/*
 * 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
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/device.h>
#include <linux/clocksource.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>

#include "tick-internal.h"
#include "timekeeping_internal.h"

/**
 * 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;
}
EXPORT_SYMBOL_GPL(clocks_calc_mult_shift);

/*[Clocksource internal variables]---------
 * curr_clocksource:
 *      currently selected clocksource.
 * suspend_clocksource:
 *      used to calculate the suspend time.
 * 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 struct clocksource *suspend_clocksource;
static LIST_HEAD(clocksource_list);
static DEFINE_MUTEX(clocksource_mutex);
static char override_name[CS_NAME_LEN];
static int finished_booting;
static u64 suspend_start;

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

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 atomic_t watchdog_reset_pending;

static void inline clocksource_watchdog_lock(unsigned long *flags)
{
        spin_lock_irqsave(&watchdog_lock, *flags);
}

static void inline clocksource_watchdog_unlock(unsigned long *flags)
{
        spin_unlock_irqrestore(&watchdog_lock, *flags);
}

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)
{
        /*
         * We cannot directly run clocksource_watchdog_kthread() here, because
         * clocksource_select() calls timekeeping_notify() which uses
         * stop_machine(). One cannot use stop_machine() from a workqueue() due
         * lock inversions wrt CPU hotplug.
         *
         * Also, we only ever run this work once or twice during the lifetime
         * of the kernel, so there is no point in creating a more permanent
         * kthread for this.
         *
         * 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 the clocksource is registered clocksource_watchdog_kthread() will
         * re-rate and re-select.
         */
        if (list_empty(&cs->list)) {
                cs->rating = 0;
                return;
        }

        if (cs->mark_unstable)
                cs->mark_unstable(cs);

        /* kick clocksource_watchdog_kthread() */
        if (finished_booting)
                schedule_work(&watchdog_work);
}

/**
 * clocksource_mark_unstable - mark clocksource unstable via watchdog
 * @cs:         clocksource to be marked unstable
 *
 * This function is called by the x86 TSC code to mark clocksources as unstable;
 * it defers demotion and re-selection to a kthread.
 */
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->list) && 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(struct timer_list *unused)
{
        struct clocksource *cs;
        u64 csnow, wdnow, cslast, wdlast, delta;
        int64_t wd_nsec, cs_nsec;
        int next_cpu, reset_pending;

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

        reset_pending = atomic_read(&watchdog_reset_pending);

        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) ||
                    atomic_read(&watchdog_reset_pending)) {
                        cs->flags |= CLOCK_SOURCE_WATCHDOG;
                        cs->wd_last = wdnow;
                        cs->cs_last = csnow;
                        continue;
                }

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

                delta = clocksource_delta(csnow, cs->cs_last, cs->mask);
                cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
                wdlast = cs->wd_last; /* save these in case we print them */
                cslast = cs->cs_last;
                cs->cs_last = csnow;
                cs->wd_last = wdnow;

                if (atomic_read(&watchdog_reset_pending))
                        continue;

                /* Check the deviation from the watchdog clocksource. */
                if (abs(cs_nsec - wd_nsec) > WATCHDOG_THRESHOLD) {
                        pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n",
                                smp_processor_id(), cs->name);
                        pr_warn("                      '%s' wd_now: %llx wd_last: %llx mask: %llx\n",
                                watchdog->name, wdnow, wdlast, watchdog->mask);
                        pr_warn("                      '%s' cs_now: %llx cs_last: %llx mask: %llx\n",
                                cs->name, csnow, cslast, cs->mask);
                        __clocksource_unstable(cs);
                        continue;
                }

                if (cs == curr_clocksource && cs->tick_stable)
                        cs->tick_stable(cs);

                if (!(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES) &&
                    (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) &&
                    (watchdog->flags & CLOCK_SOURCE_IS_CONTINUOUS)) {
                        /* Mark it valid for high-res. */
                        cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;

                        /*
                         * clocksource_done_booting() will sort it if
                         * finished_booting is not set yet.
                         */
                        if (!finished_booting)
                                continue;

                        /*
                         * If this is not the current clocksource let
                         * the watchdog thread reselect it. Due to the
                         * change to high res this clocksource might
                         * be preferred now. If it is the current
                         * clocksource let the tick code know about
                         * that change.
                         */
                        if (cs != curr_clocksource) {
                                cs->flags |= CLOCK_SOURCE_RESELECT;
                                schedule_work(&watchdog_work);
                        } else {
                                tick_clock_notify();
                        }
                }
        }

        /*
         * We only clear the watchdog_reset_pending, when we did a
         * full cycle through all clocksources.
         */
        if (reset_pending)
                atomic_dec(&watchdog_reset_pending);

        /*
         * 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;
        timer_setup(&watchdog_timer, clocksource_watchdog, 0);
        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)
{
        atomic_inc(&watchdog_reset_pending);
}

static void clocksource_enqueue_watchdog(struct clocksource *cs)
{
        INIT_LIST_HEAD(&cs->wd_list);

        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;
        }
}

static void clocksource_select_watchdog(bool fallback)
{
        struct clocksource *cs, *old_wd;
        unsigned long flags;

        spin_lock_irqsave(&watchdog_lock, flags);
        /* save current watchdog */
        old_wd = watchdog;
        if (fallback)
                watchdog = NULL;

        list_for_each_entry(cs, &clocksource_list, list) {
                /* cs is a clocksource to be watched. */
                if (cs->flags & CLOCK_SOURCE_MUST_VERIFY)
                        continue;

                /* Skip current if we were requested for a fallback. */
                if (fallback && cs == old_wd)
                        continue;

                /* Pick the best watchdog. */
                if (!watchdog || cs->rating > watchdog->rating)
                        watchdog = cs;
        }
        /* If we failed to find a fallback restore the old one. */
        if (!watchdog)
                watchdog = old_wd;

        /* If we changed the watchdog we need to reset cycles. */
        if (watchdog != old_wd)
                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)
{
        if (cs != watchdog) {
                if (cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
                        /* cs is a watched clocksource. */
                        list_del_init(&cs->wd_list);
                        /* Check if the watchdog timer needs to be stopped. */
                        clocksource_stop_watchdog();
                }
        }
}

static int __clocksource_watchdog_kthread(void)
{
        struct clocksource *cs, *tmp;
        unsigned long flags;
        int select = 0;

        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);
                        __clocksource_change_rating(cs, 0);
                        select = 1;
                }
                if (cs->flags & CLOCK_SOURCE_RESELECT) {
                        cs->flags &= ~CLOCK_SOURCE_RESELECT;
                        select = 1;
                }
        }
        /* Check if the watchdog timer needs to be stopped. */
        clocksource_stop_watchdog();
        spin_unlock_irqrestore(&watchdog_lock, flags);

        return select;
}

static int clocksource_watchdog_kthread(void *data)
{
        mutex_lock(&clocksource_mutex);
        if (__clocksource_watchdog_kthread())
                clocksource_select();
        mutex_unlock(&clocksource_mutex);
        return 0;
}

static bool clocksource_is_watchdog(struct clocksource *cs)
{
        return cs == watchdog;
}

#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 void clocksource_select_watchdog(bool fallback) { }
static inline void clocksource_dequeue_watchdog(struct clocksource *cs) { }
static inline void clocksource_resume_watchdog(void) { }
static inline int __clocksource_watchdog_kthread(void) { return 0; }
static bool clocksource_is_watchdog(struct clocksource *cs) { return false; }
void clocksource_mark_unstable(struct clocksource *cs) { }

static inline void clocksource_watchdog_lock(unsigned long *flags) { }
static inline void clocksource_watchdog_unlock(unsigned long *flags) { }

#endif /* CONFIG_CLOCKSOURCE_WATCHDOG */

static bool clocksource_is_suspend(struct clocksource *cs)
{
        return cs == suspend_clocksource;
}

static void __clocksource_suspend_select(struct clocksource *cs)
{
        /*
         * Skip the clocksource which will be stopped in suspend state.
         */
        if (!(cs->flags & CLOCK_SOURCE_SUSPEND_NONSTOP))
                return;

        /*
         * The nonstop clocksource can be selected as the suspend clocksource to
         * calculate the suspend time, so it should not supply suspend/resume
         * interfaces to suspend the nonstop clocksource when system suspends.
         */
        if (cs->suspend || cs->resume) {
                pr_warn("Nonstop clocksource %s should not supply suspend/resume interfaces\n",
                        cs->name);
        }

        /* Pick the best rating. */
        if (!suspend_clocksource || cs->rating > suspend_clocksource->rating)
                suspend_clocksource = cs;
}

/**
 * clocksource_suspend_select - Select the best clocksource for suspend timing
 * @fallback:   if select a fallback clocksource
 */
static void clocksource_suspend_select(bool fallback)
{
        struct clocksource *cs, *old_suspend;

        old_suspend = suspend_clocksource;
        if (fallback)
                suspend_clocksource = NULL;

        list_for_each_entry(cs, &clocksource_list, list) {
                /* Skip current if we were requested for a fallback. */
                if (fallback && cs == old_suspend)
                        continue;

                __clocksource_suspend_select(cs);
        }
}

/**
 * clocksource_start_suspend_timing - Start measuring the suspend timing
 * @cs:                 current clocksource from timekeeping
 * @start_cycles:       current cycles from timekeeping
 *
 * This function will save the start cycle values of suspend timer to calculate
 * the suspend time when resuming system.
 *
 * This function is called late in the suspend process from timekeeping_suspend(),
 * that means processes are freezed, non-boot cpus and interrupts are disabled
 * now. It is therefore possible to start the suspend timer without taking the
 * clocksource mutex.
 */
void clocksource_start_suspend_timing(struct clocksource *cs, u64 start_cycles)
{
        if (!suspend_clocksource)
                return;

        /*
         * If current clocksource is the suspend timer, we should use the
         * tkr_mono.cycle_last value as suspend_start to avoid same reading
         * from suspend timer.
         */
        if (clocksource_is_suspend(cs)) {
                suspend_start = start_cycles;
                return;
        }

        if (suspend_clocksource->enable &&
            suspend_clocksource->enable(suspend_clocksource)) {
                pr_warn_once("Failed to enable the non-suspend-able clocksource.\n");
                return;
        }

        suspend_start = suspend_clocksource->read(suspend_clocksource);
}

/**
 * clocksource_stop_suspend_timing - Stop measuring the suspend timing
 * @cs:         current clocksource from timekeeping
 * @cycle_now:  current cycles from timekeeping
 *
 * This function will calculate the suspend time from suspend timer.
 *
 * Returns nanoseconds since suspend started, 0 if no usable suspend clocksource.
 *
 * This function is called early in the resume process from timekeeping_resume(),
 * that means there is only one cpu, no processes are running and the interrupts
 * are disabled. It is therefore possible to stop the suspend timer without
 * taking the clocksource mutex.
 */
u64 clocksource_stop_suspend_timing(struct clocksource *cs, u64 cycle_now)
{
        u64 now, delta, nsec = 0;

        if (!suspend_clocksource)
                return 0;

        /*
         * If current clocksource is the suspend timer, we should use the
         * tkr_mono.cycle_last value from timekeeping as current cycle to
         * avoid same reading from suspend timer.
         */
        if (clocksource_is_suspend(cs))
                now = cycle_now;
        else
                now = suspend_clocksource->read(suspend_clocksource);

        if (now > suspend_start) {
                delta = clocksource_delta(now, suspend_start,
                                          suspend_clocksource->mask);
                nsec = mul_u64_u32_shr(delta, suspend_clocksource->mult,
                                       suspend_clocksource->shift);
        }

        /*
         * Disable the suspend timer to save power if current clocksource is
         * not the suspend timer.
         */
        if (!clocksource_is_suspend(cs) && suspend_clocksource->disable)
                suspend_clocksource->disable(suspend_clocksource);

        return nsec;
}

/**
 * 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_adjustment- Returns max adjustment amount
 * @cs:         Pointer to clocksource
 *
 */
static u32 clocksource_max_adjustment(struct clocksource *cs)
{
        u64 ret;
        /*
         * We won't try to correct for more than 11% adjustments (110,000 ppm),
         */
        ret = (u64)cs->mult * 11;
        do_div(ret,100);
        return (u32)ret;
}

/**
 * clocks_calc_max_nsecs - Returns maximum nanoseconds that can be converted
 * @mult:       cycle to nanosecond multiplier
 * @shift:      cycle to nanosecond divisor (power of two)
 * @maxadj:     maximum adjustment value to mult (~11%)
 * @mask:       bitmask for two's complement subtraction of non 64 bit counters
 * @max_cyc:    maximum cycle value before potential overflow (does not include
 *              any safety margin)
 *
 * NOTE: This function includes a safety margin of 50%, in other words, we
 * return half the number of nanoseconds the hardware counter can technically
 * cover. This is done so that we can potentially detect problems caused by
 * delayed timers or bad hardware, which might result in time intervals that
 * are larger than what the math used can handle without overflows.
 */
u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask, u64 *max_cyc)
{
        u64 max_nsecs, max_cycles;

        /*
         * Calculate the maximum number of cycles that we can pass to the
         * cyc2ns() function without overflowing a 64-bit result.
         */
        max_cycles = ULLONG_MAX;
        do_div(max_cycles, mult+maxadj);

        /*
         * The actual maximum number of cycles we can defer the clocksource is
         * determined by the minimum of max_cycles and mask.
         * Note: Here we subtract the maxadj to make sure we don't sleep for
         * too long if there's a large negative adjustment.
         */
        max_cycles = min(max_cycles, mask);
        max_nsecs = clocksource_cyc2ns(max_cycles, mult - maxadj, shift);

        /* return the max_cycles value as well if requested */
        if (max_cyc)
                *max_cyc = max_cycles;

        /* Return 50% of the actual maximum, so we can detect bad values */
        max_nsecs >>= 1;

        return max_nsecs;
}

/**
 * clocksource_update_max_deferment - Updates the clocksource max_idle_ns & max_cycles
 * @cs:         Pointer to clocksource to be updated
 *
 */
static inline void clocksource_update_max_deferment(struct clocksource *cs)
{
        cs->max_idle_ns = clocks_calc_max_nsecs(cs->mult, cs->shift,
                                                cs->maxadj, cs->mask,
                                                &cs->max_cycles);
}

#ifndef CONFIG_ARCH_USES_GETTIMEOFFSET

static struct clocksource *clocksource_find_best(bool oneshot, bool skipcur)
{
        struct clocksource *cs;

        if (!finished_booting || list_empty(&clocksource_list))
                return NULL;

        /*
         * We pick the clocksource with the highest rating. If oneshot
         * mode is active, we pick the highres valid clocksource with
         * the best rating.
         */
        list_for_each_entry(cs, &clocksource_list, list) {
                if (skipcur && cs == curr_clocksource)
                        continue;
                if (oneshot && !(cs->flags & CLOCK_SOURCE_VALID_FOR_HRES))
                        continue;
                return cs;
        }
        return NULL;
}

static void __clocksource_select(bool skipcur)
{
        bool oneshot = tick_oneshot_mode_active();
        struct clocksource *best, *cs;

        /* Find the best suitable clocksource */
        best = clocksource_find_best(oneshot, skipcur);
        if (!best)
                return;

        if (!strlen(override_name))
                goto found;

        /* Check for the override clocksource. */
        list_for_each_entry(cs, &clocksource_list, list) {
                if (skipcur && cs == curr_clocksource)
                        continue;
                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) && oneshot) {
                        /* Override clocksource cannot be used. */
                        if (cs->flags & CLOCK_SOURCE_UNSTABLE) {
                                pr_warn("Override clocksource %s is unstable and not HRT compatible - cannot switch while in HRT/NOHZ mode\n",
                                        cs->name);
                                override_name[0] = 0;
                        } else {
                                /*
                                 * The override cannot be currently verified.
                                 * Deferring to let the watchdog check.
                                 */
                                pr_info("Override clocksource %s is not currently HRT compatible - deferring\n",
                                        cs->name);
                        }
                } else
                        /* Override clocksource can be used. */
                        best = cs;
                break;
        }

found:
        if (curr_clocksource != best && !timekeeping_notify(best)) {
                pr_info("Switched to clocksource %s\n", best->name);
                curr_clocksource = best;
        }
}

/**
 * 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)
{
        __clocksource_select(false);
}

static void clocksource_select_fallback(void)
{
        __clocksource_select(true);
}

#else /* !CONFIG_ARCH_USES_GETTIMEOFFSET */
static inline void clocksource_select(void) { }
static inline void clocksource_select_fallback(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();
        finished_booting = 1;
        /*
         * Run the watchdog first to eliminate unstable clock sources
         */
        __clocksource_watchdog_kthread();
        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)
                        break;
                entry = &tmp->list;
        }
        list_add(&cs->list, entry);
}

/**
 * __clocksource_update_freq_scale - Used update clocksource with new freq
 * @cs:         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_update_freq_hz() or __clocksource_update_freq_khz() helper
 * functions.
 */
void __clocksource_update_freq_scale(struct clocksource *cs, u32 scale, u32 freq)
{
        u64 sec;

        /*
         * Default clocksources are *special* and self-define their mult/shift.
         * But, you're not special, so you should specify a freq value.
         */
        if (freq) {
                /*
                 * Calc the maximum number of seconds which we can run before
                 * wrapping around. For clocksources which have a mask > 32-bit
                 * 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 >= 40-bit and f >= 4GHz. That maps to
                 * ~ 0.06ppm granularity for NTP.
                 */
                sec = cs->mask;
                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);
        }
        /*
         * Ensure clocksources that have large 'mult' values don't overflow
         * when adjusted.
         */
        cs->maxadj = clocksource_max_adjustment(cs);
        while (freq && ((cs->mult + cs->maxadj < cs->mult)
                || (cs->mult - cs->maxadj > cs->mult))) {
                cs->mult >>= 1;
                cs->shift--;
                cs->maxadj = clocksource_max_adjustment(cs);
        }

        /*
         * Only warn for *special* clocksources that self-define
         * their mult/shift values and don't specify a freq.
         */
        WARN_ONCE(cs->mult + cs->maxadj < cs->mult,
                "timekeeping: Clocksource %s might overflow on 11%% adjustment\n",
                cs->name);

        clocksource_update_max_deferment(cs);

        pr_info("%s: mask: 0x%llx max_cycles: 0x%llx, max_idle_ns: %lld ns\n",
                cs->name, cs->mask, cs->max_cycles, cs->max_idle_ns);
}
EXPORT_SYMBOL_GPL(__clocksource_update_freq_scale);

/**
 * __clocksource_register_scale - Used to install new clocksources
 * @cs:         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)
{
        unsigned long flags;

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

        /* Add clocksource to the clocksource list */
        mutex_lock(&clocksource_mutex);

        clocksource_watchdog_lock(&flags);
        clocksource_enqueue(cs);
        clocksource_enqueue_watchdog(cs);
        clocksource_watchdog_unlock(&flags);

        clocksource_select();
        clocksource_select_watchdog(false);
        __clocksource_suspend_select(cs);
        mutex_unlock(&clocksource_mutex);
        return 0;
}
EXPORT_SYMBOL_GPL(__clocksource_register_scale);

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

/**
 * clocksource_change_rating - Change the rating of a registered clocksource
 * @cs:         clocksource to be changed
 * @rating:     new rating
 */
void clocksource_change_rating(struct clocksource *cs, int rating)
{
        unsigned long flags;

        mutex_lock(&clocksource_mutex);
        clocksource_watchdog_lock(&flags);
        __clocksource_change_rating(cs, rating);
        clocksource_watchdog_unlock(&flags);

        clocksource_select();
        clocksource_select_watchdog(false);
        clocksource_suspend_select(false);
        mutex_unlock(&clocksource_mutex);
}
EXPORT_SYMBOL(clocksource_change_rating);

/*
 * Unbind clocksource @cs. Called with clocksource_mutex held
 */
static int clocksource_unbind(struct clocksource *cs)
{
        unsigned long flags;

        if (clocksource_is_watchdog(cs)) {
                /* Select and try to install a replacement watchdog. */
                clocksource_select_watchdog(true);
                if (clocksource_is_watchdog(cs))
                        return -EBUSY;
        }

        if (cs == curr_clocksource) {
                /* Select and try to install a replacement clock source */
                clocksource_select_fallback();
                if (curr_clocksource == cs)
                        return -EBUSY;
        }

        if (clocksource_is_suspend(cs)) {
                /*
                 * Select and try to install a replacement suspend clocksource.
                 * If no replacement suspend clocksource, we will just let the
                 * clocksource go and have no suspend clocksource.
                 */
                clocksource_suspend_select(true);
        }

        clocksource_watchdog_lock(&flags);
        clocksource_dequeue_watchdog(cs);
        list_del_init(&cs->list);
        clocksource_watchdog_unlock(&flags);

        return 0;
}

/**
 * clocksource_unregister - remove a registered clocksource
 * @cs: clocksource to be unregistered
 */
int clocksource_unregister(struct clocksource *cs)
{
        int ret = 0;

        mutex_lock(&clocksource_mutex);
        if (!list_empty(&cs->list))
                ret = clocksource_unbind(cs);
        mutex_unlock(&clocksource_mutex);
        return ret;
}
EXPORT_SYMBOL(clocksource_unregister);

#ifdef CONFIG_SYSFS
/**
 * current_clocksource_show - sysfs interface for current clocksource
 * @dev:        unused
 * @attr:       unused
 * @buf:        char buffer to be filled with clocksource list
 *
 * Provides sysfs interface for listing current clocksource.
 */
static ssize_t current_clocksource_show(struct device *dev,
                                        struct device_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;
}

ssize_t sysfs_get_uname(const char *buf, char *dst, size_t cnt)
{
        size_t ret = cnt;

        /* strings from sysfs write are not 0 terminated! */
        if (!cnt || cnt >= CS_NAME_LEN)
                return -EINVAL;

        /* strip of \n: */
        if (buf[cnt-1] == '\n')
                cnt--;
        if (cnt > 0)
                memcpy(dst, buf, cnt);
        dst[cnt] = 0;
        return ret;
}

/**
 * current_clocksource_store - interface for manually overriding clocksource
 * @dev:        unused
 * @attr:       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 current_clocksource_store(struct device *dev,
                                         struct device_attribute *attr,
                                         const char *buf, size_t count)
{
        ssize_t ret;

        mutex_lock(&clocksource_mutex);

        ret = sysfs_get_uname(buf, override_name, count);
        if (ret >= 0)
                clocksource_select();

        mutex_unlock(&clocksource_mutex);

        return ret;
}
static DEVICE_ATTR_RW(current_clocksource);

/**
 * unbind_clocksource_store - interface for manually unbinding clocksource
 * @dev:        unused
 * @attr:       unused
 * @buf:        unused
 * @count:      length of buffer
 *
 * Takes input from sysfs interface for manually unbinding a clocksource.
 */
static ssize_t unbind_clocksource_store(struct device *dev,
                                        struct device_attribute *attr,
                                        const char *buf, size_t count)
{
        struct clocksource *cs;
        char name[CS_NAME_LEN];
        ssize_t ret;

        ret = sysfs_get_uname(buf, name, count);
        if (ret < 0)
                return ret;

        ret = -ENODEV;
        mutex_lock(&clocksource_mutex);
        list_for_each_entry(cs, &clocksource_list, list) {
                if (strcmp(cs->name, name))
                        continue;
                ret = clocksource_unbind(cs);
                break;
        }
        mutex_unlock(&clocksource_mutex);

        return ret ? ret : count;
}
static DEVICE_ATTR_WO(unbind_clocksource);

/**
 * available_clocksource_show - sysfs interface for listing clocksource
 * @dev:        unused
 * @attr:       unused
 * @buf:        char buffer to be filled with clocksource list
 *
 * Provides sysfs interface for listing registered clocksources
 */
static ssize_t available_clocksource_show(struct device *dev,
                                          struct device_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;
}
static DEVICE_ATTR_RO(available_clocksource);

static struct attribute *clocksource_attrs[] = {
        &dev_attr_current_clocksource.attr,
        &dev_attr_unbind_clocksource.attr,
        &dev_attr_available_clocksource.attr,
        NULL
};
ATTRIBUTE_GROUPS(clocksource);

static struct bus_type clocksource_subsys = {
        .name = "clocksource",
        .dev_name = "clocksource",
};

static struct device device_clocksource = {
        .id     = 0,
        .bus    = &clocksource_subsys,
        .groups = clocksource_groups,
};

static int __init init_clocksource_sysfs(void)
{
        int error = subsys_system_register(&clocksource_subsys, NULL);

        if (!error)
                error = device_register(&device_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")) {
                pr_warn("clock=pmtmr is deprecated - use clocksource=acpi_pm\n");
                return boot_override_clocksource("acpi_pm");
        }
        pr_warn("clock= boot option is deprecated - use clocksource=xyz\n");
        return boot_override_clocksource(str);
}

__setup("clock=", boot_override_clock);