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(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(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(timecounter_cyc2time);

/*[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 cycle_t watchdog_last;
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;

	wdnow = watchdog->read(watchdog);
	wd_nsec = clocksource_cyc2ns((wdnow - watchdog_last) & watchdog->mask,
				     watchdog->mult, watchdog->shift);
	watchdog_last = wdnow;

	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;
		}

		csnow = cs->read(cs);

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

		/* Check the deviation from the watchdog clocksource. */
		cs_nsec = clocksource_cyc2ns((csnow - cs->wd_last) &
					     cs->mask, cs->mult, cs->shift);
		cs->wd_last = csnow;
		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_last = watchdog->read(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;

	spin_lock_irqsave(&watchdog_lock, flags);
	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_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();

	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.
 *
 */
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);
}

#ifdef CONFIG_GENERIC_TIME

/**
 * 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_GENERIC_TIME */

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_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_select();
	clocksource_enqueue_watchdog(cs);
	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 selction.
 */
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);