Adding upstream version 5.10.209.upstream/5.10.209

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 862 insertions, 0 deletions

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
new file mode 100644
index 000000000..697199a3c
--- /dev/null
+++ b/drivers/tty/n_hdlc.c
@@ -0,0 +1,862 @@
+// 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>
+
+#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>
+#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;
+	int		  count;
+	char		  buf[];
+};
+
+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
+ * @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
+ */
+struct n_hdlc {
+	int			magic;
+	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 (n_hdlc->magic != HDLC_MAGIC) {
+		pr_warn("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;
+
+	/* 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)
+{
+	register int actual;
+	unsigned long flags;
+	struct n_hdlc_buf *tbuf;
+
+check_again:
+
+	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
+	if (n_hdlc->tbusy) {
+		n_hdlc->woke_up = true;
+		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
+		return;
+	}
+	n_hdlc->tbusy = true;
+	n_hdlc->woke_up = false;
+	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
+
+	tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
+	while (tbuf) {
+		pr_debug("sending frame %p, count=%d\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 */
+	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
+	n_hdlc->tbusy = false;
+	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
+
+	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,
+			       char *flags, int count)
+{
+	register struct n_hdlc *n_hdlc = tty->disc_data;
+	register struct n_hdlc_buf *buf;
+
+	pr_debug("%s() called count=%d\n", __func__, count);
+
+	/* verify line is using HDLC discipline */
+	if (n_hdlc->magic != HDLC_MAGIC) {
+		pr_err("line not using HDLC discipline\n");
+		return;
+	}
+
+	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(struct_size(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
+ * @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 *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 unsigned char *data, size_t count)
+{
+	struct n_hdlc *n_hdlc = tty->disc_data;
+	int error = 0;
+	DECLARE_WAITQUEUE(wait, current);
+	struct n_hdlc_buf *tbuf;
+
+	pr_debug("%s() called count=%zd\n", __func__, count);
+
+	if (n_hdlc->magic != HDLC_MAGIC)
+		return -EIO;
+
+	/* 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
+ * @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 = tty->disc_data;
+	int error = 0;
+	int count;
+	unsigned long flags;
+	struct n_hdlc_buf *buf = NULL;
+
+	pr_debug("%s() called %d\n", __func__, cmd);
+
+	/* Verify the status of the device */
+	if (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);
+		}
+		fallthrough;	/* 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
+ * @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;
+
+	if (n_hdlc->magic != HDLC_MAGIC)
+		return 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(struct_size(buf, buf, maxframe), GFP_KERNEL);
+		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(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);
+
+	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");
+
+	/* Initialize the control block */
+	n_hdlc->magic  = HDLC_MAGIC;
+
+	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 struct tty_ldisc_ops n_hdlc_ldisc = {
+	.owner		= THIS_MODULE,
+	.magic		= TTY_LDISC_MAGIC,
+	.name		= "hdlc",
+	.open		= n_hdlc_tty_open,
+	.close		= n_hdlc_tty_close,
+	.read		= n_hdlc_tty_read,
+	.write		= n_hdlc_tty_write,
+	.ioctl		= n_hdlc_tty_ioctl,
+	.poll		= n_hdlc_tty_poll,
+	.receive_buf	= n_hdlc_tty_receive,
+	.write_wakeup	= n_hdlc_tty_wakeup,
+	.flush_buffer   = flush_rx_queue,
+};
+
+static 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, &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)
+{
+	/* Release tty registration of line discipline */
+	int status = tty_unregister_ldisc(N_HDLC);
+
+	if (status)
+		pr_err("N_HDLC: can't unregister line discipline (err = %d)\n",
+				status);
+	else
+		pr_info("N_HDLC: line discipline unregistered\n");
+}
+
+module_init(n_hdlc_init);
+module_exit(n_hdlc_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Paul Fulghum paulkf@microgate.com");
+module_param(maxframe, int, 0);
+MODULE_ALIAS_LDISC(N_HDLC);