Import 2.2.8pre2

[davej-history.git] / drivers / char / 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
 *
 * 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 2-4096 bytes (2 is min HDLC frame
 *    with 1 addr byte and 1 ctrl byte).
 *
 * 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 "1.0"

#include <linux/version.h>
#include <linux/config.h>
#include <linux/module.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))

#if LINUX_VERSION_CODE < VERSION(2,1,14)
#include <linux/ioport.h>
#endif

#if LINUX_VERSION_CODE >= VERSION(2,1,23)
#include <linux/poll.h>
#endif

#include <linux/in.h>
#include <linux/malloc.h>
#include <linux/tty.h>
#include <linux/errno.h>
#include <linux/sched.h>        /* to get the struct task_struct */
#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

#if LINUX_VERSION_CODE >= VERSION(2,1,4)
#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

#if LINUX_VERSION_CODE >= VERSION(2,1,5)
#include <asm/uaccess.h>
#endif

#else  /* 2.0.x and 2.1.x before 2.1.4 */

#define GET_USER(error,value,addr)                                        \
do {                                                                      \
        error = verify_area (VERIFY_READ, (void *) addr, sizeof (value)); \
        if (error == 0)                                                   \
                value = get_user(addr);                                   \
} while (0)

#define COPY_FROM_USER(error,dest,src,size)                               \
do {                                                                      \
        error = verify_area (VERIFY_READ, (void *) src, size);            \
        if (error == 0)                                                   \
                memcpy_fromfs (dest, src, size);                          \
} while (0)

#define PUT_USER(error,value,addr)                                         \
do {                                                                       \
        error = verify_area (VERIFY_WRITE, (void *) addr, sizeof (value)); \
        if (error == 0)                                                    \
                put_user (value, addr);                                    \
} while (0)

#define COPY_TO_USER(error,dest,src,size)                                 \
do {                                                                      \
        error = verify_area (VERIFY_WRITE, (void *) dest, size);                  \
        if (error == 0)                                                   \
                memcpy_tofs (dest, src, size);                            \
} while (0)

#endif

#if LINUX_VERSION_CODE < VERSION(2,1,0)
#define __init
typedef int spinlock_t;
#define spin_lock_irqsave(a,b) {save_flags((b));cli();}
#define spin_unlock_irqrestore(a,b) {restore_flags((b));}
#define spin_lock(a)
#define spin_unlock(a)
#define schedule_timeout(a){current->timeout = jiffies + (a); schedule();}
#endif

#if LINUX_VERSION_CODE < VERSION(2,1,37)
#define test_and_set_bit(nr, addr)      set_bit(nr, addr)
#endif

#if LINUX_VERSION_CODE < VERSION(2,1,57)
#define signal_pending(p)       ((p)->signal & ~(p)->blocked)
#endif

#if LINUX_VERSION_CODE < VERSION(2,1,25)
#define net_device_stats        enet_statistics
#endif

#if LINUX_VERSION_CODE < VERSION(2,1,60)
typedef int             rw_ret_t;
typedef unsigned int    rw_count_t;
#else
typedef ssize_t         rw_ret_t;
typedef size_t          rw_count_t;
#endif

/*
 * Buffers for individual HDLC frames
 */
#define MAX_HDLC_FRAME_SIZE 4096
#define DEFAULT_RX_BUF_COUNT 10
#define MAX_RX_BUF_COUNT 30
#define DEFAULT_TX_BUF_COUNT 1

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

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 */
        struct wait_queue *read_wait;
        struct wait_queue *write_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);

#if LINUX_VERSION_CODE >= VERSION(2,1,19) 
MODULE_PARM(debuglevel, "i");
#endif

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

/* 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);
#if LINUX_VERSION_CODE < VERSION(2,1,23)
static int n_hdlc_tty_select (struct tty_struct *tty, struct inode *inode,
                      struct file *filp, int sel_type, select_table * wait);
#else
static unsigned int n_hdlc_tty_poll (struct tty_struct *tty, struct file *filp,
                                  poll_table * wait);
#endif
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->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;
                }
                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\n",__FILE__,__LINE__);
                
        /* 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;
        
        /* 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__);
                
        save_flags(flags);
        cli ();
        if (n_hdlc->tbusy) {
                n_hdlc->woke_up = 1;
                restore_flags(flags);
                return;
        }
        n_hdlc->tbusy = 1;
        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 */
                n_hdlc->woke_up = 0;
                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);
        
                        /* 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 */
                        
                        /* check if wake up code called since last write call */
                        if (n_hdlc->woke_up)
                                continue;
                                
                        /* 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 (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;
        }
        
        if (!n_hdlc->tbuf)
                tty->flags &= ~(1 << TTY_DO_WRITE_WAKEUP);
        else
                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;
        }
        
        /* 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(sizeof(N_HDLC_BUF),GFP_ATOMIC);
        }
        
        if (!buf) {
                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);
        if (n_hdlc->tty->fasync != NULL)
                kill_fasync (n_hdlc->tty->fasync, SIGIO);

}       /* 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;
                        
                /* TODO: no timeout? current->timeout = 0;*/
                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;
        struct wait_queue wait = {current, NULL};
        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 > MAX_HDLC_FRAME_SIZE) {
                if (debuglevel & DEBUG_LEVEL_INFO)
                        printk (KERN_WARNING
                                "n_hdlc_tty_write: truncating user packet "
                                "from %lu to %d\n", (unsigned long) count,
                                MAX_HDLC_FRAME_SIZE);
                count = MAX_HDLC_FRAME_SIZE;
        }
        
        /* Allocate transmit buffer */
        tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
        if (!tbuf) {
                /* sleep until transmit buffer available */             
                add_wait_queue(&n_hdlc->write_wait, &wait);
                while (!tbuf) {
                        /* TODO: no timeout? current->timeout = 0;*/
                        current->state = TASK_INTERRUPTIBLE;
                        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;
                        }
                        
                        tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
                }
                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;
        
        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) */
                        int count;
                        unsigned long flags;
                        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;
                
        default:
                error = n_tty_ioctl (tty, file, cmd, arg);
                break;
        }
        return error;
        
}       /* end of n_hdlc_tty_ioctl() */

#if LINUX_VERSION_CODE < VERSION(2,1,23)
/* n_hdlc_tty_select()
 * 
 *      Device select method. Determine if operation requires
 *      blocking and if so put appropriate wait queue in select
 *      table and return 0, otherwise return 1.
 *      
 * Arguments:
 * 
 *      tty             pointer to tty device instance data
 *      inode           pointer to inode for device
 *      filp            pointer to file object
 *      sel_type        identified the select type (read/write/exception)
 *      wait            select table for adding wait queue if appropriate
 *      
 * Return Value:
 * 
 *      1 if no need to block on operation
 *      0 if must block and wait queue added to select table
 */
static int n_hdlc_tty_select (struct tty_struct *tty, struct inode *inode,
                struct file *filp, int sel_type, select_table * wait)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
        int result = 1;
        
        if (debuglevel >= DEBUG_LEVEL_INFO)     
                printk("%s(%d)n_hdlc_tty_select() called\n",__FILE__,__LINE__);
                
        /* Verify the status of the device */
        if (!n_hdlc)
                return -EBADF;

        if (n_hdlc->magic != HDLC_MAGIC || tty != n_hdlc->tty)
                return -EBADF;

        switch (sel_type) {
        case SEL_IN:
                if (n_hdlc->rx_buf_list.head)
                        break;

        case SEL_EX:    /* Exceptions or read errors */
                /* Is this a pty link and the remote disconnected? */
                if (tty->flags & (1 << TTY_OTHER_CLOSED))
                        break;

                /* Is this a local link and the modem disconnected? */
                if (tty_hung_up_p (filp))
                        break;

                select_wait (&n_hdlc->read_wait, wait);
                result = 0;
                break;
                
        /* Write mode. A write is allowed if there is no current transmission */
        case SEL_OUT:
                if (!n_hdlc->tx_free_buf_list.head) {
                        select_wait (&n_hdlc->write_wait, wait);
                        result = 0;
                }
                break;
        }
        return result;
}       /* end of n_hdlc_tty_select() */

#else   /* 2.1.23 or later */

/* 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) */
#if LINUX_VERSION_CODE < VERSION(2,1,89)
                poll_wait(&n_hdlc->read_wait, wait);
                poll_wait(&n_hdlc->write_wait, wait);
#else
                poll_wait(filp, &n_hdlc->read_wait, wait);
                poll_wait(filp, &n_hdlc->write_wait, wait);
#endif
                /* 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() */

#endif

/* 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(sizeof(N_HDLC_BUF),GFP_KERNEL);
                if (buf)
                        n_hdlc_buf_put(&n_hdlc->rx_free_buf_list,buf);
        }
        
        /* allocate free rx buffer list */
        for(i=0;i<DEFAULT_TX_BUF_COUNT;i++) {
                buf = (N_HDLC_BUF*)kmalloc(sizeof(N_HDLC_BUF),GFP_KERNEL);
                if (buf)
                        n_hdlc_buf_put(&n_hdlc->tx_free_buf_list,buf);
        }
        
        /* Initialize the control block */
        n_hdlc->magic  = HDLC_MAGIC;

        n_hdlc->flags  = 0;
        n_hdlc->read_wait  = NULL;
        n_hdlc->write_wait = NULL;
        
        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));
                
}       /* 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() */

/* init_module()
 *
 *      called when module is loading to register line discipline
 *      
 * Arguments:           None
 * Return Value:        0 if success, otherwise error code
 */
int init_module(void)
{
        static struct tty_ldisc n_hdlc_ldisc;
        int    status;

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

        /* Register the tty discipline */
        
        memset(&n_hdlc_ldisc, 0, sizeof (n_hdlc_ldisc));
        n_hdlc_ldisc.magic              = TTY_LDISC_MAGIC;
#if LINUX_VERSION_CODE >= VERSION(2,1,28)
        n_hdlc_ldisc.name               = "hdlc";
#endif
        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;
#if LINUX_VERSION_CODE < VERSION(2,1,23)
        n_hdlc_ldisc.select             = n_hdlc_tty_select;
#else
        n_hdlc_ldisc.poll               = n_hdlc_tty_poll;
#endif
        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() */

/* cleanup_module()
 *
 *      called when module is unloading to unregister line discipline
 *      
 * Arguments:           None
 * Return Value:        None
 */
void cleanup_module(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");
}