Adding upstream version 5.10.209.upstream/5.10.209

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

1 files changed, 2555 insertions, 0 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
new file mode 100644
index 000000000..8e7931d93
--- /dev/null
+++ b/drivers/tty/n_tty.c
@@ -0,0 +1,2555 @@
+// SPDX-License-Identifier: GPL-1.0+
+/*
+ * n_tty.c --- implements the N_TTY line discipline.
+ *
+ * This code used to be in tty_io.c, but things are getting hairy
+ * enough that it made sense to split things off.  (The N_TTY
+ * processing has changed so much that it's hardly recognizable,
+ * anyway...)
+ *
+ * Note that the open routine for N_TTY is guaranteed never to return
+ * an error.  This is because Linux will fall back to setting a line
+ * to N_TTY if it can not switch to any other line discipline.
+ *
+ * Written by Theodore Ts'o, Copyright 1994.
+ *
+ * This file also contains code originally written by Linus Torvalds,
+ * Copyright 1991, 1992, 1993, and by Julian Cowley, Copyright 1994.
+ *
+ * Reduced memory usage for older ARM systems  - Russell King.
+ *
+ * 2000/01/20   Fixed SMP locking on put_tty_queue using bits of
+ *		the patch by Andrew J. Kroll <ag784@freenet.buffalo.edu>
+ *		who actually finally proved there really was a race.
+ *
+ * 2002/03/18   Implemented n_tty_wakeup to send SIGIO POLL_OUTs to
+ *		waiting writing processes-Sapan Bhatia <sapan@corewars.org>.
+ *		Also fixed a bug in BLOCKING mode where n_tty_write returns
+ *		EAGAIN
+ */
+
+#include <linux/types.h>
+#include <linux/major.h>
+#include <linux/errno.h>
+#include <linux/signal.h>
+#include <linux/fcntl.h>
+#include <linux/sched.h>
+#include <linux/interrupt.h>
+#include <linux/tty.h>
+#include <linux/timer.h>
+#include <linux/ctype.h>
+#include <linux/mm.h>
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/poll.h>
+#include <linux/bitops.h>
+#include <linux/audit.h>
+#include <linux/file.h>
+#include <linux/uaccess.h>
+#include <linux/module.h>
+#include <linux/ratelimit.h>
+#include <linux/vmalloc.h>
+#include "tty.h"
+
+/*
+ * Until this number of characters is queued in the xmit buffer, select will
+ * return "we have room for writes".
+ */
+#define WAKEUP_CHARS 256
+
+/*
+ * This defines the low- and high-watermarks for throttling and
+ * unthrottling the TTY driver.  These watermarks are used for
+ * controlling the space in the read buffer.
+ */
+#define TTY_THRESHOLD_THROTTLE		128 /* now based on remaining room */
+#define TTY_THRESHOLD_UNTHROTTLE	128
+
+/*
+ * Special byte codes used in the echo buffer to represent operations
+ * or special handling of characters.  Bytes in the echo buffer that
+ * are not part of such special blocks are treated as normal character
+ * codes.
+ */
+#define ECHO_OP_START 0xff
+#define ECHO_OP_MOVE_BACK_COL 0x80
+#define ECHO_OP_SET_CANON_COL 0x81
+#define ECHO_OP_ERASE_TAB 0x82
+
+#define ECHO_COMMIT_WATERMARK	256
+#define ECHO_BLOCK		256
+#define ECHO_DISCARD_WATERMARK	N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)
+
+
+#undef N_TTY_TRACE
+#ifdef N_TTY_TRACE
+# define n_tty_trace(f, args...)	trace_printk(f, ##args)
+#else
+# define n_tty_trace(f, args...)	no_printk(f, ##args)
+#endif
+
+struct n_tty_data {
+	/* producer-published */
+	size_t read_head;
+	size_t commit_head;
+	size_t canon_head;
+	size_t echo_head;
+	size_t echo_commit;
+	size_t echo_mark;
+	DECLARE_BITMAP(char_map, 256);
+
+	/* private to n_tty_receive_overrun (single-threaded) */
+	unsigned long overrun_time;
+	int num_overrun;
+
+	/* non-atomic */
+	bool no_room;
+
+	/* must hold exclusive termios_rwsem to reset these */
+	unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
+	unsigned char push:1;
+
+	/* shared by producer and consumer */
+	char read_buf[N_TTY_BUF_SIZE];
+	DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);
+	unsigned char echo_buf[N_TTY_BUF_SIZE];
+
+	/* consumer-published */
+	size_t read_tail;
+	size_t line_start;
+
+	/* protected by output lock */
+	unsigned int column;
+	unsigned int canon_column;
+	size_t echo_tail;
+
+	struct mutex atomic_read_lock;
+	struct mutex output_lock;
+};
+
+#define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1))
+
+static inline size_t read_cnt(struct n_tty_data *ldata)
+{
+	return ldata->read_head - ldata->read_tail;
+}
+
+static inline unsigned char read_buf(struct n_tty_data *ldata, size_t i)
+{
+	return ldata->read_buf[i & (N_TTY_BUF_SIZE - 1)];
+}
+
+static inline unsigned char *read_buf_addr(struct n_tty_data *ldata, size_t i)
+{
+	return &ldata->read_buf[i & (N_TTY_BUF_SIZE - 1)];
+}
+
+static inline unsigned char echo_buf(struct n_tty_data *ldata, size_t i)
+{
+	smp_rmb(); /* Matches smp_wmb() in add_echo_byte(). */
+	return ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
+}
+
+static inline unsigned char *echo_buf_addr(struct n_tty_data *ldata, size_t i)
+{
+	return &ldata->echo_buf[i & (N_TTY_BUF_SIZE - 1)];
+}
+
+/* If we are not echoing the data, perhaps this is a secret so erase it */
+static void zero_buffer(struct tty_struct *tty, u8 *buffer, int size)
+{
+	bool icanon = !!L_ICANON(tty);
+	bool no_echo = !L_ECHO(tty);
+
+	if (icanon && no_echo)
+		memset(buffer, 0x00, size);
+}
+
+static void tty_copy(struct tty_struct *tty, void *to, size_t tail, size_t n)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t size = N_TTY_BUF_SIZE - tail;
+	void *from = read_buf_addr(ldata, tail);
+
+	if (n > size) {
+		tty_audit_add_data(tty, from, size);
+		memcpy(to, from, size);
+		zero_buffer(tty, from, size);
+		to += size;
+		n -= size;
+		from = ldata->read_buf;
+	}
+
+	tty_audit_add_data(tty, from, n);
+	memcpy(to, from, n);
+	zero_buffer(tty, from, n);
+}
+
+/**
+ *	n_tty_kick_worker - start input worker (if required)
+ *	@tty: terminal
+ *
+ *	Re-schedules the flip buffer work if it may have stopped
+ *
+ *	Caller holds exclusive termios_rwsem
+ *	   or
+ *	n_tty_read()/consumer path:
+ *		holds non-exclusive termios_rwsem
+ */
+
+static void n_tty_kick_worker(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	/* Did the input worker stop? Restart it */
+	if (unlikely(ldata->no_room)) {
+		ldata->no_room = 0;
+
+		WARN_RATELIMIT(tty->port->itty == NULL,
+				"scheduling with invalid itty\n");
+		/* see if ldisc has been killed - if so, this means that
+		 * even though the ldisc has been halted and ->buf.work
+		 * cancelled, ->buf.work is about to be rescheduled
+		 */
+		WARN_RATELIMIT(test_bit(TTY_LDISC_HALTED, &tty->flags),
+			       "scheduling buffer work for halted ldisc\n");
+		tty_buffer_restart_work(tty->port);
+	}
+}
+
+static ssize_t chars_in_buffer(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	ssize_t n = 0;
+
+	if (!ldata->icanon)
+		n = ldata->commit_head - ldata->read_tail;
+	else
+		n = ldata->canon_head - ldata->read_tail;
+	return n;
+}
+
+/**
+ *	n_tty_write_wakeup	-	asynchronous I/O notifier
+ *	@tty: tty device
+ *
+ *	Required for the ptys, serial driver etc. since processes
+ *	that attach themselves to the master and rely on ASYNC
+ *	IO must be woken up
+ */
+
+static void n_tty_write_wakeup(struct tty_struct *tty)
+{
+	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	kill_fasync(&tty->fasync, SIGIO, POLL_OUT);
+}
+
+static void n_tty_check_throttle(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	/*
+	 * Check the remaining room for the input canonicalization
+	 * mode.  We don't want to throttle the driver if we're in
+	 * canonical mode and don't have a newline yet!
+	 */
+	if (ldata->icanon && ldata->canon_head == ldata->read_tail)
+		return;
+
+	while (1) {
+		int throttled;
+		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
+		if (N_TTY_BUF_SIZE - read_cnt(ldata) >= TTY_THRESHOLD_THROTTLE)
+			break;
+		throttled = tty_throttle_safe(tty);
+		if (!throttled)
+			break;
+	}
+	__tty_set_flow_change(tty, 0);
+}
+
+static void n_tty_check_unthrottle(struct tty_struct *tty)
+{
+	if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
+		if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
+			return;
+		n_tty_kick_worker(tty);
+		tty_wakeup(tty->link);
+		return;
+	}
+
+	/* If there is enough space in the read buffer now, let the
+	 * low-level driver know. We use chars_in_buffer() to
+	 * check the buffer, as it now knows about canonical mode.
+	 * Otherwise, if the driver is throttled and the line is
+	 * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
+	 * we won't get any more characters.
+	 */
+
+	while (1) {
+		int unthrottled;
+		tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
+		if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
+			break;
+		n_tty_kick_worker(tty);
+		unthrottled = tty_unthrottle_safe(tty);
+		if (!unthrottled)
+			break;
+	}
+	__tty_set_flow_change(tty, 0);
+}
+
+/**
+ *	put_tty_queue		-	add character to tty
+ *	@c: character
+ *	@ldata: n_tty data
+ *
+ *	Add a character to the tty read_buf queue.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		caller holds non-exclusive termios_rwsem
+ */
+
+static inline void put_tty_queue(unsigned char c, struct n_tty_data *ldata)
+{
+	*read_buf_addr(ldata, ldata->read_head) = c;
+	ldata->read_head++;
+}
+
+/**
+ *	reset_buffer_flags	-	reset buffer state
+ *	@ldata: line disc data to reset
+ *
+ *	Reset the read buffer counters and clear the flags.
+ *	Called from n_tty_open() and n_tty_flush_buffer().
+ *
+ *	Locking: caller holds exclusive termios_rwsem
+ *		 (or locking is not required)
+ */
+
+static void reset_buffer_flags(struct n_tty_data *ldata)
+{
+	ldata->read_head = ldata->canon_head = ldata->read_tail = 0;
+	ldata->commit_head = 0;
+	ldata->line_start = 0;
+
+	ldata->erasing = 0;
+	bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
+	ldata->push = 0;
+}
+
+static void n_tty_packet_mode_flush(struct tty_struct *tty)
+{
+	unsigned long flags;
+
+	if (tty->link->packet) {
+		spin_lock_irqsave(&tty->ctrl_lock, flags);
+		tty->ctrl_status |= TIOCPKT_FLUSHREAD;
+		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+		wake_up_interruptible(&tty->link->read_wait);
+	}
+}
+
+/**
+ *	n_tty_flush_buffer	-	clean input queue
+ *	@tty:	terminal device
+ *
+ *	Flush the input buffer. Called when the tty layer wants the
+ *	buffer flushed (eg at hangup) or when the N_TTY line discipline
+ *	internally has to clean the pending queue (for example some signals).
+ *
+ *	Holds termios_rwsem to exclude producer/consumer while
+ *	buffer indices are reset.
+ *
+ *	Locking: ctrl_lock, exclusive termios_rwsem
+ */
+
+static void n_tty_flush_buffer(struct tty_struct *tty)
+{
+	down_write(&tty->termios_rwsem);
+	reset_buffer_flags(tty->disc_data);
+	n_tty_kick_worker(tty);
+
+	if (tty->link)
+		n_tty_packet_mode_flush(tty);
+	up_write(&tty->termios_rwsem);
+}
+
+/**
+ *	is_utf8_continuation	-	utf8 multibyte check
+ *	@c: byte to check
+ *
+ *	Returns true if the utf8 character 'c' is a multibyte continuation
+ *	character. We use this to correctly compute the on screen size
+ *	of the character when printing
+ */
+
+static inline int is_utf8_continuation(unsigned char c)
+{
+	return (c & 0xc0) == 0x80;
+}
+
+/**
+ *	is_continuation		-	multibyte check
+ *	@c: byte to check
+ *
+ *	Returns true if the utf8 character 'c' is a multibyte continuation
+ *	character and the terminal is in unicode mode.
+ */
+
+static inline int is_continuation(unsigned char c, struct tty_struct *tty)
+{
+	return I_IUTF8(tty) && is_utf8_continuation(c);
+}
+
+/**
+ *	do_output_char			-	output one character
+ *	@c: character (or partial unicode symbol)
+ *	@tty: terminal device
+ *	@space: space available in tty driver write buffer
+ *
+ *	This is a helper function that handles one output character
+ *	(including special characters like TAB, CR, LF, etc.),
+ *	doing OPOST processing and putting the results in the
+ *	tty driver's write buffer.
+ *
+ *	Note that Linux currently ignores TABDLY, CRDLY, VTDLY, FFDLY
+ *	and NLDLY.  They simply aren't relevant in the world today.
+ *	If you ever need them, add them here.
+ *
+ *	Returns the number of bytes of buffer space used or -1 if
+ *	no space left.
+ *
+ *	Locking: should be called under the output_lock to protect
+ *		 the column state and space left in the buffer
+ */
+
+static int do_output_char(unsigned char c, struct tty_struct *tty, int space)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int	spaces;
+
+	if (!space)
+		return -1;
+
+	switch (c) {
+	case '\n':
+		if (O_ONLRET(tty))
+			ldata->column = 0;
+		if (O_ONLCR(tty)) {
+			if (space < 2)
+				return -1;
+			ldata->canon_column = ldata->column = 0;
+			tty->ops->write(tty, "\r\n", 2);
+			return 2;
+		}
+		ldata->canon_column = ldata->column;
+		break;
+	case '\r':
+		if (O_ONOCR(tty) && ldata->column == 0)
+			return 0;
+		if (O_OCRNL(tty)) {
+			c = '\n';
+			if (O_ONLRET(tty))
+				ldata->canon_column = ldata->column = 0;
+			break;
+		}
+		ldata->canon_column = ldata->column = 0;
+		break;
+	case '\t':
+		spaces = 8 - (ldata->column & 7);
+		if (O_TABDLY(tty) == XTABS) {
+			if (space < spaces)
+				return -1;
+			ldata->column += spaces;
+			tty->ops->write(tty, "        ", spaces);
+			return spaces;
+		}
+		ldata->column += spaces;
+		break;
+	case '\b':
+		if (ldata->column > 0)
+			ldata->column--;
+		break;
+	default:
+		if (!iscntrl(c)) {
+			if (O_OLCUC(tty))
+				c = toupper(c);
+			if (!is_continuation(c, tty))
+				ldata->column++;
+		}
+		break;
+	}
+
+	tty_put_char(tty, c);
+	return 1;
+}
+
+/**
+ *	process_output			-	output post processor
+ *	@c: character (or partial unicode symbol)
+ *	@tty: terminal device
+ *
+ *	Output one character with OPOST processing.
+ *	Returns -1 when the output device is full and the character
+ *	must be retried.
+ *
+ *	Locking: output_lock to protect column state and space left
+ *		 (also, this is called from n_tty_write under the
+ *		  tty layer write lock)
+ */
+
+static int process_output(unsigned char c, struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int	space, retval;
+
+	mutex_lock(&ldata->output_lock);
+
+	space = tty_write_room(tty);
+	retval = do_output_char(c, tty, space);
+
+	mutex_unlock(&ldata->output_lock);
+	if (retval < 0)
+		return -1;
+	else
+		return 0;
+}
+
+/**
+ *	process_output_block		-	block post processor
+ *	@tty: terminal device
+ *	@buf: character buffer
+ *	@nr: number of bytes to output
+ *
+ *	Output a block of characters with OPOST processing.
+ *	Returns the number of characters output.
+ *
+ *	This path is used to speed up block console writes, among other
+ *	things when processing blocks of output data. It handles only
+ *	the simple cases normally found and helps to generate blocks of
+ *	symbols for the console driver and thus improve performance.
+ *
+ *	Locking: output_lock to protect column state and space left
+ *		 (also, this is called from n_tty_write under the
+ *		  tty layer write lock)
+ */
+
+static ssize_t process_output_block(struct tty_struct *tty,
+				    const unsigned char *buf, unsigned int nr)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int	space;
+	int	i;
+	const unsigned char *cp;
+
+	mutex_lock(&ldata->output_lock);
+
+	space = tty_write_room(tty);
+	if (space <= 0) {
+		mutex_unlock(&ldata->output_lock);
+		return space;
+	}
+	if (nr > space)
+		nr = space;
+
+	for (i = 0, cp = buf; i < nr; i++, cp++) {
+		unsigned char c = *cp;
+
+		switch (c) {
+		case '\n':
+			if (O_ONLRET(tty))
+				ldata->column = 0;
+			if (O_ONLCR(tty))
+				goto break_out;
+			ldata->canon_column = ldata->column;
+			break;
+		case '\r':
+			if (O_ONOCR(tty) && ldata->column == 0)
+				goto break_out;
+			if (O_OCRNL(tty))
+				goto break_out;
+			ldata->canon_column = ldata->column = 0;
+			break;
+		case '\t':
+			goto break_out;
+		case '\b':
+			if (ldata->column > 0)
+				ldata->column--;
+			break;
+		default:
+			if (!iscntrl(c)) {
+				if (O_OLCUC(tty))
+					goto break_out;
+				if (!is_continuation(c, tty))
+					ldata->column++;
+			}
+			break;
+		}
+	}
+break_out:
+	i = tty->ops->write(tty, buf, i);
+
+	mutex_unlock(&ldata->output_lock);
+	return i;
+}
+
+/**
+ *	process_echoes	-	write pending echo characters
+ *	@tty: terminal device
+ *
+ *	Write previously buffered echo (and other ldisc-generated)
+ *	characters to the tty.
+ *
+ *	Characters generated by the ldisc (including echoes) need to
+ *	be buffered because the driver's write buffer can fill during
+ *	heavy program output.  Echoing straight to the driver will
+ *	often fail under these conditions, causing lost characters and
+ *	resulting mismatches of ldisc state information.
+ *
+ *	Since the ldisc state must represent the characters actually sent
+ *	to the driver at the time of the write, operations like certain
+ *	changes in column state are also saved in the buffer and executed
+ *	here.
+ *
+ *	A circular fifo buffer is used so that the most recent characters
+ *	are prioritized.  Also, when control characters are echoed with a
+ *	prefixed "^", the pair is treated atomically and thus not separated.
+ *
+ *	Locking: callers must hold output_lock
+ */
+
+static size_t __process_echoes(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int	space, old_space;
+	size_t tail;
+	unsigned char c;
+
+	old_space = space = tty_write_room(tty);
+
+	tail = ldata->echo_tail;
+	while (MASK(ldata->echo_commit) != MASK(tail)) {
+		c = echo_buf(ldata, tail);
+		if (c == ECHO_OP_START) {
+			unsigned char op;
+			int no_space_left = 0;
+
+			/*
+			 * Since add_echo_byte() is called without holding
+			 * output_lock, we might see only portion of multi-byte
+			 * operation.
+			 */
+			if (MASK(ldata->echo_commit) == MASK(tail + 1))
+				goto not_yet_stored;
+			/*
+			 * If the buffer byte is the start of a multi-byte
+			 * operation, get the next byte, which is either the
+			 * op code or a control character value.
+			 */
+			op = echo_buf(ldata, tail + 1);
+
+			switch (op) {
+			case ECHO_OP_ERASE_TAB: {
+				unsigned int num_chars, num_bs;
+
+				if (MASK(ldata->echo_commit) == MASK(tail + 2))
+					goto not_yet_stored;
+				num_chars = echo_buf(ldata, tail + 2);
+
+				/*
+				 * Determine how many columns to go back
+				 * in order to erase the tab.
+				 * This depends on the number of columns
+				 * used by other characters within the tab
+				 * area.  If this (modulo 8) count is from
+				 * the start of input rather than from a
+				 * previous tab, we offset by canon column.
+				 * Otherwise, tab spacing is normal.
+				 */
+				if (!(num_chars & 0x80))
+					num_chars += ldata->canon_column;
+				num_bs = 8 - (num_chars & 7);
+
+				if (num_bs > space) {
+					no_space_left = 1;
+					break;
+				}
+				space -= num_bs;
+				while (num_bs--) {
+					tty_put_char(tty, '\b');
+					if (ldata->column > 0)
+						ldata->column--;
+				}
+				tail += 3;
+				break;
+			}
+			case ECHO_OP_SET_CANON_COL:
+				ldata->canon_column = ldata->column;
+				tail += 2;
+				break;
+
+			case ECHO_OP_MOVE_BACK_COL:
+				if (ldata->column > 0)
+					ldata->column--;
+				tail += 2;
+				break;
+
+			case ECHO_OP_START:
+				/* This is an escaped echo op start code */
+				if (!space) {
+					no_space_left = 1;
+					break;
+				}
+				tty_put_char(tty, ECHO_OP_START);
+				ldata->column++;
+				space--;
+				tail += 2;
+				break;
+
+			default:
+				/*
+				 * If the op is not a special byte code,
+				 * it is a ctrl char tagged to be echoed
+				 * as "^X" (where X is the letter
+				 * representing the control char).
+				 * Note that we must ensure there is
+				 * enough space for the whole ctrl pair.
+				 *
+				 */
+				if (space < 2) {
+					no_space_left = 1;
+					break;
+				}
+				tty_put_char(tty, '^');
+				tty_put_char(tty, op ^ 0100);
+				ldata->column += 2;
+				space -= 2;
+				tail += 2;
+			}
+
+			if (no_space_left)
+				break;
+		} else {
+			if (O_OPOST(tty)) {
+				int retval = do_output_char(c, tty, space);
+				if (retval < 0)
+					break;
+				space -= retval;
+			} else {
+				if (!space)
+					break;
+				tty_put_char(tty, c);
+				space -= 1;
+			}
+			tail += 1;
+		}
+	}
+
+	/* If the echo buffer is nearly full (so that the possibility exists
+	 * of echo overrun before the next commit), then discard enough
+	 * data at the tail to prevent a subsequent overrun */
+	while (ldata->echo_commit > tail &&
+	       ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
+		if (echo_buf(ldata, tail) == ECHO_OP_START) {
+			if (echo_buf(ldata, tail + 1) == ECHO_OP_ERASE_TAB)
+				tail += 3;
+			else
+				tail += 2;
+		} else
+			tail++;
+	}
+
+ not_yet_stored:
+	ldata->echo_tail = tail;
+	return old_space - space;
+}
+
+static void commit_echoes(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t nr, old, echoed;
+	size_t head;
+
+	mutex_lock(&ldata->output_lock);
+	head = ldata->echo_head;
+	ldata->echo_mark = head;
+	old = ldata->echo_commit - ldata->echo_tail;
+
+	/* Process committed echoes if the accumulated # of bytes
+	 * is over the threshold (and try again each time another
+	 * block is accumulated) */
+	nr = head - ldata->echo_tail;
+	if (nr < ECHO_COMMIT_WATERMARK ||
+	    (nr % ECHO_BLOCK > old % ECHO_BLOCK)) {
+		mutex_unlock(&ldata->output_lock);
+		return;
+	}
+
+	ldata->echo_commit = head;
+	echoed = __process_echoes(tty);
+	mutex_unlock(&ldata->output_lock);
+
+	if (echoed && tty->ops->flush_chars)
+		tty->ops->flush_chars(tty);
+}
+
+static void process_echoes(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t echoed;
+
+	if (ldata->echo_mark == ldata->echo_tail)
+		return;
+
+	mutex_lock(&ldata->output_lock);
+	ldata->echo_commit = ldata->echo_mark;
+	echoed = __process_echoes(tty);
+	mutex_unlock(&ldata->output_lock);
+
+	if (echoed && tty->ops->flush_chars)
+		tty->ops->flush_chars(tty);
+}
+
+/* NB: echo_mark and echo_head should be equivalent here */
+static void flush_echoes(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if ((!L_ECHO(tty) && !L_ECHONL(tty)) ||
+	    ldata->echo_commit == ldata->echo_head)
+		return;
+
+	mutex_lock(&ldata->output_lock);
+	ldata->echo_commit = ldata->echo_head;
+	__process_echoes(tty);
+	mutex_unlock(&ldata->output_lock);
+}
+
+/**
+ *	add_echo_byte	-	add a byte to the echo buffer
+ *	@c: unicode byte to echo
+ *	@ldata: n_tty data
+ *
+ *	Add a character or operation byte to the echo buffer.
+ */
+
+static inline void add_echo_byte(unsigned char c, struct n_tty_data *ldata)
+{
+	*echo_buf_addr(ldata, ldata->echo_head) = c;
+	smp_wmb(); /* Matches smp_rmb() in echo_buf(). */
+	ldata->echo_head++;
+}
+
+/**
+ *	echo_move_back_col	-	add operation to move back a column
+ *	@ldata: n_tty data
+ *
+ *	Add an operation to the echo buffer to move back one column.
+ */
+
+static void echo_move_back_col(struct n_tty_data *ldata)
+{
+	add_echo_byte(ECHO_OP_START, ldata);
+	add_echo_byte(ECHO_OP_MOVE_BACK_COL, ldata);
+}
+
+/**
+ *	echo_set_canon_col	-	add operation to set the canon column
+ *	@ldata: n_tty data
+ *
+ *	Add an operation to the echo buffer to set the canon column
+ *	to the current column.
+ */
+
+static void echo_set_canon_col(struct n_tty_data *ldata)
+{
+	add_echo_byte(ECHO_OP_START, ldata);
+	add_echo_byte(ECHO_OP_SET_CANON_COL, ldata);
+}
+
+/**
+ *	echo_erase_tab	-	add operation to erase a tab
+ *	@num_chars: number of character columns already used
+ *	@after_tab: true if num_chars starts after a previous tab
+ *	@ldata: n_tty data
+ *
+ *	Add an operation to the echo buffer to erase a tab.
+ *
+ *	Called by the eraser function, which knows how many character
+ *	columns have been used since either a previous tab or the start
+ *	of input.  This information will be used later, along with
+ *	canon column (if applicable), to go back the correct number
+ *	of columns.
+ */
+
+static void echo_erase_tab(unsigned int num_chars, int after_tab,
+			   struct n_tty_data *ldata)
+{
+	add_echo_byte(ECHO_OP_START, ldata);
+	add_echo_byte(ECHO_OP_ERASE_TAB, ldata);
+
+	/* We only need to know this modulo 8 (tab spacing) */
+	num_chars &= 7;
+
+	/* Set the high bit as a flag if num_chars is after a previous tab */
+	if (after_tab)
+		num_chars |= 0x80;
+
+	add_echo_byte(num_chars, ldata);
+}
+
+/**
+ *	echo_char_raw	-	echo a character raw
+ *	@c: unicode byte to echo
+ *	@ldata: line disc data
+ *
+ *	Echo user input back onto the screen. This must be called only when
+ *	L_ECHO(tty) is true. Called from the driver receive_buf path.
+ *
+ *	This variant does not treat control characters specially.
+ */
+
+static void echo_char_raw(unsigned char c, struct n_tty_data *ldata)
+{
+	if (c == ECHO_OP_START) {
+		add_echo_byte(ECHO_OP_START, ldata);
+		add_echo_byte(ECHO_OP_START, ldata);
+	} else {
+		add_echo_byte(c, ldata);
+	}
+}
+
+/**
+ *	echo_char	-	echo a character
+ *	@c: unicode byte to echo
+ *	@tty: terminal device
+ *
+ *	Echo user input back onto the screen. This must be called only when
+ *	L_ECHO(tty) is true. Called from the driver receive_buf path.
+ *
+ *	This variant tags control characters to be echoed as "^X"
+ *	(where X is the letter representing the control char).
+ */
+
+static void echo_char(unsigned char c, struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (c == ECHO_OP_START) {
+		add_echo_byte(ECHO_OP_START, ldata);
+		add_echo_byte(ECHO_OP_START, ldata);
+	} else {
+		if (L_ECHOCTL(tty) && iscntrl(c) && c != '\t')
+			add_echo_byte(ECHO_OP_START, ldata);
+		add_echo_byte(c, ldata);
+	}
+}
+
+/**
+ *	finish_erasing		-	complete erase
+ *	@ldata: n_tty data
+ */
+
+static inline void finish_erasing(struct n_tty_data *ldata)
+{
+	if (ldata->erasing) {
+		echo_char_raw('/', ldata);
+		ldata->erasing = 0;
+	}
+}
+
+/**
+ *	eraser		-	handle erase function
+ *	@c: character input
+ *	@tty: terminal device
+ *
+ *	Perform erase and necessary output when an erase character is
+ *	present in the stream from the driver layer. Handles the complexities
+ *	of UTF-8 multibyte symbols.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		caller holds non-exclusive termios_rwsem
+ */
+
+static void eraser(unsigned char c, struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	enum { ERASE, WERASE, KILL } kill_type;
+	size_t head;
+	size_t cnt;
+	int seen_alnums;
+
+	if (ldata->read_head == ldata->canon_head) {
+		/* process_output('\a', tty); */ /* what do you think? */
+		return;
+	}
+	if (c == ERASE_CHAR(tty))
+		kill_type = ERASE;
+	else if (c == WERASE_CHAR(tty))
+		kill_type = WERASE;
+	else {
+		if (!L_ECHO(tty)) {
+			ldata->read_head = ldata->canon_head;
+			return;
+		}
+		if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
+			ldata->read_head = ldata->canon_head;
+			finish_erasing(ldata);
+			echo_char(KILL_CHAR(tty), tty);
+			/* Add a newline if ECHOK is on and ECHOKE is off. */
+			if (L_ECHOK(tty))
+				echo_char_raw('\n', ldata);
+			return;
+		}
+		kill_type = KILL;
+	}
+
+	seen_alnums = 0;
+	while (MASK(ldata->read_head) != MASK(ldata->canon_head)) {
+		head = ldata->read_head;
+
+		/* erase a single possibly multibyte character */
+		do {
+			head--;
+			c = read_buf(ldata, head);
+		} while (is_continuation(c, tty) &&
+			 MASK(head) != MASK(ldata->canon_head));
+
+		/* do not partially erase */
+		if (is_continuation(c, tty))
+			break;
+
+		if (kill_type == WERASE) {
+			/* Equivalent to BSD's ALTWERASE. */
+			if (isalnum(c) || c == '_')
+				seen_alnums++;
+			else if (seen_alnums)
+				break;
+		}
+		cnt = ldata->read_head - head;
+		ldata->read_head = head;
+		if (L_ECHO(tty)) {
+			if (L_ECHOPRT(tty)) {
+				if (!ldata->erasing) {
+					echo_char_raw('\\', ldata);
+					ldata->erasing = 1;
+				}
+				/* if cnt > 1, output a multi-byte character */
+				echo_char(c, tty);
+				while (--cnt > 0) {
+					head++;
+					echo_char_raw(read_buf(ldata, head), ldata);
+					echo_move_back_col(ldata);
+				}
+			} else if (kill_type == ERASE && !L_ECHOE(tty)) {
+				echo_char(ERASE_CHAR(tty), tty);
+			} else if (c == '\t') {
+				unsigned int num_chars = 0;
+				int after_tab = 0;
+				size_t tail = ldata->read_head;
+
+				/*
+				 * Count the columns used for characters
+				 * since the start of input or after a
+				 * previous tab.
+				 * This info is used to go back the correct
+				 * number of columns.
+				 */
+				while (MASK(tail) != MASK(ldata->canon_head)) {
+					tail--;
+					c = read_buf(ldata, tail);
+					if (c == '\t') {
+						after_tab = 1;
+						break;
+					} else if (iscntrl(c)) {
+						if (L_ECHOCTL(tty))
+							num_chars += 2;
+					} else if (!is_continuation(c, tty)) {
+						num_chars++;
+					}
+				}
+				echo_erase_tab(num_chars, after_tab, ldata);
+			} else {
+				if (iscntrl(c) && L_ECHOCTL(tty)) {
+					echo_char_raw('\b', ldata);
+					echo_char_raw(' ', ldata);
+					echo_char_raw('\b', ldata);
+				}
+				if (!iscntrl(c) || L_ECHOCTL(tty)) {
+					echo_char_raw('\b', ldata);
+					echo_char_raw(' ', ldata);
+					echo_char_raw('\b', ldata);
+				}
+			}
+		}
+		if (kill_type == ERASE)
+			break;
+	}
+	if (ldata->read_head == ldata->canon_head && L_ECHO(tty))
+		finish_erasing(ldata);
+}
+
+/**
+ *	isig		-	handle the ISIG optio
+ *	@sig: signal
+ *	@tty: terminal
+ *
+ *	Called when a signal is being sent due to terminal input.
+ *	Called from the driver receive_buf path so serialized.
+ *
+ *	Performs input and output flush if !NOFLSH. In this context, the echo
+ *	buffer is 'output'. The signal is processed first to alert any current
+ *	readers or writers to discontinue and exit their i/o loops.
+ *
+ *	Locking: ctrl_lock
+ */
+
+static void __isig(int sig, struct tty_struct *tty)
+{
+	struct pid *tty_pgrp = tty_get_pgrp(tty);
+	if (tty_pgrp) {
+		kill_pgrp(tty_pgrp, sig, 1);
+		put_pid(tty_pgrp);
+	}
+}
+
+static void isig(int sig, struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (L_NOFLSH(tty)) {
+		/* signal only */
+		__isig(sig, tty);
+
+	} else { /* signal and flush */
+		up_read(&tty->termios_rwsem);
+		down_write(&tty->termios_rwsem);
+
+		__isig(sig, tty);
+
+		/* clear echo buffer */
+		mutex_lock(&ldata->output_lock);
+		ldata->echo_head = ldata->echo_tail = 0;
+		ldata->echo_mark = ldata->echo_commit = 0;
+		mutex_unlock(&ldata->output_lock);
+
+		/* clear output buffer */
+		tty_driver_flush_buffer(tty);
+
+		/* clear input buffer */
+		reset_buffer_flags(tty->disc_data);
+
+		/* notify pty master of flush */
+		if (tty->link)
+			n_tty_packet_mode_flush(tty);
+
+		up_write(&tty->termios_rwsem);
+		down_read(&tty->termios_rwsem);
+	}
+}
+
+/**
+ *	n_tty_receive_break	-	handle break
+ *	@tty: terminal
+ *
+ *	An RS232 break event has been hit in the incoming bitstream. This
+ *	can cause a variety of events depending upon the termios settings.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		caller holds non-exclusive termios_rwsem
+ *
+ *	Note: may get exclusive termios_rwsem if flushing input buffer
+ */
+
+static void n_tty_receive_break(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (I_IGNBRK(tty))
+		return;
+	if (I_BRKINT(tty)) {
+		isig(SIGINT, tty);
+		return;
+	}
+	if (I_PARMRK(tty)) {
+		put_tty_queue('\377', ldata);
+		put_tty_queue('\0', ldata);
+	}
+	put_tty_queue('\0', ldata);
+}
+
+/**
+ *	n_tty_receive_overrun	-	handle overrun reporting
+ *	@tty: terminal
+ *
+ *	Data arrived faster than we could process it. While the tty
+ *	driver has flagged this the bits that were missed are gone
+ *	forever.
+ *
+ *	Called from the receive_buf path so single threaded. Does not
+ *	need locking as num_overrun and overrun_time are function
+ *	private.
+ */
+
+static void n_tty_receive_overrun(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	ldata->num_overrun++;
+	if (time_after(jiffies, ldata->overrun_time + HZ) ||
+			time_after(ldata->overrun_time, jiffies)) {
+		tty_warn(tty, "%d input overrun(s)\n", ldata->num_overrun);
+		ldata->overrun_time = jiffies;
+		ldata->num_overrun = 0;
+	}
+}
+
+/**
+ *	n_tty_receive_parity_error	-	error notifier
+ *	@tty: terminal device
+ *	@c: character
+ *
+ *	Process a parity error and queue the right data to indicate
+ *	the error case if necessary.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		caller holds non-exclusive termios_rwsem
+ */
+static void n_tty_receive_parity_error(struct tty_struct *tty, unsigned char c)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (I_INPCK(tty)) {
+		if (I_IGNPAR(tty))
+			return;
+		if (I_PARMRK(tty)) {
+			put_tty_queue('\377', ldata);
+			put_tty_queue('\0', ldata);
+			put_tty_queue(c, ldata);
+		} else
+			put_tty_queue('\0', ldata);
+	} else
+		put_tty_queue(c, ldata);
+}
+
+static void
+n_tty_receive_signal_char(struct tty_struct *tty, int signal, unsigned char c)
+{
+	isig(signal, tty);
+	if (I_IXON(tty))
+		start_tty(tty);
+	if (L_ECHO(tty)) {
+		echo_char(c, tty);
+		commit_echoes(tty);
+	} else
+		process_echoes(tty);
+	return;
+}
+
+/**
+ *	n_tty_receive_char	-	perform processing
+ *	@tty: terminal device
+ *	@c: character
+ *
+ *	Process an individual character of input received from the driver.
+ *	This is serialized with respect to itself by the rules for the
+ *	driver above.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		caller holds non-exclusive termios_rwsem
+ *		publishes canon_head if canonical mode is active
+ *
+ *	Returns 1 if LNEXT was received, else returns 0
+ */
+
+static int
+n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (I_IXON(tty)) {
+		if (c == START_CHAR(tty)) {
+			start_tty(tty);
+			process_echoes(tty);
+			return 0;
+		}
+		if (c == STOP_CHAR(tty)) {
+			stop_tty(tty);
+			return 0;
+		}
+	}
+
+	if (L_ISIG(tty)) {
+		if (c == INTR_CHAR(tty)) {
+			n_tty_receive_signal_char(tty, SIGINT, c);
+			return 0;
+		} else if (c == QUIT_CHAR(tty)) {
+			n_tty_receive_signal_char(tty, SIGQUIT, c);
+			return 0;
+		} else if (c == SUSP_CHAR(tty)) {
+			n_tty_receive_signal_char(tty, SIGTSTP, c);
+			return 0;
+		}
+	}
+
+	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
+		start_tty(tty);
+		process_echoes(tty);
+	}
+
+	if (c == '\r') {
+		if (I_IGNCR(tty))
+			return 0;
+		if (I_ICRNL(tty))
+			c = '\n';
+	} else if (c == '\n' && I_INLCR(tty))
+		c = '\r';
+
+	if (ldata->icanon) {
+		if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
+		    (c == WERASE_CHAR(tty) && L_IEXTEN(tty))) {
+			eraser(c, tty);
+			commit_echoes(tty);
+			return 0;
+		}
+		if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
+			ldata->lnext = 1;
+			if (L_ECHO(tty)) {
+				finish_erasing(ldata);
+				if (L_ECHOCTL(tty)) {
+					echo_char_raw('^', ldata);
+					echo_char_raw('\b', ldata);
+					commit_echoes(tty);
+				}
+			}
+			return 1;
+		}
+		if (c == REPRINT_CHAR(tty) && L_ECHO(tty) && L_IEXTEN(tty)) {
+			size_t tail = ldata->canon_head;
+
+			finish_erasing(ldata);
+			echo_char(c, tty);
+			echo_char_raw('\n', ldata);
+			while (MASK(tail) != MASK(ldata->read_head)) {
+				echo_char(read_buf(ldata, tail), tty);
+				tail++;
+			}
+			commit_echoes(tty);
+			return 0;
+		}
+		if (c == '\n') {
+			if (L_ECHO(tty) || L_ECHONL(tty)) {
+				echo_char_raw('\n', ldata);
+				commit_echoes(tty);
+			}
+			goto handle_newline;
+		}
+		if (c == EOF_CHAR(tty)) {
+			c = __DISABLED_CHAR;
+			goto handle_newline;
+		}
+		if ((c == EOL_CHAR(tty)) ||
+		    (c == EOL2_CHAR(tty) && L_IEXTEN(tty))) {
+			/*
+			 * XXX are EOL_CHAR and EOL2_CHAR echoed?!?
+			 */
+			if (L_ECHO(tty)) {
+				/* Record the column of first canon char. */
+				if (ldata->canon_head == ldata->read_head)
+					echo_set_canon_col(ldata);
+				echo_char(c, tty);
+				commit_echoes(tty);
+			}
+			/*
+			 * XXX does PARMRK doubling happen for
+			 * EOL_CHAR and EOL2_CHAR?
+			 */
+			if (c == (unsigned char) '\377' && I_PARMRK(tty))
+				put_tty_queue(c, ldata);
+
+handle_newline:
+			set_bit(ldata->read_head & (N_TTY_BUF_SIZE - 1), ldata->read_flags);
+			put_tty_queue(c, ldata);
+			smp_store_release(&ldata->canon_head, ldata->read_head);
+			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+			wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
+			return 0;
+		}
+	}
+
+	if (L_ECHO(tty)) {
+		finish_erasing(ldata);
+		if (c == '\n')
+			echo_char_raw('\n', ldata);
+		else {
+			/* Record the column of first canon char. */
+			if (ldata->canon_head == ldata->read_head)
+				echo_set_canon_col(ldata);
+			echo_char(c, tty);
+		}
+		commit_echoes(tty);
+	}
+
+	/* PARMRK doubling check */
+	if (c == (unsigned char) '\377' && I_PARMRK(tty))
+		put_tty_queue(c, ldata);
+
+	put_tty_queue(c, ldata);
+	return 0;
+}
+
+static inline void
+n_tty_receive_char_inline(struct tty_struct *tty, unsigned char c)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
+		start_tty(tty);
+		process_echoes(tty);
+	}
+	if (L_ECHO(tty)) {
+		finish_erasing(ldata);
+		/* Record the column of first canon char. */
+		if (ldata->canon_head == ldata->read_head)
+			echo_set_canon_col(ldata);
+		echo_char(c, tty);
+		commit_echoes(tty);
+	}
+	/* PARMRK doubling check */
+	if (c == (unsigned char) '\377' && I_PARMRK(tty))
+		put_tty_queue(c, ldata);
+	put_tty_queue(c, ldata);
+}
+
+static void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
+{
+	n_tty_receive_char_inline(tty, c);
+}
+
+static inline void
+n_tty_receive_char_fast(struct tty_struct *tty, unsigned char c)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) && I_IXANY(tty)) {
+		start_tty(tty);
+		process_echoes(tty);
+	}
+	if (L_ECHO(tty)) {
+		finish_erasing(ldata);
+		/* Record the column of first canon char. */
+		if (ldata->canon_head == ldata->read_head)
+			echo_set_canon_col(ldata);
+		echo_char(c, tty);
+		commit_echoes(tty);
+	}
+	put_tty_queue(c, ldata);
+}
+
+static void n_tty_receive_char_closing(struct tty_struct *tty, unsigned char c)
+{
+	if (I_ISTRIP(tty))
+		c &= 0x7f;
+	if (I_IUCLC(tty) && L_IEXTEN(tty))
+		c = tolower(c);
+
+	if (I_IXON(tty)) {
+		if (c == STOP_CHAR(tty))
+			stop_tty(tty);
+		else if (c == START_CHAR(tty) ||
+			 (tty->stopped && !tty->flow_stopped && I_IXANY(tty) &&
+			  c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
+			  c != SUSP_CHAR(tty))) {
+			start_tty(tty);
+			process_echoes(tty);
+		}
+	}
+}
+
+static void
+n_tty_receive_char_flagged(struct tty_struct *tty, unsigned char c, char flag)
+{
+	switch (flag) {
+	case TTY_BREAK:
+		n_tty_receive_break(tty);
+		break;
+	case TTY_PARITY:
+	case TTY_FRAME:
+		n_tty_receive_parity_error(tty, c);
+		break;
+	case TTY_OVERRUN:
+		n_tty_receive_overrun(tty);
+		break;
+	default:
+		tty_err(tty, "unknown flag %d\n", flag);
+		break;
+	}
+}
+
+static void
+n_tty_receive_char_lnext(struct tty_struct *tty, unsigned char c, char flag)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	ldata->lnext = 0;
+	if (likely(flag == TTY_NORMAL)) {
+		if (I_ISTRIP(tty))
+			c &= 0x7f;
+		if (I_IUCLC(tty) && L_IEXTEN(tty))
+			c = tolower(c);
+		n_tty_receive_char(tty, c);
+	} else
+		n_tty_receive_char_flagged(tty, c, flag);
+}
+
+static void
+n_tty_receive_buf_real_raw(struct tty_struct *tty, const unsigned char *cp,
+			   char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t n, head;
+
+	head = ldata->read_head & (N_TTY_BUF_SIZE - 1);
+	n = min_t(size_t, count, N_TTY_BUF_SIZE - head);
+	memcpy(read_buf_addr(ldata, head), cp, n);
+	ldata->read_head += n;
+	cp += n;
+	count -= n;
+
+	head = ldata->read_head & (N_TTY_BUF_SIZE - 1);
+	n = min_t(size_t, count, N_TTY_BUF_SIZE - head);
+	memcpy(read_buf_addr(ldata, head), cp, n);
+	ldata->read_head += n;
+}
+
+static void
+n_tty_receive_buf_raw(struct tty_struct *tty, const unsigned char *cp,
+		      char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	char flag = TTY_NORMAL;
+
+	while (count--) {
+		if (fp)
+			flag = *fp++;
+		if (likely(flag == TTY_NORMAL))
+			put_tty_queue(*cp++, ldata);
+		else
+			n_tty_receive_char_flagged(tty, *cp++, flag);
+	}
+}
+
+static void
+n_tty_receive_buf_closing(struct tty_struct *tty, const unsigned char *cp,
+			  char *fp, int count)
+{
+	char flag = TTY_NORMAL;
+
+	while (count--) {
+		if (fp)
+			flag = *fp++;
+		if (likely(flag == TTY_NORMAL))
+			n_tty_receive_char_closing(tty, *cp++);
+	}
+}
+
+static void
+n_tty_receive_buf_standard(struct tty_struct *tty, const unsigned char *cp,
+			  char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	char flag = TTY_NORMAL;
+
+	while (count--) {
+		if (fp)
+			flag = *fp++;
+		if (likely(flag == TTY_NORMAL)) {
+			unsigned char c = *cp++;
+
+			if (I_ISTRIP(tty))
+				c &= 0x7f;
+			if (I_IUCLC(tty) && L_IEXTEN(tty))
+				c = tolower(c);
+			if (L_EXTPROC(tty)) {
+				put_tty_queue(c, ldata);
+				continue;
+			}
+			if (!test_bit(c, ldata->char_map))
+				n_tty_receive_char_inline(tty, c);
+			else if (n_tty_receive_char_special(tty, c) && count) {
+				if (fp)
+					flag = *fp++;
+				n_tty_receive_char_lnext(tty, *cp++, flag);
+				count--;
+			}
+		} else
+			n_tty_receive_char_flagged(tty, *cp++, flag);
+	}
+}
+
+static void
+n_tty_receive_buf_fast(struct tty_struct *tty, const unsigned char *cp,
+		       char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	char flag = TTY_NORMAL;
+
+	while (count--) {
+		if (fp)
+			flag = *fp++;
+		if (likely(flag == TTY_NORMAL)) {
+			unsigned char c = *cp++;
+
+			if (!test_bit(c, ldata->char_map))
+				n_tty_receive_char_fast(tty, c);
+			else if (n_tty_receive_char_special(tty, c) && count) {
+				if (fp)
+					flag = *fp++;
+				n_tty_receive_char_lnext(tty, *cp++, flag);
+				count--;
+			}
+		} else
+			n_tty_receive_char_flagged(tty, *cp++, flag);
+	}
+}
+
+static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
+			  char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	bool preops = I_ISTRIP(tty) || (I_IUCLC(tty) && L_IEXTEN(tty));
+
+	if (ldata->real_raw)
+		n_tty_receive_buf_real_raw(tty, cp, fp, count);
+	else if (ldata->raw || (L_EXTPROC(tty) && !preops))
+		n_tty_receive_buf_raw(tty, cp, fp, count);
+	else if (tty->closing && !L_EXTPROC(tty))
+		n_tty_receive_buf_closing(tty, cp, fp, count);
+	else {
+		if (ldata->lnext) {
+			char flag = TTY_NORMAL;
+
+			if (fp)
+				flag = *fp++;
+			n_tty_receive_char_lnext(tty, *cp++, flag);
+			count--;
+		}
+
+		if (!preops && !I_PARMRK(tty))
+			n_tty_receive_buf_fast(tty, cp, fp, count);
+		else
+			n_tty_receive_buf_standard(tty, cp, fp, count);
+
+		flush_echoes(tty);
+		if (tty->ops->flush_chars)
+			tty->ops->flush_chars(tty);
+	}
+
+	if (ldata->icanon && !L_EXTPROC(tty))
+		return;
+
+	/* publish read_head to consumer */
+	smp_store_release(&ldata->commit_head, ldata->read_head);
+
+	if (read_cnt(ldata)) {
+		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+		wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
+	}
+}
+
+/**
+ *	n_tty_receive_buf_common	-	process input
+ *	@tty: device to receive input
+ *	@cp: input chars
+ *	@fp: flags for each char (if NULL, all chars are TTY_NORMAL)
+ *	@count: number of input chars in @cp
+ *
+ *	Called by the terminal driver when a block of characters has
+ *	been received. This function must be called from soft contexts
+ *	not from interrupt context. The driver is responsible for making
+ *	calls one at a time and in order (or using flush_to_ldisc)
+ *
+ *	Returns the # of input chars from @cp which were processed.
+ *
+ *	In canonical mode, the maximum line length is 4096 chars (including
+ *	the line termination char); lines longer than 4096 chars are
+ *	truncated. After 4095 chars, input data is still processed but
+ *	not stored. Overflow processing ensures the tty can always
+ *	receive more input until at least one line can be read.
+ *
+ *	In non-canonical mode, the read buffer will only accept 4095 chars;
+ *	this provides the necessary space for a newline char if the input
+ *	mode is switched to canonical.
+ *
+ *	Note it is possible for the read buffer to _contain_ 4096 chars
+ *	in non-canonical mode: the read buffer could already contain the
+ *	maximum canon line of 4096 chars when the mode is switched to
+ *	non-canonical.
+ *
+ *	n_tty_receive_buf()/producer path:
+ *		claims non-exclusive termios_rwsem
+ *		publishes commit_head or canon_head
+ */
+static int
+n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
+			 char *fp, int count, int flow)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int room, n, rcvd = 0, overflow;
+
+	down_read(&tty->termios_rwsem);
+
+	do {
+		/*
+		 * When PARMRK is set, each input char may take up to 3 chars
+		 * in the read buf; reduce the buffer space avail by 3x
+		 *
+		 * If we are doing input canonicalization, and there are no
+		 * pending newlines, let characters through without limit, so
+		 * that erase characters will be handled.  Other excess
+		 * characters will be beeped.
+		 *
+		 * paired with store in *_copy_from_read_buf() -- guarantees
+		 * the consumer has loaded the data in read_buf up to the new
+		 * read_tail (so this producer will not overwrite unread data)
+		 */
+		size_t tail = smp_load_acquire(&ldata->read_tail);
+
+		room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
+		if (I_PARMRK(tty))
+			room = (room + 2) / 3;
+		room--;
+		if (room <= 0) {
+			overflow = ldata->icanon && ldata->canon_head == tail;
+			if (overflow && room < 0)
+				ldata->read_head--;
+			room = overflow;
+			ldata->no_room = flow && !room;
+		} else
+			overflow = 0;
+
+		n = min(count, room);
+		if (!n)
+			break;
+
+		/* ignore parity errors if handling overflow */
+		if (!overflow || !fp || *fp != TTY_PARITY)
+			__receive_buf(tty, cp, fp, n);
+
+		cp += n;
+		if (fp)
+			fp += n;
+		count -= n;
+		rcvd += n;
+	} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
+
+	tty->receive_room = room;
+
+	/* Unthrottle if handling overflow on pty */
+	if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
+		if (overflow) {
+			tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
+			tty_unthrottle_safe(tty);
+			__tty_set_flow_change(tty, 0);
+		}
+	} else
+		n_tty_check_throttle(tty);
+
+	up_read(&tty->termios_rwsem);
+
+	return rcvd;
+}
+
+static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
+			      char *fp, int count)
+{
+	n_tty_receive_buf_common(tty, cp, fp, count, 0);
+}
+
+static int n_tty_receive_buf2(struct tty_struct *tty, const unsigned char *cp,
+			      char *fp, int count)
+{
+	return n_tty_receive_buf_common(tty, cp, fp, count, 1);
+}
+
+/**
+ *	n_tty_set_termios	-	termios data changed
+ *	@tty: terminal
+ *	@old: previous data
+ *
+ *	Called by the tty layer when the user changes termios flags so
+ *	that the line discipline can plan ahead. This function cannot sleep
+ *	and is protected from re-entry by the tty layer. The user is
+ *	guaranteed that this function will not be re-entered or in progress
+ *	when the ldisc is closed.
+ *
+ *	Locking: Caller holds tty->termios_rwsem
+ */
+
+static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
+		bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
+		ldata->line_start = ldata->read_tail;
+		if (!L_ICANON(tty) || !read_cnt(ldata)) {
+			ldata->canon_head = ldata->read_tail;
+			ldata->push = 0;
+		} else {
+			set_bit((ldata->read_head - 1) & (N_TTY_BUF_SIZE - 1),
+				ldata->read_flags);
+			ldata->canon_head = ldata->read_head;
+			ldata->push = 1;
+		}
+		ldata->commit_head = ldata->read_head;
+		ldata->erasing = 0;
+		ldata->lnext = 0;
+	}
+
+	ldata->icanon = (L_ICANON(tty) != 0);
+
+	if (I_ISTRIP(tty) || I_IUCLC(tty) || I_IGNCR(tty) ||
+	    I_ICRNL(tty) || I_INLCR(tty) || L_ICANON(tty) ||
+	    I_IXON(tty) || L_ISIG(tty) || L_ECHO(tty) ||
+	    I_PARMRK(tty)) {
+		bitmap_zero(ldata->char_map, 256);
+
+		if (I_IGNCR(tty) || I_ICRNL(tty))
+			set_bit('\r', ldata->char_map);
+		if (I_INLCR(tty))
+			set_bit('\n', ldata->char_map);
+
+		if (L_ICANON(tty)) {
+			set_bit(ERASE_CHAR(tty), ldata->char_map);
+			set_bit(KILL_CHAR(tty), ldata->char_map);
+			set_bit(EOF_CHAR(tty), ldata->char_map);
+			set_bit('\n', ldata->char_map);
+			set_bit(EOL_CHAR(tty), ldata->char_map);
+			if (L_IEXTEN(tty)) {
+				set_bit(WERASE_CHAR(tty), ldata->char_map);
+				set_bit(LNEXT_CHAR(tty), ldata->char_map);
+				set_bit(EOL2_CHAR(tty), ldata->char_map);
+				if (L_ECHO(tty))
+					set_bit(REPRINT_CHAR(tty),
+						ldata->char_map);
+			}
+		}
+		if (I_IXON(tty)) {
+			set_bit(START_CHAR(tty), ldata->char_map);
+			set_bit(STOP_CHAR(tty), ldata->char_map);
+		}
+		if (L_ISIG(tty)) {
+			set_bit(INTR_CHAR(tty), ldata->char_map);
+			set_bit(QUIT_CHAR(tty), ldata->char_map);
+			set_bit(SUSP_CHAR(tty), ldata->char_map);
+		}
+		clear_bit(__DISABLED_CHAR, ldata->char_map);
+		ldata->raw = 0;
+		ldata->real_raw = 0;
+	} else {
+		ldata->raw = 1;
+		if ((I_IGNBRK(tty) || (!I_BRKINT(tty) && !I_PARMRK(tty))) &&
+		    (I_IGNPAR(tty) || !I_INPCK(tty)) &&
+		    (tty->driver->flags & TTY_DRIVER_REAL_RAW))
+			ldata->real_raw = 1;
+		else
+			ldata->real_raw = 0;
+	}
+	/*
+	 * Fix tty hang when I_IXON(tty) is cleared, but the tty
+	 * been stopped by STOP_CHAR(tty) before it.
+	 */
+	if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow_stopped) {
+		start_tty(tty);
+		process_echoes(tty);
+	}
+
+	/* The termios change make the tty ready for I/O */
+	wake_up_interruptible(&tty->write_wait);
+	wake_up_interruptible(&tty->read_wait);
+}
+
+/**
+ *	n_tty_close		-	close the ldisc for this tty
+ *	@tty: device
+ *
+ *	Called from the terminal layer when this line discipline is
+ *	being shut down, either because of a close or becsuse of a
+ *	discipline change. The function will not be called while other
+ *	ldisc methods are in progress.
+ */
+
+static void n_tty_close(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+
+	if (tty->link)
+		n_tty_packet_mode_flush(tty);
+
+	vfree(ldata);
+	tty->disc_data = NULL;
+}
+
+/**
+ *	n_tty_open		-	open an ldisc
+ *	@tty: terminal to open
+ *
+ *	Called when this line discipline is being attached to the
+ *	terminal device. Can sleep. Called serialized so that no
+ *	other events will occur in parallel. No further open will occur
+ *	until a close.
+ */
+
+static int n_tty_open(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata;
+
+	/* Currently a malloc failure here can panic */
+	ldata = vzalloc(sizeof(*ldata));
+	if (!ldata)
+		return -ENOMEM;
+
+	ldata->overrun_time = jiffies;
+	mutex_init(&ldata->atomic_read_lock);
+	mutex_init(&ldata->output_lock);
+
+	tty->disc_data = ldata;
+	tty->closing = 0;
+	/* indicate buffer work may resume */
+	clear_bit(TTY_LDISC_HALTED, &tty->flags);
+	n_tty_set_termios(tty, NULL);
+	tty_unthrottle(tty);
+	return 0;
+}
+
+static inline int input_available_p(struct tty_struct *tty, int poll)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
+
+	if (ldata->icanon && !L_EXTPROC(tty))
+		return ldata->canon_head != ldata->read_tail;
+	else
+		return ldata->commit_head - ldata->read_tail >= amt;
+}
+
+/**
+ *	copy_from_read_buf	-	copy read data directly
+ *	@tty: terminal device
+ *	@kbp: data
+ *	@nr: size of data
+ *
+ *	Helper function to speed up n_tty_read.  It is only called when
+ *	ICANON is off; it copies characters straight from the tty queue.
+ *
+ *	Called under the ldata->atomic_read_lock sem
+ *
+ *	Returns true if it successfully copied data, but there is still
+ *	more data to be had.
+ *
+ *	n_tty_read()/consumer path:
+ *		caller holds non-exclusive termios_rwsem
+ *		read_tail published
+ */
+
+static bool copy_from_read_buf(struct tty_struct *tty,
+				      unsigned char **kbp,
+				      size_t *nr)
+
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t n;
+	bool is_eof;
+	size_t head = smp_load_acquire(&ldata->commit_head);
+	size_t tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
+
+	n = min(head - ldata->read_tail, N_TTY_BUF_SIZE - tail);
+	n = min(*nr, n);
+	if (n) {
+		unsigned char *from = read_buf_addr(ldata, tail);
+		memcpy(*kbp, from, n);
+		is_eof = n == 1 && *from == EOF_CHAR(tty);
+		tty_audit_add_data(tty, from, n);
+		zero_buffer(tty, from, n);
+		smp_store_release(&ldata->read_tail, ldata->read_tail + n);
+		/* Turn single EOF into zero-length read */
+		if (L_EXTPROC(tty) && ldata->icanon && is_eof &&
+		    (head == ldata->read_tail))
+			return false;
+		*kbp += n;
+		*nr -= n;
+
+		/* If we have more to copy, let the caller know */
+		return head != ldata->read_tail;
+	}
+	return false;
+}
+
+/**
+ *	canon_copy_from_read_buf	-	copy read data in canonical mode
+ *	@tty: terminal device
+ *	@kbp: data
+ *	@nr: size of data
+ *
+ *	Helper function for n_tty_read.  It is only called when ICANON is on;
+ *	it copies one line of input up to and including the line-delimiting
+ *	character into the result buffer.
+ *
+ *	NB: When termios is changed from non-canonical to canonical mode and
+ *	the read buffer contains data, n_tty_set_termios() simulates an EOF
+ *	push (as if C-d were input) _without_ the DISABLED_CHAR in the buffer.
+ *	This causes data already processed as input to be immediately available
+ *	as input although a newline has not been received.
+ *
+ *	Called under the atomic_read_lock mutex
+ *
+ *	n_tty_read()/consumer path:
+ *		caller holds non-exclusive termios_rwsem
+ *		read_tail published
+ */
+
+static bool canon_copy_from_read_buf(struct tty_struct *tty,
+				     unsigned char **kbp,
+				     size_t *nr)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t n, size, more, c;
+	size_t eol;
+	size_t tail, canon_head;
+	int found = 0;
+
+	/* N.B. avoid overrun if nr == 0 */
+	if (!*nr)
+		return false;
+
+	canon_head = smp_load_acquire(&ldata->canon_head);
+	n = min(*nr, canon_head - ldata->read_tail);
+
+	tail = ldata->read_tail & (N_TTY_BUF_SIZE - 1);
+	size = min_t(size_t, tail + n, N_TTY_BUF_SIZE);
+
+	n_tty_trace("%s: nr:%zu tail:%zu n:%zu size:%zu\n",
+		    __func__, *nr, tail, n, size);
+
+	eol = find_next_bit(ldata->read_flags, size, tail);
+	more = n - (size - tail);
+	if (eol == N_TTY_BUF_SIZE && more) {
+		/* scan wrapped without finding set bit */
+		eol = find_next_bit(ldata->read_flags, more, 0);
+		found = eol != more;
+	} else
+		found = eol != size;
+
+	n = eol - tail;
+	if (n > N_TTY_BUF_SIZE)
+		n += N_TTY_BUF_SIZE;
+	c = n + found;
+
+	if (!found || read_buf(ldata, eol) != __DISABLED_CHAR)
+		n = c;
+
+	n_tty_trace("%s: eol:%zu found:%d n:%zu c:%zu tail:%zu more:%zu\n",
+		    __func__, eol, found, n, c, tail, more);
+
+	tty_copy(tty, *kbp, tail, n);
+	*kbp += n;
+	*nr -= n;
+
+	if (found)
+		clear_bit(eol, ldata->read_flags);
+	smp_store_release(&ldata->read_tail, ldata->read_tail + c);
+
+	if (found) {
+		if (!ldata->push)
+			ldata->line_start = ldata->read_tail;
+		else
+			ldata->push = 0;
+		tty_audit_push();
+		return false;
+	}
+
+	/* No EOL found - do a continuation retry if there is more data */
+	return ldata->read_tail != canon_head;
+}
+
+/*
+ * If we finished a read at the exact location of an
+ * EOF (special EOL character that's a __DISABLED_CHAR)
+ * in the stream, silently eat the EOF.
+ */
+static void canon_skip_eof(struct tty_struct *tty)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	size_t tail, canon_head;
+
+	canon_head = smp_load_acquire(&ldata->canon_head);
+	tail = ldata->read_tail;
+
+	// No data?
+	if (tail == canon_head)
+		return;
+
+	// See if the tail position is EOF in the circular buffer
+	tail &= (N_TTY_BUF_SIZE - 1);
+	if (!test_bit(tail, ldata->read_flags))
+		return;
+	if (read_buf(ldata, tail) != __DISABLED_CHAR)
+		return;
+
+	// Clear the EOL bit, skip the EOF char.
+	clear_bit(tail, ldata->read_flags);
+	smp_store_release(&ldata->read_tail, ldata->read_tail + 1);
+}
+
+/**
+ *	job_control		-	check job control
+ *	@tty: tty
+ *	@file: file handle
+ *
+ *	Perform job control management checks on this file/tty descriptor
+ *	and if appropriate send any needed signals and return a negative
+ *	error code if action should be taken.
+ *
+ *	Locking: redirected write test is safe
+ *		 current->signal->tty check is safe
+ *		 ctrl_lock to safely reference tty->pgrp
+ */
+
+static int job_control(struct tty_struct *tty, struct file *file)
+{
+	/* Job control check -- must be done at start and after
+	   every sleep (POSIX.1 7.1.1.4). */
+	/* NOTE: not yet done after every sleep pending a thorough
+	   check of the logic of this change. -- jlc */
+	/* don't stop on /dev/console */
+	if (file->f_op->write_iter == redirected_tty_write)
+		return 0;
+
+	return __tty_check_change(tty, SIGTTIN);
+}
+
+
+/**
+ *	n_tty_read		-	read function for tty
+ *	@tty: tty device
+ *	@file: file object
+ *	@buf: userspace buffer pointer
+ *	@nr: size of I/O
+ *
+ *	Perform reads for the line discipline. We are guaranteed that the
+ *	line discipline will not be closed under us but we may get multiple
+ *	parallel readers and must handle this ourselves. We may also get
+ *	a hangup. Always called in user context, may sleep.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ *
+ *	n_tty_read()/consumer path:
+ *		claims non-exclusive termios_rwsem
+ *		publishes read_tail
+ */
+
+static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
+			  unsigned char *kbuf, size_t nr,
+			  void **cookie, unsigned long offset)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	unsigned char *kb = kbuf;
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	int c;
+	int minimum, time;
+	ssize_t retval = 0;
+	long timeout;
+	int packet;
+	size_t tail;
+
+	/*
+	 * Is this a continuation of a read started earler?
+	 *
+	 * If so, we still hold the atomic_read_lock and the
+	 * termios_rwsem, and can just continue to copy data.
+	 */
+	if (*cookie) {
+		if (ldata->icanon && !L_EXTPROC(tty)) {
+			/*
+			 * If we have filled the user buffer, see
+			 * if we should skip an EOF character before
+			 * releasing the lock and returning done.
+			 */
+			if (!nr)
+				canon_skip_eof(tty);
+			else if (canon_copy_from_read_buf(tty, &kb, &nr))
+				return kb - kbuf;
+		} else {
+			if (copy_from_read_buf(tty, &kb, &nr))
+				return kb - kbuf;
+		}
+
+		/* No more data - release locks and stop retries */
+		n_tty_kick_worker(tty);
+		n_tty_check_unthrottle(tty);
+		up_read(&tty->termios_rwsem);
+		mutex_unlock(&ldata->atomic_read_lock);
+		*cookie = NULL;
+		return kb - kbuf;
+	}
+
+	c = job_control(tty, file);
+	if (c < 0)
+		return c;
+
+	/*
+	 *	Internal serialization of reads.
+	 */
+	if (file->f_flags & O_NONBLOCK) {
+		if (!mutex_trylock(&ldata->atomic_read_lock))
+			return -EAGAIN;
+	} else {
+		if (mutex_lock_interruptible(&ldata->atomic_read_lock))
+			return -ERESTARTSYS;
+	}
+
+	down_read(&tty->termios_rwsem);
+
+	minimum = time = 0;
+	timeout = MAX_SCHEDULE_TIMEOUT;
+	if (!ldata->icanon) {
+		minimum = MIN_CHAR(tty);
+		if (minimum) {
+			time = (HZ / 10) * TIME_CHAR(tty);
+		} else {
+			timeout = (HZ / 10) * TIME_CHAR(tty);
+			minimum = 1;
+		}
+	}
+
+	packet = tty->packet;
+	tail = ldata->read_tail;
+
+	add_wait_queue(&tty->read_wait, &wait);
+	while (nr) {
+		/* First test for status change. */
+		if (packet && tty->link->ctrl_status) {
+			unsigned char cs;
+			if (kb != kbuf)
+				break;
+			spin_lock_irq(&tty->link->ctrl_lock);
+			cs = tty->link->ctrl_status;
+			tty->link->ctrl_status = 0;
+			spin_unlock_irq(&tty->link->ctrl_lock);
+			*kb++ = cs;
+			nr--;
+			break;
+		}
+
+		if (!input_available_p(tty, 0)) {
+			up_read(&tty->termios_rwsem);
+			tty_buffer_flush_work(tty->port);
+			down_read(&tty->termios_rwsem);
+			if (!input_available_p(tty, 0)) {
+				if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
+					retval = -EIO;
+					break;
+				}
+				if (tty_hung_up_p(file))
+					break;
+				/*
+				 * Abort readers for ttys which never actually
+				 * get hung up.  See __tty_hangup().
+				 */
+				if (test_bit(TTY_HUPPING, &tty->flags))
+					break;
+				if (!timeout)
+					break;
+				if (tty_io_nonblock(tty, file)) {
+					retval = -EAGAIN;
+					break;
+				}
+				if (signal_pending(current)) {
+					retval = -ERESTARTSYS;
+					break;
+				}
+				up_read(&tty->termios_rwsem);
+
+				timeout = wait_woken(&wait, TASK_INTERRUPTIBLE,
+						timeout);
+
+				down_read(&tty->termios_rwsem);
+				continue;
+			}
+		}
+
+		if (ldata->icanon && !L_EXTPROC(tty)) {
+			if (canon_copy_from_read_buf(tty, &kb, &nr))
+				goto more_to_be_read;
+		} else {
+			/* Deal with packet mode. */
+			if (packet && kb == kbuf) {
+				*kb++ = TIOCPKT_DATA;
+				nr--;
+			}
+
+			/*
+			 * Copy data, and if there is more to be had
+			 * and we have nothing more to wait for, then
+			 * let's mark us for retries.
+			 *
+			 * NOTE! We return here with both the termios_sem
+			 * and atomic_read_lock still held, the retries
+			 * will release them when done.
+			 */
+			if (copy_from_read_buf(tty, &kb, &nr) && kb - kbuf >= minimum) {
+more_to_be_read:
+				remove_wait_queue(&tty->read_wait, &wait);
+				*cookie = cookie;
+				return kb - kbuf;
+			}
+		}
+
+		n_tty_check_unthrottle(tty);
+
+		if (kb - kbuf >= minimum)
+			break;
+		if (time)
+			timeout = time;
+	}
+	if (tail != ldata->read_tail)
+		n_tty_kick_worker(tty);
+	up_read(&tty->termios_rwsem);
+
+	remove_wait_queue(&tty->read_wait, &wait);
+	mutex_unlock(&ldata->atomic_read_lock);
+
+	if (kb - kbuf)
+		retval = kb - kbuf;
+
+	return retval;
+}
+
+/**
+ *	n_tty_write		-	write function for tty
+ *	@tty: tty device
+ *	@file: file object
+ *	@buf: userspace buffer pointer
+ *	@nr: size of I/O
+ *
+ *	Write function of the terminal device.  This is serialized with
+ *	respect to other write callers but not to termios changes, reads
+ *	and other such events.  Since the receive code will echo characters,
+ *	thus calling driver write methods, the output_lock is used in
+ *	the output processing functions called here as well as in the
+ *	echo processing function to protect the column state and space
+ *	left in the buffer.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ *
+ *	Locking: output_lock to protect column state and space left
+ *		 (note that the process_output*() functions take this
+ *		  lock themselves)
+ */
+
+static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
+			   const unsigned char *buf, size_t nr)
+{
+	const unsigned char *b = buf;
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
+	int c;
+	ssize_t retval = 0;
+
+	/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
+	if (L_TOSTOP(tty) && file->f_op->write_iter != redirected_tty_write) {
+		retval = tty_check_change(tty);
+		if (retval)
+			return retval;
+	}
+
+	down_read(&tty->termios_rwsem);
+
+	/* Write out any echoed characters that are still pending */
+	process_echoes(tty);
+
+	add_wait_queue(&tty->write_wait, &wait);
+	while (1) {
+		if (signal_pending(current)) {
+			retval = -ERESTARTSYS;
+			break;
+		}
+		if (tty_hung_up_p(file) || (tty->link && !tty->link->count)) {
+			retval = -EIO;
+			break;
+		}
+		if (O_OPOST(tty)) {
+			while (nr > 0) {
+				ssize_t num = process_output_block(tty, b, nr);
+				if (num < 0) {
+					if (num == -EAGAIN)
+						break;
+					retval = num;
+					goto break_out;
+				}
+				b += num;
+				nr -= num;
+				if (nr == 0)
+					break;
+				c = *b;
+				if (process_output(c, tty) < 0)
+					break;
+				b++; nr--;
+			}
+			if (tty->ops->flush_chars)
+				tty->ops->flush_chars(tty);
+		} else {
+			struct n_tty_data *ldata = tty->disc_data;
+
+			while (nr > 0) {
+				mutex_lock(&ldata->output_lock);
+				c = tty->ops->write(tty, b, nr);
+				mutex_unlock(&ldata->output_lock);
+				if (c < 0) {
+					retval = c;
+					goto break_out;
+				}
+				if (!c)
+					break;
+				b += c;
+				nr -= c;
+			}
+		}
+		if (!nr)
+			break;
+		if (tty_io_nonblock(tty, file)) {
+			retval = -EAGAIN;
+			break;
+		}
+		up_read(&tty->termios_rwsem);
+
+		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
+
+		down_read(&tty->termios_rwsem);
+	}
+break_out:
+	remove_wait_queue(&tty->write_wait, &wait);
+	if (nr && tty->fasync)
+		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	up_read(&tty->termios_rwsem);
+	return (b - buf) ? b - buf : retval;
+}
+
+/**
+ *	n_tty_poll		-	poll method for N_TTY
+ *	@tty: terminal device
+ *	@file: file accessing it
+ *	@wait: poll table
+ *
+ *	Called when the line discipline is asked to poll() for data or
+ *	for special events. This code is not serialized with respect to
+ *	other events save open/close.
+ *
+ *	This code must be sure never to sleep through a hangup.
+ *	Called without the kernel lock held - fine
+ */
+
+static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
+							poll_table *wait)
+{
+	__poll_t mask = 0;
+
+	poll_wait(file, &tty->read_wait, wait);
+	poll_wait(file, &tty->write_wait, wait);
+	if (input_available_p(tty, 1))
+		mask |= EPOLLIN | EPOLLRDNORM;
+	else {
+		tty_buffer_flush_work(tty->port);
+		if (input_available_p(tty, 1))
+			mask |= EPOLLIN | EPOLLRDNORM;
+	}
+	if (tty->packet && tty->link->ctrl_status)
+		mask |= EPOLLPRI | EPOLLIN | EPOLLRDNORM;
+	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
+		mask |= EPOLLHUP;
+	if (tty_hung_up_p(file))
+		mask |= EPOLLHUP;
+	if (tty->ops->write && !tty_is_writelocked(tty) &&
+			tty_chars_in_buffer(tty) < WAKEUP_CHARS &&
+			tty_write_room(tty) > 0)
+		mask |= EPOLLOUT | EPOLLWRNORM;
+	return mask;
+}
+
+static unsigned long inq_canon(struct n_tty_data *ldata)
+{
+	size_t nr, head, tail;
+
+	if (ldata->canon_head == ldata->read_tail)
+		return 0;
+	head = ldata->canon_head;
+	tail = ldata->read_tail;
+	nr = head - tail;
+	/* Skip EOF-chars.. */
+	while (MASK(head) != MASK(tail)) {
+		if (test_bit(tail & (N_TTY_BUF_SIZE - 1), ldata->read_flags) &&
+		    read_buf(ldata, tail) == __DISABLED_CHAR)
+			nr--;
+		tail++;
+	}
+	return nr;
+}
+
+static int n_tty_ioctl(struct tty_struct *tty, struct file *file,
+		       unsigned int cmd, unsigned long arg)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int retval;
+
+	switch (cmd) {
+	case TIOCOUTQ:
+		return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
+	case TIOCINQ:
+		down_write(&tty->termios_rwsem);
+		if (L_ICANON(tty) && !L_EXTPROC(tty))
+			retval = inq_canon(ldata);
+		else
+			retval = read_cnt(ldata);
+		up_write(&tty->termios_rwsem);
+		return put_user(retval, (unsigned int __user *) arg);
+	default:
+		return n_tty_ioctl_helper(tty, file, cmd, arg);
+	}
+}
+
+static struct tty_ldisc_ops n_tty_ops = {
+	.magic           = TTY_LDISC_MAGIC,
+	.name            = "n_tty",
+	.open            = n_tty_open,
+	.close           = n_tty_close,
+	.flush_buffer    = n_tty_flush_buffer,
+	.read            = n_tty_read,
+	.write           = n_tty_write,
+	.ioctl           = n_tty_ioctl,
+	.set_termios     = n_tty_set_termios,
+	.poll            = n_tty_poll,
+	.receive_buf     = n_tty_receive_buf,
+	.write_wakeup    = n_tty_write_wakeup,
+	.receive_buf2	 = n_tty_receive_buf2,
+};
+
+/**
+ *	n_tty_inherit_ops	-	inherit N_TTY methods
+ *	@ops: struct tty_ldisc_ops where to save N_TTY methods
+ *
+ *	Enables a 'subclass' line discipline to 'inherit' N_TTY methods.
+ */
+
+void n_tty_inherit_ops(struct tty_ldisc_ops *ops)
+{
+	*ops = n_tty_ops;
+	ops->owner = NULL;
+	ops->refcount = ops->flags = 0;
+}
+EXPORT_SYMBOL_GPL(n_tty_inherit_ops);
+
+void __init n_tty_init(void)
+{
+	tty_register_ldisc(N_TTY, &n_tty_ops);
+}