[linux-block.git] / drivers / tty / n_tty.c

// SPDX-License-Identifier: GPL-1.0+
/*
 * n_tty.c --- implements the N_TTY line discipline.
 *
 * This code used to be in tty_io.c, but things are getting hairy
 * enough that it made sense to split things off.  (The N_TTY
 * processing has changed so much that it's hardly recognizable,
 * anyway...)
 *
 * Note that the open routine for N_TTY is guaranteed never to return
 * an error.  This is because Linux will fall back to setting a line
 * to N_TTY if it can not switch to any other line discipline.
 *
 * Written by Theodore Ts'o, Copyright 1994.
 *
 * This file also contains code originally written by Linus Torvalds,
 * Copyright 1991, 1992, 1993, and by Julian Cowley, Copyright 1994.
 *
 * Reduced memory usage for older ARM systems  - Russell King.
 *
 * 2000/01/20   Fixed SMP locking on put_tty_queue using bits of
 *              the patch by Andrew J. Kroll <ag784@freenet.buffalo.edu>
 *              who actually finally proved there really was a race.
 *
 * 2002/03/18   Implemented n_tty_wakeup to send SIGIO POLL_OUTs to
 *              waiting writing processes-Sapan Bhatia <sapan@corewars.org>.
 *              Also fixed a bug in BLOCKING mode where n_tty_write returns
 *              EAGAIN
 */

#include <linux/bitmap.h>
#include <linux/bitops.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/export.h>
#include <linux/fcntl.h>
#include <linux/file.h>
#include <linux/jiffies.h>
#include <linux/math.h>
#include <linux/poll.h>
#include <linux/ratelimit.h>
#include <linux/sched.h>
#include <linux/signal.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/tty.h>
#include <linux/types.h>
#include <linux/uaccess.h>
#include <linux/vmalloc.h>

#include "tty.h"

/*
 * Until this number of characters is queued in the xmit buffer, select will
 * return "we have room for writes".
 */
#define WAKEUP_CHARS 256

#define N_TTY_BUF_SIZE 4096

/*
 * This defines the low- and high-watermarks for throttling and
 * unthrottling the TTY driver.  These watermarks are used for
 * controlling the space in the read buffer.
 */
#define TTY_THRESHOLD_THROTTLE          128 /* now based on remaining room */
#define TTY_THRESHOLD_UNTHROTTLE        128

/*
 * Special byte codes used in the echo buffer to represent operations
 * or special handling of characters.  Bytes in the echo buffer that
 * are not part of such special blocks are treated as normal character
 * codes.
 */
#define ECHO_OP_START 0xff
#define ECHO_OP_MOVE_BACK_COL 0x80
#define ECHO_OP_SET_CANON_COL 0x81
#define ECHO_OP_ERASE_TAB 0x82

#define ECHO_COMMIT_WATERMARK   256
#define ECHO_BLOCK              256
#define ECHO_DISCARD_WATERMARK  N_TTY_BUF_SIZE - (ECHO_BLOCK + 32)

struct n_tty_data {
        /* producer-published */
        size_t read_head;
        size_t commit_head;
        size_t canon_head;
        size_t echo_head;
        size_t echo_commit;
        size_t echo_mark;
        DECLARE_BITMAP(char_map, 256);

        /* private to n_tty_receive_overrun (single-threaded) */
        unsigned long overrun_time;
        unsigned int num_overrun;

        /* non-atomic */
        bool no_room;

        /* must hold exclusive termios_rwsem to reset these */
        unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
        unsigned char push:1;

        /* shared by producer and consumer */
        u8 read_buf[N_TTY_BUF_SIZE];
        DECLARE_BITMAP(read_flags, N_TTY_BUF_SIZE);
        u8 echo_buf[N_TTY_BUF_SIZE];

        /* consumer-published */
        size_t read_tail;
        size_t line_start;

        /* # of chars looked ahead (to find software flow control chars) */
        size_t lookahead_count;

        /* protected by output lock */
        unsigned int column;
        unsigned int canon_column;
        size_t echo_tail;

        struct mutex atomic_read_lock;
        struct mutex output_lock;
};

#define MASK(x) ((x) & (N_TTY_BUF_SIZE - 1))

static inline size_t read_cnt(struct n_tty_data *ldata)
{
        return ldata->read_head - ldata->read_tail;
}

static inline u8 read_buf(struct n_tty_data *ldata, size_t i)
{
        return ldata->read_buf[MASK(i)];
}

static inline u8 *read_buf_addr(struct n_tty_data *ldata, size_t i)
{
        return &ldata->read_buf[MASK(i)];
}

static inline u8 echo_buf(struct n_tty_data *ldata, size_t i)
{
        smp_rmb(); /* Matches smp_wmb() in add_echo_byte(). */
        return ldata->echo_buf[MASK(i)];
}

static inline u8 *echo_buf_addr(struct n_tty_data *ldata, size_t i)
{
        return &ldata->echo_buf[MASK(i)];
}

/* If we are not echoing the data, perhaps this is a secret so erase it */
static void zero_buffer(const struct tty_struct *tty, u8 *buffer, size_t size)
{
        if (L_ICANON(tty) && !L_ECHO(tty))
                memset(buffer, 0, size);
}

static void tty_copy(const struct tty_struct *tty, void *to, size_t tail,
                     size_t n)
{
        struct n_tty_data *ldata = tty->disc_data;
        size_t size = N_TTY_BUF_SIZE - tail;
        void *from = read_buf_addr(ldata, tail);

        if (n > size) {
                tty_audit_add_data(tty, from, size);
                memcpy(to, from, size);
                zero_buffer(tty, from, size);
                to += size;
                n -= size;
                from = ldata->read_buf;
        }

        tty_audit_add_data(tty, from, n);
        memcpy(to, from, n);
        zero_buffer(tty, from, n);
}

/**
 * n_tty_kick_worker - start input worker (if required)
 * @tty: terminal
 *
 * Re-schedules the flip buffer work if it may have stopped.
 *
 * Locking:
 *  * Caller holds exclusive %termios_rwsem, or
 *  * n_tty_read()/consumer path:
 *      holds non-exclusive %termios_rwsem
 */
static void n_tty_kick_worker(const struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        /* Did the input worker stop? Restart it */
        if (unlikely(READ_ONCE(ldata->no_room))) {
                WRITE_ONCE(ldata->no_room, 0);

                WARN_RATELIMIT(tty->port->itty == NULL,
                                "scheduling with invalid itty\n");
                /* see if ldisc has been killed - if so, this means that
                 * even though the ldisc has been halted and ->buf.work
                 * cancelled, ->buf.work is about to be rescheduled
                 */
                WARN_RATELIMIT(test_bit(TTY_LDISC_HALTED, &tty->flags),
                               "scheduling buffer work for halted ldisc\n");
                tty_buffer_restart_work(tty->port);
        }
}

static ssize_t chars_in_buffer(const struct tty_struct *tty)
{
        const struct n_tty_data *ldata = tty->disc_data;
        size_t head = ldata->icanon ? ldata->canon_head : ldata->commit_head;

        return head - ldata->read_tail;
}

/**
 * n_tty_write_wakeup   -       asynchronous I/O notifier
 * @tty: tty device
 *
 * Required for the ptys, serial driver etc. since processes that attach
 * themselves to the master and rely on ASYNC IO must be woken up.
 */
static void n_tty_write_wakeup(struct tty_struct *tty)
{
        clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
        kill_fasync(&tty->fasync, SIGIO, POLL_OUT);
}

static void n_tty_check_throttle(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        /*
         * Check the remaining room for the input canonicalization
         * mode.  We don't want to throttle the driver if we're in
         * canonical mode and don't have a newline yet!
         */
        if (ldata->icanon && ldata->canon_head == ldata->read_tail)
                return;

        do {
                tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
                if (N_TTY_BUF_SIZE - read_cnt(ldata) >= TTY_THRESHOLD_THROTTLE)
                        break;
        } while (!tty_throttle_safe(tty));

        __tty_set_flow_change(tty, 0);
}

static void n_tty_check_unthrottle(struct tty_struct *tty)
{
        if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
                if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
                        return;
                n_tty_kick_worker(tty);
                tty_wakeup(tty->link);
                return;
        }

        /* If there is enough space in the read buffer now, let the
         * low-level driver know. We use chars_in_buffer() to
         * check the buffer, as it now knows about canonical mode.
         * Otherwise, if the driver is throttled and the line is
         * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
         * we won't get any more characters.
         */

        do {
                tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
                if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
                        break;

                n_tty_kick_worker(tty);
        } while (!tty_unthrottle_safe(tty));

        __tty_set_flow_change(tty, 0);
}

/**
 * put_tty_queue                -       add character to tty
 * @c: character
 * @ldata: n_tty data
 *
 * Add a character to the tty read_buf queue.
 *
 * Locking:
 *  * n_tty_receive_buf()/producer path:
 *      caller holds non-exclusive %termios_rwsem
 */
static inline void put_tty_queue(u8 c, struct n_tty_data *ldata)
{
        *read_buf_addr(ldata, ldata->read_head) = c;
        ldata->read_head++;
}

/**
 * reset_buffer_flags   -       reset buffer state
 * @ldata: line disc data to reset
 *
 * Reset the read buffer counters and clear the flags. Called from
 * n_tty_open() and n_tty_flush_buffer().
 *
 * Locking:
 *  * caller holds exclusive %termios_rwsem, or
 *  * (locking is not required)
 */
static void reset_buffer_flags(struct n_tty_data *ldata)
{
        ldata->read_head = ldata->canon_head = ldata->read_tail = 0;
        ldata->commit_head = 0;
        ldata->line_start = 0;

        ldata->erasing = 0;
        bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
        ldata->push = 0;

        ldata->lookahead_count = 0;
}

static void n_tty_packet_mode_flush(struct tty_struct *tty)
{
        unsigned long flags;

        if (tty->link->ctrl.packet) {
                spin_lock_irqsave(&tty->ctrl.lock, flags);
                tty->ctrl.pktstatus |= TIOCPKT_FLUSHREAD;
                spin_unlock_irqrestore(&tty->ctrl.lock, flags);
                wake_up_interruptible(&tty->link->read_wait);
        }
}

/**
 * n_tty_flush_buffer   -       clean input queue
 * @tty: terminal device
 *
 * Flush the input buffer. Called when the tty layer wants the buffer flushed
 * (eg at hangup) or when the %N_TTY line discipline internally has to clean
 * the pending queue (for example some signals).
 *
 * Holds %termios_rwsem to exclude producer/consumer while buffer indices are
 * reset.
 *
 * Locking: %ctrl.lock, exclusive %termios_rwsem
 */
static void n_tty_flush_buffer(struct tty_struct *tty)
{
        down_write(&tty->termios_rwsem);
        reset_buffer_flags(tty->disc_data);
        n_tty_kick_worker(tty);

        if (tty->link)
                n_tty_packet_mode_flush(tty);
        up_write(&tty->termios_rwsem);
}

/**
 * is_utf8_continuation -       utf8 multibyte check
 * @c: byte to check
 *
 * Returns: true if the utf8 character @c is a multibyte continuation
 * character. We use this to correctly compute the on-screen size of the
 * character when printing.
 */
static inline int is_utf8_continuation(u8 c)
{
        return (c & 0xc0) == 0x80;
}

/**
 * is_continuation      -       multibyte check
 * @c: byte to check
 * @tty: terminal device
 *
 * Returns: true if the utf8 character @c is a multibyte continuation character
 * and the terminal is in unicode mode.
 */
static inline int is_continuation(u8 c, const 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(u8 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(u8 c, struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        guard(mutex)(&ldata->output_lock);

        if (do_output_char(c, tty, tty_write_room(tty)) < 0)
                return -1;

        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.
 *
 * 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.
 *
 * Returns: the number of characters output.
 *
 * 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 u8 *buf, unsigned int nr)
{
        struct n_tty_data *ldata = tty->disc_data;
        unsigned int space, i;
        const u8 *cp;

        guard(mutex)(&ldata->output_lock);

        space = tty_write_room(tty);
        if (space == 0)
                return 0;

        if (nr > space)
                nr = space;

        for (i = 0, cp = buf; i < nr; i++, cp++) {
                u8 c = *cp;

                switch (c) {
                case '\n':
                        if (O_ONLRET(tty))
                                ldata->column = 0;
                        if (O_ONLCR(tty))
                                goto do_write;
                        ldata->canon_column = ldata->column;
                        break;
                case '\r':
                        if (O_ONOCR(tty) && ldata->column == 0)
                                goto do_write;
                        if (O_OCRNL(tty))
                                goto do_write;
                        ldata->canon_column = ldata->column = 0;
                        break;
                case '\t':
                        goto do_write;
                case '\b':
                        if (ldata->column > 0)
                                ldata->column--;
                        break;
                default:
                        if (!iscntrl(c)) {
                                if (O_OLCUC(tty))
                                        goto do_write;
                                if (!is_continuation(c, tty))
                                        ldata->column++;
                        }
                        break;
                }
        }
do_write:
        return tty->ops->write(tty, buf, i);
}

static int n_tty_process_echo_ops(struct tty_struct *tty, size_t *tail,
                                  int space)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 op;

        /*
         * Since add_echo_byte() is called without holding output_lock, we
         * might see only portion of multi-byte operation.
         */
        if (MASK(ldata->echo_commit) == MASK(*tail + 1))
                return -ENODATA;

        /*
         * If the buffer byte is the start of a multi-byte operation, get the
         * next byte, which is either the op code or a control character value.
         */
        op = echo_buf(ldata, *tail + 1);

        switch (op) {
        case ECHO_OP_ERASE_TAB: {
                unsigned int num_chars, num_bs;

                if (MASK(ldata->echo_commit) == MASK(*tail + 2))
                        return -ENODATA;

                num_chars = echo_buf(ldata, *tail + 2);

                /*
                 * Determine how many columns to go back in order to erase the
                 * tab. This depends on the number of columns used by other
                 * characters within the tab area. If this (modulo 8) count is
                 * from the start of input rather than from a previous tab, we
                 * offset by canon column. Otherwise, tab spacing is normal.
                 */
                if (!(num_chars & 0x80))
                        num_chars += ldata->canon_column;
                num_bs = 8 - (num_chars & 7);

                if (num_bs > space)
                        return -ENOSPC;

                space -= num_bs;
                while (num_bs--) {
                        tty_put_char(tty, '\b');
                        if (ldata->column > 0)
                                ldata->column--;
                }
                *tail += 3;
                break;
        }
        case ECHO_OP_SET_CANON_COL:
                ldata->canon_column = ldata->column;
                *tail += 2;
                break;

        case ECHO_OP_MOVE_BACK_COL:
                if (ldata->column > 0)
                        ldata->column--;
                *tail += 2;
                break;

        case ECHO_OP_START:
                /* This is an escaped echo op start code */
                if (!space)
                        return -ENOSPC;

                tty_put_char(tty, ECHO_OP_START);
                ldata->column++;
                space--;
                *tail += 2;
                break;

        default:
                /*
                 * If the op is not a special byte code, it is a ctrl char
                 * tagged to be echoed as "^X" (where X is the letter
                 * representing the control char). Note that we must ensure
                 * there is enough space for the whole ctrl pair.
                 */
                if (space < 2)
                        return -ENOSPC;

                tty_put_char(tty, '^');
                tty_put_char(tty, op ^ 0100);
                ldata->column += 2;
                space -= 2;
                *tail += 2;
                break;
        }

        return space;
}

/**
 * __process_echoes     -       write pending echo characters
 * @tty: terminal device
 *
 * Write previously buffered echo (and other ldisc-generated) characters to the
 * tty.
 *
 * Characters generated by the ldisc (including echoes) need to be buffered
 * because the driver's write buffer can fill during heavy program output.
 * Echoing straight to the driver will often fail under these conditions,
 * causing lost characters and resulting mismatches of ldisc state information.
 *
 * Since the ldisc state must represent the characters actually sent to the
 * driver at the time of the write, operations like certain changes in column
 * state are also saved in the buffer and executed here.
 *
 * A circular fifo buffer is used so that the most recent characters are
 * prioritized. Also, when control characters are echoed with a prefixed "^",
 * the pair is treated atomically and thus not separated.
 *
 * Locking: callers must hold %output_lock.
 */
static size_t __process_echoes(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;
        unsigned int space, old_space;
        size_t tail;
        u8 c;

        old_space = space = tty_write_room(tty);

        tail = ldata->echo_tail;
        while (MASK(ldata->echo_commit) != MASK(tail)) {
                c = echo_buf(ldata, tail);
                if (c == ECHO_OP_START) {
                        int ret = n_tty_process_echo_ops(tty, &tail, space);
                        if (ret == -ENODATA)
                                goto not_yet_stored;
                        if (ret < 0)
                                break;
                        space = ret;
                } else {
                        if (O_OPOST(tty)) {
                                int retval = do_output_char(c, tty, space);
                                if (retval < 0)
                                        break;
                                space -= retval;
                        } else {
                                if (!space)
                                        break;
                                tty_put_char(tty, c);
                                space -= 1;
                        }
                        tail += 1;
                }
        }

        /* If the echo buffer is nearly full (so that the possibility exists
         * of echo overrun before the next commit), then discard enough
         * data at the tail to prevent a subsequent overrun */
        while (ldata->echo_commit > tail &&
               ldata->echo_commit - tail >= ECHO_DISCARD_WATERMARK) {
                if (echo_buf(ldata, tail) == ECHO_OP_START) {
                        if (echo_buf(ldata, tail + 1) == ECHO_OP_ERASE_TAB)
                                tail += 3;
                        else
                                tail += 2;
                } else
                        tail++;
        }

 not_yet_stored:
        ldata->echo_tail = tail;
        return old_space - space;
}

static void commit_echoes(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;
        size_t nr, old, echoed;
        size_t head;

        mutex_lock(&ldata->output_lock);
        head = ldata->echo_head;
        ldata->echo_mark = head;
        old = ldata->echo_commit - ldata->echo_tail;

        /* Process committed echoes if the accumulated # of bytes
         * is over the threshold (and try again each time another
         * block is accumulated) */
        nr = head - ldata->echo_tail;
        if (nr < ECHO_COMMIT_WATERMARK ||
            (nr % ECHO_BLOCK > old % ECHO_BLOCK)) {
                mutex_unlock(&ldata->output_lock);
                return;
        }

        ldata->echo_commit = head;
        echoed = __process_echoes(tty);
        mutex_unlock(&ldata->output_lock);

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

static void process_echoes(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;
        size_t echoed;

        if (ldata->echo_mark == ldata->echo_tail)
                return;

        mutex_lock(&ldata->output_lock);
        ldata->echo_commit = ldata->echo_mark;
        echoed = __process_echoes(tty);
        mutex_unlock(&ldata->output_lock);

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

/* NB: echo_mark and echo_head should be equivalent here */
static void flush_echoes(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        if ((!L_ECHO(tty) && !L_ECHONL(tty)) ||
            ldata->echo_commit == ldata->echo_head)
                return;

        mutex_lock(&ldata->output_lock);
        ldata->echo_commit = ldata->echo_head;
        __process_echoes(tty);
        mutex_unlock(&ldata->output_lock);
}

/**
 * add_echo_byte        -       add a byte to the echo buffer
 * @c: unicode byte to echo
 * @ldata: n_tty data
 *
 * Add a character or operation byte to the echo buffer.
 */
static inline void add_echo_byte(u8 c, struct n_tty_data *ldata)
{
        *echo_buf_addr(ldata, ldata->echo_head) = c;
        smp_wmb(); /* Matches smp_rmb() in echo_buf(). */
        ldata->echo_head++;
}

/**
 * echo_move_back_col   -       add operation to move back a column
 * @ldata: n_tty data
 *
 * Add an operation to the echo buffer to move back one column.
 */
static void echo_move_back_col(struct n_tty_data *ldata)
{
        add_echo_byte(ECHO_OP_START, ldata);
        add_echo_byte(ECHO_OP_MOVE_BACK_COL, ldata);
}

/**
 * echo_set_canon_col   -       add operation to set the canon column
 * @ldata: n_tty data
 *
 * Add an operation to the echo buffer to set the canon column to the current
 * column.
 */
static void echo_set_canon_col(struct n_tty_data *ldata)
{
        add_echo_byte(ECHO_OP_START, ldata);
        add_echo_byte(ECHO_OP_SET_CANON_COL, ldata);
}

/**
 * echo_erase_tab       -       add operation to erase a tab
 * @num_chars: number of character columns already used
 * @after_tab: true if num_chars starts after a previous tab
 * @ldata: n_tty data
 *
 * Add an operation to the echo buffer to erase a tab.
 *
 * Called by the eraser function, which knows how many character columns have
 * been used since either a previous tab or the start of input. This
 * information will be used later, along with canon column (if applicable), to
 * go back the correct number of columns.
 */
static void echo_erase_tab(unsigned int num_chars, int after_tab,
                           struct n_tty_data *ldata)
{
        add_echo_byte(ECHO_OP_START, ldata);
        add_echo_byte(ECHO_OP_ERASE_TAB, ldata);

        /* We only need to know this modulo 8 (tab spacing) */
        num_chars &= 7;

        /* Set the high bit as a flag if num_chars is after a previous tab */
        if (after_tab)
                num_chars |= 0x80;

        add_echo_byte(num_chars, ldata);
}

/**
 * echo_char_raw        -       echo a character raw
 * @c: unicode byte to echo
 * @ldata: line disc data
 *
 * Echo user input back onto the screen. This must be called only when
 * L_ECHO(tty) is true. Called from the &tty_driver.receive_buf() path.
 *
 * This variant does not treat control characters specially.
 */
static void echo_char_raw(u8 c, struct n_tty_data *ldata)
{
        if (c == ECHO_OP_START) {
                add_echo_byte(ECHO_OP_START, ldata);
                add_echo_byte(ECHO_OP_START, ldata);
        } else {
                add_echo_byte(c, ldata);
        }
}

/**
 * echo_char            -       echo a character
 * @c: unicode byte to echo
 * @tty: terminal device
 *
 * Echo user input back onto the screen. This must be called only when
 * L_ECHO(tty) is true. Called from the &tty_driver.receive_buf() path.
 *
 * This variant tags control characters to be echoed as "^X" (where X is the
 * letter representing the control char).
 */
static void echo_char(u8 c, const struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

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

/**
 * finish_erasing       -       complete erase
 * @ldata: n_tty data
 */
static inline void finish_erasing(struct n_tty_data *ldata)
{
        if (ldata->erasing) {
                echo_char_raw('/', ldata);
                ldata->erasing = 0;
        }
}

/**
 * eraser               -       handle erase function
 * @c: character input
 * @tty: terminal device
 *
 * Perform erase and necessary output when an erase character is present in the
 * stream from the driver layer. Handles the complexities of UTF-8 multibyte
 * symbols.
 *
 * Locking: n_tty_receive_buf()/producer path:
 *      caller holds non-exclusive %termios_rwsem
 */
static void eraser(u8 c, const struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;
        enum { ERASE, WERASE, KILL } kill_type;
        size_t head;
        size_t cnt;
        int seen_alnums;

        if (ldata->read_head == ldata->canon_head) {
                /* process_output('\a', tty); */ /* what do you think? */
                return;
        }
        if (c == ERASE_CHAR(tty))
                kill_type = ERASE;
        else if (c == WERASE_CHAR(tty))
                kill_type = WERASE;
        else {
                if (!L_ECHO(tty)) {
                        ldata->read_head = ldata->canon_head;
                        return;
                }
                if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
                        ldata->read_head = ldata->canon_head;
                        finish_erasing(ldata);
                        echo_char(KILL_CHAR(tty), tty);
                        /* Add a newline if ECHOK is on and ECHOKE is off. */
                        if (L_ECHOK(tty))
                                echo_char_raw('\n', ldata);
                        return;
                }
                kill_type = KILL;
        }

        seen_alnums = 0;
        while (MASK(ldata->read_head) != MASK(ldata->canon_head)) {
                head = ldata->read_head;

                /* erase a single possibly multibyte character */
                do {
                        head--;
                        c = read_buf(ldata, head);
                } while (is_continuation(c, tty) &&
                         MASK(head) != MASK(ldata->canon_head));

                /* do not partially erase */
                if (is_continuation(c, tty))
                        break;

                if (kill_type == WERASE) {
                        /* Equivalent to BSD's ALTWERASE. */
                        if (isalnum(c) || c == '_')
                                seen_alnums++;
                        else if (seen_alnums)
                                break;
                }
                cnt = ldata->read_head - head;
                ldata->read_head = head;
                if (L_ECHO(tty)) {
                        if (L_ECHOPRT(tty)) {
                                if (!ldata->erasing) {
                                        echo_char_raw('\\', ldata);
                                        ldata->erasing = 1;
                                }
                                /* if cnt > 1, output a multi-byte character */
                                echo_char(c, tty);
                                while (--cnt > 0) {
                                        head++;
                                        echo_char_raw(read_buf(ldata, head), ldata);
                                        echo_move_back_col(ldata);
                                }
                        } else if (kill_type == ERASE && !L_ECHOE(tty)) {
                                echo_char(ERASE_CHAR(tty), tty);
                        } else if (c == '\t') {
                                unsigned int num_chars = 0;
                                int after_tab = 0;
                                size_t tail = ldata->read_head;

                                /*
                                 * Count the columns used for characters
                                 * since the start of input or after a
                                 * previous tab.
                                 * This info is used to go back the correct
                                 * number of columns.
                                 */
                                while (MASK(tail) != MASK(ldata->canon_head)) {
                                        tail--;
                                        c = read_buf(ldata, tail);
                                        if (c == '\t') {
                                                after_tab = 1;
                                                break;
                                        } else if (iscntrl(c)) {
                                                if (L_ECHOCTL(tty))
                                                        num_chars += 2;
                                        } else if (!is_continuation(c, tty)) {
                                                num_chars++;
                                        }
                                }
                                echo_erase_tab(num_chars, after_tab, ldata);
                        } else {
                                if (iscntrl(c) && L_ECHOCTL(tty)) {
                                        echo_char_raw('\b', ldata);
                                        echo_char_raw(' ', ldata);
                                        echo_char_raw('\b', ldata);
                                }
                                if (!iscntrl(c) || L_ECHOCTL(tty)) {
                                        echo_char_raw('\b', ldata);
                                        echo_char_raw(' ', ldata);
                                        echo_char_raw('\b', ldata);
                                }
                        }
                }
                if (kill_type == ERASE)
                        break;
        }
        if (ldata->read_head == ldata->canon_head && L_ECHO(tty))
                finish_erasing(ldata);
}


static void __isig(int sig, struct tty_struct *tty)
{
        struct pid *tty_pgrp = tty_get_pgrp(tty);
        if (tty_pgrp) {
                kill_pgrp(tty_pgrp, sig, 1);
                put_pid(tty_pgrp);
        }
}

/**
 * isig                 -       handle the ISIG optio
 * @sig: signal
 * @tty: terminal
 *
 * Called when a signal is being sent due to terminal input. Called from the
 * &tty_driver.receive_buf() path, so serialized.
 *
 * Performs input and output flush if !NOFLSH. In this context, the echo
 * buffer is 'output'. The signal is processed first to alert any current
 * readers or writers to discontinue and exit their i/o loops.
 *
 * Locking: %ctrl.lock
 */
static void isig(int sig, struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (L_NOFLSH(tty)) {
                /* signal only */
                __isig(sig, tty);

        } else { /* signal and flush */
                up_read(&tty->termios_rwsem);
                down_write(&tty->termios_rwsem);

                __isig(sig, tty);

                /* clear echo buffer */
                mutex_lock(&ldata->output_lock);
                ldata->echo_head = ldata->echo_tail = 0;
                ldata->echo_mark = ldata->echo_commit = 0;
                mutex_unlock(&ldata->output_lock);

                /* clear output buffer */
                tty_driver_flush_buffer(tty);

                /* clear input buffer */
                reset_buffer_flags(tty->disc_data);

                /* notify pty master of flush */
                if (tty->link)
                        n_tty_packet_mode_flush(tty);

                up_write(&tty->termios_rwsem);
                down_read(&tty->termios_rwsem);
        }
}

/**
 * n_tty_receive_break  -       handle break
 * @tty: terminal
 *
 * An RS232 break event has been hit in the incoming bitstream. This can cause
 * a variety of events depending upon the termios settings.
 *
 * Locking: n_tty_receive_buf()/producer path:
 *      caller holds non-exclusive termios_rwsem
 *
 * Note: may get exclusive %termios_rwsem if flushing input buffer
 */
static void n_tty_receive_break(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (I_IGNBRK(tty))
                return;
        if (I_BRKINT(tty)) {
                isig(SIGINT, tty);
                return;
        }
        if (I_PARMRK(tty)) {
                put_tty_queue('\377', ldata);
                put_tty_queue('\0', ldata);
        }
        put_tty_queue('\0', ldata);
}

/**
 * n_tty_receive_overrun        -       handle overrun reporting
 * @tty: terminal
 *
 * Data arrived faster than we could process it. While the tty driver has
 * flagged this the bits that were missed are gone forever.
 *
 * Called from the receive_buf path so single threaded. Does not need locking
 * as num_overrun and overrun_time are function private.
 */
static void n_tty_receive_overrun(const struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        ldata->num_overrun++;
        if (time_is_before_jiffies(ldata->overrun_time + HZ)) {
                tty_warn(tty, "%u input overrun(s)\n", ldata->num_overrun);
                ldata->overrun_time = jiffies;
                ldata->num_overrun = 0;
        }
}

/**
 * n_tty_receive_parity_error   -       error notifier
 * @tty: terminal device
 * @c: character
 *
 * Process a parity error and queue the right data to indicate the error case
 * if necessary.
 *
 * Locking: n_tty_receive_buf()/producer path:
 *      caller holds non-exclusive %termios_rwsem
 */
static void n_tty_receive_parity_error(const struct tty_struct *tty,
                                       u8 c)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (I_INPCK(tty)) {
                if (I_IGNPAR(tty))
                        return;
                if (I_PARMRK(tty)) {
                        put_tty_queue('\377', ldata);
                        put_tty_queue('\0', ldata);
                        put_tty_queue(c, ldata);
                } else
                        put_tty_queue('\0', ldata);
        } else
                put_tty_queue(c, ldata);
}

static void
n_tty_receive_signal_char(struct tty_struct *tty, int signal, u8 c)
{
        isig(signal, tty);
        if (I_IXON(tty))
                start_tty(tty);
        if (L_ECHO(tty)) {
                echo_char(c, tty);
                commit_echoes(tty);
        } else
                process_echoes(tty);
}

static bool n_tty_is_char_flow_ctrl(struct tty_struct *tty, u8 c)
{
        return c == START_CHAR(tty) || c == STOP_CHAR(tty);
}

/**
 * n_tty_receive_char_flow_ctrl - receive flow control chars
 * @tty: terminal device
 * @c: character
 * @lookahead_done: lookahead has processed this character already
 *
 * Receive and process flow control character actions.
 *
 * In case lookahead for flow control chars already handled the character in
 * advance to the normal receive, the actions are skipped during normal
 * receive.
 *
 * Returns true if @c is consumed as flow-control character, the character
 * must not be treated as normal character.
 */
static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, u8 c,
                                         bool lookahead_done)
{
        if (!n_tty_is_char_flow_ctrl(tty, c))
                return false;

        if (lookahead_done)
                return true;

        if (c == START_CHAR(tty)) {
                start_tty(tty);
                process_echoes(tty);
                return true;
        }

        /* STOP_CHAR */
        stop_tty(tty);
        return true;
}

static void n_tty_receive_handle_newline(struct tty_struct *tty, u8 c)
{
        struct n_tty_data *ldata = tty->disc_data;

        set_bit(MASK(ldata->read_head), ldata->read_flags);
        put_tty_queue(c, ldata);
        smp_store_release(&ldata->canon_head, ldata->read_head);
        kill_fasync(&tty->fasync, SIGIO, POLL_IN);
        wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
}

static bool n_tty_receive_char_canon(struct tty_struct *tty, u8 c)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
            (c == WERASE_CHAR(tty) && L_IEXTEN(tty))) {
                eraser(c, tty);
                commit_echoes(tty);

                return true;
        }

        if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
                ldata->lnext = 1;
                if (L_ECHO(tty)) {
                        finish_erasing(ldata);
                        if (L_ECHOCTL(tty)) {
                                echo_char_raw('^', ldata);
                                echo_char_raw('\b', ldata);
                                commit_echoes(tty);
                        }
                }

                return true;
        }

        if (c == REPRINT_CHAR(tty) && L_ECHO(tty) && L_IEXTEN(tty)) {
                size_t tail = ldata->canon_head;

                finish_erasing(ldata);
                echo_char(c, tty);
                echo_char_raw('\n', ldata);
                while (MASK(tail) != MASK(ldata->read_head)) {
                        echo_char(read_buf(ldata, tail), tty);
                        tail++;
                }
                commit_echoes(tty);

                return true;
        }

        if (c == '\n') {
                if (L_ECHO(tty) || L_ECHONL(tty)) {
                        echo_char_raw('\n', ldata);
                        commit_echoes(tty);
                }
                n_tty_receive_handle_newline(tty, c);

                return true;
        }

        if (c == EOF_CHAR(tty)) {
                c = __DISABLED_CHAR;
                n_tty_receive_handle_newline(tty, c);

                return true;
        }

        if ((c == EOL_CHAR(tty)) ||
            (c == EOL2_CHAR(tty) && L_IEXTEN(tty))) {
                /*
                 * XXX are EOL_CHAR and EOL2_CHAR echoed?!?
                 */
                if (L_ECHO(tty)) {
                        /* Record the column of first canon char. */
                        if (ldata->canon_head == ldata->read_head)
                                echo_set_canon_col(ldata);
                        echo_char(c, tty);
                        commit_echoes(tty);
                }
                /*
                 * XXX does PARMRK doubling happen for
                 * EOL_CHAR and EOL2_CHAR?
                 */
                if (c == '\377' && I_PARMRK(tty))
                        put_tty_queue(c, ldata);

                n_tty_receive_handle_newline(tty, c);

                return true;
        }

        return false;
}

static void n_tty_receive_char_special(struct tty_struct *tty, u8 c,
                                       bool lookahead_done)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c, lookahead_done))
                return;

        if (L_ISIG(tty)) {
                if (c == INTR_CHAR(tty)) {
                        n_tty_receive_signal_char(tty, SIGINT, c);
                        return;
                } else if (c == QUIT_CHAR(tty)) {
                        n_tty_receive_signal_char(tty, SIGQUIT, c);
                        return;
                } else if (c == SUSP_CHAR(tty)) {
                        n_tty_receive_signal_char(tty, SIGTSTP, c);
                        return;
                }
        }

        if (tty->flow.stopped && !tty->flow.tco_stopped && I_IXON(tty) && I_IXANY(tty)) {
                start_tty(tty);
                process_echoes(tty);
        }

        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 && n_tty_receive_char_canon(tty, c))
                return;

        if (L_ECHO(tty)) {
                finish_erasing(ldata);
                if (c == '\n')
                        echo_char_raw('\n', ldata);
                else {
                        /* Record the column of first canon char. */
                        if (ldata->canon_head == ldata->read_head)
                                echo_set_canon_col(ldata);
                        echo_char(c, tty);
                }
                commit_echoes(tty);
        }

        /* PARMRK doubling check */
        if (c == '\377' && I_PARMRK(tty))
                put_tty_queue(c, ldata);

        put_tty_queue(c, ldata);
}

/**
 * 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.
 *
 * Locking: n_tty_receive_buf()/producer path:
 *      caller holds non-exclusive %termios_rwsem
 *      publishes canon_head if canonical mode is active
 */
static void n_tty_receive_char(struct tty_struct *tty, u8 c)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (tty->flow.stopped && !tty->flow.tco_stopped && I_IXON(tty) && I_IXANY(tty)) {
                start_tty(tty);
                process_echoes(tty);
        }
        if (L_ECHO(tty)) {
                finish_erasing(ldata);
                /* Record the column of first canon char. */
                if (ldata->canon_head == ldata->read_head)
                        echo_set_canon_col(ldata);
                echo_char(c, tty);
                commit_echoes(tty);
        }
        /* PARMRK doubling check */
        if (c == '\377' && I_PARMRK(tty))
                put_tty_queue(c, ldata);
        put_tty_queue(c, ldata);
}

static void n_tty_receive_char_closing(struct tty_struct *tty, u8 c,
                                       bool lookahead_done)
{
        if (I_ISTRIP(tty))
                c &= 0x7f;
        if (I_IUCLC(tty) && L_IEXTEN(tty))
                c = tolower(c);

        if (I_IXON(tty)) {
                if (!n_tty_receive_char_flow_ctrl(tty, c, lookahead_done) &&
                    tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
                    c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
                    c != SUSP_CHAR(tty)) {
                        start_tty(tty);
                        process_echoes(tty);
                }
        }
}

static void
n_tty_receive_char_flagged(struct tty_struct *tty, u8 c, u8 flag)
{
        switch (flag) {
        case TTY_BREAK:
                n_tty_receive_break(tty);
                break;
        case TTY_PARITY:
        case TTY_FRAME:
                n_tty_receive_parity_error(tty, c);
                break;
        case TTY_OVERRUN:
                n_tty_receive_overrun(tty);
                break;
        default:
                tty_err(tty, "unknown flag %u\n", flag);
                break;
        }
}

static void
n_tty_receive_char_lnext(struct tty_struct *tty, u8 c, u8 flag)
{
        struct n_tty_data *ldata = tty->disc_data;

        ldata->lnext = 0;
        if (likely(flag == TTY_NORMAL)) {
                if (I_ISTRIP(tty))
                        c &= 0x7f;
                if (I_IUCLC(tty) && L_IEXTEN(tty))
                        c = tolower(c);
                n_tty_receive_char(tty, c);
        } else
                n_tty_receive_char_flagged(tty, c, flag);
}

/* Caller must ensure count > 0 */
static void n_tty_lookahead_flow_ctrl(struct tty_struct *tty, const u8 *cp,
                                      const u8 *fp, size_t count)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 flag = TTY_NORMAL;

        ldata->lookahead_count += count;

        if (!I_IXON(tty))
                return;

        while (count--) {
                if (fp)
                        flag = *fp++;
                if (likely(flag == TTY_NORMAL))
                        n_tty_receive_char_flow_ctrl(tty, *cp, false);
                cp++;
        }
}

static void
n_tty_receive_buf_real_raw(const struct tty_struct *tty, const u8 *cp,
                           size_t count)
{
        struct n_tty_data *ldata = tty->disc_data;

        /* handle buffer wrap-around by a loop */
        for (unsigned int i = 0; i < 2; i++) {
                size_t head = MASK(ldata->read_head);
                size_t n = min(count, N_TTY_BUF_SIZE - head);

                memcpy(read_buf_addr(ldata, head), cp, n);

                ldata->read_head += n;
                cp += n;
                count -= n;
        }
}

static void
n_tty_receive_buf_raw(struct tty_struct *tty, const u8 *cp, const u8 *fp,
                      size_t count)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 flag = TTY_NORMAL;

        while (count--) {
                if (fp)
                        flag = *fp++;
                if (likely(flag == TTY_NORMAL))
                        put_tty_queue(*cp++, ldata);
                else
                        n_tty_receive_char_flagged(tty, *cp++, flag);
        }
}

static void
n_tty_receive_buf_closing(struct tty_struct *tty, const u8 *cp, const u8 *fp,
                          size_t count, bool lookahead_done)
{
        u8 flag = TTY_NORMAL;

        while (count--) {
                if (fp)
                        flag = *fp++;
                if (likely(flag == TTY_NORMAL))
                        n_tty_receive_char_closing(tty, *cp++, lookahead_done);
        }
}

static void n_tty_receive_buf_standard(struct tty_struct *tty, const u8 *cp,
                                       const u8 *fp, size_t count,
                                       bool lookahead_done)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 flag = TTY_NORMAL;

        while (count--) {
                u8 c = *cp++;

                if (fp)
                        flag = *fp++;

                if (ldata->lnext) {
                        n_tty_receive_char_lnext(tty, c, flag);
                        continue;
                }

                if (unlikely(flag != TTY_NORMAL)) {
                        n_tty_receive_char_flagged(tty, c, flag);
                        continue;
                }

                if (I_ISTRIP(tty))
                        c &= 0x7f;
                if (I_IUCLC(tty) && L_IEXTEN(tty))
                        c = tolower(c);
                if (L_EXTPROC(tty)) {
                        put_tty_queue(c, ldata);
                        continue;
                }

                if (test_bit(c, ldata->char_map))
                        n_tty_receive_char_special(tty, c, lookahead_done);
                else
                        n_tty_receive_char(tty, c);
        }
}

static void __receive_buf(struct tty_struct *tty, const u8 *cp, const u8 *fp,
                          size_t count)
{
        struct n_tty_data *ldata = tty->disc_data;
        bool preops = I_ISTRIP(tty) || (I_IUCLC(tty) && L_IEXTEN(tty));
        size_t la_count = min(ldata->lookahead_count, count);

        if (ldata->real_raw)
                n_tty_receive_buf_real_raw(tty, cp, count);
        else if (ldata->raw || (L_EXTPROC(tty) && !preops))
                n_tty_receive_buf_raw(tty, cp, fp, count);
        else if (tty->closing && !L_EXTPROC(tty)) {
                if (la_count > 0) {
                        n_tty_receive_buf_closing(tty, cp, fp, la_count, true);
                        cp += la_count;
                        if (fp)
                                fp += la_count;
                        count -= la_count;
                }
                if (count > 0)
                        n_tty_receive_buf_closing(tty, cp, fp, count, false);
        } else {
                if (la_count > 0) {
                        n_tty_receive_buf_standard(tty, cp, fp, la_count, true);
                        cp += la_count;
                        if (fp)
                                fp += la_count;
                        count -= la_count;
                }
                if (count > 0)
                        n_tty_receive_buf_standard(tty, cp, fp, count, false);

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

        ldata->lookahead_count -= la_count;

        if (ldata->icanon && !L_EXTPROC(tty))
                return;

        /* publish read_head to consumer */
        smp_store_release(&ldata->commit_head, ldata->read_head);

        if (read_cnt(ldata)) {
                kill_fasync(&tty->fasync, SIGIO, POLL_IN);
                wake_up_interruptible_poll(&tty->read_wait, EPOLLIN | EPOLLRDNORM);
        }
}

/**
 * n_tty_receive_buf_common     -       process input
 * @tty: device to receive input
 * @cp: input chars
 * @fp: flags for each char (if %NULL, all chars are %TTY_NORMAL)
 * @count: number of input chars in @cp
 * @flow: enable flow control
 *
 * Called by the terminal driver when a block of characters has been received.
 * This function must be called from soft contexts not from interrupt context.
 * The driver is responsible for making calls one at a time and in order (or
 * using flush_to_ldisc()).
 *
 * Returns: the # of input chars from @cp which were processed.
 *
 * In canonical mode, the maximum line length is 4096 chars (including the line
 * termination char); lines longer than 4096 chars are truncated. After 4095
 * chars, input data is still processed but not stored. Overflow processing
 * ensures the tty can always receive more input until at least one line can be
 * read.
 *
 * In non-canonical mode, the read buffer will only accept 4095 chars; this
 * provides the necessary space for a newline char if the input mode is
 * switched to canonical.
 *
 * Note it is possible for the read buffer to _contain_ 4096 chars in
 * non-canonical mode: the read buffer could already contain the maximum canon
 * line of 4096 chars when the mode is switched to non-canonical.
 *
 * Locking: n_tty_receive_buf()/producer path:
 *      claims non-exclusive %termios_rwsem
 *      publishes commit_head or canon_head
 */
static size_t
n_tty_receive_buf_common(struct tty_struct *tty, const u8 *cp, const u8 *fp,
                         size_t count, bool flow)
{
        struct n_tty_data *ldata = tty->disc_data;
        size_t n, rcvd = 0;
        int room, overflow;

        down_read(&tty->termios_rwsem);

        do {
                /*
                 * When PARMRK is set, each input char may take up to 3 chars
                 * in the read buf; reduce the buffer space avail by 3x
                 *
                 * If we are doing input canonicalization, and there are no
                 * pending newlines, let characters through without limit, so
                 * that erase characters will be handled.  Other excess
                 * characters will be beeped.
                 *
                 * paired with store in *_copy_from_read_buf() -- guarantees
                 * the consumer has loaded the data in read_buf up to the new
                 * read_tail (so this producer will not overwrite unread data)
                 */
                size_t tail = smp_load_acquire(&ldata->read_tail);

                room = N_TTY_BUF_SIZE - (ldata->read_head - tail);
                if (I_PARMRK(tty))
                        room = DIV_ROUND_UP(room, 3);
                room--;
                if (room <= 0) {
                        overflow = ldata->icanon && ldata->canon_head == tail;
                        if (overflow && room < 0)
                                ldata->read_head--;
                        room = overflow;
                        WRITE_ONCE(ldata->no_room, flow && !room);
                } else
                        overflow = 0;

                n = min_t(size_t, count, room);
                if (!n)
                        break;

                /* ignore parity errors if handling overflow */
                if (!overflow || !fp || *fp != TTY_PARITY)
                        __receive_buf(tty, cp, fp, n);

                cp += n;
                if (fp)
                        fp += n;
                count -= n;
                rcvd += n;
        } while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));

        tty->receive_room = room;

        /* Unthrottle if handling overflow on pty */
        if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
                if (overflow) {
                        tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
                        tty_unthrottle_safe(tty);
                        __tty_set_flow_change(tty, 0);
                }
        } else
                n_tty_check_throttle(tty);

        if (unlikely(ldata->no_room)) {
                /*
                 * Barrier here is to ensure to read the latest read_tail in
                 * chars_in_buffer() and to make sure that read_tail is not loaded
                 * before ldata->no_room is set.
                 */
                smp_mb();
                if (!chars_in_buffer(tty))
                        n_tty_kick_worker(tty);
        }

        up_read(&tty->termios_rwsem);

        return rcvd;
}

static void n_tty_receive_buf(struct tty_struct *tty, const u8 *cp,
                              const u8 *fp, size_t count)
{
        n_tty_receive_buf_common(tty, cp, fp, count, false);
}

static size_t n_tty_receive_buf2(struct tty_struct *tty, const u8 *cp,
                                 const u8 *fp, size_t count)
{
        return n_tty_receive_buf_common(tty, cp, fp, count, true);
}

/**
 * n_tty_set_termios    -       termios data changed
 * @tty: terminal
 * @old: previous data
 *
 * Called by the tty layer when the user changes termios flags so that the line
 * discipline can plan ahead. This function cannot sleep and is protected from
 * re-entry by the tty layer. The user is guaranteed that this function will
 * not be re-entered or in progress when the ldisc is closed.
 *
 * Locking: Caller holds @tty->termios_rwsem
 */
static void n_tty_set_termios(struct tty_struct *tty, const struct ktermios *old)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (!old || (old->c_lflag ^ tty->termios.c_lflag) & (ICANON | EXTPROC)) {
                bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
                ldata->line_start = ldata->read_tail;
                if (!L_ICANON(tty) || !read_cnt(ldata)) {
                        ldata->canon_head = ldata->read_tail;
                        ldata->push = 0;
                } else {
                        set_bit(MASK(ldata->read_head - 1), ldata->read_flags);
                        ldata->canon_head = ldata->read_head;
                        ldata->push = 1;
                }
                ldata->commit_head = ldata->read_head;
                ldata->erasing = 0;
                ldata->lnext = 0;
        }

        ldata->icanon = (L_ICANON(tty) != 0);

        if (I_ISTRIP(tty) || I_IUCLC(tty) || I_IGNCR(tty) ||
            I_ICRNL(tty) || I_INLCR(tty) || L_ICANON(tty) ||
            I_IXON(tty) || L_ISIG(tty) || L_ECHO(tty) ||
            I_PARMRK(tty)) {
                bitmap_zero(ldata->char_map, 256);

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

                if (L_ICANON(tty)) {
                        set_bit(ERASE_CHAR(tty), ldata->char_map);
                        set_bit(KILL_CHAR(tty), ldata->char_map);
                        set_bit(EOF_CHAR(tty), ldata->char_map);
                        set_bit('\n', ldata->char_map);
                        set_bit(EOL_CHAR(tty), ldata->char_map);
                        if (L_IEXTEN(tty)) {
                                set_bit(WERASE_CHAR(tty), ldata->char_map);
                                set_bit(LNEXT_CHAR(tty), ldata->char_map);
                                set_bit(EOL2_CHAR(tty), ldata->char_map);
                                if (L_ECHO(tty))
                                        set_bit(REPRINT_CHAR(tty),
                                                ldata->char_map);
                        }
                }
                if (I_IXON(tty)) {
                        set_bit(START_CHAR(tty), ldata->char_map);
                        set_bit(STOP_CHAR(tty), ldata->char_map);
                }
                if (L_ISIG(tty)) {
                        set_bit(INTR_CHAR(tty), ldata->char_map);
                        set_bit(QUIT_CHAR(tty), ldata->char_map);
                        set_bit(SUSP_CHAR(tty), ldata->char_map);
                }
                clear_bit(__DISABLED_CHAR, ldata->char_map);
                ldata->raw = 0;
                ldata->real_raw = 0;
        } else {
                ldata->raw = 1;
                if ((I_IGNBRK(tty) || (!I_BRKINT(tty) && !I_PARMRK(tty))) &&
                    (I_IGNPAR(tty) || !I_INPCK(tty)) &&
                    (tty->driver->flags & TTY_DRIVER_REAL_RAW))
                        ldata->real_raw = 1;
                else
                        ldata->real_raw = 0;
        }
        /*
         * Fix tty hang when I_IXON(tty) is cleared, but the tty
         * been stopped by STOP_CHAR(tty) before it.
         */
        if (!I_IXON(tty) && old && (old->c_iflag & IXON) && !tty->flow.tco_stopped) {
                start_tty(tty);
                process_echoes(tty);
        }

        /* The termios change make the tty ready for I/O */
        wake_up_interruptible(&tty->write_wait);
        wake_up_interruptible(&tty->read_wait);
}

/**
 * n_tty_close          -       close the ldisc for this tty
 * @tty: device
 *
 * Called from the terminal layer when this line discipline is being shut down,
 * either because of a close or becsuse of a discipline change. The function
 * will not be called while other ldisc methods are in progress.
 */
static void n_tty_close(struct tty_struct *tty)
{
        struct n_tty_data *ldata = tty->disc_data;

        if (tty->link)
                n_tty_packet_mode_flush(tty);

        down_write(&tty->termios_rwsem);
        vfree(ldata);
        tty->disc_data = NULL;
        up_write(&tty->termios_rwsem);
}

/**
 * n_tty_open           -       open an ldisc
 * @tty: terminal to open
 *
 * Called when this line discipline is being attached to the terminal device.
 * Can sleep. Called serialized so that no other events will occur in parallel.
 * No further open will occur until a close.
 */
static int n_tty_open(struct tty_struct *tty)
{
        struct n_tty_data *ldata;

        /* Currently a malloc failure here can panic */
        ldata = vzalloc(sizeof(*ldata));
        if (!ldata)
                return -ENOMEM;

        ldata->overrun_time = jiffies;
        mutex_init(&ldata->atomic_read_lock);
        mutex_init(&ldata->output_lock);

        tty->disc_data = ldata;
        tty->closing = 0;
        /* indicate buffer work may resume */
        clear_bit(TTY_LDISC_HALTED, &tty->flags);
        n_tty_set_termios(tty, NULL);
        tty_unthrottle(tty);
        return 0;
}

static inline int input_available_p(const struct tty_struct *tty, int poll)
{
        const struct n_tty_data *ldata = tty->disc_data;
        int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;

        if (ldata->icanon && !L_EXTPROC(tty))
                return ldata->canon_head != ldata->read_tail;
        else
                return ldata->commit_head - ldata->read_tail >= amt;
}

/**
 * copy_from_read_buf   -       copy read data directly
 * @tty: terminal device
 * @kbp: data
 * @nr: size of data
 *
 * Helper function to speed up n_tty_read(). It is only called when %ICANON is
 * off; it copies characters straight from the tty queue.
 *
 * Returns: true if it successfully copied data, but there is still more data
 * to be had.
 *
 * Locking:
 *  * called under the @ldata->atomic_read_lock sem
 *  * n_tty_read()/consumer path:
 *              caller holds non-exclusive %termios_rwsem;
 *              read_tail published
 */
static bool copy_from_read_buf(const struct tty_struct *tty, u8 **kbp,
                               size_t *nr)

{
        struct n_tty_data *ldata = tty->disc_data;
        size_t n;
        bool is_eof;
        size_t head = smp_load_acquire(&ldata->commit_head);
        size_t tail = MASK(ldata->read_tail);

        n = min3(head - ldata->read_tail, N_TTY_BUF_SIZE - tail, *nr);
        if (!n)
                return false;

        u8 *from = read_buf_addr(ldata, tail);
        memcpy(*kbp, from, n);
        is_eof = n == 1 && *from == EOF_CHAR(tty);
        tty_audit_add_data(tty, from, n);
        zero_buffer(tty, from, n);
        smp_store_release(&ldata->read_tail, ldata->read_tail + n);

        /* Turn single EOF into zero-length read */
        if (L_EXTPROC(tty) && ldata->icanon && is_eof &&
            head == ldata->read_tail)
                return false;

        *kbp += n;
        *nr -= n;

        /* If we have more to copy, let the caller know */
        return head != ldata->read_tail;
}

/**
 * canon_copy_from_read_buf     -       copy read data in canonical mode
 * @tty: terminal device
 * @kbp: data
 * @nr: size of data
 *
 * Helper function for n_tty_read(). It is only called when %ICANON is on; it
 * copies one line of input up to and including the line-delimiting character
 * into the result buffer.
 *
 * Note: When termios is changed from non-canonical to canonical mode and the
 * read buffer contains data, n_tty_set_termios() simulates an EOF push (as if
 * C-d were input) _without_ the %DISABLED_CHAR in the buffer. This causes data
 * already processed as input to be immediately available as input although a
 * newline has not been received.
 *
 * Locking:
 *  * called under the %atomic_read_lock mutex
 *  * n_tty_read()/consumer path:
 *      caller holds non-exclusive %termios_rwsem;
 *      read_tail published
 */
static bool canon_copy_from_read_buf(const struct tty_struct *tty, u8 **kbp,
                                     size_t *nr)
{
        struct n_tty_data *ldata = tty->disc_data;
        size_t n, size, more, c;
        size_t eol;
        size_t tail, canon_head;
        int found = 0;

        /* N.B. avoid overrun if nr == 0 */
        if (!*nr)
                return false;

        canon_head = smp_load_acquire(&ldata->canon_head);
        n = min(*nr, canon_head - ldata->read_tail);

        tail = MASK(ldata->read_tail);
        size = min_t(size_t, tail + n, N_TTY_BUF_SIZE);

        eol = find_next_bit(ldata->read_flags, size, tail);
        more = n - (size - tail);
        if (eol == N_TTY_BUF_SIZE && more) {
                /* scan wrapped without finding set bit */
                eol = find_first_bit(ldata->read_flags, more);
                found = eol != more;
        } else
                found = eol != size;

        n = eol - tail;
        if (n > N_TTY_BUF_SIZE)
                n += N_TTY_BUF_SIZE;
        c = n + found;

        if (!found || read_buf(ldata, eol) != __DISABLED_CHAR)
                n = c;

        tty_copy(tty, *kbp, tail, n);
        *kbp += n;
        *nr -= n;

        if (found)
                clear_bit(eol, ldata->read_flags);
        smp_store_release(&ldata->read_tail, ldata->read_tail + c);

        if (found) {
                if (!ldata->push)
                        ldata->line_start = ldata->read_tail;
                else
                        ldata->push = 0;
                tty_audit_push();
                return false;
        }

        /* No EOL found - do a continuation retry if there is more data */
        return ldata->read_tail != canon_head;
}

/*
 * If we finished a read at the exact location of an
 * EOF (special EOL character that's a __DISABLED_CHAR)
 * in the stream, silently eat the EOF.
 */
static void canon_skip_eof(struct n_tty_data *ldata)
{
        size_t tail, canon_head;

        canon_head = smp_load_acquire(&ldata->canon_head);
        tail = ldata->read_tail;

        // No data?
        if (tail == canon_head)
                return;

        // See if the tail position is EOF in the circular buffer
        tail &= (N_TTY_BUF_SIZE - 1);
        if (!test_bit(tail, ldata->read_flags))
                return;
        if (read_buf(ldata, tail) != __DISABLED_CHAR)
                return;

        // Clear the EOL bit, skip the EOF char.
        clear_bit(tail, ldata->read_flags);
        smp_store_release(&ldata->read_tail, ldata->read_tail + 1);
}

/**
 * job_control          -       check job control
 * @tty: tty
 * @file: file handle
 *
 * Perform job control management checks on this @file/@tty descriptor and if
 * appropriate send any needed signals and return a negative error code if
 * action should be taken.
 *
 * Locking:
 *  * redirected write test is safe
 *  * current->signal->tty check is safe
 *  * ctrl.lock to safely reference @tty->ctrl.pgrp
 */
static int job_control(struct tty_struct *tty, struct file *file)
{
        /* Job control check -- must be done at start and after
           every sleep (POSIX.1 7.1.1.4). */
        /* NOTE: not yet done after every sleep pending a thorough
           check of the logic of this change. -- jlc */
        /* don't stop on /dev/console */
        if (file->f_op->write_iter == redirected_tty_write)
                return 0;

        return __tty_check_change(tty, SIGTTIN);
}

/*
 * We still hold the atomic_read_lock and the termios_rwsem, and can just
 * continue to copy data.
 */
static ssize_t n_tty_continue_cookie(struct tty_struct *tty, u8 *kbuf,
                                   size_t nr, void **cookie)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 *kb = kbuf;

        if (ldata->icanon && !L_EXTPROC(tty)) {
                /*
                 * If we have filled the user buffer, see if we should skip an
                 * EOF character before releasing the lock and returning done.
                 */
                if (!nr)
                        canon_skip_eof(ldata);
                else if (canon_copy_from_read_buf(tty, &kb, &nr))
                        return kb - kbuf;
        } else {
                if (copy_from_read_buf(tty, &kb, &nr))
                        return kb - kbuf;
        }

        /* No more data - release locks and stop retries */
        n_tty_kick_worker(tty);
        n_tty_check_unthrottle(tty);
        up_read(&tty->termios_rwsem);
        mutex_unlock(&ldata->atomic_read_lock);
        *cookie = NULL;

        return kb - kbuf;
}

static int n_tty_wait_for_input(struct tty_struct *tty, struct file *file,
                                struct wait_queue_entry *wait, long *timeout)
{
        if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                return -EIO;
        if (tty_hung_up_p(file))
                return 0;
        /*
         * Abort readers for ttys which never actually get hung up.
         * See __tty_hangup().
         */
        if (test_bit(TTY_HUPPING, &tty->flags))
                return 0;
        if (!*timeout)
                return 0;
        if (tty_io_nonblock(tty, file))
                return -EAGAIN;
        if (signal_pending(current))
                return -ERESTARTSYS;

        up_read(&tty->termios_rwsem);
        *timeout = wait_woken(wait, TASK_INTERRUPTIBLE, *timeout);
        down_read(&tty->termios_rwsem);

        return 1;
}

/**
 * n_tty_read           -       read function for tty
 * @tty: tty device
 * @file: file object
 * @kbuf: kernelspace buffer pointer
 * @nr: size of I/O
 * @cookie: if non-%NULL, this is a continuation read
 * @offset: where to continue reading from (unused in n_tty)
 *
 * 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.
 *
 * Locking: n_tty_read()/consumer path:
 *      claims non-exclusive termios_rwsem;
 *      publishes read_tail
 */
static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, u8 *kbuf,
                          size_t nr, void **cookie, unsigned long offset)
{
        struct n_tty_data *ldata = tty->disc_data;
        u8 *kb = kbuf;
        DEFINE_WAIT_FUNC(wait, woken_wake_function);
        int minimum, time;
        ssize_t retval;
        long timeout;
        bool packet;
        size_t old_tail;

        /* Is this a continuation of a read started earlier? */
        if (*cookie)
                return n_tty_continue_cookie(tty, kbuf, nr, cookie);

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

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

        down_read(&tty->termios_rwsem);

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

        packet = tty->ctrl.packet;
        old_tail = ldata->read_tail;

        add_wait_queue(&tty->read_wait, &wait);
        while (nr) {
                /* First test for status change. */
                if (packet && tty->link->ctrl.pktstatus) {
                        u8 cs;
                        if (kb != kbuf)
                                break;
                        spin_lock_irq(&tty->link->ctrl.lock);
                        cs = tty->link->ctrl.pktstatus;
                        tty->link->ctrl.pktstatus = 0;
                        spin_unlock_irq(&tty->link->ctrl.lock);
                        *kb++ = cs;
                        nr--;
                        break;
                }

                if (!input_available_p(tty, 0)) {
                        up_read(&tty->termios_rwsem);
                        tty_buffer_flush_work(tty->port);
                        down_read(&tty->termios_rwsem);
                        if (!input_available_p(tty, 0)) {
                                int ret = n_tty_wait_for_input(tty, file, &wait,
                                                               &timeout);
                                if (ret <= 0) {
                                        retval = ret;
                                        break;
                                }
                                continue;
                        }
                }

                if (ldata->icanon && !L_EXTPROC(tty)) {
                        if (canon_copy_from_read_buf(tty, &kb, &nr))
                                goto more_to_be_read;
                } else {
                        /* Deal with packet mode. */
                        if (packet && kb == kbuf) {
                                *kb++ = TIOCPKT_DATA;
                                nr--;
                        }

                        if (copy_from_read_buf(tty, &kb, &nr) && kb - kbuf >= minimum)
                                goto more_to_be_read;
                }

                n_tty_check_unthrottle(tty);

                if (kb - kbuf >= minimum)
                        break;
                if (time)
                        timeout = time;
        }
        if (old_tail != ldata->read_tail) {
                /*
                 * Make sure no_room is not read in n_tty_kick_worker()
                 * before setting ldata->read_tail in copy_from_read_buf().
                 */
                smp_mb();
                n_tty_kick_worker(tty);
        }
        up_read(&tty->termios_rwsem);

        remove_wait_queue(&tty->read_wait, &wait);
        mutex_unlock(&ldata->atomic_read_lock);

        if (kb - kbuf)
                retval = kb - kbuf;

        return retval;
more_to_be_read:
        /*
         * There is more to be had and we have nothing more to wait for, so
         * let's mark us for retries.
         *
         * NOTE! We return here with both the termios_sem and atomic_read_lock
         * still held, the retries will release them when done.
         */
        remove_wait_queue(&tty->read_wait, &wait);
        *cookie = cookie;

        return kb - kbuf;
}

/**
 * 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 u8 *buf, size_t nr)
{
        const u8 *b = buf;
        DEFINE_WAIT_FUNC(wait, woken_wake_function);
        ssize_t num, retval = 0;

        /* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
        if (L_TOSTOP(tty) && file->f_op->write_iter != redirected_tty_write) {
                retval = tty_check_change(tty);
                if (retval)
                        return retval;
        }

        down_read(&tty->termios_rwsem);

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

        add_wait_queue(&tty->write_wait, &wait);
        while (1) {
                if (signal_pending(current)) {
                        retval = -ERESTARTSYS;
                        break;
                }
                if (tty_hung_up_p(file) || (tty->link && !tty->link->count)) {
                        retval = -EIO;
                        break;
                }
                if (O_OPOST(tty)) {
                        while (nr > 0) {
                                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;
                                if (process_output(*b, tty) < 0)
                                        break;
                                b++; nr--;
                        }
                        if (tty->ops->flush_chars)
                                tty->ops->flush_chars(tty);
                } else {
                        struct n_tty_data *ldata = tty->disc_data;

                        while (nr > 0) {
                                mutex_lock(&ldata->output_lock);
                                num = tty->ops->write(tty, b, nr);
                                mutex_unlock(&ldata->output_lock);
                                if (num < 0) {
                                        retval = num;
                                        goto break_out;
                                }
                                if (!num)
                                        break;
                                b += num;
                                nr -= num;
                        }
                }
                if (!nr)
                        break;
                if (tty_io_nonblock(tty, file)) {
                        retval = -EAGAIN;
                        break;
                }
                up_read(&tty->termios_rwsem);

                wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);

                down_read(&tty->termios_rwsem);
        }
break_out:
        remove_wait_queue(&tty->write_wait, &wait);
        if (nr && tty->fasync)
                set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
        up_read(&tty->termios_rwsem);
        return (b - buf) ? b - buf : retval;
}

/**
 * n_tty_poll           -       poll method for N_TTY
 * @tty: terminal device
 * @file: file accessing it
 * @wait: poll table
 *
 * Called when the line discipline is asked to poll() for data or for special
 * events. This code is not serialized with respect to other events save
 * open/close.
 *
 * This code must be sure never to sleep through a hangup.
 *
 * Locking: called without the kernel lock held -- fine.
 */
static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
                                                        poll_table *wait)
{
        __poll_t mask = 0;

        poll_wait(file, &tty->read_wait, wait);
        poll_wait(file, &tty->write_wait, wait);
        if (input_available_p(tty, 1))
                mask |= EPOLLIN | EPOLLRDNORM;
        else {
                tty_buffer_flush_work(tty->port);
                if (input_available_p(tty, 1))
                        mask |= EPOLLIN | EPOLLRDNORM;
        }
        if (tty->ctrl.packet && tty->link->ctrl.pktstatus)
                mask |= EPOLLPRI | EPOLLIN | EPOLLRDNORM;
        if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                mask |= EPOLLHUP;
        if (tty_hung_up_p(file))
                mask |= EPOLLHUP;
        if (tty->ops->write && !tty_is_writelocked(tty) &&
                        tty_chars_in_buffer(tty) < WAKEUP_CHARS &&
                        tty_write_room(tty) > 0)
                mask |= EPOLLOUT | EPOLLWRNORM;
        return mask;
}

static unsigned long inq_canon(struct n_tty_data *ldata)
{
        size_t nr, head, tail;

        if (ldata->canon_head == ldata->read_tail)
                return 0;
        head = ldata->canon_head;
        tail = ldata->read_tail;
        nr = head - tail;
        /* Skip EOF-chars.. */
        while (MASK(head) != MASK(tail)) {
                if (test_bit(MASK(tail), ldata->read_flags) &&
                    read_buf(ldata, tail) == __DISABLED_CHAR)
                        nr--;
                tail++;
        }
        return nr;
}

static int n_tty_ioctl(struct tty_struct *tty, unsigned int cmd,
                       unsigned long arg)
{
        struct n_tty_data *ldata = tty->disc_data;
        unsigned int num;

        switch (cmd) {
        case TIOCOUTQ:
                return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
        case TIOCINQ:
                down_write(&tty->termios_rwsem);
                if (L_ICANON(tty) && !L_EXTPROC(tty))
                        num = inq_canon(ldata);
                else
                        num = read_cnt(ldata);
                up_write(&tty->termios_rwsem);
                return put_user(num, (unsigned int __user *) arg);
        default:
                return n_tty_ioctl_helper(tty, cmd, arg);
        }
}

static struct tty_ldisc_ops n_tty_ops = {
        .owner           = THIS_MODULE,
        .num             = N_TTY,
        .name            = "n_tty",
        .open            = n_tty_open,
        .close           = n_tty_close,
        .flush_buffer    = n_tty_flush_buffer,
        .read            = n_tty_read,
        .write           = n_tty_write,
        .ioctl           = n_tty_ioctl,
        .set_termios     = n_tty_set_termios,
        .poll            = n_tty_poll,
        .receive_buf     = n_tty_receive_buf,
        .write_wakeup    = n_tty_write_wakeup,
        .receive_buf2    = n_tty_receive_buf2,
        .lookahead_buf   = n_tty_lookahead_flow_ctrl,
};

/**
 *      n_tty_inherit_ops       -       inherit N_TTY methods
 *      @ops: struct tty_ldisc_ops where to save N_TTY methods
 *
 *      Enables a 'subclass' line discipline to 'inherit' N_TTY methods.
 */

void n_tty_inherit_ops(struct tty_ldisc_ops *ops)
{
        *ops = n_tty_ops;
        ops->owner = NULL;
}
EXPORT_SYMBOL_GPL(n_tty_inherit_ops);

void __init n_tty_init(void)
{
        tty_register_ldisc(&n_tty_ops);
}