// 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.
 */

#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>

#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 <linux/uaccess.h>
#include "tty.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;
        size_t            count;
        u8                buf[];
};

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

/**
 * struct n_hdlc - per device instance data structure
 * @tbusy: reentrancy flag for tx wakeup code
 * @woke_up: tx wakeup needs to be run again as it was called while @tbusy
 * @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
 * @write_work: work struct for deferred frame transmission
 * @tty_for_write_work: pointer to tty instance used by the @write_work
 */
struct n_hdlc {
        bool                    tbusy;
        bool                    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;
        struct work_struct      write_work;
        struct tty_struct       *tty_for_write_work;
};

/*
 * 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);
static void n_hdlc_tty_write_work(struct work_struct *work);

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

static void flush_rx_queue(struct tty_struct *tty)
{
        struct n_hdlc *n_hdlc = tty->disc_data;
        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 = tty->disc_data;
        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 void n_hdlc_free_buf_list(struct n_hdlc_buf_list *list)
{
        struct n_hdlc_buf *buf;

        do {
                buf = n_hdlc_buf_get(list);
                kfree(buf);
        } while (buf);
}

/**
 * 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 = tty->disc_data;

#if defined(TTY_NO_WRITE_SPLIT)
        clear_bit(TTY_NO_WRITE_SPLIT, &tty->flags);
#endif
        tty->disc_data = NULL;

        /* Ensure that the n_hdlcd process is not hanging on select()/poll() */
        wake_up_interruptible(&tty->read_wait);
        wake_up_interruptible(&tty->write_wait);

        cancel_work_sync(&n_hdlc->write_work);

        n_hdlc_free_buf_list(&n_hdlc->rx_free_buf_list);
        n_hdlc_free_buf_list(&n_hdlc->tx_free_buf_list);
        n_hdlc_free_buf_list(&n_hdlc->rx_buf_list);
        n_hdlc_free_buf_list(&n_hdlc->tx_buf_list);
        kfree(n_hdlc);
}       /* 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 = tty->disc_data;

        pr_debug("%s() called (device=%s)\n", __func__, tty->name);

        /* There should not be an existing table for this slot. */
        if (n_hdlc) {
                pr_err("%s: tty already associated!\n", __func__);
                return -EEXIST;
        }

        n_hdlc = n_hdlc_alloc();
        if (!n_hdlc) {
                pr_err("%s: n_hdlc_alloc failed\n", __func__);
                return -ENFILE;
        }

        INIT_WORK(&n_hdlc->write_work, n_hdlc_tty_write_work);
        n_hdlc->tty_for_write_work = tty;
        tty->disc_data = n_hdlc;
        tty->receive_room = 65536;

        /* change tty_io write() to not split large writes into 8K chunks */
        set_bit(TTY_NO_WRITE_SPLIT, &tty->flags);

        /* flush receive data from driver */
        tty_driver_flush_buffer(tty);

        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)
{
        struct n_hdlc_buf *tbuf;
        ssize_t actual;

check_again:
        scoped_guard(spinlock_irqsave, &n_hdlc->tx_buf_list.spinlock) {
                if (n_hdlc->tbusy) {
                        n_hdlc->woke_up = true;
                        return;
                }
                n_hdlc->tbusy = true;
                n_hdlc->woke_up = false;
        }

        tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
        while (tbuf) {
                pr_debug("sending frame %p, count=%zu\n", 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) {
                        pr_debug("frame %p completed\n", 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 {
                        pr_debug("frame %p pending\n", 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 */
        scoped_guard(spinlock_irqsave, &n_hdlc->tx_buf_list.spinlock)
                n_hdlc->tbusy = false;

        if (n_hdlc->woke_up)
                goto check_again;
}       /* end of n_hdlc_send_frames() */

/**
 * n_hdlc_tty_write_work - Asynchronous callback for transmit wakeup
 * @work: pointer to work_struct
 *
 * Called when low level device driver can accept more send data.
 */
static void n_hdlc_tty_write_work(struct work_struct *work)
{
        struct n_hdlc *n_hdlc = container_of(work, struct n_hdlc, write_work);
        struct tty_struct *tty = n_hdlc->tty_for_write_work;

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

/**
 * 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 = tty->disc_data;

        schedule_work(&n_hdlc->write_work);
}       /* 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,
                               const u8 *flags, size_t count)
{
        register struct n_hdlc *n_hdlc = tty->disc_data;
        register struct n_hdlc_buf *buf;

        pr_debug("%s() called count=%zu\n", __func__, count);

        if (count > maxframe) {
                pr_debug("rx count>maxframesize, data discarded\n");
                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_flex(*buf, buf, maxframe, GFP_ATOMIC);
        }

        if (!buf) {
                pr_debug("no more rx buffers, data discarded\n");
                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 (tty->fasync != NULL)
                kill_fasync(&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
 * @kbuf: pointer to returned data buffer
 * @nr: size of returned data buffer
 * @cookie: stored rbuf from previous run
 * @offset: offset into the 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 *kbuf, size_t nr, void **cookie,
                               unsigned long offset)
{
        struct n_hdlc *n_hdlc = tty->disc_data;
        int ret = 0;
        struct n_hdlc_buf *rbuf;
        DECLARE_WAITQUEUE(wait, current);

        /* Is this a repeated call for an rbuf we already found earlier? */
        rbuf = *cookie;
        if (rbuf)
                goto have_rbuf;

        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)
                        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);

        if (!rbuf)
                return ret;
        *cookie = rbuf;

have_rbuf:
        /* Have we used it up entirely? */
        if (offset >= rbuf->count)
                goto done_with_rbuf;

        /* More data to go, but can't copy any more? EOVERFLOW */
        ret = -EOVERFLOW;
        if (!nr)
                goto done_with_rbuf;

        /* Copy as much data as possible */
        ret = rbuf->count - offset;
        if (ret > nr)
                ret = nr;
        memcpy(kbuf, rbuf->buf+offset, ret);
        offset += ret;

        /* If we still have data left, we leave the rbuf in the cookie */
        if (offset < rbuf->count)
                return ret;

done_with_rbuf:
        *cookie = NULL;

        if (n_hdlc->rx_free_buf_list.count > DEFAULT_RX_BUF_COUNT)
                kfree(rbuf);
        else
                n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, rbuf);

        return ret;

}       /* end of n_hdlc_tty_read() */

/**
 * n_hdlc_tty_write - write a single frame of data to device
 * @tty: pointer to associated tty device instance data
 * @file: pointer to file object data
 * @data: pointer to transmit data (one frame)
 * @count: size of transmit frame in bytes
 *
 * Returns the number of bytes written (or error code).
 */
static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
                                const u8 *data, size_t count)
{
        struct n_hdlc *n_hdlc = tty->disc_data;
        DECLARE_WAITQUEUE(wait, current);
        struct n_hdlc_buf *tbuf;
        ssize_t error = 0;

        pr_debug("%s() called count=%zd\n", __func__, count);

        /* verify frame size */
        if (count > maxframe) {
                pr_debug("%s: truncating user packet from %zu to %d\n",
                                __func__, 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();

                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
 * @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, unsigned int cmd,
                            unsigned long arg)
{
        struct n_hdlc *n_hdlc = tty->disc_data;
        int count;
        struct n_hdlc_buf *buf = NULL;

        pr_debug("%s() called %d\n", __func__, cmd);

        switch (cmd) {
        case FIONREAD:
                /* report count of read data available */
                /* in next available frame (if any) */
                scoped_guard(spinlock_irqsave, &n_hdlc->rx_buf_list.spinlock) {
                        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;
                }
                return put_user(count, (int __user *)arg);

        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 */
                scoped_guard(spinlock_irqsave, &n_hdlc->tx_buf_list.spinlock) {
                        buf = list_first_entry_or_null(&n_hdlc->tx_buf_list.list,
                                                       struct n_hdlc_buf, list_item);
                        if (buf)
                                count += buf->count;
                }
                return put_user(count, (int __user *)arg);

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

        default:
                return n_tty_ioctl_helper(tty, cmd, arg);
        }
}       /* 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
 * @wait: 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 = tty->disc_data;
        __poll_t mask = 0;

        /*
         * queue the 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() */

static void n_hdlc_alloc_buf(struct n_hdlc_buf_list *list, unsigned int count,
                const char *name)
{
        struct n_hdlc_buf *buf;
        unsigned int i;

        for (i = 0; i < count; i++) {
                buf = kmalloc_flex(*buf, buf, maxframe);
                if (!buf) {
                        pr_debug("%s(), kmalloc() failed for %s buffer %u\n",
                                        __func__, name, i);
                        return;
                }
                n_hdlc_buf_put(list, buf);
        }
}

/**
 * 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 *n_hdlc = kzalloc_obj(*n_hdlc);

        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);

        n_hdlc_alloc_buf(&n_hdlc->rx_free_buf_list, DEFAULT_RX_BUF_COUNT, "rx");
        n_hdlc_alloc_buf(&n_hdlc->tx_free_buf_list, DEFAULT_TX_BUF_COUNT, "tx");

        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)
{
        guard(spinlock_irqsave)(&buf_list->spinlock);

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

/**
 * 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)
{
        guard(spinlock_irqsave)(&buf_list->spinlock);

        list_add_tail(&buf->list_item, &buf_list->list);
        buf_list->count++;
}       /* 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)
{
        struct n_hdlc_buf *buf;

        guard(spinlock_irqsave)(&buf_list->spinlock);

        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--;
        }

        return buf;
}       /* end of n_hdlc_buf_get() */

static struct tty_ldisc_ops n_hdlc_ldisc = {
        .owner          = THIS_MODULE,
        .num            = N_HDLC,
        .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 int __init n_hdlc_init(void)
{
        int status;

        /* range check maxframe arg */
        maxframe = clamp(maxframe, 4096, MAX_HDLC_FRAME_SIZE);

        status = tty_register_ldisc(&n_hdlc_ldisc);
        if (!status)
                pr_info("N_HDLC line discipline registered with maxframe=%d\n",
                                maxframe);
        else
                pr_err("N_HDLC: error registering line discipline: %d\n",
                                status);

        return status;

}       /* end of init_module() */

static void __exit n_hdlc_exit(void)
{
        tty_unregister_ldisc(&n_hdlc_ldisc);
}

module_init(n_hdlc_init);
module_exit(n_hdlc_exit);

MODULE_DESCRIPTION("HDLC line discipline support");
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Paul Fulghum paulkf@microgate.com");
module_param(maxframe, int, 0);
MODULE_ALIAS_LDISC(N_HDLC);