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* linux/kernel/time/jiffies.c
* This file contains the jiffies based clocksource.
* Copyright (C) 2004, 2005 IBM, John Stultz (
* 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
* 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.
#include <linux/clocksource.h>
#include <linux/jiffies.h>
#include <linux/init.h>
/* 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 reccomended
* for "tick-less" systems.
#define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
/* Since jiffies uses a simple NSEC_PER_JIFFY 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. NSEC_PER_JIFFY grows as
* HZ shrinks, so values greater than 8 overflow 32bits when
* HZ=100.
static cycle_t jiffies_read(struct clocksource *cs)
return (cycle_t) jiffies;
struct clocksource clocksource_jiffies = {
.name = "jiffies",
.rating = 1, /* lowest valid rating*/
.read = jiffies_read,
.mask = 0xffffffff, /*32bits*/
.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
static int __init init_jiffies_clocksource(void)
return clocksource_register(&clocksource_jiffies);
struct clocksource * __init __weak clocksource_default_clock(void)
return &clocksource_jiffies;