY_DRIVER_HARDWARE_BREAK);
	if (IS_ERR(gsm_tty_driver)) {
		pr_err("gsm_init: tty allocation failed.\n");
		status = PTR_ERR(gsm_tty_driver);
		goto err_unreg_ldisc;
	}
	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->init_termios	= tty_std_termios;
	/* Fixme */
	gsm_tty_driver->init_termios.c_lflag &= ~ECHO;
	tty_set_operations(gsm_tty_driver, &gsmtty_ops);

	if (tty_register_driver(gsm_tty_driver)) {
		pr_err("gsm_init: tty registration failed.\n");
		status = -EBUSY;
		goto err_put_driver;
	}
	pr_debug("gsm_init: loaded as %d,%d.\n",
			gsm_tty_driver->major, gsm_tty_driver->minor_start);
	return 0;
err_put_driver:
	tty_driver_kref_put(gsm_tty_driver);
err_unreg_ldisc:
	tty_unregister_ldisc(&tty_ldisc_packet);
	return status;
}

static void __exit gsm_exit(void)
{
	tty_unregister_ldisc(&tty_ldisc_packet);
	tty_unregister_driver(gsm_tty_driver);
	tty_driver_kref_put(gsm_tty_driver);
}

module_init(gsm_init);
module_exit(gsm_exit);


MODULE_LICENSE("GPL");
MODULE_ALIAS_LDISC(N_GSM0710);