text: /*
   * 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/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;
  }
  
  /*[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[CS_NAME_LEN];
  static int finished_booting;
  
  #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 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, 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);
  		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)) {
  			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)) {
  			/* 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;
  	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)
  {
  	atomic_inc(&watchdog_reset_pending);
  }
  
  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)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&watchdog_lock, flags);
  	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();
  		}
  	}
  	spin_unlock_irqrestore(&watchdog_lock, flags);
  }
  
  static int __clocksource_watchdog_kthread(void)
  {
  	struct clocksource *cs, *tmp;
  	unsigned long flags;
  	LIST_HEAD(unstable);
  	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);
  			list_add(&cs->wd_list, &unstable);
  			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);
  
  	/* 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);
  	}
  	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 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) { }
  
  #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_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
   */
  u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask)
  {
  	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/(mult+maxadj)
  	 * which is equivalent to the below.
  	 * max_cycles < (2^63)/(mult + maxadj)
  	 * max_cycles < 2^(log2((2^63)/(mult + maxadj)))
  	 * max_cycles < 2^(log2(2^63) - log2(mult + maxadj))
  	 * max_cycles < 2^(63 - log2(mult + maxadj))
  	 * max_cycles < 1 << (63 - log2(mult + maxadj))
  	 * 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(mult + maxadj) + 1));
  
  	/*
  	 * 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 max_nsecs;
  }
  
  /**
   * 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_nsecs = clocks_calc_max_nsecs(cs->mult, cs->shift, cs->maxadj,
  					  cs->mask);
  	/*
  	 * 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 >> 3);
  }
  
  #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;
  
  	/* 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. */
  			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 && !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)
  {
  	return __clocksource_select(false);
  }
  
  static void clocksource_select_fallback(void)
  {
  	return __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)
  			entry = &tmp->list;
  	list_add(&cs->list, entry);
  }
  
  /**
   * __clocksource_updatefreq_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_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 >> 3));
  	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);
  
  	/*
  	 * for clocksources that have large mults, to avoid overflow.
  	 * Since mult may be adjusted by ntp, add an safety extra margin
  	 *
  	 */
  	cs->maxadj = clocksource_max_adjustment(cs);
  	while ((cs->mult + cs->maxadj < cs->mult)
  		|| (cs->mult - cs->maxadj > cs->mult)) {
  		cs->mult >>= 1;
  		cs->shift--;
  		cs->maxadj = clocksource_max_adjustment(cs);
  	}
  
  	cs->max_idle_ns = clocksource_max_deferment(cs);
  }
  EXPORT_SYMBOL_GPL(__clocksource_updatefreq_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)
  {
  
  	/* 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
   * @cs:		clocksource to be registered
   *
   * Returns -EBUSY if registration fails, zero otherwise.
   */
  int clocksource_register(struct clocksource *cs)
  {
  	/* calculate max adjustment for given mult/shift */
  	cs->maxadj = clocksource_max_adjustment(cs);
  	WARN_ONCE(cs->mult + cs->maxadj < cs->mult,
  		"Clocksource %s might overflow on 11%% adjustment\n",
  		cs->name);
  
  	/* 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_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)
  {
  	mutex_lock(&clocksource_mutex);
  	__clocksource_change_rating(cs, rating);
  	clocksource_select();
  	mutex_unlock(&clocksource_mutex);
  }
  EXPORT_SYMBOL(clocksource_change_rating);
  
  /*
   * Unbind clocksource @cs. Called with clocksource_mutex held
   */
  static int clocksource_unbind(struct clocksource *cs)
  {
  	/*
  	 * I really can't convince myself to support this on hardware
  	 * designed by lobotomized monkeys.
  	 */
  	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;
  	}
  	clocksource_dequeue_watchdog(cs);
  	list_del_init(&cs->list);
  	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
  /**
   * sysfs_show_current_clocksources - 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
  sysfs_show_current_clocksources(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;
  }
  
  /**
   * sysfs_override_clocksource - 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 sysfs_override_clocksource(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;
  }
  
  /**
   * sysfs_unbind_current_clocksource - 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 sysfs_unbind_clocksource(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;
  }
  
  /**
   * sysfs_show_available_clocksources - 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
  sysfs_show_available_clocksources(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;
  }
  
  /*
   * Sysfs setup bits:
   */
  static DEVICE_ATTR(current_clocksource, 0644, sysfs_show_current_clocksources,
  		   sysfs_override_clocksource);
  
  static DEVICE_ATTR(unbind_clocksource, 0200, NULL, sysfs_unbind_clocksource);
  
  static DEVICE_ATTR(available_clocksource, 0444,
  		   sysfs_show_available_clocksources, NULL);
  
  static struct bus_type clocksource_subsys = {
  	.name = "clocksource",
  	.dev_name = "clocksource",
  };
  
  static struct device device_clocksource = {
  	.id	= 0,
  	.bus	= &clocksource_subsys,
  };
  
  static int __init init_clocksource_sysfs(void)
  {
  	int error = subsys_system_register(&clocksource_subsys, NULL);
  
  	if (!error)
  		error = device_register(&device_clocksource);
  	if (!error)
  		error = device_create_file(
  				&device_clocksource,
  				&dev_attr_current_clocksource);
  	if (!error)
  		error = device_create_file(&device_clocksource,
  					   &dev_attr_unbind_clocksource);
  	if (!error)
  		error = device_create_file(
  				&device_clocksource,
  				&dev_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);
  