hvc_console.c

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/*
 * Copyright (C) 2001 Anton Blanchard <[email protected]>, IBM
 * Copyright (C) 2001 Paul Mackerras <[email protected]>, IBM
 * Copyright (C) 2004 Benjamin Herrenschmidt <[email protected]>, IBM Corp.
 * Copyright (C) 2004 IBM Corporation
 *
 * Additional Author(s):
 *  Ryan S. Arnold <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
 */

#include <linux/console.h>
#include <linux/cpumask.h>
#include <linux/init.h>
#include <linux/kbd_kern.h>
#include <linux/kernel.h>
#include <linux/kthread.h>
#include <linux/list.h>
#include <linux/module.h>
#include <linux/major.h>
#include <linux/sysrq.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/delay.h>
#include <linux/freezer.h>
#include <linux/slab.h>
#include <linux/serial_core.h>

#include <asm/uaccess.h>

#include "hvc_console.h"

#define HVC_MAJOR   229
#define HVC_MINOR   0

/*
 * Wait this long per iteration while trying to push buffered data to the
 * hypervisor before allowing the tty to complete a close operation.
 */
#define HVC_CLOSE_WAIT (HZ/100) /* 1/10 of a second */

/*
 * These sizes are most efficient for vio, because they are the
 * native transfer size. We could make them selectable in the
 * future to better deal with backends that want other buffer sizes.
 */
#define N_OUTBUF    16
#define N_INBUF     16

#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))

static struct tty_driver *hvc_driver;
static struct task_struct *hvc_task;

/* Picks up late kicks after list walk but before schedule() */
static int hvc_kicked;

static int hvc_init(void);

#ifdef CONFIG_MAGIC_SYSRQ
static int sysrq_pressed;
#endif

/* dynamic list of hvc_struct instances */
static LIST_HEAD(hvc_structs);

/*
 * Protect the list of hvc_struct instances from inserts and removals during
 * list traversal.
 */
static DEFINE_SPINLOCK(hvc_structs_lock);

/*
 * This value is used to assign a tty->index value to a hvc_struct based
 * upon order of exposure via hvc_probe(), when we can not match it to
 * a console candidate registered with hvc_instantiate().
 */
static int last_hvc = -1;

/*
 * Do not call this function with either the hvc_structs_lock or the hvc_struct
 * lock held.  If successful, this function increments the kref reference
 * count against the target hvc_struct so it should be released when finished.
 */
static struct hvc_struct *hvc_get_by_index(int index)
{
    struct hvc_struct *hp;
    unsigned long flags;

    spin_lock(&hvc_structs_lock);

    list_for_each_entry(hp, &hvc_structs, next) {
        spin_lock_irqsave(&hp->lock, flags);
        if (hp->index == index) {
            tty_port_get(&hp->port);
            spin_unlock_irqrestore(&hp->lock, flags);
            spin_unlock(&hvc_structs_lock);
            return hp;
        }
        spin_unlock_irqrestore(&hp->lock, flags);
    }
    hp = NULL;

    spin_unlock(&hvc_structs_lock);
    return hp;
}


/*
 * Initial console vtermnos for console API usage prior to full console
 * initialization.  Any vty adapter outside this range will not have usable
 * console interfaces but can still be used as a tty device.  This has to be
 * static because kmalloc will not work during early console init.
 */
static const struct hv_ops *cons_ops[MAX_NR_HVC_CONSOLES];
static uint32_t vtermnos[MAX_NR_HVC_CONSOLES] =
    {[0 ... MAX_NR_HVC_CONSOLES - 1] = -1};

/*
 * Console APIs, NOT TTY.  These APIs are available immediately when
 * hvc_console_setup() finds adapters.
 */

static void hvc_console_print(struct console *co, const char *b,
                  unsigned count)
{
    char c[N_OUTBUF] __ALIGNED__;
    unsigned i = 0, n = 0;
    int r, donecr = 0, index = co->index;

    /* Console access attempt outside of acceptable console range. */
    if (index >= MAX_NR_HVC_CONSOLES)
        return;

    /* This console adapter was removed so it is not usable. */
    if (vtermnos[index] == -1)
        return;

    while (count > 0 || i > 0) {
        if (count > 0 && i < sizeof(c)) {
            if (b[n] == '\n' && !donecr) {
                c[i++] = '\r';
                donecr = 1;
            } else {
                c[i++] = b[n++];
                donecr = 0;
                --count;
            }
        } else {
            r = cons_ops[index]->put_chars(vtermnos[index], c, i);
            if (r <= 0) {
                /* throw away characters on error
                 * but spin in case of -EAGAIN */
                if (r != -EAGAIN)
                    i = 0;
            } else if (r > 0) {
                i -= r;
                if (i > 0)
                    memmove(c, c+r, i);
            }
        }
    }
}

static struct tty_driver *hvc_console_device(struct console *c, int *index)
{
    if (vtermnos[c->index] == -1)
        return NULL;

    *index = c->index;
    return hvc_driver;
}

static int __init hvc_console_setup(struct console *co, char *options)
{   
    if (co->index < 0 || co->index >= MAX_NR_HVC_CONSOLES)
        return -ENODEV;

    if (vtermnos[co->index] == -1)
        return -ENODEV;

    return 0;
}

static struct console hvc_console = {
    .name       = "hvc",
    .write      = hvc_console_print,
    .device     = hvc_console_device,
    .setup      = hvc_console_setup,
    .flags      = CON_PRINTBUFFER,
    .index      = -1,
};

/*
 * Early console initialization.  Precedes driver initialization.
 *
 * (1) we are first, and the user specified another driver
 * -- index will remain -1
 * (2) we are first and the user specified no driver
 * -- index will be set to 0, then we will fail setup.
 * (3)  we are first and the user specified our driver
 * -- index will be set to user specified driver, and we will fail
 * (4) we are after driver, and this initcall will register us
 * -- if the user didn't specify a driver then the console will match
 *
 * Note that for cases 2 and 3, we will match later when the io driver
 * calls hvc_instantiate() and call register again.
 */
static int __init hvc_console_init(void)
{
    register_console(&hvc_console);
    return 0;
}
console_initcall(hvc_console_init);

/* callback when the kboject ref count reaches zero. */
static void hvc_port_destruct(struct tty_port *port)
{
    struct hvc_struct *hp = container_of(port, struct hvc_struct, port);
    unsigned long flags;

    spin_lock(&hvc_structs_lock);

    spin_lock_irqsave(&hp->lock, flags);
    list_del(&(hp->next));
    spin_unlock_irqrestore(&hp->lock, flags);

    spin_unlock(&hvc_structs_lock);

    kfree(hp);
}

static void hvc_check_console(int index)
{
    /* Already enabled, bail out */
    if (hvc_console.flags & CON_ENABLED)
        return;

    /* If this index is what the user requested, then register
     * now (setup won't fail at this point).  It's ok to just
     * call register again if previously .setup failed.
     */
    if (index == hvc_console.index)
        register_console(&hvc_console);
}

/*
 * hvc_instantiate() is an early console discovery method which locates
 * consoles * prior to the vio subsystem discovering them.  Hotplugged
 * vty adapters do NOT get an hvc_instantiate() callback since they
 * appear after early console init.
 */
int hvc_instantiate(uint32_t vtermno, int index, const struct hv_ops *ops)
{
    struct hvc_struct *hp;

    if (index < 0 || index >= MAX_NR_HVC_CONSOLES)
        return -1;

    if (vtermnos[index] != -1)
        return -1;

    /* make sure no no tty has been registered in this index */
    hp = hvc_get_by_index(index);
    if (hp) {
        tty_port_put(&hp->port);
        return -1;
    }

    vtermnos[index] = vtermno;
    cons_ops[index] = ops;

    /* reserve all indices up to and including this index */
    if (last_hvc < index)
        last_hvc = index;

    /* check if we need to re-register the kernel console */
    hvc_check_console(index);

    return 0;
}
EXPORT_SYMBOL_GPL(hvc_instantiate);

/* Wake the sleeping khvcd */
void hvc_kick(void)
{
    hvc_kicked = 1;
    wake_up_process(hvc_task);
}
EXPORT_SYMBOL_GPL(hvc_kick);

static void hvc_unthrottle(struct tty_struct *tty)
{
    hvc_kick();
}

static int hvc_install(struct tty_driver *driver, struct tty_struct *tty)
{
    struct hvc_struct *hp;
    int rc;

    /* Auto increments kref reference if found. */
    if (!(hp = hvc_get_by_index(tty->index)))
        return -ENODEV;

    tty->driver_data = hp;

    rc = tty_port_install(&hp->port, driver, tty);
    if (rc)
        tty_port_put(&hp->port);
    return rc;
}

/*
 * The TTY interface won't be used until after the vio layer has exposed the vty
 * adapter to the kernel.
 */
static int hvc_open(struct tty_struct *tty, struct file * filp)
{
    struct hvc_struct *hp = tty->driver_data;
    unsigned long flags;
    int rc = 0;

    spin_lock_irqsave(&hp->port.lock, flags);
    /* Check and then increment for fast path open. */
    if (hp->port.count++ > 0) {
        spin_unlock_irqrestore(&hp->port.lock, flags);
        hvc_kick();
        return 0;
    } /* else count == 0 */
    spin_unlock_irqrestore(&hp->port.lock, flags);

    tty_port_tty_set(&hp->port, tty);

    if (hp->ops->notifier_add)
        rc = hp->ops->notifier_add(hp, hp->data);

    /*
     * If the notifier fails we return an error.  The tty layer
     * will call hvc_close() after a failed open but we don't want to clean
     * up there so we'll clean up here and clear out the previously set
     * tty fields and return the kref reference.
     */
    if (rc) {
        tty_port_tty_set(&hp->port, NULL);
        tty->driver_data = NULL;
        tty_port_put(&hp->port);
        printk(KERN_ERR "hvc_open: request_irq failed with rc %d.\n", rc);
    }
    /* Force wakeup of the polling thread */
    hvc_kick();

    return rc;
}

static void hvc_close(struct tty_struct *tty, struct file * filp)
{
    struct hvc_struct *hp;
    unsigned long flags;

    if (tty_hung_up_p(filp))
        return;

    /*
     * No driver_data means that this close was issued after a failed
     * hvc_open by the tty layer's release_dev() function and we can just
     * exit cleanly because the kref reference wasn't made.
     */
    if (!tty->driver_data)
        return;

    hp = tty->driver_data;

    spin_lock_irqsave(&hp->port.lock, flags);

    if (--hp->port.count == 0) {
        spin_unlock_irqrestore(&hp->port.lock, flags);
        /* We are done with the tty pointer now. */
        tty_port_tty_set(&hp->port, NULL);

        if (hp->ops->notifier_del)
            hp->ops->notifier_del(hp, hp->data);

        /* cancel pending tty resize work */
        cancel_work_sync(&hp->tty_resize);

        /*
         * Chain calls chars_in_buffer() and returns immediately if
         * there is no buffered data otherwise sleeps on a wait queue
         * waking periodically to check chars_in_buffer().
         */
        tty_wait_until_sent_from_close(tty, HVC_CLOSE_WAIT);
    } else {
        if (hp->port.count < 0)
            printk(KERN_ERR "hvc_close %X: oops, count is %d\n",
                hp->vtermno, hp->port.count);
        spin_unlock_irqrestore(&hp->port.lock, flags);
    }
}

static void hvc_cleanup(struct tty_struct *tty)
{
    struct hvc_struct *hp = tty->driver_data;

    tty_port_put(&hp->port);
}

static void hvc_hangup(struct tty_struct *tty)
{
    struct hvc_struct *hp = tty->driver_data;
    unsigned long flags;

    if (!hp)
        return;

    /* cancel pending tty resize work */
    cancel_work_sync(&hp->tty_resize);

    spin_lock_irqsave(&hp->port.lock, flags);

    /*
     * The N_TTY line discipline has problems such that in a close vs
     * open->hangup case this can be called after the final close so prevent
     * that from happening for now.
     */
    if (hp->port.count <= 0) {
        spin_unlock_irqrestore(&hp->port.lock, flags);
        return;
    }

    hp->port.count = 0;
    spin_unlock_irqrestore(&hp->port.lock, flags);
    tty_port_tty_set(&hp->port, NULL);

    hp->n_outbuf = 0;

    if (hp->ops->notifier_hangup)
        hp->ops->notifier_hangup(hp, hp->data);
}

/*
 * Push buffered characters whether they were just recently buffered or waiting
 * on a blocked hypervisor.  Call this function with hp->lock held.
 */
static int hvc_push(struct hvc_struct *hp)
{
    int n;

    n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
    if (n <= 0) {
        if (n == 0 || n == -EAGAIN) {
            hp->do_wakeup = 1;
            return 0;
        }
        /* throw away output on error; this happens when
           there is no session connected to the vterm. */
        hp->n_outbuf = 0;
    } else
        hp->n_outbuf -= n;
    if (hp->n_outbuf > 0)
        memmove(hp->outbuf, hp->outbuf + n, hp->n_outbuf);
    else
        hp->do_wakeup = 1;

    return n;
}

static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count)
{
    struct hvc_struct *hp = tty->driver_data;
    unsigned long flags;
    int rsize, written = 0;

    /* This write was probably executed during a tty close. */
    if (!hp)
        return -EPIPE;

    /* FIXME what's this (unprotected) check for? */
    if (hp->port.count <= 0)
        return -EIO;

    spin_lock_irqsave(&hp->lock, flags);

    /* Push pending writes */
    if (hp->n_outbuf > 0)
        hvc_push(hp);

    while (count > 0 && (rsize = hp->outbuf_size - hp->n_outbuf) > 0) {
        if (rsize > count)
            rsize = count;
        memcpy(hp->outbuf + hp->n_outbuf, buf, rsize);
        count -= rsize;
        buf += rsize;
        hp->n_outbuf += rsize;
        written += rsize;
        hvc_push(hp);
    }
    spin_unlock_irqrestore(&hp->lock, flags);

    /*
     * Racy, but harmless, kick thread if there is still pending data.
     */
    if (hp->n_outbuf)
        hvc_kick();

    return written;
}

static void hvc_set_winsz(struct work_struct *work)
{
    struct hvc_struct *hp;
    unsigned long hvc_flags;
    struct tty_struct *tty;
    struct winsize ws;

    hp = container_of(work, struct hvc_struct, tty_resize);

    tty = tty_port_tty_get(&hp->port);
    if (!tty)
        return;

    spin_lock_irqsave(&hp->lock, hvc_flags);
    ws = hp->ws;
    spin_unlock_irqrestore(&hp->lock, hvc_flags);

    tty_do_resize(tty, &ws);
    tty_kref_put(tty);
}

/*
 * This is actually a contract between the driver and the tty layer outlining
 * how much write room the driver can guarantee will be sent OR BUFFERED.  This
 * driver MUST honor the return value.
 */
static int hvc_write_room(struct tty_struct *tty)
{
    struct hvc_struct *hp = tty->driver_data;

    if (!hp)
        return 0;

    return hp->outbuf_size - hp->n_outbuf;
}

static int hvc_chars_in_buffer(struct tty_struct *tty)
{
    struct hvc_struct *hp = tty->driver_data;

    if (!hp)
        return 0;
    return hp->n_outbuf;
}

/*
 * timeout will vary between the MIN and MAX values defined here.  By default
 * and during console activity we will use a default MIN_TIMEOUT of 10.  When
 * the console is idle, we increase the timeout value on each pass through
 * msleep until we reach the max.  This may be noticeable as a brief (average
 * one second) delay on the console before the console responds to input when
 * there has been no input for some time.
 */
#define MIN_TIMEOUT     (10)
#define MAX_TIMEOUT     (2000)
static u32 timeout = MIN_TIMEOUT;

#define HVC_POLL_READ   0x00000001
#define HVC_POLL_WRITE  0x00000002

int hvc_poll(struct hvc_struct *hp)
{
    struct tty_struct *tty;
    int i, n, poll_mask = 0;
    char buf[N_INBUF] __ALIGNED__;
    unsigned long flags;
    int read_total = 0;
    int written_total = 0;

    spin_lock_irqsave(&hp->lock, flags);

    /* Push pending writes */
    if (hp->n_outbuf > 0)
        written_total = hvc_push(hp);

    /* Reschedule us if still some write pending */
    if (hp->n_outbuf > 0) {
        poll_mask |= HVC_POLL_WRITE;
        /* If hvc_push() was not able to write, sleep a few msecs */
        timeout = (written_total) ? 0 : MIN_TIMEOUT;
    }

    /* No tty attached, just skip */
    tty = tty_port_tty_get(&hp->port);
    if (tty == NULL)
        goto bail;

    /* Now check if we can get data (are we throttled ?) */
    if (test_bit(TTY_THROTTLED, &tty->flags))
        goto throttled;

    /* If we aren't notifier driven and aren't throttled, we always
     * request a reschedule
     */
    if (!hp->irq_requested)
        poll_mask |= HVC_POLL_READ;

    /* Read data if any */
    for (;;) {
        int count = tty_buffer_request_room(tty, N_INBUF);

        /* If flip is full, just reschedule a later read */
        if (count == 0) {
            poll_mask |= HVC_POLL_READ;
            break;
        }

        n = hp->ops->get_chars(hp->vtermno, buf, count);
        if (n <= 0) {
            /* Hangup the tty when disconnected from host */
            if (n == -EPIPE) {
                spin_unlock_irqrestore(&hp->lock, flags);
                tty_hangup(tty);
                spin_lock_irqsave(&hp->lock, flags);
            } else if ( n == -EAGAIN ) {
                /*
                 * Some back-ends can only ensure a certain min
                 * num of bytes read, which may be > 'count'.
                 * Let the tty clear the flip buff to make room.
                 */
                poll_mask |= HVC_POLL_READ;
            }
            break;
        }
        for (i = 0; i < n; ++i) {
#ifdef CONFIG_MAGIC_SYSRQ
            if (hp->index == hvc_console.index) {
                /* Handle the SysRq Hack */
                /* XXX should support a sequence */
                if (buf[i] == '\x0f') { /* ^O */
                    /* if ^O is pressed again, reset
                     * sysrq_pressed and flip ^O char */
                    sysrq_pressed = !sysrq_pressed;
                    if (sysrq_pressed)
                        continue;
                } else if (sysrq_pressed) {
                    handle_sysrq(buf[i]);
                    sysrq_pressed = 0;
                    continue;
                }
            }
#endif /* CONFIG_MAGIC_SYSRQ */
            tty_insert_flip_char(tty, buf[i], 0);
        }

        read_total += n;
    }
 throttled:
    /* Wakeup write queue if necessary */
    if (hp->do_wakeup) {
        hp->do_wakeup = 0;
        tty_wakeup(tty);
    }
 bail:
    spin_unlock_irqrestore(&hp->lock, flags);

    if (read_total) {
        /* Activity is occurring, so reset the polling backoff value to
           a minimum for performance. */
        timeout = MIN_TIMEOUT;

        tty_flip_buffer_push(tty);
    }
    tty_kref_put(tty);

    return poll_mask;
}
EXPORT_SYMBOL_GPL(hvc_poll);

void __hvc_resize(struct hvc_struct *hp, struct winsize ws)
{
    hp->ws = ws;
    schedule_work(&hp->tty_resize);
}
EXPORT_SYMBOL_GPL(__hvc_resize);

/*
 * This kthread is either polling or interrupt driven.  This is determined by
 * calling hvc_poll() who determines whether a console adapter support
 * interrupts.
 */
static int khvcd(void *unused)
{
    int poll_mask;
    struct hvc_struct *hp;

    set_freezable();
    do {
        poll_mask = 0;
        hvc_kicked = 0;
        try_to_freeze();
        wmb();
        if (!cpus_are_in_xmon()) {
            spin_lock(&hvc_structs_lock);
            list_for_each_entry(hp, &hvc_structs, next) {
                poll_mask |= hvc_poll(hp);
            }
            spin_unlock(&hvc_structs_lock);
        } else
            poll_mask |= HVC_POLL_READ;
        if (hvc_kicked)
            continue;
        set_current_state(TASK_INTERRUPTIBLE);
        if (!hvc_kicked) {
            if (poll_mask == 0)
                schedule();
            else {
                if (timeout < MAX_TIMEOUT)
                    timeout += (timeout >> 6) + 1;

                msleep_interruptible(timeout);
            }
        }
        __set_current_state(TASK_RUNNING);
    } while (!kthread_should_stop());

    return 0;
}

static int hvc_tiocmget(struct tty_struct *tty)
{
    struct hvc_struct *hp = tty->driver_data;

    if (!hp || !hp->ops->tiocmget)
        return -EINVAL;
    return hp->ops->tiocmget(hp);
}

static int hvc_tiocmset(struct tty_struct *tty,
            unsigned int set, unsigned int clear)
{
    struct hvc_struct *hp = tty->driver_data;

    if (!hp || !hp->ops->tiocmset)
        return -EINVAL;
    return hp->ops->tiocmset(hp, set, clear);
}

#ifdef CONFIG_CONSOLE_POLL
int hvc_poll_init(struct tty_driver *driver, int line, char *options)
{
    return 0;
}

static int hvc_poll_get_char(struct tty_driver *driver, int line)
{
    struct tty_struct *tty = driver->ttys[0];
    struct hvc_struct *hp = tty->driver_data;
    int n;
    char ch;

    n = hp->ops->get_chars(hp->vtermno, &ch, 1);

    if (n == 0)
        return NO_POLL_CHAR;

    return ch;
}

static void hvc_poll_put_char(struct tty_driver *driver, int line, char ch)
{
    struct tty_struct *tty = driver->ttys[0];
    struct hvc_struct *hp = tty->driver_data;
    int n;

    do {
        n = hp->ops->put_chars(hp->vtermno, &ch, 1);
    } while (n <= 0);
}
#endif

static const struct tty_operations hvc_ops = {
    .install = hvc_install,
    .open = hvc_open,
    .close = hvc_close,
    .cleanup = hvc_cleanup,
    .write = hvc_write,
    .hangup = hvc_hangup,
    .unthrottle = hvc_unthrottle,
    .write_room = hvc_write_room,
    .chars_in_buffer = hvc_chars_in_buffer,
    .tiocmget = hvc_tiocmget,
    .tiocmset = hvc_tiocmset,
#ifdef CONFIG_CONSOLE_POLL
    .poll_init = hvc_poll_init,
    .poll_get_char = hvc_poll_get_char,
    .poll_put_char = hvc_poll_put_char,
#endif
};

static const struct tty_port_operations hvc_port_ops = {
    .destruct = hvc_port_destruct,
};

struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
                 const struct hv_ops *ops,
                 int outbuf_size)
{
    struct hvc_struct *hp;
    int i;

    /* We wait until a driver actually comes along */
    if (!hvc_driver) {
        int err = hvc_init();
        if (err)
            return ERR_PTR(err);
    }

    hp = kzalloc(ALIGN(sizeof(*hp), sizeof(long)) + outbuf_size,
            GFP_KERNEL);
    if (!hp)
        return ERR_PTR(-ENOMEM);

    hp->vtermno = vtermno;
    hp->data = data;
    hp->ops = ops;
    hp->outbuf_size = outbuf_size;
    hp->outbuf = &((char *)hp)[ALIGN(sizeof(*hp), sizeof(long))];

    tty_port_init(&hp->port);
    hp->port.ops = &hvc_port_ops;

    INIT_WORK(&hp->tty_resize, hvc_set_winsz);
    spin_lock_init(&hp->lock);
    spin_lock(&hvc_structs_lock);

    /*
     * find index to use:
     * see if this vterm id matches one registered for console.
     */
    for (i=0; i < MAX_NR_HVC_CONSOLES; i++)
        if (vtermnos[i] == hp->vtermno &&
            cons_ops[i] == hp->ops)
            break;

    /* no matching slot, just use a counter */
    if (i >= MAX_NR_HVC_CONSOLES)
        i = ++last_hvc;

    hp->index = i;
    cons_ops[i] = ops;
    vtermnos[i] = vtermno;

    list_add_tail(&(hp->next), &hvc_structs);
    spin_unlock(&hvc_structs_lock);

    /* check if we need to re-register the kernel console */
    hvc_check_console(i);

    return hp;
}
EXPORT_SYMBOL_GPL(hvc_alloc);

int hvc_remove(struct hvc_struct *hp)
{
    unsigned long flags;
    struct tty_struct *tty;

    tty = tty_port_tty_get(&hp->port);

    spin_lock_irqsave(&hp->lock, flags);
    if (hp->index < MAX_NR_HVC_CONSOLES) {
        console_lock();
        vtermnos[hp->index] = -1;
        cons_ops[hp->index] = NULL;
        console_unlock();
    }

    /* Don't whack hp->irq because tty_hangup() will need to free the irq. */

    spin_unlock_irqrestore(&hp->lock, flags);

    /*
     * We 'put' the instance that was grabbed when the kref instance
     * was initialized using kref_init().  Let the last holder of this
     * kref cause it to be removed, which will probably be the tty_vhangup
     * below.
     */
    tty_port_put(&hp->port);

    /*
     * This function call will auto chain call hvc_hangup.
     */
    if (tty) {
        tty_vhangup(tty);
        tty_kref_put(tty);
    }
    return 0;
}
EXPORT_SYMBOL_GPL(hvc_remove);

/* Driver initialization: called as soon as someone uses hvc_alloc(). */
static int hvc_init(void)
{
    struct tty_driver *drv;
    int err;

    /* We need more than hvc_count adapters due to hotplug additions. */
    drv = alloc_tty_driver(HVC_ALLOC_TTY_ADAPTERS);
    if (!drv) {
        err = -ENOMEM;
        goto out;
    }

    drv->driver_name = "hvc";
    drv->name = "hvc";
    drv->major = HVC_MAJOR;
    drv->minor_start = HVC_MINOR;
    drv->type = TTY_DRIVER_TYPE_SYSTEM;
    drv->init_termios = tty_std_termios;
    drv->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_RESET_TERMIOS;
    tty_set_operations(drv, &hvc_ops);

    /* Always start the kthread because there can be hotplug vty adapters
     * added later. */
    hvc_task = kthread_run(khvcd, NULL, "khvcd");
    if (IS_ERR(hvc_task)) {
        printk(KERN_ERR "Couldn't create kthread for console.\n");
        err = PTR_ERR(hvc_task);
        goto put_tty;
    }

    err = tty_register_driver(drv);
    if (err) {
        printk(KERN_ERR "Couldn't register hvc console driver\n");
        goto stop_thread;
    }

    /*
     * Make sure tty is fully registered before allowing it to be
     * found by hvc_console_device.
     */
    smp_mb();
    hvc_driver = drv;
    return 0;

stop_thread:
    kthread_stop(hvc_task);
    hvc_task = NULL;
put_tty:
    put_tty_driver(drv);
out:
    return err;
}

/* This isn't particularly necessary due to this being a console driver
 * but it is nice to be thorough.
 */
static void __exit hvc_exit(void)
{
    if (hvc_driver) {
        kthread_stop(hvc_task);

        tty_unregister_driver(hvc_driver);
        /* return tty_struct instances allocated in hvc_init(). */
        put_tty_driver(hvc_driver);
        unregister_console(&hvc_console);
    }
}
module_exit(hvc_exit);