// SPDX-License-Identifier: GPL-1.0+
/* 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 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 <linux/uaccess.h>

/*
 * Buffers for individual HDLC frames
 */
#define MAX_HDLC_FRAME_SIZE 65535 
#define DEFAULT_RX_BUF_COUNT 10
#define MAX_RX_BUF_COUNT 60
#define DEFAULT_TX_BUF_COUNT 3

struct n_hdlc_buf {
        struct list_head  list_item;
        int               count;
        char              buf[1];
};

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

struct n_hdlc_buf_list {
        struct list_head  list;
        int               count;
        spinlock_t        spinlock;
};

/**
 * struct n_hdlc - per device instance data structure
 * @magic - magic value for structure
 * @flags - miscellaneous control flags
 * @tty - ptr to TTY structure
 * @backup_tty - TTY to use if tty gets closed
 * @tbusy - reentrancy flag for tx wakeup code
 * @woke_up - FIXME: describe this field
 * @tx_buf_list - list of pending transmit frame buffers
 * @rx_buf_list - list of received frame buffers
 * @tx_free_buf_list - list unused transmit frame buffers
 * @rx_free_buf_list - list unused received frame buffers
 */
struct n_hdlc {
        int                     magic;
        __u32                   flags;
        struct tty_struct       *tty;
        struct tty_struct       *backup_tty;
        int                     tbusy;
        int                     woke_up;
        struct n_hdlc_buf_list  tx_buf_list;
        struct n_hdlc_buf_list  rx_buf_list;
        struct n_hdlc_buf_list  tx_free_buf_list;
        struct n_hdlc_buf_list  rx_free_buf_list;
};

/*
 * HDLC buffer list manipulation functions
 */
static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
                                                struct n_hdlc_buf *buf);
static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
                           struct n_hdlc_buf *buf);
static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *list);

/* Local functions */

static struct n_hdlc *n_hdlc_alloc (void);

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

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

/* TTY callbacks */

static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
                           __u8 __user *buf, size_t nr);
static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
                            const unsigned char *buf, size_t nr);
static int n_hdlc_tty_ioctl(struct tty_struct *tty, struct file *file,
                            unsigned int cmd, unsigned long arg);
static __poll_t n_hdlc_tty_poll(struct tty_struct *tty, struct file *filp,
                                    poll_table *wait);
static int n_hdlc_tty_open(struct tty_struct *tty);
static void n_hdlc_tty_close(struct tty_struct *tty);
static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *cp,
                               char *fp, int count);
static void n_hdlc_tty_wakeup(struct tty_struct *tty);

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

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

static void flush_rx_queue(struct tty_struct *tty)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
        struct n_hdlc_buf *buf;

        while ((buf = n_hdlc_buf_get(&n_hdlc->rx_buf_list)))
                n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, buf);
}

static void flush_tx_queue(struct tty_struct *tty)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
        struct n_hdlc_buf *buf;

        while ((buf = n_hdlc_buf_get(&n_hdlc->tx_buf_list)))
                n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, buf);
}

static struct tty_ldisc_ops n_hdlc_ldisc = {
        .owner          = THIS_MODULE,
        .magic          = TTY_LDISC_MAGIC,
        .name           = "hdlc",
        .open           = n_hdlc_tty_open,
        .close          = n_hdlc_tty_close,
        .read           = n_hdlc_tty_read,
        .write          = n_hdlc_tty_write,
        .ioctl          = n_hdlc_tty_ioctl,
        .poll           = n_hdlc_tty_poll,
        .receive_buf    = n_hdlc_tty_receive,
        .write_wakeup   = n_hdlc_tty_wakeup,
        .flush_buffer   = flush_rx_queue,
};

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

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

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

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

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

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

        if (debuglevel >= DEBUG_LEVEL_INFO)     
                printk("%s(%d)n_hdlc_tty_open() called (device=%s)\n",
                __FILE__,__LINE__,
                tty->name);
                
        /* There should not be an existing table for this slot. */
        if (n_hdlc) {
                printk (KERN_ERR"n_hdlc_tty_open:tty already associated!\n" );
                return -EEXIST;
        }
        
        n_hdlc = n_hdlc_alloc();
        if (!n_hdlc) {
                printk (KERN_ERR "n_hdlc_alloc failed\n");
                return -ENFILE;
        }
                
        tty->disc_data = n_hdlc;
        n_hdlc->tty    = tty;
        tty->receive_room = 65536;
        
#if defined(TTY_NO_WRITE_SPLIT)
        /* change tty_io write() to not split large writes into 8K chunks */
        set_bit(TTY_NO_WRITE_SPLIT,&tty->flags);
#endif
        
        /* flush receive data from driver */
        tty_driver_flush_buffer(tty);
                
        if (debuglevel >= DEBUG_LEVEL_INFO)     
                printk("%s(%d)n_hdlc_tty_open() success\n",__FILE__,__LINE__);
                
        return 0;
        
}       /* end of n_tty_hdlc_open() */

/**
 * n_hdlc_send_frames - send frames on pending send buffer list
 * @n_hdlc - pointer to ldisc instance data
 * @tty - pointer to tty instance data
 *
 * Send frames on pending send buffer list until the driver does not accept a
 * frame (busy) this function is called after adding a frame to the send buffer
 * list and by the tty wakeup callback.
 */
static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
{
        register int actual;
        unsigned long flags;
        struct n_hdlc_buf *tbuf;

        if (debuglevel >= DEBUG_LEVEL_INFO)     
                printk("%s(%d)n_hdlc_send_frames() called\n",__FILE__,__LINE__);
 check_again:
                
        spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
        if (n_hdlc->tbusy) {
                n_hdlc->woke_up = 1;
                spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
                return;
        }
        n_hdlc->tbusy = 1;
        n_hdlc->woke_up = 0;
        spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);

        tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
        while (tbuf) {
                if (debuglevel >= DEBUG_LEVEL_INFO)     
                        printk("%s(%d)sending frame %p, count=%d\n",
                                __FILE__,__LINE__,tbuf,tbuf->count);
                        
                /* Send the next block of data to device */
                set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
                actual = tty->ops->write(tty, tbuf->buf, tbuf->count);

                /* rollback was possible and has been done */
                if (actual == -ERESTARTSYS) {
                        n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
                        break;
                }
                /* if transmit error, throw frame away by */
                /* pretending it was accepted by driver */
                if (actual < 0)
                        actual = tbuf->count;
                
                if (actual == tbuf->count) {
                        if (debuglevel >= DEBUG_LEVEL_INFO)     
                                printk("%s(%d)frame %p completed\n",
                                        __FILE__,__LINE__,tbuf);
                                        
                        /* free current transmit buffer */
                        n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, tbuf);

                        /* wait up sleeping writers */
                        wake_up_interruptible(&tty->write_wait);
        
                        /* get next pending transmit buffer */
                        tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
                } else {
                        if (debuglevel >= DEBUG_LEVEL_INFO)     
                                printk("%s(%d)frame %p pending\n",
                                        __FILE__,__LINE__,tbuf);

                        /*
                         * the buffer was not accepted by driver,
                         * return it back into tx queue
                         */
                        n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
                        break;
                }
        }
        
        if (!tbuf)
                clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
        
        /* Clear the re-entry flag */
        spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
        n_hdlc->tbusy = 0;
        spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags); 
        
        if (n_hdlc->woke_up)
          goto check_again;

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

/**
 * n_hdlc_tty_wakeup - Callback for transmit wakeup
 * @tty - pointer to associated tty instance data
 *
 * Called when low level device driver can accept more send data.
 */
static void n_hdlc_tty_wakeup(struct tty_struct *tty)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc(tty);

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

        if (tty != n_hdlc->tty) {
                clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
                return;
        }

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

/**
 * n_hdlc_tty_receive - Called by tty driver when receive data is available
 * @tty - pointer to tty instance data
 * @data - pointer to received data
 * @flags - pointer to flags for data
 * @count - count of received data in bytes
 *
 * Called by tty low level driver when receive data is available. Data is
 * interpreted as one HDLC frame.
 */
static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
                               char *flags, int count)
{
        register struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
        register struct n_hdlc_buf *buf;

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

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

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

}       /* end of n_hdlc_tty_receive() */

/**
 * n_hdlc_tty_read - Called to retrieve one frame of data (if available)
 * @tty - pointer to tty instance data
 * @file - pointer to open file object
 * @buf - pointer to returned data buffer
 * @nr - size of returned data buffer
 *      
 * Returns the number of bytes returned or error code.
 */
static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
                           __u8 __user *buf, size_t nr)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc(tty);
        int ret = 0;
        struct n_hdlc_buf *rbuf;
        DECLARE_WAITQUEUE(wait, current);

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

        /* verify user access to buffer */
        if (!access_ok(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 {
                                __set_current_state(TASK_RUNNING);
                                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 (tty_io_nonblock(tty, file)) {
                        ret = -EAGAIN;
                        break;
                }

                schedule();

                if (signal_pending(current)) {
                        ret = -EINTR;
                        break;
                }
        }

        remove_wait_queue(&tty->read_wait, &wait);
        __set_current_state(TASK_RUNNING);

        return ret;
        
}       /* end of n_hdlc_tty_read() */

/**
 * n_hdlc_tty_write - write a single frame of data to device
 * @tty - pointer to associated tty device instance data
 * @file - pointer to file object data
 * @data - pointer to transmit data (one frame)
 * @count - size of transmit frame in bytes
 *              
 * Returns the number of bytes written (or error code).
 */
static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
                            const unsigned char *data, size_t count)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
        int error = 0;
        DECLARE_WAITQUEUE(wait, current);
        struct n_hdlc_buf *tbuf;

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

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

        /* verify frame size */
        if (count > maxframe ) {
                if (debuglevel & DEBUG_LEVEL_INFO)
                        printk (KERN_WARNING
                                "n_hdlc_tty_write: truncating user packet "
                                "from %lu to %d\n", (unsigned long) count,
                                maxframe );
                count = maxframe;
        }
        
        add_wait_queue(&tty->write_wait, &wait);

        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);
        
                tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
                if (tbuf)
                        break;

                if (tty_io_nonblock(tty, file)) {
                        error = -EAGAIN;
                        break;
                }
                schedule();
                        
                n_hdlc = tty2n_hdlc (tty);
                if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC || 
                    tty != n_hdlc->tty) {
                        printk("n_hdlc_tty_write: %p invalid after wait!\n", n_hdlc);
                        error = -EIO;
                        break;
                }
                        
                if (signal_pending(current)) {
                        error = -EINTR;
                        break;
                }
        }

        __set_current_state(TASK_RUNNING);
        remove_wait_queue(&tty->write_wait, &wait);

        if (!error) {           
                /* Retrieve the user's buffer */
                memcpy(tbuf->buf, data, count);

                /* Send the data */
                tbuf->count = error = count;
                n_hdlc_buf_put(&n_hdlc->tx_buf_list,tbuf);
                n_hdlc_send_frames(n_hdlc,tty);
        }

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

/**
 * n_hdlc_tty_ioctl - process IOCTL system call for the tty device.
 * @tty - pointer to tty instance data
 * @file - pointer to open file object for device
 * @cmd - IOCTL command code
 * @arg - argument for IOCTL call (cmd dependent)
 *
 * Returns command dependent result.
 */
static int n_hdlc_tty_ioctl(struct tty_struct *tty, struct file *file,
                            unsigned int cmd, unsigned long arg)
{
        struct n_hdlc *n_hdlc = tty2n_hdlc (tty);
        int error = 0;
        int count;
        unsigned long flags;
        struct n_hdlc_buf *buf = NULL;

        if (debuglevel >= DEBUG_LEVEL_INFO)     
                printk("%s(%d)n_hdlc_tty_ioctl() called %d\n",
                        __FILE__,__LINE__,cmd);
                
        /* Verify the status of the device */
        if (!n_hdlc || n_hdlc->magic != HDLC_MAGIC)
                return -EBADF;

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

        case TIOCOUTQ:
                /* get the pending tx byte count in the driver */
                count = tty_chars_in_buffer(tty);
                /* add size of next output frame in queue */
                spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock,flags);
                buf = list_first_entry_or_null(&n_hdlc->tx_buf_list.list,
                                                struct n_hdlc_buf, list_item);
                if (buf)
                        count += buf->count;
                spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock,flags);
                error = put_user(count, (int __user *)arg);
                break;

        case TCFLSH:
                switch (arg) {
                case TCIOFLUSH:
                case TCOFLUSH:
                        flush_tx_queue(tty);
                }
                /* fall through - to default */

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

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

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

                poll_wait(filp, &tty->read_wait, wait);
                poll_wait(filp, &tty->write_wait, wait);

                /* set bits for operations that won't block */
                if (!list_empty(&n_hdlc->rx_buf_list.list))
                        mask |= EPOLLIN | EPOLLRDNORM;  /* readable */
                if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                        mask |= EPOLLHUP;
                if (tty_hung_up_p(filp))
                        mask |= EPOLLHUP;
                if (!tty_is_writelocked(tty) &&
                                !list_empty(&n_hdlc->tx_free_buf_list.list))
                        mask |= EPOLLOUT | EPOLLWRNORM; /* 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 = kzalloc(sizeof(*n_hdlc), GFP_KERNEL);

        if (!n_hdlc)
                return NULL;

        spin_lock_init(&n_hdlc->rx_free_buf_list.spinlock);
        spin_lock_init(&n_hdlc->tx_free_buf_list.spinlock);
        spin_lock_init(&n_hdlc->rx_buf_list.spinlock);
        spin_lock_init(&n_hdlc->tx_buf_list.spinlock);

        INIT_LIST_HEAD(&n_hdlc->rx_free_buf_list.list);
        INIT_LIST_HEAD(&n_hdlc->tx_free_buf_list.list);
        INIT_LIST_HEAD(&n_hdlc->rx_buf_list.list);
        INIT_LIST_HEAD(&n_hdlc->tx_buf_list.list);

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

/**
 * n_hdlc_buf_return - put the HDLC buffer after the head of the specified list
 * @buf_list - pointer to the buffer list
 * @buf - pointer to the buffer
 */
static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
                                                struct n_hdlc_buf *buf)
{
        unsigned long flags;

        spin_lock_irqsave(&buf_list->spinlock, flags);

        list_add(&buf->list_item, &buf_list->list);
        buf_list->count++;

        spin_unlock_irqrestore(&buf_list->spinlock, flags);
}

/**
 * n_hdlc_buf_put - add specified HDLC buffer to tail of specified list
 * @buf_list - pointer to buffer list
 * @buf - pointer to buffer
 */
static void n_hdlc_buf_put(struct n_hdlc_buf_list *buf_list,
                           struct n_hdlc_buf *buf)
{
        unsigned long flags;

        spin_lock_irqsave(&buf_list->spinlock, flags);

        list_add_tail(&buf->list_item, &buf_list->list);
        buf_list->count++;

        spin_unlock_irqrestore(&buf_list->spinlock, flags);
}       /* end of n_hdlc_buf_put() */

/**
 * n_hdlc_buf_get - remove and return an HDLC buffer from list
 * @buf_list - pointer to HDLC buffer list
 * 
 * Remove and return an HDLC buffer from the head of the specified HDLC buffer
 * list.
 * Returns a pointer to HDLC buffer if available, otherwise %NULL.
 */
static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *buf_list)
{
        unsigned long flags;
        struct n_hdlc_buf *buf;

        spin_lock_irqsave(&buf_list->spinlock, flags);

        buf = list_first_entry_or_null(&buf_list->list,
                                                struct n_hdlc_buf, list_item);
        if (buf) {
                list_del(&buf->list_item);
                buf_list->count--;
        }

        spin_unlock_irqrestore(&buf_list->spinlock, flags);
        return buf;
}       /* end of n_hdlc_buf_get() */

static const char hdlc_banner[] __initconst =
        KERN_INFO "HDLC line discipline maxframe=%u\n";
static const char hdlc_register_ok[] __initconst =
        KERN_INFO "N_HDLC line discipline registered.\n";
static const char hdlc_register_fail[] __initconst =
        KERN_ERR "error registering line discipline: %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);

        return status;
        
}       /* end of init_module() */

static const char hdlc_unregister_ok[] __exitdata =
        KERN_INFO "N_HDLC: line discipline unregistered\n";
static const 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);