clocksource: Make watchdog robust vs. interruption

[mirror_ubuntu-bionic-kernel.git] / kernel / time / clocksource.c

/*
 * linux/kernel/time/clocksource.c
 *
 * This file contains the functions which manage clocksource drivers.
 *
 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@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., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * TODO WishList:
 *   o Allow clocksource drivers to be unregistered
 */

#include <linux/clocksource.h>
#include <linux/sysdev.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
#include <linux/tick.h>
#include <linux/kthread.h>

void timecounter_init(struct timecounter *tc,
                      const struct cyclecounter *cc,
                      u64 start_tstamp)
{
        tc->cc = cc;
        tc->cycle_last = cc->read(cc);
        tc->nsec = start_tstamp;
}
EXPORT_SYMBOL_GPL(timecounter_init);

/**
 * timecounter_read_delta - get nanoseconds since last call of this function
 * @tc:         Pointer to time counter
 *
 * When the underlying cycle counter runs over, this will be handled
 * correctly as long as it does not run over more than once between
 * calls.
 *
 * The first call to this function for a new time counter initializes
 * the time tracking and returns an undefined result.
 */
static u64 timecounter_read_delta(struct timecounter *tc)
{
        cycle_t cycle_now, cycle_delta;
        u64 ns_offset;

        /* read cycle counter: */
        cycle_now = tc->cc->read(tc->cc);

        /* calculate the delta since the last timecounter_read_delta(): */
        cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;

        /* convert to nanoseconds: */
        ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta);

        /* update time stamp of timecounter_read_delta() call: */
        tc->cycle_last = cycle_now;

        return ns_offset;
}

u64 timecounter_read(struct timecounter *tc)
{
        u64 nsec;

        /* increment time by nanoseconds since last call */
        nsec = timecounter_read_delta(tc);
        nsec += tc->nsec;
        tc->nsec = nsec;

        return nsec;
}
EXPORT_SYMBOL_GPL(timecounter_read);

u64 timecounter_cyc2time(struct timecounter *tc,
                         cycle_t cycle_tstamp)
{
        u64 cycle_delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
        u64 nsec;

        /*
         * Instead of always treating cycle_tstamp as more recent
         * than tc->cycle_last, detect when it is too far in the
         * future and treat it as old time stamp instead.
         */
        if (cycle_delta > tc->cc->mask / 2) {
                cycle_delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
                nsec = tc->nsec - cyclecounter_cyc2ns(tc->cc, cycle_delta);
        } else {
                nsec = cyclecounter_cyc2ns(tc->cc, cycle_delta) + tc->nsec;
        }

        return nsec;
}
EXPORT_SYMBOL_GPL(timecounter_cyc2time);

/**
 * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks
 * @mult:       pointer to mult variable
 * @shift:      pointer to shift variable
 * @from:       frequency to convert from
 * @to:         frequency to convert to
 * @maxsec:     guaranteed runtime conversion range in seconds
 *
 * The function evaluates the shift/mult pair for the scaled math
 * operations of clocksources and clockevents.
 *
 * @to and @from are frequency values in HZ. For clock sources @to is
 * NSEC_PER_SEC == 1GHz and @from is the counter frequency. For clock
 * event @to is the counter frequency and @from is NSEC_PER_SEC.
 *
 * The @maxsec conversion range argument controls the time frame in
 * seconds which must be covered by the runtime conversion with the
 * calculated mult and shift factors. This guarantees that no 64bit
 * overflow happens when the input value of the conversion is
 * multiplied with the calculated mult factor. Larger ranges may
 * reduce the conversion accuracy by chosing smaller mult and shift
 * factors.
 */
void
clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
{
        u64 tmp;
        u32 sft, sftacc= 32;

        /*
         * Calculate the shift factor which is limiting the conversion
         * range:
         */
        tmp = ((u64)maxsec * from) >> 32;
        while (tmp) {
                tmp >>=1;
                sftacc--;
        }

        /*
         * Find the conversion shift/mult pair which has the best
         * accuracy and fits the maxsec conversion range:
         */
        for (sft = 32; sft > 0; sft--) {
                tmp = (u64) to << sft;
                tmp += from / 2;
                do_div(tmp, from);
                if ((tmp >> sftacc) == 0)
                        break;
        }
        *mult = tmp;
        *shift = sft;
}

/*[Clocksource internal variables]---------
 * curr_clocksource:
 *      currently selected clocksource.
 * clocksource_list:
 *      linked list with the registered clocksources
 * clocksource_mutex:
 *      protects manipulations to curr_clocksource and the clocksource_list
 * override_name:
 *      Name of the user-specified clocksource.
 */
static struct clocksource *curr_clocksource;
static LIST_HEAD(clocksource_list);
static DEFINE_MUTEX(clocksource_mutex);
static char override_name[32];
static int finished_booting;

#ifdef CONFIG_CLOCKSOURCE_WATCHDOG
static void clocksource_watchdog_work(struct work_struct *work);

static LIST_HEAD(watchdog_list);
static struct clocksource *watchdog;
static struct timer_list watchdog_timer;
static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
static DEFINE_SPINLOCK(watchdog_lock);
static int watchdog_running;

static int clocksource_watchdog_kthread(void *data);
static void __clocksource_change_rating(struct clocksource *cs, int rating);

/*
 * Interval: 0.5sec Threshold: 0.0625s
 */
#define WATCHDOG_INTERVAL (HZ >> 1)
#define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)

static void clocksource_watchdog_work(struct work_struct *work)
{
        /*
         * If kthread_run fails the next watchdog scan over the
         * watchdog_list will find the unstable clock again.
         */
        kthread_run(clocksource_watchdog_kthread, NULL, "kwatchdog");
}

static void __clocksource_unstable(struct clocksource *cs)
{
        cs->flags &= ~(CLOCK_SOURCE_VALID_FOR_HRES | CLOCK_SOURCE_WATCHDOG);
        cs->flags |= CLOCK_SOURCE_UNSTABLE;
        if (finished_booting)
                schedule_work(&watchdog_work);
}

static void clocksource_unstable(struct clocksource *cs, int64_t delta)
{
        printk(KERN_WARNING "Clocksource %s unstable (delta = %Ld ns)\n",
               cs->name, delta);
        __clocksource_unstable(cs);
}

/**
 * clocksource_mark_unstable - mark clocksource unstable via watchdog
 * @cs:         clocksource to be marked unstable
 *
 * This function is called instead of clocksource_change_rating from
 * cpu hotplug code to avoid a deadlock between the clocksource mutex
 * and the cpu hotplug mutex. It defers the update of the clocksource
 * to the watchdog thread.
 */
void clocksource_mark_unstable(struct clocksource *cs)
{
        unsigned long flags;

        spin_lock_irqsave(&watchdog_lock, flags);
        if (!(cs->flags & CLOCK_SOURCE_UNSTABLE)) {
                if (list_empty(&cs->wd_list))
                        list_add(&cs->wd_list, &watchdog_list);
                __clocksource_unstable(cs);
        }
        spin_unlock_irqrestore(&watchdog_lock, flags);
}

static void clocksource_watchdog(unsigned long data)
{
        struct clocksource *cs;
        cycle_t csnow, wdnow;
        int64_t wd_nsec, cs_nsec;
        int next_cpu;

        spin_lock(&watchdog_lock);
        if (!watchdog_running)
                goto out;

        list_for_each_entry(cs, &watchdog_list, wd_list) {

                /* Clocksource already marked unstable? */
                if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
                        if (finished_booting)
                                schedule_work(&watchdog_work);
                        continue;
                }

                local_irq_disable();
                csnow = cs->read(cs);
                wdnow = watchdog->read(watchdog);
                local_irq_enable();

                /* Clocksource initialized ? */
                if (!(cs->flags & CLOCK_SOURCE_WATCHDOG)) {
                        cs->flags |= CLOCK_SOURCE_WATCHDOG;
                        cs->wd_last = wdnow;
                        cs->cs_last = csnow;
                        continue;
                }

                wd_nsec = clocksource_cyc2ns((wdnow - cs->wd_last) & watchdog->mask,
                                             watchdog->mult, watchdog->shift);

                cs_nsec = clocksource_cyc2ns((csnow - cs->cs_last) &
                                             cs->mask, cs->mult, cs->shift);
                cs->cs_last = csnow;
                cs->wd_last = wdnow;

                /* Check the deviation from the watchdog clocksource. */
                if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
                        clocksource_unstable(cs, cs_nsec - wd_nsec);
                        continue;
                }

                if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) &&
                    (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) &&
                    (watchdog->flags & CLOCK_SOURCE_IS_CONTINUOUS)) {
                        cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
                        /*
                         * We just marked the clocksource as highres-capable,
                         * notify the rest of the system as well so that we
                         * transition into high-res mode:
                         */
                        tick_clock_notify();
                }
        }

        /*
         * Cycle through CPUs to check if the CPUs stay synchronized
         * to each other.
         */
        next_cpu = cpumask_next(raw_smp_processor_id(), cpu_online_mask);
        if (next_cpu >= nr_cpu_ids)
                next_cpu = cpumask_first(cpu_online_mask);
        watchdog_timer.expires += WATCHDOG_INTERVAL;
        add_timer_on(&watchdog_timer, next_cpu);
out:
        spin_unlock(&watchdog_lock);
}

static inline void clocksource_start_watchdog(void)
{
        if (watchdog_running || !watchdog || list_empty(&watchdog_list))
                return;
        init_timer(&watchdog_timer);
        watchdog_timer.function = clocksource_watchdog;
        watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
        add_timer_on(&watchdog_timer, cpumask_first(cpu_online_mask));
        watchdog_running = 1;
}

static inline void clocksource_stop_watchdog(void)
{
        if (!watchdog_running || (watchdog && !list_empty(&watchdog_list)))
                return;
        del_timer(&watchdog_timer);
        watchdog_running = 0;
}

static inline void clocksource_reset_watchdog(void)
{
        struct clocksource *cs;

        list_for_each_entry(cs, &watchdog_list, wd_list)
                cs->flags &= ~CLOCK_SOURCE_WATCHDOG;
}

static void clocksource_resume_watchdog(void)
{
        unsigned long flags;

        /*
         * We use trylock here to avoid a potential dead lock when
         * kgdb calls this code after the kernel has been stopped with
         * watchdog_lock held. When watchdog_lock is held we just
         * return and accept, that the watchdog might trigger and mark
         * the monitored clock source (usually TSC) unstable.
         *
         * This does not affect the other caller clocksource_resume()
         * because at this point the kernel is UP, interrupts are
         * disabled and nothing can hold watchdog_lock.
         */
        if (!spin_trylock_irqsave(&watchdog_lock, flags))
                return;
        clocksource_reset_watchdog();
        spin_unlock_irqrestore(&watchdog_lock, flags);
}

static void clocksource_enqueue_watchdog(struct clocksource *cs)
{
        unsigned long flags;

        spin_lock_irqsave(&watchdog_lock, flags);
        if (cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
                /* cs is a clocksource to be watched. */
                list_add(&cs->wd_list, &watchdog_list);
                cs->flags &= ~CLOCK_SOURCE_WATCHDOG;
        } else {
                /* cs is a watchdog. */
                if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
                        cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
                /* Pick the best watchdog. */
                if (!watchdog || cs->rating > watchdog->rating) {
                        watchdog = cs;
                        /* Reset watchdog cycles */
                        clocksource_reset_watchdog();
                }
        }
        /* Check if the watchdog timer needs to be started. */
        clocksource_start_watchdog();
        spin_unlock_irqrestore(&watchdog_lock, flags);
}

static void clocksource_dequeue_watchdog(struct clocksource *cs)
{
        struct clocksource *tmp;
        unsigned long flags;

        spin_lock_irqsave(&watchdog_lock, flags);
        if (cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
                /* cs is a watched clocksource. */
                list_del_init(&cs->wd_list);
        } else if (cs == watchdog) {
                /* Reset watchdog cycles */
                clocksource_reset_watchdog();
                /* Current watchdog is removed. Find an alternative. */
                watchdog = NULL;
                list_for_each_entry(tmp, &clocksource_list, list) {
                        if (tmp == cs || tmp->flags & CLOCK_SOURCE_MUST_VERIFY)
                                continue;
                        if (!watchdog || tmp->rating > watchdog->rating)
                                watchdog = tmp;
                }
        }
        cs->flags &= ~CLOCK_SOURCE_WATCHDOG;
        /* Check if the watchdog timer needs to be stopped. */
        clocksource_stop_watchdog();
        spin_unlock_irqrestore(&watchdog_lock, flags);
}

static int clocksource_watchdog_kthread(void *data)
{
        struct clocksource *cs, *tmp;
        unsigned long flags;
        LIST_HEAD(unstable);

        mutex_lock(&clocksource_mutex);
        spin_lock_irqsave(&watchdog_lock, flags);
        list_for_each_entry_safe(cs, tmp, &watchdog_list, wd_list)
                if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
                        list_del_init(&cs->wd_list);
                        list_add(&cs->wd_list, &unstable);
                }
        /* Check if the watchdog timer needs to be stopped. */
        clocksource_stop_watchdog();
        spin_unlock_irqrestore(&watchdog_lock, flags);

        /* Needs to be done outside of watchdog lock */
        list_for_each_entry_safe(cs, tmp, &unstable, wd_list) {
                list_del_init(&cs->wd_list);
                __clocksource_change_rating(cs, 0);
        }
        mutex_unlock(&clocksource_mutex);
        return 0;
}

#else /* CONFIG_CLOCKSOURCE_WATCHDOG */

static void clocksource_enqueue_watchdog(struct clocksource *cs)
{
        if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
                cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
}

static inline void clocksource_dequeue_watchdog(struct clocksource *cs) { }
static inline void clocksource_resume_watchdog(void) { }
static inline int clocksource_watchdog_kthread(void *data) { return 0; }

#endif /* CONFIG_CLOCKSOURCE_WATCHDOG */

/**
 * clocksource_suspend - suspend the clocksource(s)
 */
void clocksource_suspend(void)
{
        struct clocksource *cs;

        list_for_each_entry_reverse(cs, &clocksource_list, list)
                if (cs->suspend)
                        cs->suspend(cs);
}

/**
 * clocksource_resume - resume the clocksource(s)
 */
void clocksource_resume(void)
{
        struct clocksource *cs;

        list_for_each_entry(cs, &clocksource_list, list)
                if (cs->resume)
                        cs->resume(cs);

        clocksource_resume_watchdog();
}

/**
 * clocksource_touch_watchdog - Update watchdog
 *
 * Update the watchdog after exception contexts such as kgdb so as not
 * to incorrectly trip the watchdog. This might fail when the kernel
 * was stopped in code which holds watchdog_lock.
 */
void clocksource_touch_watchdog(void)
{
        clocksource_resume_watchdog();
}

/**
 * clocksource_max_deferment - Returns max time the clocksource can be deferred
 * @cs:         Pointer to clocksource
 *
 */
static u64 clocksource_max_deferment(struct clocksource *cs)
{
        u64 max_nsecs, max_cycles;

        /*
         * Calculate the maximum number of cycles that we can pass to the
         * cyc2ns function without overflowing a 64-bit signed result. The
         * maximum number of cycles is equal to ULLONG_MAX/cs->mult which
         * is equivalent to the below.
         * max_cycles < (2^63)/cs->mult
         * max_cycles < 2^(log2((2^63)/cs->mult))
         * max_cycles < 2^(log2(2^63) - log2(cs->mult))
         * max_cycles < 2^(63 - log2(cs->mult))
         * max_cycles < 1 << (63 - log2(cs->mult))
         * Please note that we add 1 to the result of the log2 to account for
         * any rounding errors, ensure the above inequality is satisfied and
         * no overflow will occur.
         */
        max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));

        /*
         * The actual maximum number of cycles we can defer the clocksource is
         * determined by the minimum of max_cycles and cs->mask.
         */
        max_cycles = min_t(u64, max_cycles, (u64) cs->mask);
        max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);

        /*
         * To ensure that the clocksource does not wrap whilst we are idle,
         * limit the time the clocksource can be deferred by 12.5%. Please
         * note a margin of 12.5% is used because this can be computed with
         * a shift, versus say 10% which would require division.
         */
        return max_nsecs - (max_nsecs >> 5);
}

#ifndef CONFIG_ARCH_USES_GETTIMEOFFSET

/**
 * clocksource_select - Select the best clocksource available
 *
 * Private function. Must hold clocksource_mutex when called.
 *
 * Select the clocksource with the best rating, or the clocksource,
 * which is selected by userspace override.
 */
static void clocksource_select(void)
{
        struct clocksource *best, *cs;

        if (!finished_booting || list_empty(&clocksource_list))
                return;
        /* First clocksource on the list has the best rating. */
        best = list_first_entry(&clocksource_list, struct clocksource, list);
        /* Check for the override clocksource. */
        list_for_each_entry(cs, &clocksource_list, list) {
                if (strcmp(cs->name, override_name) != 0)
                        continue;
                /*
                 * Check to make sure we don't switch to a non-highres
                 * capable clocksource if the tick code is in oneshot
                 * mode (highres or nohz)
                 */
                if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) &&
                    tick_oneshot_mode_active()) {
                        /* Override clocksource cannot be used. */
                        printk(KERN_WARNING "Override clocksource %s is not "
                               "HRT compatible. Cannot switch while in "
                               "HRT/NOHZ mode\n", cs->name);
                        override_name[0] = 0;
                } else
                        /* Override clocksource can be used. */
                        best = cs;
                break;
        }
        if (curr_clocksource != best) {
                printk(KERN_INFO "Switching to clocksource %s\n", best->name);
                curr_clocksource = best;
                timekeeping_notify(curr_clocksource);
        }
}

#else /* !CONFIG_ARCH_USES_GETTIMEOFFSET */

static inline void clocksource_select(void) { }

#endif

/*
 * clocksource_done_booting - Called near the end of core bootup
 *
 * Hack to avoid lots of clocksource churn at boot time.
 * We use fs_initcall because we want this to start before
 * device_initcall but after subsys_initcall.
 */
static int __init clocksource_done_booting(void)
{
        mutex_lock(&clocksource_mutex);
        curr_clocksource = clocksource_default_clock();
        mutex_unlock(&clocksource_mutex);

        finished_booting = 1;

        /*
         * Run the watchdog first to eliminate unstable clock sources
         */
        clocksource_watchdog_kthread(NULL);

        mutex_lock(&clocksource_mutex);
        clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;
}
fs_initcall(clocksource_done_booting);

/*
 * Enqueue the clocksource sorted by rating
 */
static void clocksource_enqueue(struct clocksource *cs)
{
        struct list_head *entry = &clocksource_list;
        struct clocksource *tmp;

        list_for_each_entry(tmp, &clocksource_list, list)
                /* Keep track of the place, where to insert */
                if (tmp->rating >= cs->rating)
                        entry = &tmp->list;
        list_add(&cs->list, entry);
}

/**
 * __clocksource_updatefreq_scale - Used update clocksource with new freq
 * @t:          clocksource to be registered
 * @scale:      Scale factor multiplied against freq to get clocksource hz
 * @freq:       clocksource frequency (cycles per second) divided by scale
 *
 * This should only be called from the clocksource->enable() method.
 *
 * This *SHOULD NOT* be called directly! Please use the
 * clocksource_updatefreq_hz() or clocksource_updatefreq_khz helper functions.
 */
void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)
{
        u64 sec;

        /*
         * Calc the maximum number of seconds which we can run before
         * wrapping around. For clocksources which have a mask > 32bit
         * we need to limit the max sleep time to have a good
         * conversion precision. 10 minutes is still a reasonable
         * amount. That results in a shift value of 24 for a
         * clocksource with mask >= 40bit and f >= 4GHz. That maps to
         * ~ 0.06ppm granularity for NTP. We apply the same 12.5%
         * margin as we do in clocksource_max_deferment()
         */
        sec = (cs->mask - (cs->mask >> 5));
        do_div(sec, freq);
        do_div(sec, scale);
        if (!sec)
                sec = 1;
        else if (sec > 600 && cs->mask > UINT_MAX)
                sec = 600;

        clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
                               NSEC_PER_SEC / scale, sec * scale);
        cs->max_idle_ns = clocksource_max_deferment(cs);
}
EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);

/**
 * __clocksource_register_scale - Used to install new clocksources
 * @t:          clocksource to be registered
 * @scale:      Scale factor multiplied against freq to get clocksource hz
 * @freq:       clocksource frequency (cycles per second) divided by scale
 *
 * Returns -EBUSY if registration fails, zero otherwise.
 *
 * This *SHOULD NOT* be called directly! Please use the
 * clocksource_register_hz() or clocksource_register_khz helper functions.
 */
int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
{

        /* Initialize mult/shift and max_idle_ns */
        __clocksource_updatefreq_scale(cs, scale, freq);

        /* Add clocksource to the clcoksource list */
        mutex_lock(&clocksource_mutex);
        clocksource_enqueue(cs);
        clocksource_enqueue_watchdog(cs);
        clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;
}
EXPORT_SYMBOL_GPL(__clocksource_register_scale);


/**
 * clocksource_register - Used to install new clocksources
 * @t:          clocksource to be registered
 *
 * Returns -EBUSY if registration fails, zero otherwise.
 */
int clocksource_register(struct clocksource *cs)
{
        /* calculate max idle time permitted for this clocksource */
        cs->max_idle_ns = clocksource_max_deferment(cs);

        mutex_lock(&clocksource_mutex);
        clocksource_enqueue(cs);
        clocksource_enqueue_watchdog(cs);
        clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;
}
EXPORT_SYMBOL(clocksource_register);

static void __clocksource_change_rating(struct clocksource *cs, int rating)
{
        list_del(&cs->list);
        cs->rating = rating;
        clocksource_enqueue(cs);
        clocksource_select();
}

/**
 * clocksource_change_rating - Change the rating of a registered clocksource
 */
void clocksource_change_rating(struct clocksource *cs, int rating)
{
        mutex_lock(&clocksource_mutex);
        __clocksource_change_rating(cs, rating);
        mutex_unlock(&clocksource_mutex);
}
EXPORT_SYMBOL(clocksource_change_rating);

/**
 * clocksource_unregister - remove a registered clocksource
 */
void clocksource_unregister(struct clocksource *cs)
{
        mutex_lock(&clocksource_mutex);
        clocksource_dequeue_watchdog(cs);
        list_del(&cs->list);
        clocksource_select();
        mutex_unlock(&clocksource_mutex);
}
EXPORT_SYMBOL(clocksource_unregister);

#ifdef CONFIG_SYSFS
/**
 * sysfs_show_current_clocksources - sysfs interface for current clocksource
 * @dev:        unused
 * @buf:        char buffer to be filled with clocksource list
 *
 * Provides sysfs interface for listing current clocksource.
 */
static ssize_t
sysfs_show_current_clocksources(struct sys_device *dev,
                                struct sysdev_attribute *attr, char *buf)
{
        ssize_t count = 0;

        mutex_lock(&clocksource_mutex);
        count = snprintf(buf, PAGE_SIZE, "%s\n", curr_clocksource->name);
        mutex_unlock(&clocksource_mutex);

        return count;
}

/**
 * sysfs_override_clocksource - interface for manually overriding clocksource
 * @dev:        unused
 * @buf:        name of override clocksource
 * @count:      length of buffer
 *
 * Takes input from sysfs interface for manually overriding the default
 * clocksource selection.
 */
static ssize_t sysfs_override_clocksource(struct sys_device *dev,
                                          struct sysdev_attribute *attr,
                                          const char *buf, size_t count)
{
        size_t ret = count;

        /* strings from sysfs write are not 0 terminated! */
        if (count >= sizeof(override_name))
                return -EINVAL;

        /* strip of \n: */
        if (buf[count-1] == '\n')
                count--;

        mutex_lock(&clocksource_mutex);

        if (count > 0)
                memcpy(override_name, buf, count);
        override_name[count] = 0;
        clocksource_select();

        mutex_unlock(&clocksource_mutex);

        return ret;
}

/**
 * sysfs_show_available_clocksources - sysfs interface for listing clocksource
 * @dev:        unused
 * @buf:        char buffer to be filled with clocksource list
 *
 * Provides sysfs interface for listing registered clocksources
 */
static ssize_t
sysfs_show_available_clocksources(struct sys_device *dev,
                                  struct sysdev_attribute *attr,
                                  char *buf)
{
        struct clocksource *src;
        ssize_t count = 0;

        mutex_lock(&clocksource_mutex);
        list_for_each_entry(src, &clocksource_list, list) {
                /*
                 * Don't show non-HRES clocksource if the tick code is
                 * in one shot mode (highres=on or nohz=on)
                 */
                if (!tick_oneshot_mode_active() ||
                    (src->flags & CLOCK_SOURCE_VALID_FOR_HRES))
                        count += snprintf(buf + count,
                                  max((ssize_t)PAGE_SIZE - count, (ssize_t)0),
                                  "%s ", src->name);
        }
        mutex_unlock(&clocksource_mutex);

        count += snprintf(buf + count,
                          max((ssize_t)PAGE_SIZE - count, (ssize_t)0), "\n");

        return count;
}

/*
 * Sysfs setup bits:
 */
static SYSDEV_ATTR(current_clocksource, 0644, sysfs_show_current_clocksources,
                   sysfs_override_clocksource);

static SYSDEV_ATTR(available_clocksource, 0444,
                   sysfs_show_available_clocksources, NULL);

static struct sysdev_class clocksource_sysclass = {
        .name = "clocksource",
};

static struct sys_device device_clocksource = {
        .id     = 0,
        .cls    = &clocksource_sysclass,
};

static int __init init_clocksource_sysfs(void)
{
        int error = sysdev_class_register(&clocksource_sysclass);

        if (!error)
                error = sysdev_register(&device_clocksource);
        if (!error)
                error = sysdev_create_file(
                                &device_clocksource,
                                &attr_current_clocksource);
        if (!error)
                error = sysdev_create_file(
                                &device_clocksource,
                                &attr_available_clocksource);
        return error;
}

device_initcall(init_clocksource_sysfs);
#endif /* CONFIG_SYSFS */

/**
 * boot_override_clocksource - boot clock override
 * @str:        override name
 *
 * Takes a clocksource= boot argument and uses it
 * as the clocksource override name.
 */
static int __init boot_override_clocksource(char* str)
{
        mutex_lock(&clocksource_mutex);
        if (str)
                strlcpy(override_name, str, sizeof(override_name));
        mutex_unlock(&clocksource_mutex);
        return 1;
}

__setup("clocksource=", boot_override_clocksource);

/**
 * boot_override_clock - Compatibility layer for deprecated boot option
 * @str:        override name
 *
 * DEPRECATED! Takes a clock= boot argument and uses it
 * as the clocksource override name
 */
static int __init boot_override_clock(char* str)
{
        if (!strcmp(str, "pmtmr")) {
                printk("Warning: clock=pmtmr is deprecated. "
                        "Use clocksource=acpi_pm.\n");
                return boot_override_clocksource("acpi_pm");
        }
        printk("Warning! clock= boot option is deprecated. "
                "Use clocksource=xyz\n");
        return boot_override_clocksource(str);
}

__setup("clock=", boot_override_clock);