KVM: Remove unnecessary initialization and checks in mark_page_dirty()

[linux-2.6/mini2440.git] / kernel / printk.c

/*
 *  linux/kernel/printk.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 * Modified to make sys_syslog() more flexible: added commands to
 * return the last 4k of kernel messages, regardless of whether
 * they've been read or not.  Added option to suppress kernel printk's
 * to the console.  Added hook for sending the console messages
 * elsewhere, in preparation for a serial line console (someday).
 * Ted Ts'o, 2/11/93.
 * Modified for sysctl support, 1/8/97, Chris Horn.
 * Fixed SMP synchronization, 08/08/99, Manfred Spraul
 *     manfred@colorfullife.com
 * Rewrote bits to get rid of console_lock
 *      01Mar01 Andrew Morton <andrewm@uow.edu.au>
 */

#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/console.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/interrupt.h>                    /* For in_interrupt() */
#include <linux/delay.h>
#include <linux/smp.h>
#include <linux/security.h>
#include <linux/bootmem.h>
#include <linux/syscalls.h>
#include <linux/jiffies.h>

#include <asm/uaccess.h>

#define __LOG_BUF_LEN   (1 << CONFIG_LOG_BUF_SHIFT)

/* printk's without a loglevel use this.. */
#define DEFAULT_MESSAGE_LOGLEVEL 4 /* KERN_WARNING */

/* We show everything that is MORE important than this.. */
#define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */
#define DEFAULT_CONSOLE_LOGLEVEL 7 /* anything MORE serious than KERN_DEBUG */

DECLARE_WAIT_QUEUE_HEAD(log_wait);

int console_printk[4] = {
        DEFAULT_CONSOLE_LOGLEVEL,       /* console_loglevel */
        DEFAULT_MESSAGE_LOGLEVEL,       /* default_message_loglevel */
        MINIMUM_CONSOLE_LOGLEVEL,       /* minimum_console_loglevel */
        DEFAULT_CONSOLE_LOGLEVEL,       /* default_console_loglevel */
};

/*
 * Low level drivers may need that to know if they can schedule in
 * their unblank() callback or not. So let's export it.
 */
int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);

/*
 * console_sem protects the console_drivers list, and also
 * provides serialisation for access to the entire console
 * driver system.
 */
static DECLARE_MUTEX(console_sem);
static DECLARE_MUTEX(secondary_console_sem);
struct console *console_drivers;
/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it are racing, but it helps tracking those weird code
 * path in the console code where we end up in places I want
 * locked without the console sempahore held
 */
static int console_locked, console_suspended;

/*
 * logbuf_lock protects log_buf, log_start, log_end, con_start and logged_chars
 * It is also used in interesting ways to provide interlocking in
 * release_console_sem().
 */
static DEFINE_SPINLOCK(logbuf_lock);

#define LOG_BUF_MASK    (log_buf_len-1)
#define LOG_BUF(idx) (log_buf[(idx) & LOG_BUF_MASK])

/*
 * The indices into log_buf are not constrained to log_buf_len - they
 * must be masked before subscripting
 */
static unsigned long log_start; /* Index into log_buf: next char to be read by syslog() */
static unsigned long con_start; /* Index into log_buf: next char to be sent to consoles */
static unsigned long log_end;   /* Index into log_buf: most-recently-written-char + 1 */

/*
 *      Array of consoles built from command line options (console=)
 */
struct console_cmdline
{
        char    name[8];                        /* Name of the driver       */
        int     index;                          /* Minor dev. to use        */
        char    *options;                       /* Options for the driver   */
};

#define MAX_CMDLINECONSOLES 8

static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
static int selected_console = -1;
static int preferred_console = -1;

/* Flag: console code may call schedule() */
static int console_may_schedule;

#ifdef CONFIG_PRINTK

static char __log_buf[__LOG_BUF_LEN];
static char *log_buf = __log_buf;
static int log_buf_len = __LOG_BUF_LEN;
static unsigned long logged_chars; /* Number of chars produced since last read+clear operation */

static int __init log_buf_len_setup(char *str)
{
        unsigned long size = memparse(str, &str);
        unsigned long flags;

        if (size)
                size = roundup_pow_of_two(size);
        if (size > log_buf_len) {
                unsigned long start, dest_idx, offset;
                char *new_log_buf;

                new_log_buf = alloc_bootmem(size);
                if (!new_log_buf) {
                        printk(KERN_WARNING "log_buf_len: allocation failed\n");
                        goto out;
                }

                spin_lock_irqsave(&logbuf_lock, flags);
                log_buf_len = size;
                log_buf = new_log_buf;

                offset = start = min(con_start, log_start);
                dest_idx = 0;
                while (start != log_end) {
                        log_buf[dest_idx] = __log_buf[start & (__LOG_BUF_LEN - 1)];
                        start++;
                        dest_idx++;
                }
                log_start -= offset;
                con_start -= offset;
                log_end -= offset;
                spin_unlock_irqrestore(&logbuf_lock, flags);

                printk(KERN_NOTICE "log_buf_len: %d\n", log_buf_len);
        }
out:
        return 1;
}

__setup("log_buf_len=", log_buf_len_setup);

/*
 * Commands to do_syslog:
 *
 *      0 -- Close the log.  Currently a NOP.
 *      1 -- Open the log. Currently a NOP.
 *      2 -- Read from the log.
 *      3 -- Read all messages remaining in the ring buffer.
 *      4 -- Read and clear all messages remaining in the ring buffer
 *      5 -- Clear ring buffer.
 *      6 -- Disable printk's to console
 *      7 -- Enable printk's to console
 *      8 -- Set level of messages printed to console
 *      9 -- Return number of unread characters in the log buffer
 *     10 -- Return size of the log buffer
 */
int do_syslog(int type, char __user *buf, int len)
{
        unsigned long i, j, limit, count;
        int do_clear = 0;
        char c;
        int error = 0;

        error = security_syslog(type);
        if (error)
                return error;

        switch (type) {
        case 0:         /* Close log */
                break;
        case 1:         /* Open log */
                break;
        case 2:         /* Read from log */
                error = -EINVAL;
                if (!buf || len < 0)
                        goto out;
                error = 0;
                if (!len)
                        goto out;
                if (!access_ok(VERIFY_WRITE, buf, len)) {
                        error = -EFAULT;
                        goto out;
                }
                error = wait_event_interruptible(log_wait,
                                                        (log_start - log_end));
                if (error)
                        goto out;
                i = 0;
                spin_lock_irq(&logbuf_lock);
                while (!error && (log_start != log_end) && i < len) {
                        c = LOG_BUF(log_start);
                        log_start++;
                        spin_unlock_irq(&logbuf_lock);
                        error = __put_user(c,buf);
                        buf++;
                        i++;
                        cond_resched();
                        spin_lock_irq(&logbuf_lock);
                }
                spin_unlock_irq(&logbuf_lock);
                if (!error)
                        error = i;
                break;
        case 4:         /* Read/clear last kernel messages */
                do_clear = 1;
                /* FALL THRU */
        case 3:         /* Read last kernel messages */
                error = -EINVAL;
                if (!buf || len < 0)
                        goto out;
                error = 0;
                if (!len)
                        goto out;
                if (!access_ok(VERIFY_WRITE, buf, len)) {
                        error = -EFAULT;
                        goto out;
                }
                count = len;
                if (count > log_buf_len)
                        count = log_buf_len;
                spin_lock_irq(&logbuf_lock);
                if (count > logged_chars)
                        count = logged_chars;
                if (do_clear)
                        logged_chars = 0;
                limit = log_end;
                /*
                 * __put_user() could sleep, and while we sleep
                 * printk() could overwrite the messages
                 * we try to copy to user space. Therefore
                 * the messages are copied in reverse. <manfreds>
                 */
                for (i = 0; i < count && !error; i++) {
                        j = limit-1-i;
                        if (j + log_buf_len < log_end)
                                break;
                        c = LOG_BUF(j);
                        spin_unlock_irq(&logbuf_lock);
                        error = __put_user(c,&buf[count-1-i]);
                        cond_resched();
                        spin_lock_irq(&logbuf_lock);
                }
                spin_unlock_irq(&logbuf_lock);
                if (error)
                        break;
                error = i;
                if (i != count) {
                        int offset = count-error;
                        /* buffer overflow during copy, correct user buffer. */
                        for (i = 0; i < error; i++) {
                                if (__get_user(c,&buf[i+offset]) ||
                                    __put_user(c,&buf[i])) {
                                        error = -EFAULT;
                                        break;
                                }
                                cond_resched();
                        }
                }
                break;
        case 5:         /* Clear ring buffer */
                logged_chars = 0;
                break;
        case 6:         /* Disable logging to console */
                console_loglevel = minimum_console_loglevel;
                break;
        case 7:         /* Enable logging to console */
                console_loglevel = default_console_loglevel;
                break;
        case 8:         /* Set level of messages printed to console */
                error = -EINVAL;
                if (len < 1 || len > 8)
                        goto out;
                if (len < minimum_console_loglevel)
                        len = minimum_console_loglevel;
                console_loglevel = len;
                error = 0;
                break;
        case 9:         /* Number of chars in the log buffer */
                error = log_end - log_start;
                break;
        case 10:        /* Size of the log buffer */
                error = log_buf_len;
                break;
        default:
                error = -EINVAL;
                break;
        }
out:
        return error;
}

asmlinkage long sys_syslog(int type, char __user *buf, int len)
{
        return do_syslog(type, buf, len);
}

/*
 * Call the console drivers on a range of log_buf
 */
static void __call_console_drivers(unsigned long start, unsigned long end)
{
        struct console *con;

        for (con = console_drivers; con; con = con->next) {
                if ((con->flags & CON_ENABLED) && con->write &&
                                (cpu_online(smp_processor_id()) ||
                                (con->flags & CON_ANYTIME)))
                        con->write(con, &LOG_BUF(start), end - start);
        }
}

static int __read_mostly ignore_loglevel;

static int __init ignore_loglevel_setup(char *str)
{
        ignore_loglevel = 1;
        printk(KERN_INFO "debug: ignoring loglevel setting.\n");

        return 1;
}

__setup("ignore_loglevel", ignore_loglevel_setup);

/*
 * Write out chars from start to end - 1 inclusive
 */
static void _call_console_drivers(unsigned long start,
                                unsigned long end, int msg_log_level)
{
        if ((msg_log_level < console_loglevel || ignore_loglevel) &&
                        console_drivers && start != end) {
                if ((start & LOG_BUF_MASK) > (end & LOG_BUF_MASK)) {
                        /* wrapped write */
                        __call_console_drivers(start & LOG_BUF_MASK,
                                                log_buf_len);
                        __call_console_drivers(0, end & LOG_BUF_MASK);
                } else {
                        __call_console_drivers(start, end);
                }
        }
}

/*
 * Call the console drivers, asking them to write out
 * log_buf[start] to log_buf[end - 1].
 * The console_sem must be held.
 */
static void call_console_drivers(unsigned long start, unsigned long end)
{
        unsigned long cur_index, start_print;
        static int msg_level = -1;

        BUG_ON(((long)(start - end)) > 0);

        cur_index = start;
        start_print = start;
        while (cur_index != end) {
                if (msg_level < 0 && ((end - cur_index) > 2) &&
                                LOG_BUF(cur_index + 0) == '<' &&
                                LOG_BUF(cur_index + 1) >= '0' &&
                                LOG_BUF(cur_index + 1) <= '7' &&
                                LOG_BUF(cur_index + 2) == '>') {
                        msg_level = LOG_BUF(cur_index + 1) - '0';
                        cur_index += 3;
                        start_print = cur_index;
                }
                while (cur_index != end) {
                        char c = LOG_BUF(cur_index);

                        cur_index++;
                        if (c == '\n') {
                                if (msg_level < 0) {
                                        /*
                                         * printk() has already given us loglevel tags in
                                         * the buffer.  This code is here in case the
                                         * log buffer has wrapped right round and scribbled
                                         * on those tags
                                         */
                                        msg_level = default_message_loglevel;
                                }
                                _call_console_drivers(start_print, cur_index, msg_level);
                                msg_level = -1;
                                start_print = cur_index;
                                break;
                        }
                }
        }
        _call_console_drivers(start_print, end, msg_level);
}

static void emit_log_char(char c)
{
        LOG_BUF(log_end) = c;
        log_end++;
        if (log_end - log_start > log_buf_len)
                log_start = log_end - log_buf_len;
        if (log_end - con_start > log_buf_len)
                con_start = log_end - log_buf_len;
        if (logged_chars < log_buf_len)
                logged_chars++;
}

/*
 * Zap console related locks when oopsing. Only zap at most once
 * every 10 seconds, to leave time for slow consoles to print a
 * full oops.
 */
static void zap_locks(void)
{
        static unsigned long oops_timestamp;

        if (time_after_eq(jiffies, oops_timestamp) &&
                        !time_after(jiffies, oops_timestamp + 30 * HZ))
                return;

        oops_timestamp = jiffies;

        /* If a crash is occurring, make sure we can't deadlock */
        spin_lock_init(&logbuf_lock);
        /* And make sure that we print immediately */
        init_MUTEX(&console_sem);
}

#if defined(CONFIG_PRINTK_TIME)
static int printk_time = 1;
#else
static int printk_time = 0;
#endif
module_param(printk_time, int, S_IRUGO | S_IWUSR);

static int __init printk_time_setup(char *str)
{
        if (*str)
                return 0;
        printk_time = 1;
        return 1;
}

__setup("time", printk_time_setup);

__attribute__((weak)) unsigned long long printk_clock(void)
{
        return sched_clock();
}

/* Check if we have any console registered that can be called early in boot. */
static int have_callable_console(void)
{
        struct console *con;

        for (con = console_drivers; con; con = con->next)
                if (con->flags & CON_ANYTIME)
                        return 1;

        return 0;
}

/**
 * printk - print a kernel message
 * @fmt: format string
 *
 * This is printk().  It can be called from any context.  We want it to work.
 *
 * We try to grab the console_sem.  If we succeed, it's easy - we log the output and
 * call the console drivers.  If we fail to get the semaphore we place the output
 * into the log buffer and return.  The current holder of the console_sem will
 * notice the new output in release_console_sem() and will send it to the
 * consoles before releasing the semaphore.
 *
 * One effect of this deferred printing is that code which calls printk() and
 * then changes console_loglevel may break. This is because console_loglevel
 * is inspected when the actual printing occurs.
 *
 * See also:
 * printf(3)
 */

asmlinkage int printk(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);
        r = vprintk(fmt, args);
        va_end(args);

        return r;
}

/* cpu currently holding logbuf_lock */
static volatile unsigned int printk_cpu = UINT_MAX;

asmlinkage int vprintk(const char *fmt, va_list args)
{
        unsigned long flags;
        int printed_len;
        char *p;
        static char printk_buf[1024];
        static int log_level_unknown = 1;

        preempt_disable();
        if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())
                /* If a crash is occurring during printk() on this CPU,
                 * make sure we can't deadlock */
                zap_locks();

        /* This stops the holder of console_sem just where we want him */
        raw_local_irq_save(flags);
        lockdep_off();
        spin_lock(&logbuf_lock);
        printk_cpu = smp_processor_id();

        /* Emit the output into the temporary buffer */
        printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);

        /*
         * Copy the output into log_buf.  If the caller didn't provide
         * appropriate log level tags, we insert them here
         */
        for (p = printk_buf; *p; p++) {
                if (log_level_unknown) {
                        /* log_level_unknown signals the start of a new line */
                        if (printk_time) {
                                int loglev_char;
                                char tbuf[50], *tp;
                                unsigned tlen;
                                unsigned long long t;
                                unsigned long nanosec_rem;

                                /*
                                 * force the log level token to be
                                 * before the time output.
                                 */
                                if (p[0] == '<' && p[1] >='0' &&
                                   p[1] <= '7' && p[2] == '>') {
                                        loglev_char = p[1];
                                        p += 3;
                                        printed_len -= 3;
                                } else {
                                        loglev_char = default_message_loglevel
                                                + '0';
                                }
                                t = printk_clock();
                                nanosec_rem = do_div(t, 1000000000);
                                tlen = sprintf(tbuf,
                                                "<%c>[%5lu.%06lu] ",
                                                loglev_char,
                                                (unsigned long)t,
                                                nanosec_rem/1000);

                                for (tp = tbuf; tp < tbuf + tlen; tp++)
                                        emit_log_char(*tp);
                                printed_len += tlen;
                        } else {
                                if (p[0] != '<' || p[1] < '0' ||
                                   p[1] > '7' || p[2] != '>') {
                                        emit_log_char('<');
                                        emit_log_char(default_message_loglevel
                                                + '0');
                                        emit_log_char('>');
                                        printed_len += 3;
                                }
                        }
                        log_level_unknown = 0;
                        if (!*p)
                                break;
                }
                emit_log_char(*p);
                if (*p == '\n')
                        log_level_unknown = 1;
        }

        if (!down_trylock(&console_sem)) {
                /*
                 * We own the drivers.  We can drop the spinlock and
                 * let release_console_sem() print the text, maybe ...
                 */
                console_locked = 1;
                printk_cpu = UINT_MAX;
                spin_unlock(&logbuf_lock);

                /*
                 * Console drivers may assume that per-cpu resources have
                 * been allocated. So unless they're explicitly marked as
                 * being able to cope (CON_ANYTIME) don't call them until
                 * this CPU is officially up.
                 */
                if (cpu_online(smp_processor_id()) || have_callable_console()) {
                        console_may_schedule = 0;
                        release_console_sem();
                } else {
                        /* Release by hand to avoid flushing the buffer. */
                        console_locked = 0;
                        up(&console_sem);
                }
                lockdep_on();
                raw_local_irq_restore(flags);
        } else {
                /*
                 * Someone else owns the drivers.  We drop the spinlock, which
                 * allows the semaphore holder to proceed and to call the
                 * console drivers with the output which we just produced.
                 */
                printk_cpu = UINT_MAX;
                spin_unlock(&logbuf_lock);
                lockdep_on();
                raw_local_irq_restore(flags);
        }

        preempt_enable();
        return printed_len;
}
EXPORT_SYMBOL(printk);
EXPORT_SYMBOL(vprintk);

#else

asmlinkage long sys_syslog(int type, char __user *buf, int len)
{
        return -ENOSYS;
}

static void call_console_drivers(unsigned long start, unsigned long end)
{
}

#endif

/*
 * Set up a list of consoles.  Called from init/main.c
 */
static int __init console_setup(char *str)
{
        char name[sizeof(console_cmdline[0].name)];
        char *s, *options;
        int idx;

        /*
         * Decode str into name, index, options.
         */
        if (str[0] >= '0' && str[0] <= '9') {
                strcpy(name, "ttyS");
                strncpy(name + 4, str, sizeof(name) - 5);
        } else {
                strncpy(name, str, sizeof(name) - 1);
        }
        name[sizeof(name) - 1] = 0;
        if ((options = strchr(str, ',')) != NULL)
                *(options++) = 0;
#ifdef __sparc__
        if (!strcmp(str, "ttya"))
                strcpy(name, "ttyS0");
        if (!strcmp(str, "ttyb"))
                strcpy(name, "ttyS1");
#endif
        for (s = name; *s; s++)
                if ((*s >= '0' && *s <= '9') || *s == ',')
                        break;
        idx = simple_strtoul(s, NULL, 10);
        *s = 0;

        add_preferred_console(name, idx, options);
        return 1;
}
__setup("console=", console_setup);

/**
 * add_preferred_console - add a device to the list of preferred consoles.
 * @name: device name
 * @idx: device index
 * @options: options for this console
 *
 * The last preferred console added will be used for kernel messages
 * and stdin/out/err for init.  Normally this is used by console_setup
 * above to handle user-supplied console arguments; however it can also
 * be used by arch-specific code either to override the user or more
 * commonly to provide a default console (ie from PROM variables) when
 * the user has not supplied one.
 */
int __init add_preferred_console(char *name, int idx, char *options)
{
        struct console_cmdline *c;
        int i;

        /*
         *      See if this tty is not yet registered, and
         *      if we have a slot free.
         */
        for(i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)
                if (strcmp(console_cmdline[i].name, name) == 0 &&
                          console_cmdline[i].index == idx) {
                                selected_console = i;
                                return 0;
                }
        if (i == MAX_CMDLINECONSOLES)
                return -E2BIG;
        selected_console = i;
        c = &console_cmdline[i];
        memcpy(c->name, name, sizeof(c->name));
        c->name[sizeof(c->name) - 1] = 0;
        c->options = options;
        c->index = idx;
        return 0;
}

#ifndef CONFIG_DISABLE_CONSOLE_SUSPEND
/**
 * suspend_console - suspend the console subsystem
 *
 * This disables printk() while we go into suspend states
 */
void suspend_console(void)
{
        printk("Suspending console(s)\n");
        acquire_console_sem();
        console_suspended = 1;
}

void resume_console(void)
{
        console_suspended = 0;
        release_console_sem();
}
#endif /* CONFIG_DISABLE_CONSOLE_SUSPEND */

/**
 * acquire_console_sem - lock the console system for exclusive use.
 *
 * Acquires a semaphore which guarantees that the caller has
 * exclusive access to the console system and the console_drivers list.
 *
 * Can sleep, returns nothing.
 */
void acquire_console_sem(void)
{
        BUG_ON(in_interrupt());
        if (console_suspended) {
                down(&secondary_console_sem);
                return;
        }
        down(&console_sem);
        console_locked = 1;
        console_may_schedule = 1;
}
EXPORT_SYMBOL(acquire_console_sem);

int try_acquire_console_sem(void)
{
        if (down_trylock(&console_sem))
                return -1;
        console_locked = 1;
        console_may_schedule = 0;
        return 0;
}
EXPORT_SYMBOL(try_acquire_console_sem);

int is_console_locked(void)
{
        return console_locked;
}

void wake_up_klogd(void)
{
        if (!oops_in_progress && waitqueue_active(&log_wait))
                wake_up_interruptible(&log_wait);
}

/**
 * release_console_sem - unlock the console system
 *
 * Releases the semaphore which the caller holds on the console system
 * and the console driver list.
 *
 * While the semaphore was held, console output may have been buffered
 * by printk().  If this is the case, release_console_sem() emits
 * the output prior to releasing the semaphore.
 *
 * If there is output waiting for klogd, we wake it up.
 *
 * release_console_sem() may be called from any context.
 */
void release_console_sem(void)
{
        unsigned long flags;
        unsigned long _con_start, _log_end;
        unsigned long wake_klogd = 0;

        if (console_suspended) {
                up(&secondary_console_sem);
                return;
        }

        console_may_schedule = 0;

        for ( ; ; ) {
                spin_lock_irqsave(&logbuf_lock, flags);
                wake_klogd |= log_start - log_end;
                if (con_start == log_end)
                        break;                  /* Nothing to print */
                _con_start = con_start;
                _log_end = log_end;
                con_start = log_end;            /* Flush */
                spin_unlock(&logbuf_lock);
                call_console_drivers(_con_start, _log_end);
                local_irq_restore(flags);
        }
        console_locked = 0;
        up(&console_sem);
        spin_unlock_irqrestore(&logbuf_lock, flags);
        if (wake_klogd)
                wake_up_klogd();
}
EXPORT_SYMBOL(release_console_sem);

/**
 * console_conditional_schedule - yield the CPU if required
 *
 * If the console code is currently allowed to sleep, and
 * if this CPU should yield the CPU to another task, do
 * so here.
 *
 * Must be called within acquire_console_sem().
 */
void __sched console_conditional_schedule(void)
{
        if (console_may_schedule)
                cond_resched();
}
EXPORT_SYMBOL(console_conditional_schedule);

void console_print(const char *s)
{
        printk(KERN_EMERG "%s", s);
}
EXPORT_SYMBOL(console_print);

void console_unblank(void)
{
        struct console *c;

        /*
         * console_unblank can no longer be called in interrupt context unless
         * oops_in_progress is set to 1..
         */
        if (oops_in_progress) {
                if (down_trylock(&console_sem) != 0)
                        return;
        } else
                acquire_console_sem();

        console_locked = 1;
        console_may_schedule = 0;
        for (c = console_drivers; c != NULL; c = c->next)
                if ((c->flags & CON_ENABLED) && c->unblank)
                        c->unblank();
        release_console_sem();
}

/*
 * Return the console tty driver structure and its associated index
 */
struct tty_driver *console_device(int *index)
{
        struct console *c;
        struct tty_driver *driver = NULL;

        acquire_console_sem();
        for (c = console_drivers; c != NULL; c = c->next) {
                if (!c->device)
                        continue;
                driver = c->device(c, index);
                if (driver)
                        break;
        }
        release_console_sem();
        return driver;
}

/*
 * Prevent further output on the passed console device so that (for example)
 * serial drivers can disable console output before suspending a port, and can
 * re-enable output afterwards.
 */
void console_stop(struct console *console)
{
        acquire_console_sem();
        console->flags &= ~CON_ENABLED;
        release_console_sem();
}
EXPORT_SYMBOL(console_stop);

void console_start(struct console *console)
{
        acquire_console_sem();
        console->flags |= CON_ENABLED;
        release_console_sem();
}
EXPORT_SYMBOL(console_start);

/*
 * The console driver calls this routine during kernel initialization
 * to register the console printing procedure with printk() and to
 * print any messages that were printed by the kernel before the
 * console driver was initialized.
 */
void register_console(struct console *console)
{
        int i;
        unsigned long flags;
        struct console *bootconsole = NULL;

        if (console_drivers) {
                if (console->flags & CON_BOOT)
                        return;
                if (console_drivers->flags & CON_BOOT)
                        bootconsole = console_drivers;
        }

        if (preferred_console < 0 || bootconsole || !console_drivers)
                preferred_console = selected_console;

        /*
         *      See if we want to use this console driver. If we
         *      didn't select a console we take the first one
         *      that registers here.
         */
        if (preferred_console < 0) {
                if (console->index < 0)
                        console->index = 0;
                if (console->setup == NULL ||
                    console->setup(console, NULL) == 0) {
                        console->flags |= CON_ENABLED | CON_CONSDEV;
                        preferred_console = 0;
                }
        }

        /*
         *      See if this console matches one we selected on
         *      the command line.
         */
        for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];
                        i++) {
                if (strcmp(console_cmdline[i].name, console->name) != 0)
                        continue;
                if (console->index >= 0 &&
                    console->index != console_cmdline[i].index)
                        continue;
                if (console->index < 0)
                        console->index = console_cmdline[i].index;
                if (console->setup &&
                    console->setup(console, console_cmdline[i].options) != 0)
                        break;
                console->flags |= CON_ENABLED;
                console->index = console_cmdline[i].index;
                if (i == selected_console) {
                        console->flags |= CON_CONSDEV;
                        preferred_console = selected_console;
                }
                break;
        }

        if (!(console->flags & CON_ENABLED))
                return;

        if (bootconsole) {
                printk(KERN_INFO "console handover: boot [%s%d] -> real [%s%d]\n",
                       bootconsole->name, bootconsole->index,
                       console->name, console->index);
                unregister_console(bootconsole);
                console->flags &= ~CON_PRINTBUFFER;
        }

        /*
         *      Put this console in the list - keep the
         *      preferred driver at the head of the list.
         */
        acquire_console_sem();
        if ((console->flags & CON_CONSDEV) || console_drivers == NULL) {
                console->next = console_drivers;
                console_drivers = console;
                if (console->next)
                        console->next->flags &= ~CON_CONSDEV;
        } else {
                console->next = console_drivers->next;
                console_drivers->next = console;
        }
        if (console->flags & CON_PRINTBUFFER) {
                /*
                 * release_console_sem() will print out the buffered messages
                 * for us.
                 */
                spin_lock_irqsave(&logbuf_lock, flags);
                con_start = log_start;
                spin_unlock_irqrestore(&logbuf_lock, flags);
        }
        release_console_sem();
}
EXPORT_SYMBOL(register_console);

int unregister_console(struct console *console)
{
        struct console *a, *b;
        int res = 1;

        acquire_console_sem();
        if (console_drivers == console) {
                console_drivers=console->next;
                res = 0;
        } else if (console_drivers) {
                for (a=console_drivers->next, b=console_drivers ;
                     a; b=a, a=b->next) {
                        if (a == console) {
                                b->next = a->next;
                                res = 0;
                                break;
                        }
                }
        }

        /*
         * If this isn't the last console and it has CON_CONSDEV set, we
         * need to set it on the next preferred console.
         */
        if (console_drivers != NULL && console->flags & CON_CONSDEV)
                console_drivers->flags |= CON_CONSDEV;

        release_console_sem();
        return res;
}
EXPORT_SYMBOL(unregister_console);

/**
 * tty_write_message - write a message to a certain tty, not just the console.
 * @tty: the destination tty_struct
 * @msg: the message to write
 *
 * This is used for messages that need to be redirected to a specific tty.
 * We don't put it into the syslog queue right now maybe in the future if
 * really needed.
 */
void tty_write_message(struct tty_struct *tty, char *msg)
{
        if (tty && tty->driver->write)
                tty->driver->write(tty, msg, strlen(msg));
        return;
}

/*
 * printk rate limiting, lifted from the networking subsystem.
 *
 * This enforces a rate limit: not more than one kernel message
 * every printk_ratelimit_jiffies to make a denial-of-service
 * attack impossible.
 */
int __printk_ratelimit(int ratelimit_jiffies, int ratelimit_burst)
{
        static DEFINE_SPINLOCK(ratelimit_lock);
        static unsigned long toks = 10 * 5 * HZ;
        static unsigned long last_msg;
        static int missed;
        unsigned long flags;
        unsigned long now = jiffies;

        spin_lock_irqsave(&ratelimit_lock, flags);
        toks += now - last_msg;
        last_msg = now;
        if (toks > (ratelimit_burst * ratelimit_jiffies))
                toks = ratelimit_burst * ratelimit_jiffies;
        if (toks >= ratelimit_jiffies) {
                int lost = missed;

                missed = 0;
                toks -= ratelimit_jiffies;
                spin_unlock_irqrestore(&ratelimit_lock, flags);
                if (lost)
                        printk(KERN_WARNING "printk: %d messages suppressed.\n", lost);
                return 1;
        }
        missed++;
        spin_unlock_irqrestore(&ratelimit_lock, flags);
        return 0;
}
EXPORT_SYMBOL(__printk_ratelimit);

/* minimum time in jiffies between messages */
int printk_ratelimit_jiffies = 5 * HZ;

/* number of messages we send before ratelimiting */
int printk_ratelimit_burst = 10;

int printk_ratelimit(void)
{
        return __printk_ratelimit(printk_ratelimit_jiffies,
                                printk_ratelimit_burst);
}
EXPORT_SYMBOL(printk_ratelimit);

/**
 * printk_timed_ratelimit - caller-controlled printk ratelimiting
 * @caller_jiffies: pointer to caller's state
 * @interval_msecs: minimum interval between prints
 *
 * printk_timed_ratelimit() returns true if more than @interval_msecs
 * milliseconds have elapsed since the last time printk_timed_ratelimit()
 * returned true.
 */
bool printk_timed_ratelimit(unsigned long *caller_jiffies,
                        unsigned int interval_msecs)
{
        if (*caller_jiffies == 0 || time_after(jiffies, *caller_jiffies)) {
                *caller_jiffies = jiffies + msecs_to_jiffies(interval_msecs);
                return true;
        }
        return false;
}
EXPORT_SYMBOL(printk_timed_ratelimit);