text: /* 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
   *
   * 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 separate
   *    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
  
  #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/ioctl.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 <linux/if.h>
  #include <linux/bitops.h>
  
  #include <asm/termios.h>
  #include <asm/uaccess.h>
  
  /*
   * 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 3
  
  struct n_hdlc_buf {
  	struct list_head  list_item;
  	int		  count;
  	char		  buf[1];
  };
  
  #define	N_HDLC_BUF_SIZE	(sizeof(struct n_hdlc_buf) + maxframe)
  
  struct n_hdlc_buf_list {
  	struct list_head  list;
  	int		  count;
  	spinlock_t	  spinlock;
  };
  
  /**
   * struct n_hdlc - per device instance data structure
   * @magic - magic value for structure
   * @flags - miscellaneous control flags
   * @tty - ptr to TTY structure
   * @backup_tty - TTY to use if tty gets closed
   * @tbusy - reentrancy flag for tx wakeup code
   * @woke_up - FIXME: describe this field
   * @tx_buf_list - list of pending transmit frame buffers
   * @rx_buf_list - list of received frame buffers
   * @tx_free_buf_list - list unused transmit frame buffers
   * @rx_free_buf_list - list unused received frame buffers
   */
  struct n_hdlc {
  	int			magic;
  	__u32			flags;
  	struct tty_struct	*tty;
  	struct tty_struct	*backup_tty;
  	int			tbusy;
  	int			woke_up;
  	struct n_hdlc_buf_list	tx_buf_list;
  	struct n_hdlc_buf_list	rx_buf_list;
  	struct n_hdlc_buf_list	tx_free_buf_list;
  	struct n_hdlc_buf_list	rx_free_buf_list;
  };
  
  /*
   * HDLC buffer list manipulation functions
   */
  static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
  						struct n_hdlc_buf *buf);
  static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
  			   struct n_hdlc_buf *buf);
  static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *list);
  
  /* Local functions */
  
  static struct n_hdlc *n_hdlc_alloc (void);
  
  /* debug level can be set by insmod for debugging purposes */
  #define DEBUG_LEVEL_INFO	1
  static int debuglevel;
  
  /* max frame size for memory allocations */
  static int maxframe = 4096;
  
  /* TTY callbacks */
  
  static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
  			   __u8 __user *buf, size_t nr);
  static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
  			    const unsigned char *buf, size_t nr);
  static int n_hdlc_tty_ioctl(struct tty_struct *tty, struct file *file,
  			    unsigned int cmd, unsigned long arg);
  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 *tty);
  static void n_hdlc_tty_close(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)
  
  static void flush_rx_queue(struct tty_struct *tty)
  {
  	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
  	struct n_hdlc_buf *buf;
  
  	while ((buf = n_hdlc_buf_get(&n_hdlc->rx_buf_list)))
  		n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, buf);
  }
  
  static void flush_tx_queue(struct tty_struct *tty)
  {
  	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
  	struct n_hdlc_buf *buf;
  
  	while ((buf = n_hdlc_buf_get(&n_hdlc->tx_buf_list)))
  		n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, buf);
  }
  
  static struct tty_ldisc_ops n_hdlc_ldisc = {
  	.owner		= THIS_MODULE,
  	.magic		= TTY_LDISC_MAGIC,
  	.name		= "hdlc",
  	.open		= n_hdlc_tty_open,
  	.close		= n_hdlc_tty_close,
  	.read		= n_hdlc_tty_read,
  	.write		= n_hdlc_tty_write,
  	.ioctl		= n_hdlc_tty_ioctl,
  	.poll		= n_hdlc_tty_poll,
  	.receive_buf	= n_hdlc_tty_receive,
  	.write_wakeup	= n_hdlc_tty_wakeup,
  	.flush_buffer   = flush_rx_queue,
  };
  
  /**
   * n_hdlc_release - release an n_hdlc per device line discipline info structure
   * @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);
  	struct 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 (&tty->read_wait);
  	wake_up_interruptible (&tty->write_wait);
  
  	if (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 - line discipline close
   * @tty - pointer to tty info structure
   *
   * 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 = NULL;
  		if (tty != n_hdlc->tty)
  			return;
  		if (n_hdlc->backup_tty) {
  			n_hdlc->tty = n_hdlc->backup_tty;
  		} else {
  			n_hdlc_release (n_hdlc);
  		}
  	}
  	
  	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
   * @tty - pointer to tty info structure
   *
   * Returns 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 (device=%s)\n",
  		__FILE__,__LINE__,
  		tty->name);
  		
  	/* 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;
  	tty->receive_room = 65536;
  	
  #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 receive data from driver */
  	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
   * @n_hdlc - pointer to ldisc instance data
   * @tty - pointer to tty instance data
   *
   * 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.
   */
  static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
  {
  	register int actual;
  	unsigned long flags;
  	struct n_hdlc_buf *tbuf;
  
  	if (debuglevel >= DEBUG_LEVEL_INFO)	
  		printk("%s(%d)n_hdlc_send_frames() called\n",__FILE__,__LINE__);
   check_again:
  		
   	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
  	if (n_hdlc->tbusy) {
  		n_hdlc->woke_up = 1;
   		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
  		return;
  	}
  	n_hdlc->tbusy = 1;
  	n_hdlc->woke_up = 0;
  	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
  
  	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 */
  		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
  		actual = tty->ops->write(tty, tbuf->buf, tbuf->count);
  
  		/* rollback was possible and has been done */
  		if (actual == -ERESTARTSYS) {
  			n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
  			break;
  		}
  		/* 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);
  
  			/* wait up sleeping writers */
  			wake_up_interruptible(&tty->write_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);
  
  			/*
  			 * the buffer was not accepted by driver,
  			 * return it back into tx queue
  			 */
  			n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
  			break;
  		}
  	}
  	
  	if (!tbuf)
  		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
  	
  	/* Clear the re-entry flag */
  	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
  	n_hdlc->tbusy = 0;
  	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, 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
   * @tty	- pointer to associated tty instance data
   *
   * Called when low level device driver can accept more send data.
   */
  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) {
  		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
  		return;
  	}
  
  	n_hdlc_send_frames (n_hdlc, tty);
  		
  }	/* end of n_hdlc_tty_wakeup() */
  
  /**
   * n_hdlc_tty_receive - Called by tty driver when receive data is available
   * @tty	- pointer to tty instance data
   * @data - pointer to received data
   * @flags - pointer to flags for data
   * @count - count of received data in bytes
   *
   * Called by tty low level driver when receive data is available. Data is
   * interpreted as one HDLC frame.
   */
  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 struct 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 || 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 = 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 (&tty->read_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 retrieve one frame of data (if available)
   * @tty - pointer to tty instance data
   * @file - pointer to open file object
   * @buf - pointer to returned data buffer
   * @nr - size of returned data buffer
   * 	
   * Returns the number of bytes returned or error code.
   */
  static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
  			   __u8 __user *buf, size_t nr)
  {
  	struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
  	int ret = 0;
  	struct n_hdlc_buf *rbuf;
  	DECLARE_WAITQUEUE(wait, current);
  
  	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 */
  	if (!access_ok(VERIFY_WRITE, buf, nr)) {
  		printk(KERN_WARNING "%s(%d) n_hdlc_tty_read() can't verify user "
  		"buffer\n", __FILE__, __LINE__);
  		return -EFAULT;
  	}
  
  	add_wait_queue(&tty->read_wait, &wait);
  
  	for (;;) {
  		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
  			ret = -EIO;
  			break;
  		}
  		if (tty_hung_up_p(file))
  			break;
  
  		set_current_state(TASK_INTERRUPTIBLE);
  
  		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
  		if (rbuf) {
  			if (rbuf->count > nr) {
  				/* too large for caller's buffer */
  				ret = -EOVERFLOW;
  			} else {
  				if (copy_to_user(buf, rbuf->buf, rbuf->count))
  					ret = -EFAULT;
  				else
  					ret = rbuf->count;
  			}
  
  			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);
  			break;
  		}
  			
  		/* no data */
  		if (file->f_flags & O_NONBLOCK) {
  			ret = -EAGAIN;
  			break;
  		}
  
  		schedule();
  
  		if (signal_pending(current)) {
  			ret = -EINTR;
  			break;
  		}
  	}
  
  	remove_wait_queue(&tty->read_wait, &wait);
  	__set_current_state(TASK_RUNNING);
  
  	return ret;
  	
  }	/* end of n_hdlc_tty_read() */
  
  /**
   * n_hdlc_tty_write - write a single frame of data to device
   * @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
   * 		
   * Returns the number of bytes written (or error code).
   */
  static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
  			    const unsigned char *data, size_t count)
  {
  	struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
  	int error = 0;
  	DECLARE_WAITQUEUE(wait, current);
  	struct n_hdlc_buf *tbuf;
  
  	if (debuglevel >= DEBUG_LEVEL_INFO)	
  		printk("%s(%d)n_hdlc_tty_write() called count=%Zd\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(&tty->write_wait, &wait);
  
  	for (;;) {
  		set_current_state(TASK_INTERRUPTIBLE);
  	
  		tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
  		if (tbuf)
  			break;
  
  		if (file->f_flags & O_NONBLOCK) {
  			error = -EAGAIN;
  			break;
  		}
  		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(&tty->write_wait, &wait);
  
  	if (!error) {		
  		/* Retrieve the user's buffer */
  		memcpy(tbuf->buf, data, count);
  
  		/* 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.
   * @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)
   *
   * Returns command dependent result.
   */
  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;
  	struct n_hdlc_buf *buf = NULL;
  
  	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);
  		buf = list_first_entry_or_null(&n_hdlc->rx_buf_list.list,
  						struct n_hdlc_buf, list_item);
  		if (buf)
  			count = buf->count;
  		else
  			count = 0;
  		spin_unlock_irqrestore(&n_hdlc->rx_buf_list.spinlock,flags);
  		error = put_user(count, (int __user *)arg);
  		break;
  
  	case TIOCOUTQ:
  		/* get the pending tx byte count in the driver */
  		count = tty_chars_in_buffer(tty);
  		/* add size of next output frame in queue */
  		spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock,flags);
  		buf = list_first_entry_or_null(&n_hdlc->tx_buf_list.list,
  						struct n_hdlc_buf, list_item);
  		if (buf)
  			count += buf->count;
  		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock,flags);
  		error = put_user(count, (int __user *)arg);
  		break;
  
  	case TCFLSH:
  		switch (arg) {
  		case TCIOFLUSH:
  		case TCOFLUSH:
  			flush_tx_queue(tty);
  		}
  		/* fall through to default */
  
  	default:
  		error = n_tty_ioctl_helper(tty, file, cmd, arg);
  		break;
  	}
  	return error;
  	
  }	/* end of n_hdlc_tty_ioctl() */
  
  /**
   * n_hdlc_tty_poll - TTY callback for poll system call
   * @tty - pointer to tty instance data
   * @filp - pointer to open file object for device
   * @poll_table - wait queue for operations
   * 
   * Determine which operations (read/write) will not block and return info
   * to caller.
   * Returns a 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, &tty->read_wait, wait);
  		poll_wait(filp, &tty->write_wait, wait);
  
  		/* set bits for operations that won't block */
  		if (!list_empty(&n_hdlc->rx_buf_list.list))
  			mask |= POLLIN | POLLRDNORM;	/* readable */
  		if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
  			mask |= POLLHUP;
  		if (tty_hung_up_p(filp))
  			mask |= POLLHUP;
  		if (!tty_is_writelocked(tty) &&
  				!list_empty(&n_hdlc->tx_free_buf_list.list))
  			mask |= POLLOUT | POLLWRNORM;	/* writable */
  	}
  	return mask;
  }	/* end of n_hdlc_tty_poll() */
  
  /**
   * n_hdlc_alloc - allocate an n_hdlc instance data structure
   *
   * Returns a pointer to newly created structure if success, otherwise %NULL
   */
  static struct n_hdlc *n_hdlc_alloc(void)
  {
  	struct n_hdlc_buf *buf;
  	int i;
  	struct n_hdlc *n_hdlc = kmalloc(sizeof(*n_hdlc), GFP_KERNEL);
  
  	if (!n_hdlc)
  		return NULL;
  
  	memset(n_hdlc, 0, sizeof(*n_hdlc));
  
  	spin_lock_init(&n_hdlc->rx_free_buf_list.spinlock);
  	spin_lock_init(&n_hdlc->tx_free_buf_list.spinlock);
  	spin_lock_init(&n_hdlc->rx_buf_list.spinlock);
  	spin_lock_init(&n_hdlc->tx_buf_list.spinlock);
  
  	INIT_LIST_HEAD(&n_hdlc->rx_free_buf_list.list);
  	INIT_LIST_HEAD(&n_hdlc->tx_free_buf_list.list);
  	INIT_LIST_HEAD(&n_hdlc->rx_buf_list.list);
  	INIT_LIST_HEAD(&n_hdlc->tx_buf_list.list);
  
  	/* allocate free rx buffer list */
  	for(i=0;i<DEFAULT_RX_BUF_COUNT;i++) {
  		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 = 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;
  	
  	return n_hdlc;
  	
  }	/* end of n_hdlc_alloc() */
  
  /**
   * n_hdlc_buf_return - put the HDLC buffer after the head of the specified list
   * @buf_list - pointer to the buffer list
   * @buf - pointer to the buffer
   */
  static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
  						struct n_hdlc_buf *buf)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&buf_list->spinlock, flags);
  
  	list_add(&buf->list_item, &buf_list->list);
  	buf_list->count++;
  
  	spin_unlock_irqrestore(&buf_list->spinlock, flags);
  }
  
  /**
   * n_hdlc_buf_put - add specified HDLC buffer to tail of specified list
   * @buf_list - pointer to buffer list
   * @buf	- pointer to buffer
   */
  static void n_hdlc_buf_put(struct n_hdlc_buf_list *buf_list,
  			   struct n_hdlc_buf *buf)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&buf_list->spinlock, flags);
  
  	list_add_tail(&buf->list_item, &buf_list->list);
  	buf_list->count++;
  
  	spin_unlock_irqrestore(&buf_list->spinlock, flags);
  }	/* end of n_hdlc_buf_put() */
  
  /**
   * n_hdlc_buf_get - remove and return an HDLC buffer from list
   * @buf_list - pointer to HDLC buffer list
   * 
   * Remove and return an HDLC buffer from the head of the specified HDLC buffer
   * list.
   * Returns a pointer to HDLC buffer if available, otherwise %NULL.
   */
  static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *buf_list)
  {
  	unsigned long flags;
  	struct n_hdlc_buf *buf;
  
  	spin_lock_irqsave(&buf_list->spinlock, flags);
  
  	buf = list_first_entry_or_null(&buf_list->list,
  						struct n_hdlc_buf, list_item);
  	if (buf) {
  		list_del(&buf->list_item);
  		buf_list->count--;
  	}
  
  	spin_unlock_irqrestore(&buf_list->spinlock, flags);
  	return buf;
  }	/* end of n_hdlc_buf_get() */
  
  static char hdlc_banner[] __initdata =
  	KERN_INFO "HDLC line discipline maxframe=%u\n";
  static char hdlc_register_ok[] __initdata =
  	KERN_INFO "N_HDLC line discipline registered.\n";
  static char hdlc_register_fail[] __initdata =
  	KERN_ERR "error registering line discipline: %d\n";
  static char hdlc_init_fail[] __initdata =
  	KERN_INFO "N_HDLC: init failure %d\n";
  
  static int __init n_hdlc_init(void)
  {
  	int status;
  
  	/* range check maxframe arg */
  	if (maxframe < 4096)
  		maxframe = 4096;
  	else if (maxframe > 65535)
  		maxframe = 65535;
  
  	printk(hdlc_banner, maxframe);
  
  	status = tty_register_ldisc(N_HDLC, &n_hdlc_ldisc);
  	if (!status)
  		printk(hdlc_register_ok);
  	else
  		printk(hdlc_register_fail, status);
  
  	if (status)
  		printk(hdlc_init_fail, status);
  	return status;
  	
  }	/* end of init_module() */
  
  static char hdlc_unregister_ok[] __exitdata =
  	KERN_INFO "N_HDLC: line discipline unregistered\n";
  static char hdlc_unregister_fail[] __exitdata =
  	KERN_ERR "N_HDLC: can't unregister line discipline (err = %d)\n";
  
  static void __exit n_hdlc_exit(void)
  {
  	/* Release tty registration of line discipline */
  	int status = tty_unregister_ldisc(N_HDLC);
  
  	if (status)
  		printk(hdlc_unregister_fail, status);
  	else
  		printk(hdlc_unregister_ok);
  }
  
  module_init(n_hdlc_init);
  module_exit(n_hdlc_exit);
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Paul Fulghum paulkf@microgate.com");
  module_param(debuglevel, int, 0);
  module_param(maxframe, int, 0);
  MODULE_ALIAS_LDISC(N_HDLC);
  