linux/kernel/time/jiffies.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * This file contains the jiffies based clocksource.
 *
 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
 */
#include <linux/clocksource.h>
#include <linux/jiffies.h>
#include <linux/module.h>
#include <linux/init.h>

#include "timekeeping.h"


/* Since jiffies uses a simple TICK_NSEC multiplier
 * conversion, the .shift value could be zero. However
 * this would make NTP adjustments impossible as they are
 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
 * shift both the nominator and denominator the same
 * amount, and give ntp adjustments in units of 1/2^8
 *
 * The value 8 is somewhat carefully chosen, as anything
 * larger can result in overflows. TICK_NSEC grows as HZ
 * shrinks, so values greater than 8 overflow 32bits when
 * HZ=100.
 */
#if HZ < 34
#define JIFFIES_SHIFT	6
#elif HZ < 67
#define JIFFIES_SHIFT	7
#else
#define JIFFIES_SHIFT	8
#endif

static u64 jiffies_read(struct clocksource *cs)
{
	return (u64) jiffies;
}

/*
 * The Jiffies based clocksource is the lowest common
 * denominator clock source which should function on
 * all systems. It has the same coarse resolution as
 * the timer interrupt frequency HZ and it suffers
 * inaccuracies caused by missed or lost timer
 * interrupts and the inability for the timer
 * interrupt hardware to accuratly tick at the
 * requested HZ value. It is also not recommended
 * for "tick-less" systems.
 */
static struct clocksource clocksource_jiffies = {
	.name		= "jiffies",
	.rating		= 1, /* lowest valid rating*/
	.read		= jiffies_read,
	.mask		= CLOCKSOURCE_MASK(32),
	.mult		= TICK_NSEC << JIFFIES_SHIFT, /* details above */
	.shift		= JIFFIES_SHIFT,
	.max_cycles	= 10,
};

__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);

#if (BITS_PER_LONG < 64)
u64 get_jiffies_64(void)
{
	unsigned int seq;
	u64 ret;

	do {
		seq = read_seqbegin(&jiffies_lock);
		ret = jiffies_64;
	} while (read_seqretry(&jiffies_lock, seq));
	return ret;
}
EXPORT_SYMBOL(get_jiffies_64);
#endif

EXPORT_SYMBOL(jiffies);

static int __init init_jiffies_clocksource(void)
{
	return __clocksource_register(&clocksource_jiffies);
}

core_initcall(init_jiffies_clocksource);

struct clocksource * __init __weak clocksource_default_clock(void)
{
	return &clocksource_jiffies;
}

static struct clocksource refined_jiffies;

int register_refined_jiffies(long cycles_per_second)
{
	u64 nsec_per_tick, shift_hz;
	long cycles_per_tick;



	refined_jiffies = clocksource_jiffies;
	refined_jiffies.name = "refined-jiffies";
	refined_jiffies.rating++;

	/* Calc cycles per tick */
	cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
	/* shift_hz stores hz<<8 for extra accuracy */
	shift_hz = (u64)cycles_per_second << 8;
	shift_hz += cycles_per_tick/2;
	do_div(shift_hz, cycles_per_tick);
	/* Calculate nsec_per_tick using shift_hz */
	nsec_per_tick = (u64)NSEC_PER_SEC << 8;
	nsec_per_tick += (u32)shift_hz/2;
	do_div(nsec_per_tick, (u32)shift_hz);

	refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;

	__clocksource_register(&refined_jiffies);
	return 0;
}