/* generic HDLC line discipline for Linux
 *
 * Written by Paul Fulghum paulkf@microgate.com
 * for Microgate Corporation
 *
 * Microgate and SyncLink are registered trademarks of Microgate Corporation
 *
 * Adapted from ppp.c, written by Michael Callahan <callahan@maths.ox.ac.uk>,
 *	Al Longyear <longyear@netcom.com>, Paul Mackerras <Paul.Mackerras@cs.anu.edu.au>
 *
 * Original release 01/11/99
 * $Id: n_hdlc.c,v 3.2 2000/11/06 22:34:38 paul Exp $
 *
 * This code is released under the GNU General Public License (GPL)
 *
 * This module implements the tty line discipline N_HDLC for use with
 * tty device drivers that support bit-synchronous HDLC communications.
 *
 * All HDLC data is frame oriented which means:
 *
 * 1. tty write calls represent one complete transmit frame of data
 *    The device driver should accept the complete frame or none of 
 *    the frame (busy) in the write method. Each write call should have
 *    a byte count in the range of 2-65535 bytes (2 is min HDLC frame
 *    with 1 addr byte and 1 ctrl byte). The max byte count of 65535
 *    should include any crc bytes required. For example, when using
 *    CCITT CRC32, 4 crc bytes are required, so the maximum size frame
 *    the application may transmit is limited to 65531 bytes. For CCITT
 *    CRC16, the maximum application frame size would be 65533.
 *
 *
 * 2. receive callbacks from the device driver represents
 *    one received frame. The device driver should bypass
 *    the tty flip buffer and call the line discipline receive
 *    callback directly to avoid fragmenting or concatenating
 *    multiple frames into a single receive callback.
 *
 *    The HDLC line discipline queues the receive frames in seperate
 *    buffers so complete receive frames can be returned by the
 *    tty read calls.
 *
 * 3. tty read calls returns an entire frame of data or nothing.
 *    
 * 4. all send and receive data is considered raw. No processing
 *    or translation is performed by the line discipline, regardless
 *    of the tty flags
 *
 * 5. When line discipline is queried for the amount of receive
 *    data available (FIOC), 0 is returned if no data available,
 *    otherwise the count of the next available frame is returned.
 *    (instead of the sum of all received frame counts).
 *
 * These conventions allow the standard tty programming interface
 * to be used for synchronous HDLC applications when used with
 * this line discipline (or another line discipline that is frame
 * oriented such as N_PPP).
 *
 * The SyncLink driver (synclink.c) implements both asynchronous
 * (using standard line discipline N_TTY) and synchronous HDLC
 * (using N_HDLC) communications, with the latter using the above
 * conventions.
 *
 * This implementation is very basic and does not maintain
 * any statistics. The main point is to enforce the raw data
 * and frame orientation of HDLC communications.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#define HDLC_MAGIC 0x239e
#define HDLC_VERSION "3.2"

#include <linux/version.h>
#include <linux/config.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/fcntl.h>
#include <linux/interrupt.h>
#include <linux/ptrace.h>

#undef VERSION
#define VERSION(major,minor,patch) (((((major)<<8)+(minor))<<8)+(patch))

#include <linux/poll.h>
#include <linux/in.h>
#include <linux/slab.h>
#include <linux/tty.h>
#include <linux/errno.h>
#include <linux/string.h>	/* used in new tty drivers */
#include <linux/signal.h>	/* used in new tty drivers */
#include <asm/system.h>
#include <asm/bitops.h>
#include <asm/termios.h>
#include <linux/if.h>

#include <linux/ioctl.h>

#ifdef CONFIG_KERNELD
#include <linux/kerneld.h>
#endif

#include <asm/segment.h>
#define GET_USER(error,value,addr) error = get_user(value,addr)
#define COPY_FROM_USER(error,dest,src,size) error = copy_from_user(dest,src,size) ? -EFAULT : 0
#define PUT_USER(error,value,addr) error = put_user(value,addr)
#define COPY_TO_USER(error,dest,src,size) error = copy_to_user(dest,src,size) ? -EFAULT : 0

#include <asm/uaccess.h>

typedef ssize_t		rw_ret_t;
typedef size_t		rw_count_t;

/*
 * Buffers for individual HDLC frames
 */
#define MAX_HDLC_FRAME_SIZE 65535 
#define DEFAULT_RX_BUF_COUNT 10
#define MAX_RX_BUF_COUNT 60
#define DEFAULT_TX_BUF_COUNT 1


typedef struct _n_hdlc_buf
{
	struct _n_hdlc_buf *link;
	int count;
	char buf[1];
} N_HDLC_BUF;

#define	N_HDLC_BUF_SIZE	(sizeof(N_HDLC_BUF)+maxframe)

typedef struct _n_hdlc_buf_list
{
	N_HDLC_BUF *head;
	N_HDLC_BUF *tail;
	int count;
	spinlock_t spinlock;
	
} N_HDLC_BUF_LIST;

/*
 * Per device instance data structure
 */
struct n_hdlc {
	int		magic;		/* magic value for structure	*/
	__u32		flags;		/* miscellaneous control flags	*/
	
	struct tty_struct *tty;		/* ptr to TTY structure	*/
	struct tty_struct *backup_tty;	/* TTY to use if tty gets closed */
	
	/* Queues for select() functionality */
	wait_queue_head_t read_wait;
	wait_queue_head_t write_wait;
	wait_queue_head_t poll_wait;

	int		tbusy;		/* reentrancy flag for tx wakeup code */
	int		woke_up;
	N_HDLC_BUF	*tbuf;		/* currently transmitting tx buffer */
	N_HDLC_BUF_LIST tx_buf_list;	/* list of pending transmit frame buffers */	
	N_HDLC_BUF_LIST	rx_buf_list;	/* list of received frame buffers */
	N_HDLC_BUF_LIST tx_free_buf_list;	/* list unused transmit frame buffers */	
	N_HDLC_BUF_LIST	rx_free_buf_list;	/* list unused received frame buffers */
};

/*
 * HDLC buffer list manipulation functions
 */
void n_hdlc_buf_list_init(N_HDLC_BUF_LIST *list);
void n_hdlc_buf_put(N_HDLC_BUF_LIST *list,N_HDLC_BUF *buf);
N_HDLC_BUF* n_hdlc_buf_get(N_HDLC_BUF_LIST *list);

/* Local functions */

static struct n_hdlc *n_hdlc_alloc (void);

MODULE_PARM(debuglevel, "i");
MODULE_PARM(maxframe, "i");


/* debug level can be set by insmod for debugging purposes */
#define DEBUG_LEVEL_INFO	1
int debuglevel=0;

/* max frame size for memory allocations */
ssize_t	maxframe=4096;

/* TTY callbacks */

static rw_ret_t n_hdlc_tty_read(struct tty_struct *,
	struct file *, __u8 *, rw_count_t);
static rw_ret_t n_hdlc_tty_write(struct tty_struct *,
	struct file *, const __u8 *, rw_count_t);
static int n_hdlc_tty_ioctl(struct tty_struct *,
	struct file *, unsigned int, unsigned long);
static unsigned int n_hdlc_tty_poll (struct tty_struct *tty, struct file *filp,
				  poll_table * wait);
static int n_hdlc_tty_open (struct tty_struct *);
static void n_hdlc_tty_close (struct tty_struct *);
static int n_hdlc_tty_room (struct tty_struct *tty);
static void n_hdlc_tty_receive (struct tty_struct *tty,
	const __u8 * cp, char *fp, int count);
static void n_hdlc_tty_wakeup (struct tty_struct *tty);

#define bset(p,b)	((p)[(b) >> 5] |= (1 << ((b) & 0x1f)))

#define tty2n_hdlc(tty)	((struct n_hdlc *) ((tty)->disc_data))
#define n_hdlc2tty(n_hdlc)	((n_hdlc)->tty)

/* Define this string only once for all macro invocations */
static char szVersion[] = HDLC_VERSION;

/* n_hdlc_release()
 *
 *	release an n_hdlc per device line discipline info structure
 *
 */
static void n_hdlc_release (struct n_hdlc *n_hdlc)
{
	struct tty_struct *tty = n_hdlc2tty (n_hdlc);
	N_HDLC_BUF *buf;
	
	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_release() called\n",__FILE__,__LINE__);
		
	/* Ensure that the n_hdlcd process is not hanging on select()/poll() */
	wake_up_interruptible (&n_hdlc->read_wait);
	wake_up_interruptible (&n_hdlc->poll_wait);
	wake_up_interruptible (&n_hdlc->write_wait);

	if (tty != NULL && tty->disc_data == n_hdlc)
		tty->disc_data = NULL;	/* Break the tty->n_hdlc link */

	/* Release transmit and receive buffers */
	for(;;) {
		buf = n_hdlc_buf_get(&n_hdlc->rx_free_buf_list);
		if (buf) {
			kfree(buf);
		} else
			break;
	}
	for(;;) {
		buf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
		if (buf) {
			kfree(buf);
		} else
			break;
	}
	for(;;) {
		buf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
		if (buf) {
			kfree(buf);
		} else
			break;
	}
	for(;;) {
		buf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
		if (buf) {
			kfree(buf);
		} else
			break;
	}
	
	kfree(n_hdlc);
	
}	/* end of n_hdlc_release() */

/* n_hdlc_tty_close()
 *
 *	Called when the line discipline is changed to something
 *	else, the tty is closed, or the tty detects a hangup.
 */
static void n_hdlc_tty_close(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_close() called\n",__FILE__,__LINE__);
		
	if (n_hdlc != NULL) {
		if (n_hdlc->magic != HDLC_MAGIC) {
			printk (KERN_WARNING"n_hdlc: trying to close unopened tty!\n");
			return;
		}
#if defined(TTY_NO_WRITE_SPLIT)
		clear_bit(TTY_NO_WRITE_SPLIT,&tty->flags);
#endif
		tty->disc_data = NULL;
		if (tty == n_hdlc->backup_tty)
			n_hdlc->backup_tty = 0;
		if (tty != n_hdlc->tty)
			return;
		if (n_hdlc->backup_tty) {
			n_hdlc->tty = n_hdlc->backup_tty;
		} else {
			n_hdlc_release (n_hdlc);
			MOD_DEC_USE_COUNT;
		}
	}
	
	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_close() success\n",__FILE__,__LINE__);
		
}	/* end of n_hdlc_tty_close() */

/* n_hdlc_tty_open
 * 
 * 	called when line discipline changed to n_hdlc
 * 	
 * Arguments:	tty	pointer to tty info structure
 * Return Value:	0 if success, otherwise error code
 */
static int n_hdlc_tty_open (struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_open() called (major=%u,minor=%u)\n",
		__FILE__,__LINE__,
		MAJOR(tty->device), MINOR(tty->device));
		
	/* There should not be an existing table for this slot. */
	if (n_hdlc) {
		printk (KERN_ERR"n_hdlc_tty_open:tty already associated!\n" );
		return -EEXIST;
	}
	
	n_hdlc = n_hdlc_alloc();
	if (!n_hdlc) {
		printk (KERN_ERR "n_hdlc_alloc failed\n");
		return -ENFILE;
	}
		
	tty->disc_data = n_hdlc;
	n_hdlc->tty    = tty;
	
	MOD_INC_USE_COUNT;
	
#if defined(TTY_NO_WRITE_SPLIT)
	/* change tty_io write() to not split large writes into 8K chunks */
	set_bit(TTY_NO_WRITE_SPLIT,&tty->flags);
#endif
	
	/* Flush any pending characters in the driver and discipline. */
	
	if (tty->ldisc.flush_buffer)
		tty->ldisc.flush_buffer (tty);

	if (tty->driver.flush_buffer)
		tty->driver.flush_buffer (tty);
		
	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_open() success\n",__FILE__,__LINE__);
		
	return 0;
	
}	/* end of n_tty_hdlc_open() */

/* n_hdlc_send_frames()
 * 
 * 	send frames on pending send buffer list until the
 * 	driver does not accept a frame (busy)
 * 	this function is called after adding a frame to the
 * 	send buffer list and by the tty wakeup callback
 * 	
 * Arguments:		n_hdlc		pointer to ldisc instance data
 * 			tty		pointer to tty instance data
 * Return Value:	None
 */
static void n_hdlc_send_frames (struct n_hdlc *n_hdlc, struct tty_struct *tty)
{
	register int actual;
	unsigned long flags;
	N_HDLC_BUF *tbuf;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_send_frames() called\n",__FILE__,__LINE__);
 check_again:
		
	save_flags(flags);
	cli ();
	if (n_hdlc->tbusy) {
		n_hdlc->woke_up = 1;
		restore_flags(flags);
		return;
	}
	n_hdlc->tbusy = 1;
	n_hdlc->woke_up = 0;
	restore_flags(flags);

	/* get current transmit buffer or get new transmit */
	/* buffer from list of pending transmit buffers */
		
	tbuf = n_hdlc->tbuf;
	if (!tbuf)
		tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
		
	while (tbuf) {
		if (debuglevel >= DEBUG_LEVEL_INFO)	
			printk("%s(%d)sending frame %p, count=%d\n",
				__FILE__,__LINE__,tbuf,tbuf->count);
			
		/* Send the next block of data to device */
		tty->flags |= (1 << TTY_DO_WRITE_WAKEUP);
		actual = tty->driver.write(tty, 0, tbuf->buf, tbuf->count);
		    
		/* if transmit error, throw frame away by */
		/* pretending it was accepted by driver */
		if (actual < 0)
			actual = tbuf->count;
		
		if (actual == tbuf->count) {
			if (debuglevel >= DEBUG_LEVEL_INFO)	
				printk("%s(%d)frame %p completed\n",
					__FILE__,__LINE__,tbuf);
					
			/* free current transmit buffer */
			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,tbuf);
			
			/* this tx buffer is done */
			n_hdlc->tbuf = NULL;
			
			/* wait up sleeping writers */
			wake_up_interruptible(&n_hdlc->write_wait);
			wake_up_interruptible(&n_hdlc->poll_wait);
	
			/* get next pending transmit buffer */
			tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
		} else {
			if (debuglevel >= DEBUG_LEVEL_INFO)	
				printk("%s(%d)frame %p pending\n",
					__FILE__,__LINE__,tbuf);
					
			/* buffer not accepted by driver */
			/* set this buffer as pending buffer */
			n_hdlc->tbuf = tbuf;
			break;
		}
	}
	
	if (!tbuf)
		tty->flags  &= ~(1 << TTY_DO_WRITE_WAKEUP);
	
	/* Clear the re-entry flag */
	save_flags(flags);
	cli ();
	n_hdlc->tbusy = 0;
	restore_flags(flags);
	
        if (n_hdlc->woke_up)
	  goto check_again;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_send_frames() exit\n",__FILE__,__LINE__);
		
}	/* end of n_hdlc_send_frames() */

/* n_hdlc_tty_wakeup()
 *
 *	Callback for transmit wakeup. Called when low level
 *	device driver can accept more send data.
 *
 * Arguments:		tty	pointer to associated tty instance data
 * Return Value:	None
 */
static void n_hdlc_tty_wakeup (struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_wakeup() called\n",__FILE__,__LINE__);
		
	if (!n_hdlc)
		return;

	if (tty != n_hdlc->tty) {
		tty->flags &= ~(1 << TTY_DO_WRITE_WAKEUP);
		return;
	}

	n_hdlc_send_frames (n_hdlc, tty);
		
}	/* end of n_hdlc_tty_wakeup() */

/* n_hdlc_tty_room()
 * 
 *	Callback function from tty driver. Return the amount of 
 *	space left in the receiver's buffer to decide if remote
 *	transmitter is to be throttled.
 *
 * Arguments:		tty	pointer to associated tty instance data
 * Return Value:	number of bytes left in receive buffer
 */
static int n_hdlc_tty_room (struct tty_struct *tty)
{
	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_room() called\n",__FILE__,__LINE__);
	/* always return a larger number to prevent */
	/* throttling of remote transmitter. */
	return 65536;
}	/* end of n_hdlc_tty_root() */

/* n_hdlc_tty_receive()
 * 
 * 	Called by tty low level driver when receive data is
 * 	available. Data is interpreted as one HDLC frame.
 * 	
 * Arguments:	 	tty		pointer to tty isntance data
 * 			data		pointer to received data
 * 			flags		pointer to flags for data
 * 			count		count of received data in bytes
 * 	
 * Return Value:	None
 */
static void n_hdlc_tty_receive(struct tty_struct *tty,
	const __u8 * data, char *flags, int count)
{
	register struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
	register N_HDLC_BUF *buf;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_receive() called count=%d\n",
			__FILE__,__LINE__, count);
		
	/* This can happen if stuff comes in on the backup tty */
	if (n_hdlc == 0 || tty != n_hdlc->tty)
		return;
		
	/* verify line is using HDLC discipline */
	if (n_hdlc->magic != HDLC_MAGIC) {
		printk("%s(%d) line not using HDLC discipline\n",
			__FILE__,__LINE__);
		return;
	}
	
	if ( count>maxframe ) {
		if (debuglevel >= DEBUG_LEVEL_INFO)	
			printk("%s(%d) rx count>maxframesize, data discarded\n",
			       __FILE__,__LINE__);
		return;
	}

	/* get a free HDLC buffer */	
	buf = n_hdlc_buf_get(&n_hdlc->rx_free_buf_list);
	if (!buf) {
		/* no buffers in free list, attempt to allocate another rx buffer */
		/* unless the maximum count has been reached */
		if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
			buf = (N_HDLC_BUF*)kmalloc(N_HDLC_BUF_SIZE,GFP_ATOMIC);
	}
	
	if (!buf) {
		if (debuglevel >= DEBUG_LEVEL_INFO)	
			printk("%s(%d) no more rx buffers, data discarded\n",
			       __FILE__,__LINE__);
		return;
	}
		
	/* copy received data to HDLC buffer */
	memcpy(buf->buf,data,count);
	buf->count=count;

	/* add HDLC buffer to list of received frames */
	n_hdlc_buf_put(&n_hdlc->rx_buf_list,buf);
	
	/* wake up any blocked reads and perform async signalling */
	wake_up_interruptible (&n_hdlc->read_wait);
	wake_up_interruptible (&n_hdlc->poll_wait);
	if (n_hdlc->tty->fasync != NULL)
		kill_fasync (&n_hdlc->tty->fasync, SIGIO, POLL_IN);

}	/* end of n_hdlc_tty_receive() */

/* n_hdlc_tty_read()
 * 
 * 	Called to retreive one frame of data (if available)
 * 	
 * Arguments:
 * 
 * 	tty		pointer to tty instance data
 * 	file		pointer to open file object
 * 	buf		pointer to returned data buffer
 * 	nr		size of returned data buffer
 * 	
 * Return Value:
 * 
 * 	Number of bytes returned or error code
 */
static rw_ret_t n_hdlc_tty_read (struct tty_struct *tty,
	struct file *file, __u8 * buf, rw_count_t nr)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
	int error;
	rw_ret_t ret;
	N_HDLC_BUF *rbuf;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_read() called\n",__FILE__,__LINE__);
		
	/* Validate the pointers */
	if (!n_hdlc)
		return -EIO;

	/* verify user access to buffer */
	error = verify_area (VERIFY_WRITE, buf, nr);
	if (error != 0) {
		printk(KERN_WARNING"%s(%d) n_hdlc_tty_read() can't verify user "
		"buffer\n",__FILE__,__LINE__);
		return (error);
	}

	for (;;) {
		n_hdlc = tty2n_hdlc (tty);
		if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC ||
			 tty != n_hdlc->tty)
			return 0;

		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
		if (rbuf)
			break;
			
		/* no data */
		if (file->f_flags & O_NONBLOCK)
			return -EAGAIN;
			
		interruptible_sleep_on (&n_hdlc->read_wait);
		if (signal_pending(current))
			return -EINTR;
	}
		
	if (rbuf->count > nr) {
		/* frame too large for caller's buffer (discard frame) */
		ret = (rw_ret_t)-EOVERFLOW;
	} else {
		/* Copy the data to the caller's buffer */
		COPY_TO_USER(error,buf,rbuf->buf,rbuf->count);
		if (error)
			ret = (rw_ret_t)error;
		else
			ret = (rw_ret_t)rbuf->count;
	}
	
	/* return HDLC buffer to free list unless the free list */
	/* count has exceeded the default value, in which case the */
	/* buffer is freed back to the OS to conserve memory */
	if (n_hdlc->rx_free_buf_list.count > DEFAULT_RX_BUF_COUNT)
		kfree(rbuf);
	else	
		n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,rbuf);
	
	return ret;
	
}	/* end of n_hdlc_tty_read() */

/* n_hdlc_tty_write()
 * 
 * 	write a single frame of data to device
 * 	
 * Arguments:	tty	pointer to associated tty device instance data
 * 		file	pointer to file object data
 * 		data	pointer to transmit data (one frame)
 * 		count	size of transmit frame in bytes
 * 		
 * Return Value:	number of bytes written (or error code)
 */
static rw_ret_t n_hdlc_tty_write (struct tty_struct *tty, struct file *file,
	const __u8 * data, rw_count_t count)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
	int error = 0;
	DECLARE_WAITQUEUE(wait, current);
	N_HDLC_BUF *tbuf;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_write() called count=%d\n",
			__FILE__,__LINE__,count);
		
	/* Verify pointers */
	if (!n_hdlc)
		return -EIO;

	if (n_hdlc->magic != HDLC_MAGIC)
		return -EIO;

	/* verify frame size */
	if (count > maxframe ) {
		if (debuglevel & DEBUG_LEVEL_INFO)
			printk (KERN_WARNING
				"n_hdlc_tty_write: truncating user packet "
				"from %lu to %d\n", (unsigned long) count,
				maxframe );
		count = maxframe;
	}
	
	add_wait_queue(&n_hdlc->write_wait, &wait);
	set_current_state(TASK_INTERRUPTIBLE);
	
	/* Allocate transmit buffer */
	/* sleep until transmit buffer available */		
	while (!(tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list))) {
		schedule();
			
		n_hdlc = tty2n_hdlc (tty);
		if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC || 
		    tty != n_hdlc->tty) {
			printk("n_hdlc_tty_write: %p invalid after wait!\n", n_hdlc);
			error = -EIO;
			break;
		}
			
		if (signal_pending(current)) {
			error = -EINTR;
			break;
		}
	}

	set_current_state(TASK_RUNNING);
	remove_wait_queue(&n_hdlc->write_wait, &wait);

	if (!error) {		
		/* Retrieve the user's buffer */
		COPY_FROM_USER (error, tbuf->buf, data, count);
		if (error) {
			/* return tx buffer to free list */
			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,tbuf);
		} else {
			/* Send the data */
			tbuf->count = error = count;
			n_hdlc_buf_put(&n_hdlc->tx_buf_list,tbuf);
			n_hdlc_send_frames(n_hdlc,tty);
		}
	}

	return error;
	
}	/* end of n_hdlc_tty_write() */

/* n_hdlc_tty_ioctl()
 *
 *	Process IOCTL system call for the tty device.
 *
 * Arguments:
 *
 *	tty		pointer to tty instance data
 *	file		pointer to open file object for device
 *	cmd		IOCTL command code
 *	arg		argument for IOCTL call (cmd dependent)
 *
 * Return Value:	Command dependent
 */
static int n_hdlc_tty_ioctl (struct tty_struct *tty, struct file * file,
               unsigned int cmd, unsigned long arg)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
	int error = 0;
	int count;
	unsigned long flags;
	
	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_ioctl() called %d\n",
			__FILE__,__LINE__,cmd);
		
	/* Verify the status of the device */
	if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC)
		return -EBADF;

	switch (cmd) {
	case FIONREAD:
		/* report count of read data available */
		/* in next available frame (if any) */
		spin_lock_irqsave(&n_hdlc->rx_buf_list.spinlock,flags);
		if (n_hdlc->rx_buf_list.head)
			count = n_hdlc->rx_buf_list.head->count;
		else
			count = 0;
		spin_unlock_irqrestore(&n_hdlc->rx_buf_list.spinlock,flags);
		PUT_USER (error, count, (int *) arg);
		break;

	case TIOCOUTQ:
		/* get the pending tx byte count in the driver */
		count = tty->driver.chars_in_buffer ?
				tty->driver.chars_in_buffer(tty) : 0;
		/* add size of next output frame in queue */
		spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock,flags);
		if (n_hdlc->tx_buf_list.head)
			count += n_hdlc->tx_buf_list.head->count;
		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock,flags);
		PUT_USER (error, count, (int*)arg);
		break;

	default:
		error = n_tty_ioctl (tty, file, cmd, arg);
		break;
	}
	return error;
	
}	/* end of n_hdlc_tty_ioctl() */

/* n_hdlc_tty_poll()
 * 
 * 	TTY callback for poll system call. Determine which 
 * 	operations (read/write) will not block and return
 * 	info to caller.
 * 	
 * Arguments:
 * 
 * 	tty		pointer to tty instance data
 * 	filp		pointer to open file object for device
 * 	poll_table	wait queue for operations
 * 
 * Return Value:
 * 
 * 	bit mask containing info on which ops will not block
 */
static unsigned int n_hdlc_tty_poll (struct tty_struct *tty,
	 struct file *filp, poll_table * wait)
{
	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
	unsigned int mask = 0;

	if (debuglevel >= DEBUG_LEVEL_INFO)	
		printk("%s(%d)n_hdlc_tty_poll() called\n",__FILE__,__LINE__);
		
	if (n_hdlc && n_hdlc->magic == HDLC_MAGIC && tty == n_hdlc->tty) {
		/* queue current process into any wait queue that */
		/* may awaken in the future (read and write) */
		poll_wait(filp, &n_hdlc->poll_wait, wait);

		/* set bits for operations that wont block */
		if(n_hdlc->rx_buf_list.head)
			mask |= POLLIN | POLLRDNORM;	/* readable */
		if(tty->flags & (1 << TTY_OTHER_CLOSED))
			mask |= POLLHUP;
		if(tty_hung_up_p(filp))
			mask |= POLLHUP;
		if(n_hdlc->tx_free_buf_list.head)
			mask |= POLLOUT | POLLWRNORM;	/* writable */
	}
	return mask;
}	/* end of n_hdlc_tty_poll() */

/* n_hdlc_alloc()
 * 
 * 	Allocate an n_hdlc instance data structure
 *
 * Arguments:		None
 * Return Value:	pointer to structure if success, otherwise 0	
 */
static struct n_hdlc *n_hdlc_alloc (void)
{
	struct n_hdlc	*n_hdlc;
	N_HDLC_BUF	*buf;
	int		i;
	
	n_hdlc = (struct n_hdlc *)kmalloc(sizeof(struct n_hdlc), GFP_KERNEL);
	if (!n_hdlc)
		return 0;

	memset(n_hdlc, 0, sizeof(*n_hdlc));

	n_hdlc_buf_list_init(&n_hdlc->rx_free_buf_list);
	n_hdlc_buf_list_init(&n_hdlc->tx_free_buf_list);
	n_hdlc_buf_list_init(&n_hdlc->rx_buf_list);
	n_hdlc_buf_list_init(&n_hdlc->tx_buf_list);
	
	/* allocate free rx buffer list */
	for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
		buf = (N_HDLC_BUF*)kmalloc(N_HDLC_BUF_SIZE,GFP_KERNEL);
		if (buf)
			n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
		else if (debuglevel >= DEBUG_LEVEL_INFO)	
			printk("%s(%d)n_hdlc_alloc(), kalloc() failed for rx buffer %d\n",__FILE__,__LINE__, i);
	}
	
	/* allocate free tx buffer list */
	for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
		buf = (N_HDLC_BUF*)kmalloc(N_HDLC_BUF_SIZE,GFP_KERNEL);
		if (buf)
			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
		else if (debuglevel >= DEBUG_LEVEL_INFO)	
			printk("%s(%d)n_hdlc_alloc(), kalloc() failed for tx buffer %d\n",__FILE__,__LINE__, i);
	}
	
	/* Initialize the control block */
	n_hdlc->magic  = HDLC_MAGIC;

	n_hdlc->flags  = 0;
	init_waitqueue_head(&n_hdlc->read_wait);
	init_waitqueue_head(&n_hdlc->poll_wait);
	init_waitqueue_head(&n_hdlc->write_wait);
	
	return n_hdlc;
	
}	/* end of n_hdlc_alloc() */

/* n_hdlc_buf_list_init()
 * 
 * 	initialize specified HDLC buffer list
 * 	
 * Arguments:	 	list	pointer to buffer list
 * Return Value:	None	
 */
void n_hdlc_buf_list_init(N_HDLC_BUF_LIST *list)
{
	memset(list,0,sizeof(N_HDLC_BUF_LIST));
	spin_lock_init(&list->spinlock);
}	/* end of n_hdlc_buf_list_init() */

/* n_hdlc_buf_put()
 * 
 * 	add specified HDLC buffer to tail of specified list
 * 	
 * Arguments:
 * 
 * 	list	pointer to buffer list
 * 	buf	pointer to buffer
 * 
 * Return Value:	None	
 */
void n_hdlc_buf_put(N_HDLC_BUF_LIST *list,N_HDLC_BUF *buf)
{
	unsigned long flags;
	spin_lock_irqsave(&list->spinlock,flags);
	
	buf->link=NULL;
	if(list->tail)
		list->tail->link = buf;
	else
		list->head = buf;
	list->tail = buf;
	(list->count)++;
	
	spin_unlock_irqrestore(&list->spinlock,flags);
	
}	/* end of n_hdlc_buf_put() */

/* n_hdlc_buf_get()
 * 
 * 	remove and return an HDLC buffer from the
 * 	head of the specified HDLC buffer list
 * 	
 * Arguments:
 * 
 * 	list	pointer to HDLC buffer list
 * 	
 * Return Value:
 * 
 * 	pointer to HDLC buffer if available, otherwise NULL
 */
N_HDLC_BUF* n_hdlc_buf_get(N_HDLC_BUF_LIST *list)
{
	unsigned long flags;
	N_HDLC_BUF *buf;
	spin_lock_irqsave(&list->spinlock,flags);
	
	buf = list->head;
	if (buf) {
		list->head = buf->link;
		(list->count)--;
	}
	if (!list->head)
		list->tail = NULL;
	
	spin_unlock_irqrestore(&list->spinlock,flags);
	return buf;
	
}	/* end of n_hdlc_buf_get() */

static int __init n_hdlc_init(void)
{
	static struct tty_ldisc	n_hdlc_ldisc;
	int    status;

	/* range check maxframe arg */
	if ( maxframe<4096)
		maxframe=4096;
	else if ( maxframe>65535)
		maxframe=65535;

	printk("HDLC line discipline: version %s, maxframe=%u\n", 
		szVersion, maxframe);

	/* Register the tty discipline */
	
	memset(&n_hdlc_ldisc, 0, sizeof (n_hdlc_ldisc));
	n_hdlc_ldisc.magic		= TTY_LDISC_MAGIC;
	n_hdlc_ldisc.name          	= "hdlc";
	n_hdlc_ldisc.open		= n_hdlc_tty_open;
	n_hdlc_ldisc.close		= n_hdlc_tty_close;
	n_hdlc_ldisc.read		= n_hdlc_tty_read;
	n_hdlc_ldisc.write		= n_hdlc_tty_write;
	n_hdlc_ldisc.ioctl		= n_hdlc_tty_ioctl;
	n_hdlc_ldisc.poll		= n_hdlc_tty_poll;
	n_hdlc_ldisc.receive_room	= n_hdlc_tty_room;
	n_hdlc_ldisc.receive_buf	= n_hdlc_tty_receive;
	n_hdlc_ldisc.write_wakeup	= n_hdlc_tty_wakeup;

	status = tty_register_ldisc(N_HDLC, &n_hdlc_ldisc);
	if (!status)
		printk (KERN_INFO"N_HDLC line discipline registered.\n");
	else
		printk (KERN_ERR"error registering line discipline: %d\n",status);

	if (status)
		printk(KERN_INFO"N_HDLC: init failure %d\n", status);
	return (status);
	
}	/* end of init_module() */

static void __exit n_hdlc_exit(void)
{
	int status;
	/* Release tty registration of line discipline */
	if ((status = tty_register_ldisc(N_HDLC, NULL)))
		printk("N_HDLC: can't unregister line discipline (err = %d)\n", status);
	else
		printk("N_HDLC: line discipline unregistered\n");
}

module_init(n_hdlc_init);
module_exit(n_hdlc_exit);
