linux/arch/arm/kernel/sched_clock.c

/*
 * sched_clock.c: support for extending counters to full 64-bit ns counter
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#include <linux/clocksource.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/timer.h>

#include <asm/sched_clock.h>

static void sched_clock_poll(unsigned long wrap_ticks);
static DEFINE_TIMER(sched_clock_timer, sched_clock_poll, 0, 0);
static void (*sched_clock_update_fn)(void);

static void sched_clock_poll(unsigned long wrap_ticks)
{
	mod_timer(&sched_clock_timer, round_jiffies(jiffies + wrap_ticks));
	sched_clock_update_fn();
}

void __init init_sched_clock(struct clock_data *cd, void (*update)(void),
	unsigned int clock_bits, unsigned long rate)
{
	unsigned long r, w;
	u64 res, wrap;
	char r_unit;

	sched_clock_update_fn = update;

	/* calculate the mult/shift to convert counter ticks to ns. */
	clocks_calc_mult_shift(&cd->mult, &cd->shift, rate, NSEC_PER_SEC, 60);

	r = rate;
	if (r >= 4000000) {
		r /= 1000000;
		r_unit = 'M';
	} else {
		r /= 1000;
		r_unit = 'k';
	}

	/* calculate how many ns until we wrap */
	wrap = cyc_to_ns((1ULL << clock_bits) - 1, cd->mult, cd->shift);
	do_div(wrap, NSEC_PER_MSEC);
	w = wrap;

	/* calculate the ns resolution of this counter */
	res = cyc_to_ns(1ULL, cd->mult, cd->shift);
	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lums\n",
		clock_bits, r, r_unit, res, w);

	/*
	 * Start the timer to keep sched_clock() properly updated and
	 * sets the initial epoch.
	 */
	sched_clock_timer.data = msecs_to_jiffies(w - (w / 10));
	sched_clock_poll(sched_clock_timer.data);

	/*
	 * Ensure that sched_clock() starts off at 0ns
	 */
	cd->epoch_ns = 0;
}