n_hdlc.c

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/* generic HDLC line discipline for Linux
 *
 * Written by Paul Fulghum [email protected]
 * for Microgate Corporation
 *
 * Microgate and SyncLink are registered trademarks of Microgate Corporation
 *
 * Adapted from ppp.c, written by Michael Callahan <[email protected]>,
 *  Al Longyear <[email protected]>,
 *  Paul Mackerras <[email protected]>
 *
 * 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 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 {
    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 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;
    unsigned long flags;

    while ((buf = n_hdlc_buf_get(&n_hdlc->tx_buf_list)))
        n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, buf);
    spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
    if (n_hdlc->tbuf) {
        n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, n_hdlc->tbuf);
        n_hdlc->tbuf = NULL;
    }
    spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
}

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,
};

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() */

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() */

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() */

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 */
        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->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)
        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() */

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() */

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() */

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() */

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() */

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_chars_in_buffer(tty);
        /* 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;

    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() */

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() */

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() */

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() */

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() */

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 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 [email protected]");
module_param(debuglevel, int, 0);
module_param(maxframe, int, 0);
MODULE_ALIAS_LDISC(N_HDLC);