[PATCH] TTY layer buffering revamp

[deliverable/linux.git] / drivers / char / hvc_console.c

/*
 * Copyright (C) 2001 Anton Blanchard <anton@au.ibm.com>, IBM
 * Copyright (C) 2001 Paul Mackerras <paulus@au.ibm.com>, IBM
 * Copyright (C) 2004 Benjamin Herrenschmidt <benh@kernel.crashing.org>, IBM Corp.
 * Copyright (C) 2004 IBM Corporation
 *
 * Additional Author(s):
 *  Ryan S. Arnold <rsa@us.ibm.com>
 *
 * 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/config.h>
#include <linux/console.h>
#include <linux/cpumask.h>
#include <linux/init.h>
#include <linux/kbd_kern.h>
#include <linux/kernel.h>
#include <linux/kobject.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 <asm/uaccess.h>
#include <asm/hvconsole.h>

#define HVC_MAJOR       229
#define HVC_MINOR       0

#define TIMEOUT         (10)

/*
 * 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 */

/*
 * The Linux TTY code does not support dynamic addition of tty derived devices
 * so we need to know how many tty devices we might need when space is allocated
 * for the tty device.  Since this driver supports hotplug of vty adapters we
 * need to make sure we have enough allocated.
 */
#define HVC_ALLOC_TTY_ADAPTERS  8

#define N_OUTBUF        16
#define N_INBUF         16

#define __ALIGNED__     __attribute__((__aligned__(8)))

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;

#ifdef CONFIG_MAGIC_SYSRQ
static int sysrq_pressed;
#endif

struct hvc_struct {
        spinlock_t lock;
        int index;
        struct tty_struct *tty;
        unsigned int count;
        int do_wakeup;
        char outbuf[N_OUTBUF] __ALIGNED__;
        int n_outbuf;
        uint32_t vtermno;
        struct hv_ops *ops;
        int irq_requested;
        int irq;
        struct list_head next;
        struct kobject kobj; /* ref count & hvc_struct lifetime */
};

/* dynamic list of hvc_struct instances */
static struct list_head hvc_structs = LIST_HEAD_INIT(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 canidate registered with hvc_instantiate().
 */
static int last_hvc = -1;

/*
 * Do not call this function with either the hvc_strucst_lock or the hvc_struct
 * lock held.  If successful, this function increments the kobject reference
 * count against the target hvc_struct so it should be released when finished.
 */
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) {
                        kobject_get(&hp->kobj);
                        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 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.
 */

void hvc_console_print(struct console *co, const char *b, unsigned count)
{
        char c[16] __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 useable. */
        if (vtermnos[index] < 0)
                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 chars on error */
                                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;
}

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

/*
 * Early console initialization.  Preceeds 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_con_driver);
        return 0;
}
console_initcall(hvc_console_init);

/*
 * 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, 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 registerd in this index */
        hp = hvc_get_by_index(index);
        if (hp) {
                kobject_put(&hp->kobj);
                return -1;
        }

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

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

        /* 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_con_driver.index)
                register_console(&hvc_con_driver);

        return 0;
}
EXPORT_SYMBOL(hvc_instantiate);

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

static int hvc_poll(struct hvc_struct *hp);

/*
 * NOTE: This API isn't used if the console adapter doesn't support interrupts.
 * In this case the console is poll driven.
 */
static irqreturn_t hvc_handle_interrupt(int irq, void *dev_instance, struct pt_regs *regs)
{
        /* if hvc_poll request a repoll, then kick the hvcd thread */
        if (hvc_poll(dev_instance))
                hvc_kick();
        return IRQ_HANDLED;
}

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

/*
 * 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;
        unsigned long flags;
        int irq = NO_IRQ;
        int rc = 0;
        struct kobject *kobjp;

        /* Auto increments kobject reference if found. */
        if (!(hp = hvc_get_by_index(tty->index))) {
                printk(KERN_WARNING "hvc_console: tty open failed, no vty associated with tty.\n");
                return -ENODEV;
        }

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

        tty->driver_data = hp;
        hp->tty = tty;
        /* Save for request_irq outside of spin_lock. */
        irq = hp->irq;
        if (irq != NO_IRQ)
                hp->irq_requested = 1;

        kobjp = &hp->kobj;

        spin_unlock_irqrestore(&hp->lock, flags);
        /* check error, fallback to non-irq */
        if (irq != NO_IRQ)
                rc = request_irq(irq, hvc_handle_interrupt, SA_INTERRUPT, "hvc_console", hp);

        /*
         * If the request_irq() fails and 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 kobject reference.
         */
        if (rc) {
                spin_lock_irqsave(&hp->lock, flags);
                hp->tty = NULL;
                hp->irq_requested = 0;
                spin_unlock_irqrestore(&hp->lock, flags);
                tty->driver_data = NULL;
                kobject_put(kobjp);
                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;
        struct kobject *kobjp;
        int irq = NO_IRQ;
        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 kobject reference wasn't made.
         */
        if (!tty->driver_data)
                return;

        hp = tty->driver_data;
        spin_lock_irqsave(&hp->lock, flags);

        kobjp = &hp->kobj;
        if (--hp->count == 0) {
                if (hp->irq_requested)
                        irq = hp->irq;
                hp->irq_requested = 0;

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

                /*
                 * 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(tty, HVC_CLOSE_WAIT);

                if (irq != NO_IRQ)
                        free_irq(irq, hp);

        } else {
                if (hp->count < 0)
                        printk(KERN_ERR "hvc_close %X: oops, count is %d\n",
                                hp->vtermno, hp->count);
                spin_unlock_irqrestore(&hp->lock, flags);
        }

        kobject_put(kobjp);
}

static void hvc_hangup(struct tty_struct *tty)
{
        struct hvc_struct *hp = tty->driver_data;
        unsigned long flags;
        int irq = NO_IRQ;
        int temp_open_count;
        struct kobject *kobjp;

        if (!hp)
                return;

        spin_lock_irqsave(&hp->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->count <= 0) {
                spin_unlock_irqrestore(&hp->lock, flags);
                return;
        }

        kobjp = &hp->kobj;
        temp_open_count = hp->count;
        hp->count = 0;
        hp->n_outbuf = 0;
        hp->tty = NULL;
        if (hp->irq_requested)
                /* Saved for use outside of spin_lock. */
                irq = hp->irq;
        hp->irq_requested = 0;
        spin_unlock_irqrestore(&hp->lock, flags);
        if (irq != NO_IRQ)
                free_irq(irq, hp);
        while(temp_open_count) {
                --temp_open_count;
                kobject_put(kobjp);
        }
}

/*
 * Push buffered characters whether they were just recently buffered or waiting
 * on a blocked hypervisor.  Call this function with hp->lock held.
 */
static void 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)
                        return;
                /* 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;
}

static inline int __hvc_write_kernel(struct hvc_struct *hp,
                                   const unsigned char *buf, int count)
{
        unsigned long flags;
        int rsize, written = 0;

        spin_lock_irqsave(&hp->lock, flags);

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

        while (count > 0 && (rsize = N_OUTBUF - 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);

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

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

        if (hp->count <= 0)
                return -EIO;

        written = __hvc_write_kernel(hp, buf, count);

        /*
         * Racy, but harmless, kick thread if there is still pending data.
         * There really is nothing wrong with kicking the thread, even if there
         * is no buffered data.
         */
        if (hp->n_outbuf)
                hvc_kick();

        return written;
}

/*
 * This is actually a contract between the driver and the tty layer outlining
 * how much write room the driver can guarentee 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 -1;

        return N_OUTBUF - hp->n_outbuf;
}

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

        if (!hp)
                return -1;
        return hp->n_outbuf;
}

#define HVC_POLL_READ   0x00000001
#define HVC_POLL_WRITE  0x00000002
#define HVC_POLL_QUICK  0x00000004

static 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;

        spin_lock_irqsave(&hp->lock, flags);

        /* Push pending writes */
        if (hp->n_outbuf > 0)
                hvc_push(hp);
        /* Reschedule us if still some write pending */
        if (hp->n_outbuf > 0)
                poll_mask |= HVC_POLL_WRITE;

        /* No tty attached, just skip */
        tty = hp->tty;
        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 interrupt driven and aren't throttled, we always
         * request a reschedule
         */
        if (hp->irq == NO_IRQ)
                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);
                        }
                        break;
                }
                for (i = 0; i < n; ++i) {
#ifdef CONFIG_MAGIC_SYSRQ
                        if (hp->index == hvc_con_driver.index) {
                                /* Handle the SysRq Hack */
                                /* XXX should support a sequence */
                                if (buf[i] == '\x0f') { /* ^O */
                                        sysrq_pressed = 1;
                                        continue;
                                } else if (sysrq_pressed) {
                                        handle_sysrq(buf[i], NULL, tty);
                                        sysrq_pressed = 0;
                                        continue;
                                }
                        }
#endif /* CONFIG_MAGIC_SYSRQ */
                        tty_insert_flip_char(tty, buf[i], 0);
                }

                if (count)
                        tty_schedule_flip(tty);

                /*
                 * Account for the total amount read in one loop, and if above
                 * 64 bytes, we do a quick schedule loop to let the tty grok
                 * the data and eventually throttle us.
                 */
                read_total += n;
                if (read_total >= 64) {
                        poll_mask |= HVC_POLL_QUICK;
                        break;
                }
        }
 throttled:
        /* Wakeup write queue if necessary */
        if (hp->do_wakeup) {
                hp->do_wakeup = 0;
                tty_wakeup(tty);
        }
 bail:
        spin_unlock_irqrestore(&hp->lock, flags);

        return poll_mask;
}

#if defined(CONFIG_XMON) && defined(CONFIG_SMP)
extern cpumask_t cpus_in_xmon;
#else
static const cpumask_t cpus_in_xmon = CPU_MASK_NONE;
#endif

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

        __set_current_state(TASK_RUNNING);
        do {
                poll_mask = 0;
                hvc_kicked = 0;
                wmb();
                if (cpus_empty(cpus_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;
                if (poll_mask & HVC_POLL_QUICK) {
                        yield();
                        continue;
                }
                set_current_state(TASK_INTERRUPTIBLE);
                if (!hvc_kicked) {
                        if (poll_mask == 0)
                                schedule();
                        else
                                msleep_interruptible(TIMEOUT);
                }
                __set_current_state(TASK_RUNNING);
        } while (!kthread_should_stop());

        return 0;
}

static struct tty_operations hvc_ops = {
        .open = hvc_open,
        .close = hvc_close,
        .write = hvc_write,
        .hangup = hvc_hangup,
        .unthrottle = hvc_unthrottle,
        .write_room = hvc_write_room,
        .chars_in_buffer = hvc_chars_in_buffer,
};

/* callback when the kboject ref count reaches zero. */
static void destroy_hvc_struct(struct kobject *kobj)
{
        struct hvc_struct *hp = container_of(kobj, struct hvc_struct, kobj);
        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 struct kobj_type hvc_kobj_type = {
        .release = destroy_hvc_struct,
};

struct hvc_struct __devinit *hvc_alloc(uint32_t vtermno, int irq,
                                        struct hv_ops *ops)
{
        struct hvc_struct *hp;
        int i;

        hp = kmalloc(sizeof(*hp), GFP_KERNEL);
        if (!hp)
                return ERR_PTR(-ENOMEM);

        memset(hp, 0x00, sizeof(*hp));

        hp->vtermno = vtermno;
        hp->irq = irq;
        hp->ops = ops;

        kobject_init(&hp->kobj);
        hp->kobj.ktype = &hvc_kobj_type;

        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)
                        break;

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

        hp->index = i;

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

        return hp;
}
EXPORT_SYMBOL(hvc_alloc);

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

        spin_lock_irqsave(&hp->lock, flags);
        tty = hp->tty;
        kobjp = &hp->kobj;

        if (hp->index < MAX_NR_HVC_CONSOLES)
                vtermnos[hp->index] = -1;

        /* 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 kobject instance
         * was intialized using kobject_init().  Let the last holder of this
         * kobject cause it to be removed, which will probably be the tty_hangup
         * below.
         */
        kobject_put(kobjp);

        /*
         * This function call will auto chain call hvc_hangup.  The tty should
         * always be valid at this time unless a simultaneous tty close already
         * cleaned up the hvc_struct.
         */
        if (tty)
                tty_hangup(tty);
        return 0;
}
EXPORT_SYMBOL(hvc_remove);

/* Driver initialization.  Follow console initialization.  This is where the TTY
 * interfaces start to become available. */
int __init hvc_init(void)
{
        /* We need more than hvc_count adapters due to hotplug additions. */
        hvc_driver = alloc_tty_driver(HVC_ALLOC_TTY_ADAPTERS);
        if (!hvc_driver)
                return -ENOMEM;

        hvc_driver->owner = THIS_MODULE;
        hvc_driver->devfs_name = "hvc/";
        hvc_driver->driver_name = "hvc";
        hvc_driver->name = "hvc";
        hvc_driver->major = HVC_MAJOR;
        hvc_driver->minor_start = HVC_MINOR;
        hvc_driver->type = TTY_DRIVER_TYPE_SYSTEM;
        hvc_driver->init_termios = tty_std_termios;
        hvc_driver->flags = TTY_DRIVER_REAL_RAW;
        tty_set_operations(hvc_driver, &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)) {
                panic("Couldn't create kthread for console.\n");
                put_tty_driver(hvc_driver);
                return -EIO;
        }

        if (tty_register_driver(hvc_driver))
                panic("Couldn't register hvc console driver\n");

        return 0;
}
module_init(hvc_init);

/* This isn't particularily necessary due to this being a console driver
 * but it is nice to be thorough.
 */
static void __exit hvc_exit(void)
{
        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_con_driver);
}
module_exit(hvc_exit);