[deliverable/linux.git] / drivers / char / 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 <asm/uaccess.h>
#include <asm/system.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

static inline unsigned char *alloc_buf(void)
{
        gfp_t prio = in_interrupt() ? GFP_ATOMIC : GFP_KERNEL;

        if (PAGE_SIZE != N_TTY_BUF_SIZE)
                return kmalloc(N_TTY_BUF_SIZE, prio);
        else
                return (unsigned char *)__get_free_page(prio);
}

static inline void free_buf(unsigned char *buf)
{
        if (PAGE_SIZE != N_TTY_BUF_SIZE)
                kfree(buf);
        else
                free_page((unsigned long) buf);
}

static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
                               unsigned char __user *ptr)
{
        tty_audit_add_data(tty, &x, 1);
        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)
{
        /* tty->read_cnt is not read locked ? */
        int     left = N_TTY_BUF_SIZE - tty->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 = tty->icanon && !tty->canon_data;
        tty->receive_room = left;
}

static void put_tty_queue_nolock(unsigned char c, struct tty_struct *tty)
{
        if (tty->read_cnt < N_TTY_BUF_SIZE) {
                tty->read_buf[tty->read_head] = c;
                tty->read_head = (tty->read_head + 1) & (N_TTY_BUF_SIZE-1);
                tty->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)
{
        unsigned long flags;

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

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

        tty->canon_head = tty->canon_data = tty->erasing = 0;
        memset(&tty->read_flags, 0, sizeof tty->read_flags);
        n_tty_set_room(tty);
        check_unthrottle(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)
{
        unsigned long flags;
        ssize_t n = 0;

        spin_lock_irqsave(&tty->read_lock, flags);
        if (!tty->icanon) {
                n = tty->read_cnt;
        } else if (tty->canon_data) {
                n = (tty->canon_head > tty->read_tail) ?
                        tty->canon_head - tty->read_tail :
                        tty->canon_head + (N_TTY_BUF_SIZE - tty->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.),
 *      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)
{
        int     spaces;

        if (!space)
                return -1;
        
        switch (c) {
        case '\n':
                if (O_ONLRET(tty))
                        tty->column = 0;
                if (O_ONLCR(tty)) {
                        if (space < 2)
                                return -1;
                        tty->canon_column = tty->column = 0;
                        tty_put_char(tty, '\r');
                        tty_put_char(tty, c);
                        return 2;
                }
                tty->canon_column = tty->column;
                break;
        case '\r':
                if (O_ONOCR(tty) && tty->column == 0)
                        return 0;
                if (O_OCRNL(tty)) {
                        c = '\n';
                        if (O_ONLRET(tty))
                                tty->canon_column = tty->column = 0;
                        break;
                }
                tty->canon_column = tty->column = 0;
                break;
        case '\t':
                spaces = 8 - (tty->column & 7);
                if (O_TABDLY(tty) == XTABS) {
                        if (space < spaces)
                                return -1;
                        tty->column += spaces;
                        tty->ops->write(tty, "        ", spaces);
                        return spaces;
                }
                tty->column += spaces;
                break;
        case '\b':
                if (tty->column > 0)
                        tty->column--;
                break;
        default:
                if (O_OLCUC(tty))
                        c = toupper(c);
                if (!iscntrl(c) && !is_continuation(c, tty))
                        tty->column++;
                break;
        }

        tty_put_char(tty, c);
        return 1;
}

/**
 *      process_output                  -       output post processor
 *      @c: character (or partial unicode symbol)
 *      @tty: terminal device
 *
 *      Perform 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
 *      @inbuf: user buffer
 *      @nr: number of bytes
 *
 *      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)
{
        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++) {
                switch (*cp) {
                case '\n':
                        if (O_ONLRET(tty))
                                tty->column = 0;
                        if (O_ONLCR(tty))
                                goto break_out;
                        tty->canon_column = tty->column;
                        break;
                case '\r':
                        if (O_ONOCR(tty) && tty->column == 0)
                                goto break_out;
                        if (O_OCRNL(tty))
                                goto break_out;
                        tty->canon_column = tty->column = 0;
                        break;
                case '\t':
                        goto break_out;
                case '\b':
                        if (tty->column > 0)
                                tty->column--;
                        break;
                default:
                        if (O_OLCUC(tty))
                                goto break_out;
                        if (!iscntrl(*cp))
                                tty->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)
{
        int     space, nr;
        unsigned char c;
        unsigned char *cp, *buf_end;

        if (!tty->echo_cnt)
                return;

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

        space = tty_write_room(tty);

        buf_end = tty->echo_buf + N_TTY_BUF_SIZE;
        cp = tty->echo_buf + tty->echo_pos;
        nr = tty->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 += tty->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 (tty->column > 0)
                                                tty->column--;
                                }
                                cp += 3;
                                nr -= 3;
                                break;

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

                        case ECHO_OP_MOVE_BACK_COL:
                                if (tty->column > 0)
                                        tty->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);
                                tty->column++;
                                space--;
                                cp += 2;
                                nr -= 2;
                                break;

                        default:
                                if (iscntrl(op)) {
                                        if (L_ECHOCTL(tty)) {
                                                /*
                                                 * 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);
                                                tty->column += 2;
                                                space -= 2;
                                        } else {
                                                if (!space) {
                                                        no_space_left = 1;
                                                        break;
                                                }
                                                tty_put_char(tty, op);
                                                space--;
                                        }
                                }
                                /*
                                 * If above falls through, this was an
                                 * undefined op.
                                 */
                                cp += 2;
                                nr -= 2;
                        }

                        if (no_space_left)
                                break;
                } else {
                        int retval;

                        if ((retval = do_output_char(c, tty, space)) < 0)
                                break;
                        space -= retval;
                        cp += 1;
                        nr -= 1;
                }

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

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

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

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

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

        tty->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 possibly echoed as
 *      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 (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)
{
        if (tty->erasing) {
                echo_char_raw('/', tty);
                tty->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)
{
        enum { ERASE, WERASE, KILL } kill_type;
        int head, seen_alnums, cnt;
        unsigned long flags;

        /* FIXME: locking needed ? */
        if (tty->read_head == tty->canon_head) {
                /* echo_char_raw('\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);
                        tty->read_cnt -= ((tty->read_head - tty->canon_head) &
                                          (N_TTY_BUF_SIZE - 1));
                        tty->read_head = tty->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);
                        tty->read_cnt -= ((tty->read_head - tty->canon_head) &
                                          (N_TTY_BUF_SIZE - 1));
                        tty->read_head = tty->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 (tty->read_head != tty->canon_head) {
                head = tty->read_head;

                /* erase a single possibly multibyte character */
                do {
                        head = (head - 1) & (N_TTY_BUF_SIZE-1);
                        c = tty->read_buf[head];
                } while (is_continuation(c, tty) && head != tty->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 = (tty->read_head - head) & (N_TTY_BUF_SIZE-1);
                spin_lock_irqsave(&tty->read_lock, flags);
                tty->read_head = head;
                tty->read_cnt -= cnt;
                spin_unlock_irqrestore(&tty->read_lock, flags);
                if (L_ECHO(tty)) {
                        if (L_ECHOPRT(tty)) {
                                if (!tty->erasing) {
                                        echo_char_raw('\\', tty);
                                        tty->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(tty->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 = tty->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 != tty->canon_head) {
                                        tail = (tail-1) & (N_TTY_BUF_SIZE-1);
                                        c = tty->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 (tty->read_head == tty->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)
{
        char buf[64];

        tty->num_overrun++;
        if (time_before(tty->overrun_time, jiffies - HZ) ||
                        time_after(tty->overrun_time, jiffies)) {
                printk(KERN_WARNING "%s: %d input overrun(s)\n",
                        tty_name(tty, buf),
                        tty->num_overrun);
                tty->overrun_time = jiffies;
                tty->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)
{
        unsigned long flags;

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

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

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

                        finish_erasing(tty);
                        echo_char(c, tty);
                        echo_char_raw('\n', tty);
                        while (tail != tty->read_head) {
                                echo_char(tty->read_buf[tail], tty);
                                tail = (tail+1) & (N_TTY_BUF_SIZE-1);
                        }
                        process_echoes(tty);
                        return;
                }
                if (c == '\n') {
                        if (L_ECHO(tty) || L_ECHONL(tty)) {
                                if (tty->read_cnt >= N_TTY_BUF_SIZE-1)
                                        echo_char_raw('\a', tty);
                                echo_char_raw('\n', tty);
                                process_echoes(tty);
                        }
                        goto handle_newline;
                }
                if (c == EOF_CHAR(tty)) {
                        if (tty->canon_head != tty->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))) {
                        /*
                         * XXX are EOL_CHAR and EOL2_CHAR echoed?!?
                         */
                        if (L_ECHO(tty)) {
                                if (tty->read_cnt >= N_TTY_BUF_SIZE-1)
                                        echo_char_raw('\a', tty);
                                /* Record the column of first canon char. */
                                if (tty->canon_head == tty->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 (I_PARMRK(tty) && c == (unsigned char) '\377')
                                put_tty_queue(c, tty);

handle_newline:
                        spin_lock_irqsave(&tty->read_lock, flags);
                        set_bit(tty->read_head, tty->read_flags);
                        put_tty_queue_nolock(c, tty);
                        tty->canon_head = tty->read_head;
                        tty->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;
                }
        }

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

        if (I_PARMRK(tty) && c == (unsigned char) '\377')
                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)
{
        /* Write out any echoed characters that are still pending */
        process_echoes(tty);
        
        if (tty->fasync) {
                set_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)
{
        const unsigned char *p;
        char *f, flags = TTY_NORMAL;
        int     i;
        char    buf[64];
        unsigned long cpuflags;

        if (!tty->read_buf)
                return;

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

                i = min(N_TTY_BUF_SIZE - tty->read_cnt,
                        N_TTY_BUF_SIZE - tty->read_head);
                i = min(count, i);
                memcpy(tty->read_buf + tty->read_head, cp, i);
                tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
                tty->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 (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
                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);
}

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)
{
        int canon_change = 1;
        BUG_ON(!tty);

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

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

        tty->icanon = (L_ICANON(tty) != 0);
        if (test_bit(TTY_HW_COOK_IN, &tty->flags)) {
                tty->raw = 1;
                tty->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)) {
                memset(tty->process_char_map, 0, 256/8);

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

                if (L_ICANON(tty)) {
                        set_bit(ERASE_CHAR(tty), tty->process_char_map);
                        set_bit(KILL_CHAR(tty), tty->process_char_map);
                        set_bit(EOF_CHAR(tty), tty->process_char_map);
                        set_bit('\n', tty->process_char_map);
                        set_bit(EOL_CHAR(tty), tty->process_char_map);
                        if (L_IEXTEN(tty)) {
                                set_bit(WERASE_CHAR(tty),
                                        tty->process_char_map);
                                set_bit(LNEXT_CHAR(tty),
                                        tty->process_char_map);
                                set_bit(EOL2_CHAR(tty),
                                        tty->process_char_map);
                                if (L_ECHO(tty))
                                        set_bit(REPRINT_CHAR(tty),
                                                tty->process_char_map);
                        }
                }
                if (I_IXON(tty)) {
                        set_bit(START_CHAR(tty), tty->process_char_map);
                        set_bit(STOP_CHAR(tty), tty->process_char_map);
                }
                if (L_ISIG(tty)) {
                        set_bit(INTR_CHAR(tty), tty->process_char_map);
                        set_bit(QUIT_CHAR(tty), tty->process_char_map);
                        set_bit(SUSP_CHAR(tty), tty->process_char_map);
                }
                clear_bit(__DISABLED_CHAR, tty->process_char_map);
                tty->raw = 0;
                tty->real_raw = 0;
        } else {
                tty->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))
                        tty->real_raw = 1;
                else
                        tty->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)
{
        n_tty_flush_buffer(tty);
        if (tty->read_buf) {
                free_buf(tty->read_buf);
                tty->read_buf = NULL;
        }
        if (tty->echo_buf) {
                free_buf(tty->echo_buf);
                tty->echo_buf = 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)
{
        if (!tty)
                return -EINVAL;

        /* These are ugly. Currently a malloc failure here can panic */
        if (!tty->read_buf) {
                tty->read_buf = alloc_buf();
                if (!tty->read_buf)
                        return -ENOMEM;
        }
        if (!tty->echo_buf) {
                tty->echo_buf = alloc_buf();
                if (!tty->echo_buf)
                        return -ENOMEM;
        }
        memset(tty->read_buf, 0, N_TTY_BUF_SIZE);
        memset(tty->echo_buf, 0, N_TTY_BUF_SIZE);
        reset_buffer_flags(tty);
        tty->column = 0;
        n_tty_set_termios(tty, NULL);
        tty->minimum_to_wake = 1;
        tty->closing = 0;
        return 0;
}

static inline int input_available_p(struct tty_struct *tty, int amt)
{
        if (tty->icanon) {
                if (tty->canon_data)
                        return 1;
        } else if (tty->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)

{
        int retval;
        size_t n;
        unsigned long flags;

        retval = 0;
        spin_lock_irqsave(&tty->read_lock, flags);
        n = min(tty->read_cnt, N_TTY_BUF_SIZE - tty->read_tail);
        n = min(*nr, n);
        spin_unlock_irqrestore(&tty->read_lock, flags);
        if (n) {
                retval = copy_to_user(*b, &tty->read_buf[tty->read_tail], n);
                n -= retval;
                tty_audit_add_data(tty, &tty->read_buf[tty->read_tail], n);
                spin_lock_irqsave(&tty->read_lock, flags);
                tty->read_tail = (tty->read_tail + n) & (N_TTY_BUF_SIZE-1);
                tty->read_cnt -= n;
                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)
{
        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:

        BUG_ON(!tty->read_buf);

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

        minimum = time = 0;
        timeout = MAX_SCHEDULE_TIMEOUT;
        if (!tty->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 (tty->icanon) {
                        /* N.B. avoid overrun if nr == 0 */
                        while (nr && tty->read_cnt) {
                                int eol;

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

        if (!tty->canon_data)
                return 0;
        head = tty->canon_head;
        tail = tty->read_tail;
        nr = (head - tail) & (N_TTY_BUF_SIZE-1);
        /* Skip EOF-chars.. */
        while (head != tail) {
                if (test_bit(tail, tty->read_flags) &&
                    tty->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)
{
        int retval;

        switch (cmd) {
        case TIOCOUTQ:
                return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
        case TIOCINQ:
                /* FIXME: Locking */
                retval = tty->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
};