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n_hdlc.c

/* 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 4.8 2003/05/06 21:18:51 paulkf 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 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
#define HDLC_VERSION "$Revision: 4.8 $"

#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/system.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 1

struct n_hdlc_buf {
      struct n_hdlc_buf *link;
      int           count;
      char          buf[1];
};

#define     N_HDLC_BUF_SIZE   (sizeof(struct n_hdlc_buf) + maxframe)

struct n_hdlc_buf_list {
      struct n_hdlc_buf *head;
      struct n_hdlc_buf *tail;
      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
 * @tbuf - currently transmitting tx buffer
 * @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
 */
00148 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 *tbuf;
      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_list_init(struct n_hdlc_buf_list *list);
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 struct tty_ldisc 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,
};

/**
 * 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->tbuf);
      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 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
 * @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);

      /* 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, tbuf->buf, tbuf->count);

            /* rollback was possible and has been done */
            if (actual == -ERESTARTSYS) {
                  n_hdlc->tbuf = 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);
                  
                  /* this tx buffer is done */
                  n_hdlc->tbuf = NULL;
                  
                  /* 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);
                              
                  /* 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 */
      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) {
            tty->flags &= ~(1 << TTY_DO_WRITE_WAKEUP);
            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 == 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 = 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;
      struct 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 */
      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;
      }

      for (;;) {
            if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                  return -EIO;

            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 (&tty->read_wait);
            if (signal_pending(current))
                  return -EINTR;
      }
            
      if (rbuf->count > nr)
            /* frame too large for caller's buffer (discard frame) */
            ret = -EOVERFLOW;
      else {
            /* Copy the data to the caller's buffer */
            if (copy_to_user(buf, rbuf->buf, rbuf->count))
                  ret = -EFAULT;
            else
                  ret = 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
 * @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);
      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(&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;
      
      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);
            error = put_user(count, (int __user *)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);
            error = put_user(count, (int __user *)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
 * @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 (n_hdlc->rx_buf_list.head)
                  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) &&
                        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
 *
 * 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));

      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 = 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_list_init - initialize specified HDLC buffer list
 * @list - pointer to buffer list
 */
static void n_hdlc_buf_list_init(struct n_hdlc_buf_list *list)
{
      memset(list, 0, sizeof(*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
 * @list - pointer to buffer list
 * @buf     - pointer to buffer
 */
static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
                     struct 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 list
 * @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 *list)
{
      unsigned long flags;
      struct 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 char hdlc_banner[] __initdata =
      KERN_INFO "HDLC line discipline: version " HDLC_VERSION
      ", 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);

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