drivers/tty/n_tty.c

/*
 * 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.
 *
 * This file may be redistributed under the terms of the GNU General Public
 * License.
 *
 * 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>


/* number of characters left in xmit buffer before select has we have room */
#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

struct n_tty_data {
	unsigned int column;
	unsigned long overrun_time;
	int num_overrun;

	unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
	unsigned char echo_overrun:1;

	DECLARE_BITMAP(process_char_map, 256);
	DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);

	char *read_buf;
	int read_head;
	int read_tail;
	int read_cnt;

	unsigned char *echo_buf;
	unsigned int echo_pos;
	unsigned int echo_cnt;

	int canon_data;
	unsigned long canon_head;
	unsigned int canon_column;
};

static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
			       unsigned char __user *ptr)
{
	struct n_tty_data *ldata = tty->disc_data;

	tty_audit_add_data(tty, &x, 1, ldata->icanon);
	return put_user(x, ptr);
}

/**
 *	n_tty_set__room	-	receive space
 *	@tty: terminal
 *
 *	Called by the driver to find out how much data it is
 *	permitted to feed to the line discipline without any being lost
 *	and thus to manage flow control. Not serialized. Answers for the
 *	"instant".
 */

static void n_tty_set_room(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	int left;
	int old_left;

	/* ldata->read_cnt is not read locked ? */
	if (I_PARMRK(tty)) {
		/* Multiply read_cnt by 3, since each byte might take up to
		 * three times as many spaces when PARMRK is set (depending on
		 * its flags, e.g. parity error). */
		left = N_TTY_BUF_SIZE - ldata->read_cnt * 3 - 1;
	} else
		left = N_TTY_BUF_SIZE - ldata->read_cnt - 1;

	/*
	 * 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.
	 */
	if (left <= 0)
		left = ldata->icanon && !ldata->canon_data;
	old_left = tty->receive_room;
	tty->receive_room = left;

	/* Did this open up the receive buffer? We may need to flip */
	if (left && !old_left)
		schedule_work(&tty->buf.work);
}

static void put_tty_queue_nolock(unsigned char c, struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;

	if (ldata->read_cnt < N_TTY_BUF_SIZE) {
		ldata->read_buf[ldata->read_head] = c;
		ldata->read_head = (ldata->read_head + 1) & (N_TTY_BUF_SIZE-1);
		ldata->read_cnt++;
	}
}

/**
 *	put_tty_queue		-	add character to tty
 *	@c: character
 *	@tty: tty device
 *
 *	Add a character to the tty read_buf queue. This is done under the
 *	read_lock to serialize character addition and also to protect us
 *	against parallel reads or flushes
 */

static void put_tty_queue(unsigned char c, struct tty_struct *tty)
{
	unsigned long flags;
	/*
	 *	The problem of stomping on the buffers ends here.
	 *	Why didn't anyone see this one coming? --AJK
	*/
	spin_lock_irqsave(&tty->read_lock, flags);
	put_tty_queue_nolock(c, tty);
	spin_unlock_irqrestore(&tty->read_lock, flags);
}

/**
 *	check_unthrottle	-	allow new receive data
 *	@tty; tty device
 *
 *	Check whether to call the driver unthrottle functions
 *
 *	Can sleep, may be called under the atomic_read_lock mutex but
 *	this is not guaranteed.
 */
static void check_unthrottle(struct tty_struct *tty)
{
	if (tty->count)
		tty_unthrottle(tty);
}

/**
 *	reset_buffer_flags	-	reset buffer state
 *	@tty: terminal to reset
 *
 *	Reset the read buffer counters, clear the flags,
 *	and make sure the driver is unthrottled. Called
 *	from n_tty_open() and n_tty_flush_buffer().
 *
 *	Locking: tty_read_lock for read fields.
 */

static void reset_buffer_flags(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	unsigned long flags;

	spin_lock_irqsave(&tty->read_lock, flags);
	ldata->read_head = ldata->read_tail = ldata->read_cnt = 0;
	spin_unlock_irqrestore(&tty->read_lock, flags);

	mutex_lock(&tty->echo_lock);
	ldata->echo_pos = ldata->echo_cnt = ldata->echo_overrun = 0;
	mutex_unlock(&tty->echo_lock);

	ldata->canon_head = ldata->canon_data = ldata->erasing = 0;
	bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
	n_tty_set_room(tty);
}

/**
 *	n_tty_flush_buffer	-	clean input queue
 *	@tty:	terminal device
 *
 *	Flush the input buffer. Called when the line discipline is
 *	being closed, 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).
 *
 *	Locking: ctrl_lock, read_lock.
 */

static void n_tty_flush_buffer(struct tty_struct *tty)
{
	unsigned long flags;
	/* clear everything and unthrottle the driver */
	reset_buffer_flags(tty);

	if (!tty->link)
		return;

	spin_lock_irqsave(&tty->ctrl_lock, flags);
	if (tty->link->packet) {
		tty->ctrl_status |= TIOCPKT_FLUSHREAD;
		wake_up_interruptible(&tty->link->read_wait);
	}
	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
}

/**
 *	n_tty_chars_in_buffer	-	report available bytes
 *	@tty: tty device
 *
 *	Report the number of characters buffered to be delivered to user
 *	at this instant in time.
 *
 *	Locking: read_lock
 */

static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	unsigned long flags;
	ssize_t n = 0;

	spin_lock_irqsave(&tty->read_lock, flags);
	if (!ldata->icanon) {
		n = ldata->read_cnt;
	} else if (ldata->canon_data) {
		n = (ldata->canon_head > ldata->read_tail) ?
			ldata->canon_head - ldata->read_tail :
			ldata->canon_head + (N_TTY_BUF_SIZE - ldata->read_tail);
	}
	spin_unlock_irqrestore(&tty->read_lock, flags);
	return n;
}

/**
 *	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)
{
	int	space, retval;

	mutex_lock(&tty->output_lock);

	space = tty_write_room(tty);
	retval = do_output_char(c, tty, space);

	mutex_unlock(&tty->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(&tty->output_lock);

	space = tty_write_room(tty);
	if (!space) {
		mutex_unlock(&tty->output_lock);
		return 0;
	}
	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(&tty->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: output_lock to protect column state and space left,
 *		 echo_lock to protect the echo buffer
 */

static void process_echoes(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	int	space, nr;
	unsigned char c;
	unsigned char *cp, *buf_end;

	if (!ldata->echo_cnt)
		return;

	mutex_lock(&tty->output_lock);
	mutex_lock(&tty->echo_lock);

	space = tty_write_room(tty);

	buf_end = ldata->echo_buf + N_TTY_BUF_SIZE;
	cp = ldata->echo_buf + ldata->echo_pos;
	nr = ldata->echo_cnt;
	while (nr > 0) {
		c = *cp;
		if (c == ECHO_OP_START) {
			unsigned char op;
			unsigned char *opp;
			int no_space_left = 0;

			/*
			 * 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.
			 */
			opp = cp + 1;
			if (opp == buf_end)
				opp -= N_TTY_BUF_SIZE;
			op = *opp;

			switch (op) {
				unsigned int num_chars, num_bs;

			case ECHO_OP_ERASE_TAB:
				if (++opp == buf_end)
					opp -= N_TTY_BUF_SIZE;
				num_chars = *opp;

				/*
				 * 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--;
				}
				cp += 3;
				nr -= 3;
				break;

			case ECHO_OP_SET_CANON_COL:
				ldata->canon_column = ldata->column;
				cp += 2;
				nr -= 2;
				break;

			case ECHO_OP_MOVE_BACK_COL:
				if (ldata->column > 0)
					ldata->column--;
				cp += 2;
				nr -= 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--;
				cp += 2;
				nr -= 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;
				cp += 2;
				nr -= 2;
			}

			if (no_space_left)
				break;
		} else {
			if (O_OPOST(tty) &&
			    !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
				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;
			}
			cp += 1;
			nr -= 1;
		}

		/* When end of circular buffer reached, wrap around */
		if (cp >= buf_end)
			cp -= N_TTY_BUF_SIZE;
	}

	if (nr == 0) {
		ldata->echo_pos = 0;
		ldata->echo_cnt = 0;
		ldata->echo_overrun = 0;
	} else {
		int num_processed = ldata->echo_cnt - nr;
		ldata->echo_pos += num_processed;
		ldata->echo_pos &= N_TTY_BUF_SIZE - 1;
		ldata->echo_cnt = nr;
		if (num_processed > 0)
			ldata->echo_overrun = 0;
	}

	mutex_unlock(&tty->echo_lock);
	mutex_unlock(&tty->output_lock);

	if (tty->ops->flush_chars)
		tty->ops->flush_chars(tty);
}

/**
 *	add_echo_byte	-	add a byte to the echo buffer
 *	@c: unicode byte to echo
 *	@tty: terminal device
 *
 *	Add a character or operation byte to the echo buffer.
 *
 *	Should be called under the echo lock to protect the echo buffer.
 */

static void add_echo_byte(unsigned char c, struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	int	new_byte_pos;

	if (ldata->echo_cnt == N_TTY_BUF_SIZE) {
		/* Circular buffer is already at capacity */
		new_byte_pos = ldata->echo_pos;

		/*
		 * Since the buffer start position needs to be advanced,
		 * be sure to step by a whole operation byte group.
		 */
		if (ldata->echo_buf[ldata->echo_pos] == ECHO_OP_START) {
			if (ldata->echo_buf[(ldata->echo_pos + 1) &
					  (N_TTY_BUF_SIZE - 1)] ==
						ECHO_OP_ERASE_TAB) {
				ldata->echo_pos += 3;
				ldata->echo_cnt -= 2;
			} else {
				ldata->echo_pos += 2;
				ldata->echo_cnt -= 1;
			}
		} else {
			ldata->echo_pos++;
		}
		ldata->echo_pos &= N_TTY_BUF_SIZE - 1;

		ldata->echo_overrun = 1;
	} else {
		new_byte_pos = ldata->echo_pos + ldata->echo_cnt;
		new_byte_pos &= N_TTY_BUF_SIZE - 1;
		ldata->echo_cnt++;
	}

	ldata->echo_buf[new_byte_pos] = c;
}

/**
 *	echo_move_back_col	-	add operation to move back a column
 *	@tty: terminal device
 *
 *	Add an operation to the echo buffer to move back one column.
 *
 *	Locking: echo_lock to protect the echo buffer
 */

static void echo_move_back_col(struct tty_struct *tty)
{
	mutex_lock(&tty->echo_lock);

	add_echo_byte(ECHO_OP_START, tty);
	add_echo_byte(ECHO_OP_MOVE_BACK_COL, tty);

	mutex_unlock(&tty->echo_lock);
}

/**
 *	echo_set_canon_col	-	add operation to set the canon column
 *	@tty: terminal device
 *
 *	Add an operation to the echo buffer to set the canon column
 *	to the current column.
 *
 *	Locking: echo_lock to protect the echo buffer
 */

static void echo_set_canon_col(struct tty_struct *tty)
{
	mutex_lock(&tty->echo_lock);

	add_echo_byte(ECHO_OP_START, tty);
	add_echo_byte(ECHO_OP_SET_CANON_COL, tty);

	mutex_unlock(&tty->echo_lock);
}

/**
 *	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
 *	@tty: terminal device
 *
 *	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.
 *
 *	Locking: echo_lock to protect the echo buffer
 */

static void echo_erase_tab(unsigned int num_chars, int after_tab,
			   struct tty_struct *tty)
{
	mutex_lock(&tty->echo_lock);

	add_echo_byte(ECHO_OP_START, tty);
	add_echo_byte(ECHO_OP_ERASE_TAB, tty);

	/* 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, tty);

	mutex_unlock(&tty->echo_lock);
}

/**
 *	echo_char_raw	-	echo a character raw
 *	@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 does not treat control characters specially.
 *
 *	Locking: echo_lock to protect the echo buffer
 */

static void echo_char_raw(unsigned char c, struct tty_struct *tty)
{
	mutex_lock(&tty->echo_lock);

	if (c == ECHO_OP_START) {
		add_echo_byte(ECHO_OP_START, tty);
		add_echo_byte(ECHO_OP_START, tty);
	} else {
		add_echo_byte(c, tty);
	}

	mutex_unlock(&tty->echo_lock);
}

/**
 *	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).
 *
 *	Locking: echo_lock to protect the echo buffer
 */

static void echo_char(unsigned char c, struct tty_struct *tty)
{
	mutex_lock(&tty->echo_lock);

	if (c == ECHO_OP_START) {
		add_echo_byte(ECHO_OP_START, tty);
		add_echo_byte(ECHO_OP_START, tty);
	} else {
		if (L_ECHOCTL(tty) && iscntrl(c) && c != '\t')
			add_echo_byte(ECHO_OP_START, tty);
		add_echo_byte(c, tty);
	}

	mutex_unlock(&tty->echo_lock);
}

/**
 *	finish_erasing		-	complete erase
 *	@tty: tty doing the erase
 */

static inline void finish_erasing(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	if (ldata->erasing) {
		echo_char_raw('/', tty);
		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.
 *
 *	Locking: read_lock for tty buffers
 */

static void eraser(unsigned char c, struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	enum { ERASE, WERASE, KILL } kill_type;
	int head, seen_alnums, cnt;
	unsigned long flags;

	/* FIXME: locking needed ? */
	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)) {
			spin_lock_irqsave(&tty->read_lock, flags);
			ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
					  (N_TTY_BUF_SIZE - 1));
			ldata->read_head = ldata->canon_head;
			spin_unlock_irqrestore(&tty->read_lock, flags);
			return;
		}
		if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
			spin_lock_irqsave(&tty->read_lock, flags);
			ldata->read_cnt -= ((ldata->read_head - ldata->canon_head) &
					  (N_TTY_BUF_SIZE - 1));
			ldata->read_head = ldata->canon_head;
			spin_unlock_irqrestore(&tty->read_lock, flags);
			finish_erasing(tty);
			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', tty);
			return;
		}
		kill_type = KILL;
	}

	seen_alnums = 0;
	/* FIXME: Locking ?? */
	while (ldata->read_head != ldata->canon_head) {
		head = ldata->read_head;

		/* erase a single possibly multibyte character */
		do {
			head = (head - 1) & (N_TTY_BUF_SIZE-1);
			c = ldata->read_buf[head];
		} while (is_continuation(c, tty) && head != 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) & (N_TTY_BUF_SIZE-1);
		spin_lock_irqsave(&tty->read_lock, flags);
		ldata->read_head = head;
		ldata->read_cnt -= cnt;
		spin_unlock_irqrestore(&tty->read_lock, flags);
		if (L_ECHO(tty)) {
			if (L_ECHOPRT(tty)) {
				if (!ldata->erasing) {
					echo_char_raw('\\', tty);
					ldata->erasing = 1;
				}
				/* if cnt > 1, output a multi-byte character */
				echo_char(c, tty);
				while (--cnt > 0) {
					head = (head+1) & (N_TTY_BUF_SIZE-1);
					echo_char_raw(ldata->read_buf[head], tty);
					echo_move_back_col(tty);
				}
			} 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;
				unsigned long 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 (tail != ldata->canon_head) {
					tail = (tail-1) & (N_TTY_BUF_SIZE-1);
					c = ldata->read_buf[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, tty);
			} else {
				if (iscntrl(c) && L_ECHOCTL(tty)) {
					echo_char_raw('\b', tty);
					echo_char_raw(' ', tty);
					echo_char_raw('\b', tty);
				}
				if (!iscntrl(c) || L_ECHOCTL(tty)) {
					echo_char_raw('\b', tty);
					echo_char_raw(' ', tty);
					echo_char_raw('\b', tty);
				}
			}
		}
		if (kill_type == ERASE)
			break;
	}
	if (ldata->read_head == ldata->canon_head && L_ECHO(tty))
		finish_erasing(tty);
}

/**
 *	isig		-	handle the ISIG optio
 *	@sig: signal
 *	@tty: terminal
 *	@flush: force flush
 *
 *	Called when a signal is being sent due to terminal input. This
 *	may caus terminal flushing to take place according to the termios
 *	settings and character used. Called from the driver receive_buf
 *	path so serialized.
 *
 *	Locking: ctrl_lock, read_lock (both via flush buffer)
 */

static inline void isig(int sig, struct tty_struct *tty, int flush)
{
	if (tty->pgrp)
		kill_pgrp(tty->pgrp, sig, 1);
	if (flush || !L_NOFLSH(tty)) {
		n_tty_flush_buffer(tty);
		tty_driver_flush_buffer(tty);
	}
}

/**
 *	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.
 *
 *	Called from the receive_buf path so single threaded.
 */

static inline void n_tty_receive_break(struct tty_struct *tty)
{
	if (I_IGNBRK(tty))
		return;
	if (I_BRKINT(tty)) {
		isig(SIGINT, tty, 1);
		return;
	}
	if (I_PARMRK(tty)) {
		put_tty_queue('\377', tty);
		put_tty_queue('\0', tty);
	}
	put_tty_queue('\0', tty);
	wake_up_interruptible(&tty->read_wait);
}

/**
 *	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 inline void n_tty_receive_overrun(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	char buf[64];

	ldata->num_overrun++;
	if (time_after(jiffies, ldata->overrun_time + HZ) ||
			time_after(ldata->overrun_time, jiffies)) {
		printk(KERN_WARNING "%s: %d input overrun(s)\n",
			tty_name(tty, buf),
			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. Locking as per n_tty_receive_buf.
 */
static inline void n_tty_receive_parity_error(struct tty_struct *tty,
					      unsigned char c)
{
	if (I_IGNPAR(tty))
		return;
	if (I_PARMRK(tty)) {
		put_tty_queue('\377', tty);
		put_tty_queue('\0', tty);
		put_tty_queue(c, tty);
	} else	if (I_INPCK(tty))
		put_tty_queue('\0', tty);
	else
		put_tty_queue(c, tty);
	wake_up_interruptible(&tty->read_wait);
}

/**
 *	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.
 */

static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
{
	struct n_tty_data *ldata = tty->disc_data;
	unsigned long flags;
	int parmrk;

	if (ldata->raw) {
		put_tty_queue(c, tty);
		return;
	}

	if (I_ISTRIP(tty))
		c &= 0x7f;
	if (I_IUCLC(tty) && L_IEXTEN(tty))
		c = tolower(c);

	if (L_EXTPROC(tty)) {
		put_tty_queue(c, tty);
		return;
	}

	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) &&
	    I_IXANY(tty) && c != START_CHAR(tty) && c != STOP_CHAR(tty) &&
	    c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) && c != SUSP_CHAR(tty)) {
		start_tty(tty);
		process_echoes(tty);
	}

	if (tty->closing) {
		if (I_IXON(tty)) {
			if (c == START_CHAR(tty)) {
				start_tty(tty);
				process_echoes(tty);
			} else if (c == STOP_CHAR(tty))
				stop_tty(tty);
		}
		return;
	}

	/*
	 * If the previous character was LNEXT, or we know that this
	 * character is not one of the characters that we'll have to
	 * handle specially, do shortcut processing to speed things
	 * up.
	 */
	if (!test_bit(c, ldata->process_char_map) || ldata->lnext) {
		ldata->lnext = 0;
		parmrk = (c == (unsigned char) '\377' && I_PARMRK(tty)) ? 1 : 0;
		if (ldata->read_cnt >= (N_TTY_BUF_SIZE - parmrk - 1)) {
			/* beep if no space */
			if (L_ECHO(tty))
				process_output('\a', tty);
			return;
		}
		if (L_ECHO(tty)) {
			finish_erasing(tty);
			/* Record the column of first canon char. */
			if (ldata->canon_head == ldata->read_head)
				echo_set_canon_col(tty);
			echo_char(c, tty);
			process_echoes(tty);
		}
		if (parmrk)
			put_tty_queue(c, tty);
		put_tty_queue(c, tty);
		return;
	}

	if (I_IXON(tty)) {
		if (c == START_CHAR(tty)) {
			start_tty(tty);
			process_echoes(tty);
			return;
		}
		if (c == STOP_CHAR(tty)) {
			stop_tty(tty);
			return;
		}
	}

	if (L_ISIG(tty)) {
		int signal;
		signal = SIGINT;
		if (c == INTR_CHAR(tty))
			goto send_signal;
		signal = SIGQUIT;
		if (c == QUIT_CHAR(tty))
			goto send_signal;
		signal = SIGTSTP;
		if (c == SUSP_CHAR(tty)) {
send_signal:
			/*
			 * Note that we do not use isig() here because we want
			 * the order to be:
			 * 1) flush, 2) echo, 3) signal
			 */
			if (!L_NOFLSH(tty)) {
				n_tty_flush_buffer(tty);
				tty_driver_flush_buffer(tty);
			}
			if (I_IXON(tty))
				start_tty(tty);
			if (L_ECHO(tty)) {
				echo_char(c, tty);
				process_echoes(tty);
			}
			if (tty->pgrp)
				kill_pgrp(tty->pgrp, signal, 1);
			return;
		}
	}

	if (c == '\r') {
		if (I_IGNCR(tty))
			return;
		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);
			process_echoes(tty);
			return;
		}
		if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
			ldata->lnext = 1;
			if (L_ECHO(tty)) {
				finish_erasing(tty);
				if (L_ECHOCTL(tty)) {
					echo_char_raw('^', tty);
					echo_char_raw('\b', tty);
					process_echoes(tty);
				}
			}
			return;
		}
		if (c == REPRINT_CHAR(tty) && L_ECHO(tty) &&
		    L_IEXTEN(tty)) {
			unsigned long tail = ldata->canon_head;

			finish_erasing(tty);
			echo_char(c, tty);
			echo_char_raw('\n', tty);
			while (tail != ldata->read_head) {
				echo_char(ldata->read_buf[tail], tty);
				tail = (tail+1) & (N_TTY_BUF_SIZE-1);
			}
			process_echoes(tty);
			return;
		}
		if (c == '\n') {
			if (ldata->read_cnt >= N_TTY_BUF_SIZE) {
				if (L_ECHO(tty))
					process_output('\a', tty);
				return;
			}
			if (L_ECHO(tty) || L_ECHONL(tty)) {
				echo_char_raw('\n', tty);
				process_echoes(tty);
			}
			goto handle_newline;
		}
		if (c == EOF_CHAR(tty)) {
			if (ldata->read_cnt >= N_TTY_BUF_SIZE)
				return;
			if (ldata->canon_head != ldata->read_head)
				set_bit(TTY_PUSH, &tty->flags);
			c = __DISABLED_CHAR;
			goto handle_newline;
		}
		if ((c == EOL_CHAR(tty)) ||
		    (c == EOL2_CHAR(tty) && L_IEXTEN(tty))) {
			parmrk = (c == (unsigned char) '\377' && I_PARMRK(tty))
				 ? 1 : 0;
			if (ldata->read_cnt >= (N_TTY_BUF_SIZE - parmrk)) {
				if (L_ECHO(tty))
					process_output('\a', tty);
				return;
			}
			/*
			 * 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(tty);
				echo_char(c, tty);
				process_echoes(tty);
			}
			/*
			 * XXX does PARMRK doubling happen for
			 * EOL_CHAR and EOL2_CHAR?
			 */
			if (parmrk)
				put_tty_queue(c, tty);

handle_newline:
			spin_lock_irqsave(&tty->read_lock, flags);
			set_bit(ldata->read_head, ldata->read_flags);
			put_tty_queue_nolock(c, tty);
			ldata->canon_head = ldata->read_head;
			ldata->canon_data++;
			spin_unlock_irqrestore(&tty->read_lock, flags);
			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
			if (waitqueue_active(&tty->read_wait))
				wake_up_interruptible(&tty->read_wait);
			return;
		}
	}

	parmrk = (c == (unsigned char) '\377' && I_PARMRK(tty)) ? 1 : 0;
	if (ldata->read_cnt >= (N_TTY_BUF_SIZE - parmrk - 1)) {
		/* beep if no space */
		if (L_ECHO(tty))
			process_output('\a', tty);
		return;
	}
	if (L_ECHO(tty)) {
		finish_erasing(tty);
		if (c == '\n')
			echo_char_raw('\n', tty);
		else {
			/* Record the column of first canon char. */
			if (ldata->canon_head == ldata->read_head)
				echo_set_canon_col(tty);
			echo_char(c, tty);
		}
		process_echoes(tty);
	}

	if (parmrk)
		put_tty_queue(c, tty);

	put_tty_queue(c, tty);
}


/**
 *	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)
{
	if (tty->fasync && test_and_clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags))
		kill_fasync(&tty->fasync, SIGIO, POLL_OUT);
}

/**
 *	n_tty_receive_buf	-	data receive
 *	@tty: terminal device
 *	@cp: buffer
 *	@fp: flag buffer
 *	@count: characters
 *
 *	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)
 */

static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
			      char *fp, int count)
{
	struct n_tty_data *ldata = tty->disc_data;
	const unsigned char *p;
	char *f, flags = TTY_NORMAL;
	int	i;
	char	buf[64];
	unsigned long cpuflags;

	if (ldata->real_raw) {
		spin_lock_irqsave(&tty->read_lock, cpuflags);
		i = min(N_TTY_BUF_SIZE - ldata->read_cnt,
			N_TTY_BUF_SIZE - ldata->read_head);
		i = min(count, i);
		memcpy(ldata->read_buf + ldata->read_head, cp, i);
		ldata->read_head = (ldata->read_head + i) & (N_TTY_BUF_SIZE-1);
		ldata->read_cnt += i;
		cp += i;
		count -= i;

		i = min(N_TTY_BUF_SIZE - ldata->read_cnt,
			N_TTY_BUF_SIZE - ldata->read_head);
		i = min(count, i);
		memcpy(ldata->read_buf + ldata->read_head, cp, i);
		ldata->read_head = (ldata->read_head + i) & (N_TTY_BUF_SIZE-1);
		ldata->read_cnt += i;
		spin_unlock_irqrestore(&tty->read_lock, cpuflags);
	} else {
		for (i = count, p = cp, f = fp; i; i--, p++) {
			if (f)
				flags = *f++;
			switch (flags) {
			case TTY_NORMAL:
				n_tty_receive_char(tty, *p);
				break;
			case TTY_BREAK:
				n_tty_receive_break(tty);
				break;
			case TTY_PARITY:
			case TTY_FRAME:
				n_tty_receive_parity_error(tty, *p);
				break;
			case TTY_OVERRUN:
				n_tty_receive_overrun(tty);
				break;
			default:
				printk(KERN_ERR "%s: unknown flag %d\n",
				       tty_name(tty, buf), flags);
				break;
			}
		}
		if (tty->ops->flush_chars)
			tty->ops->flush_chars(tty);
	}

	n_tty_set_room(tty);

	if ((!ldata->icanon && (ldata->read_cnt >= tty->minimum_to_wake)) ||
		L_EXTPROC(tty)) {
		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
		if (waitqueue_active(&tty->read_wait))
			wake_up_interruptible(&tty->read_wait);
	}

	/*
	 * 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 (tty->receive_room < TTY_THRESHOLD_THROTTLE)
		tty_throttle(tty);

        /* FIXME: there is a tiny race here if the receive room check runs
           before the other work executes and empties the buffer (upping
           the receiving room and unthrottling. We then throttle and get
           stuck. This has been observed and traced down by Vincent Pillet/
           We need to address this when we sort out out the rx path locking */
}

int is_ignored(int sig)
{
	return (sigismember(&current->blocked, sig) ||
		current->sighand->action[sig-1].sa.sa_handler == SIG_IGN);
}

/**
 *	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_mutex
 */

static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
{
	struct n_tty_data *ldata = tty->disc_data;
	int canon_change = 1;

	if (old)
		canon_change = (old->c_lflag ^ tty->termios.c_lflag) & ICANON;
	if (canon_change) {
		bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
		ldata->canon_head = ldata->read_tail;
		ldata->canon_data = 0;
		ldata->erasing = 0;
	}

	if (canon_change && !L_ICANON(tty) && ldata->read_cnt)
		wake_up_interruptible(&tty->read_wait);

	ldata->icanon = (L_ICANON(tty) != 0);
	if (test_bit(TTY_HW_COOK_IN, &tty->flags)) {
		ldata->raw = 1;
		ldata->real_raw = 1;
		n_tty_set_room(tty);
		return;
	}
	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->process_char_map, 256);

		if (I_IGNCR(tty) || I_ICRNL(tty))
			set_bit('\r', ldata->process_char_map);
		if (I_INLCR(tty))
			set_bit('\n', ldata->process_char_map);

		if (L_ICANON(tty)) {
			set_bit(ERASE_CHAR(tty), ldata->process_char_map);
			set_bit(KILL_CHAR(tty), ldata->process_char_map);
			set_bit(EOF_CHAR(tty), ldata->process_char_map);
			set_bit('\n', ldata->process_char_map);
			set_bit(EOL_CHAR(tty), ldata->process_char_map);
			if (L_IEXTEN(tty)) {
				set_bit(WERASE_CHAR(tty),
					ldata->process_char_map);
				set_bit(LNEXT_CHAR(tty),
					ldata->process_char_map);
				set_bit(EOL2_CHAR(tty),
					ldata->process_char_map);
				if (L_ECHO(tty))
					set_bit(REPRINT_CHAR(tty),
						ldata->process_char_map);
			}
		}
		if (I_IXON(tty)) {
			set_bit(START_CHAR(tty), ldata->process_char_map);
			set_bit(STOP_CHAR(tty), ldata->process_char_map);
		}
		if (L_ISIG(tty)) {
			set_bit(INTR_CHAR(tty), ldata->process_char_map);
			set_bit(QUIT_CHAR(tty), ldata->process_char_map);
			set_bit(SUSP_CHAR(tty), ldata->process_char_map);
		}
		clear_bit(__DISABLED_CHAR, ldata->process_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;
	}
	n_tty_set_room(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;

	n_tty_flush_buffer(tty);
	kfree(ldata->read_buf);
	kfree(ldata->echo_buf);
	kfree(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;

	ldata = kzalloc(sizeof(*ldata), GFP_KERNEL);
	if (!ldata)
		goto err;

	ldata->overrun_time = jiffies;

	/* These are ugly. Currently a malloc failure here can panic */
	ldata->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
	ldata->echo_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
	if (!ldata->read_buf || !ldata->echo_buf)
		goto err_free_bufs;

	tty->disc_data = ldata;
	reset_buffer_flags(tty);
	tty_unthrottle(tty);
	ldata->column = 0;
	n_tty_set_termios(tty, NULL);
	tty->minimum_to_wake = 1;
	tty->closing = 0;

	return 0;
err_free_bufs:
	kfree(ldata->read_buf);
	kfree(ldata->echo_buf);
	kfree(ldata);
err:
	return -ENOMEM;
}

static inline int input_available_p(struct tty_struct *tty, int amt)
{
	struct n_tty_data *ldata = tty->disc_data;

	tty_flush_to_ldisc(tty);
	if (ldata->icanon && !L_EXTPROC(tty)) {
		if (ldata->canon_data)
			return 1;
	} else if (ldata->read_cnt >= (amt ? amt : 1))
		return 1;

	return 0;
}

/**
 *	copy_from_read_buf	-	copy read data directly
 *	@tty: terminal device
 *	@b: user 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 to
 *	user space directly.  It can be profitably called twice; once to
 *	drain the space from the tail pointer to the (physical) end of the
 *	buffer, and once to drain the space from the (physical) beginning of
 *	the buffer to head pointer.
 *
 *	Called under the tty->atomic_read_lock sem
 *
 */

static int copy_from_read_buf(struct tty_struct *tty,
				      unsigned char __user **b,
				      size_t *nr)

{
	struct n_tty_data *ldata = tty->disc_data;
	int retval;
	size_t n;
	unsigned long flags;
	bool is_eof;

	retval = 0;
	spin_lock_irqsave(&tty->read_lock, flags);
	n = min(ldata->read_cnt, N_TTY_BUF_SIZE - ldata->read_tail);
	n = min(*nr, n);
	spin_unlock_irqrestore(&tty->read_lock, flags);
	if (n) {
		retval = copy_to_user(*b, &ldata->read_buf[ldata->read_tail], n);
		n -= retval;
		is_eof = n == 1 &&
			ldata->read_buf[ldata->read_tail] == EOF_CHAR(tty);
		tty_audit_add_data(tty, &ldata->read_buf[ldata->read_tail], n,
				ldata->icanon);
		spin_lock_irqsave(&tty->read_lock, flags);
		ldata->read_tail = (ldata->read_tail + n) & (N_TTY_BUF_SIZE-1);
		ldata->read_cnt -= n;
		/* Turn single EOF into zero-length read */
		if (L_EXTPROC(tty) && ldata->icanon && is_eof && !ldata->read_cnt)
			n = 0;
		spin_unlock_irqrestore(&tty->read_lock, flags);
		*b += n;
		*nr -= n;
	}
	return retval;
}

extern ssize_t redirected_tty_write(struct file *, const char __user *,
							size_t, loff_t *);

/**
 *	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.
 *
 *	FIXME:
 *	Locking: None - redirected write test is safe, testing
 *	current->signal should possibly lock current->sighand
 *	pgrp locking ?
 */

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 != redirected_tty_write &&
	    current->signal->tty == tty) {
		if (!tty->pgrp)
			printk(KERN_ERR "n_tty_read: no tty->pgrp!\n");
		else if (task_pgrp(current) != tty->pgrp) {
			if (is_ignored(SIGTTIN) ||
			    is_current_pgrp_orphaned())
				return -EIO;
			kill_pgrp(task_pgrp(current), SIGTTIN, 1);
			set_thread_flag(TIF_SIGPENDING);
			return -ERESTARTSYS;
		}
	}
	return 0;
}


/**
 *	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.
 */

static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
			 unsigned char __user *buf, size_t nr)
{
	struct n_tty_data *ldata = tty->disc_data;
	unsigned char __user *b = buf;
	DECLARE_WAITQUEUE(wait, current);
	int c;
	int minimum, time;
	ssize_t retval = 0;
	ssize_t size;
	long timeout;
	unsigned long flags;
	int packet;

do_it_again:
	c = job_control(tty, file);
	if (c < 0)
		return c;

	minimum = time = 0;
	timeout = MAX_SCHEDULE_TIMEOUT;
	if (!ldata->icanon) {
		time = (HZ / 10) * TIME_CHAR(tty);
		minimum = MIN_CHAR(tty);
		if (minimum) {
			if (time)
				tty->minimum_to_wake = 1;
			else if (!waitqueue_active(&tty->read_wait) ||
				 (tty->minimum_to_wake > minimum))
				tty->minimum_to_wake = minimum;
		} else {
			timeout = 0;
			if (time) {
				timeout = time;
				time = 0;
			}
			tty->minimum_to_wake = minimum = 1;
		}
	}

	/*
	 *	Internal serialization of reads.
	 */
	if (file->f_flags & O_NONBLOCK) {
		if (!mutex_trylock(&tty->atomic_read_lock))
			return -EAGAIN;
	} else {
		if (mutex_lock_interruptible(&tty->atomic_read_lock))
			return -ERESTARTSYS;
	}
	packet = tty->packet;

	add_wait_queue(&tty->read_wait, &wait);
	while (nr) {
		/* First test for status change. */
		if (packet && tty->link->ctrl_status) {
			unsigned char cs;
			if (b != buf)
				break;
			spin_lock_irqsave(&tty->link->ctrl_lock, flags);
			cs = tty->link->ctrl_status;
			tty->link->ctrl_status = 0;
			spin_unlock_irqrestore(&tty->link->ctrl_lock, flags);
			if (tty_put_user(tty, cs, b++)) {
				retval = -EFAULT;
				b--;
				break;
			}
			nr--;
			break;
		}
		/* This statement must be first before checking for input
		   so that any interrupt will set the state back to
		   TASK_RUNNING. */
		set_current_state(TASK_INTERRUPTIBLE);

		if (((minimum - (b - buf)) < tty->minimum_to_wake) &&
		    ((minimum - (b - buf)) >= 1))
			tty->minimum_to_wake = (minimum - (b - buf));

		if (!input_available_p(tty, 0)) {
			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
				retval = -EIO;
				break;
			}
			if (tty_hung_up_p(file))
				break;
			if (!timeout)
				break;
			if (file->f_flags & O_NONBLOCK) {
				retval = -EAGAIN;
				break;
			}
			if (signal_pending(current)) {
				retval = -ERESTARTSYS;
				break;
			}
			/* FIXME: does n_tty_set_room need locking ? */
			n_tty_set_room(tty);
			timeout = schedule_timeout(timeout);
			continue;
		}
		__set_current_state(TASK_RUNNING);

		/* Deal with packet mode. */
		if (packet && b == buf) {
			if (tty_put_user(tty, TIOCPKT_DATA, b++)) {
				retval = -EFAULT;
				b--;
				break;
			}
			nr--;
		}

		if (ldata->icanon && !L_EXTPROC(tty)) {
			/* N.B. avoid overrun if nr == 0 */
			spin_lock_irqsave(&tty->read_lock, flags);
			while (nr && ldata->read_cnt) {
				int eol;

				eol = test_and_clear_bit(ldata->read_tail,
						ldata->read_flags);
				c = ldata->read_buf[ldata->read_tail];
				ldata->read_tail = ((ldata->read_tail+1) &
						  (N_TTY_BUF_SIZE-1));
				ldata->read_cnt--;
				if (eol) {
					/* this test should be redundant:
					 * we shouldn't be reading data if
					 * canon_data is 0
					 */
					if (--ldata->canon_data < 0)
						ldata->canon_data = 0;
				}
				spin_unlock_irqrestore(&tty->read_lock, flags);

				if (!eol || (c != __DISABLED_CHAR)) {
					if (tty_put_user(tty, c, b++)) {
						retval = -EFAULT;
						b--;
						spin_lock_irqsave(&tty->read_lock, flags);
						break;
					}
					nr--;
				}
				if (eol) {
					tty_audit_push(tty);
					spin_lock_irqsave(&tty->read_lock, flags);
					break;
				}
				spin_lock_irqsave(&tty->read_lock, flags);
			}
			spin_unlock_irqrestore(&tty->read_lock, flags);
			if (retval)
				break;
		} else {
			int uncopied;
			/* The copy function takes the read lock and handles
			   locking internally for this case */
			uncopied = copy_from_read_buf(tty, &b, &nr);
			uncopied += copy_from_read_buf(tty, &b, &nr);
			if (uncopied) {
				retval = -EFAULT;
				break;
			}
		}

		/* If there is enough space in the read buffer now, let the
		 * low-level driver know. We use n_tty_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.
		 */
		if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
			n_tty_set_room(tty);
			check_unthrottle(tty);
		}

		if (b - buf >= minimum)
			break;
		if (time)
			timeout = time;
	}
	mutex_unlock(&tty->atomic_read_lock);
	remove_wait_queue(&tty->read_wait, &wait);

	if (!waitqueue_active(&tty->read_wait))
		tty->minimum_to_wake = minimum;

	__set_current_state(TASK_RUNNING);
	size = b - buf;
	if (size) {
		retval = size;
		if (nr)
			clear_bit(TTY_PUSH, &tty->flags);
	} else if (test_and_clear_bit(TTY_PUSH, &tty->flags))
		goto do_it_again;

	n_tty_set_room(tty);
	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;
	DECLARE_WAITQUEUE(wait, current);
	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 != redirected_tty_write) {
		retval = tty_check_change(tty);
		if (retval)
			return retval;
	}

	/* Write out any echoed characters that are still pending */
	process_echoes(tty);

	add_wait_queue(&tty->write_wait, &wait);
	while (1) {
		set_current_state(TASK_INTERRUPTIBLE);
		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) && !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
			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 {
			while (nr > 0) {
				c = tty->ops->write(tty, b, nr);
				if (c < 0) {
					retval = c;
					goto break_out;
				}
				if (!c)
					break;
				b += c;
				nr -= c;
			}
		}
		if (!nr)
			break;
		if (file->f_flags & O_NONBLOCK) {
			retval = -EAGAIN;
			break;
		}
		schedule();
	}
break_out:
	__set_current_state(TASK_RUNNING);
	remove_wait_queue(&tty->write_wait, &wait);
	if (b - buf != nr && tty->fasync)
		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
	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 unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
							poll_table *wait)
{
	unsigned int mask = 0;

	poll_wait(file, &tty->read_wait, wait);
	poll_wait(file, &tty->write_wait, wait);
	if (input_available_p(tty, TIME_CHAR(tty) ? 0 : MIN_CHAR(tty)))
		mask |= POLLIN | POLLRDNORM;
	if (tty->packet && tty->link->ctrl_status)
		mask |= POLLPRI | POLLIN | POLLRDNORM;
	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
		mask |= POLLHUP;
	if (tty_hung_up_p(file))
		mask |= POLLHUP;
	if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {
		if (MIN_CHAR(tty) && !TIME_CHAR(tty))
			tty->minimum_to_wake = MIN_CHAR(tty);
		else
			tty->minimum_to_wake = 1;
	}
	if (tty->ops->write && !tty_is_writelocked(tty) &&
			tty_chars_in_buffer(tty) < WAKEUP_CHARS &&
			tty_write_room(tty) > 0)
		mask |= POLLOUT | POLLWRNORM;
	return mask;
}

static unsigned long inq_canon(struct tty_struct *tty)
{
	struct n_tty_data *ldata = tty->disc_data;
	int nr, head, tail;

	if (!ldata->canon_data)
		return 0;
	head = ldata->canon_head;
	tail = ldata->read_tail;
	nr = (head - tail) & (N_TTY_BUF_SIZE-1);
	/* Skip EOF-chars.. */
	while (head != tail) {
		if (test_bit(tail, ldata->read_flags) &&
		    ldata->read_buf[tail] == __DISABLED_CHAR)
			nr--;
		tail = (tail+1) & (N_TTY_BUF_SIZE-1);
	}
	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:
		/* FIXME: Locking */
		retval = ldata->read_cnt;
		if (L_ICANON(tty))
			retval = inq_canon(tty);
		return put_user(retval, (unsigned int __user *) arg);
	default:
		return n_tty_ioctl_helper(tty, file, cmd, arg);
	}
}

struct tty_ldisc_ops tty_ldisc_N_TTY = {
	.magic           = TTY_LDISC_MAGIC,
	.name            = "n_tty",
	.open            = n_tty_open,
	.close           = n_tty_close,
	.flush_buffer    = n_tty_flush_buffer,
	.chars_in_buffer = n_tty_chars_in_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
};

/**
 *	n_tty_inherit_ops	-	inherit N_TTY methods
 *	@ops: struct tty_ldisc_ops where to save N_TTY methods
 *
 *	Used by a generic struct tty_ldisc_ops to easily inherit N_TTY
 *	methods.
 */

void n_tty_inherit_ops(struct tty_ldisc_ops *ops)
{
	*ops = tty_ldisc_N_TTY;
	ops->owner = NULL;
	ops->refcount = ops->flags = 0;
}
EXPORT_SYMBOL_GPL(n_tty_inherit_ops);