[linux.git] / kernel / time / jiffies.c

/***********************************************************************
* linux/kernel/time/jiffies.c
*
* This file contains the jiffies based clocksource.
*
* Copyright (C) 2004, 2005 IBM, John Stultz ([email protected])
*
* 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.
*
************************************************************************/
#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.
 */
#define JIFFIES_SHIFT	8

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 */
	.mult_orig	= NSEC_PER_JIFFY << JIFFIES_SHIFT,
	.shift		= JIFFIES_SHIFT,
};

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

core_initcall(init_jiffies_clocksource);
Commit	Line	Data
734efb46 JS	1	/***********************************************************************
	2	* linux/kernel/time/jiffies.c
	3	*
	4	* This file contains the jiffies based clocksource.
	5	*
	6	* Copyright (C) 2004, 2005 IBM, John Stultz ([email protected])
	7	*
	8	* This program is free software; you can redistribute it and/or modify
	9	* it under the terms of the GNU General Public License as published by
	10	* the Free Software Foundation; either version 2 of the License, or
	11	* (at your option) any later version.
	12	*
	13	* This program is distributed in the hope that it will be useful,
	14	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	15	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	16	* GNU General Public License for more details.
	17	*
	18	* You should have received a copy of the GNU General Public License
	19	* along with this program; if not, write to the Free Software
	20	* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
	21	*
	22	************************************************************************/
	23	#include <linux/clocksource.h>
	24	#include <linux/jiffies.h>
	25	#include <linux/init.h>
	26
	27	/* The Jiffies based clocksource is the lowest common
	28	* denominator clock source which should function on
	29	* all systems. It has the same coarse resolution as
	30	* the timer interrupt frequency HZ and it suffers
	31	* inaccuracies caused by missed or lost timer
	32	* interrupts and the inability for the timer
	33	* interrupt hardware to accuratly tick at the
	34	* requested HZ value. It is also not reccomended
	35	* for "tick-less" systems.
	36	*/
	37	#define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
	38
	39	/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
	40	* conversion, the .shift value could be zero. However
	41	* this would make NTP adjustments impossible as they are
	42	* in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
	43	* shift both the nominator and denominator the same
	44	* amount, and give ntp adjustments in units of 1/2^8
	45	*
	46	* The value 8 is somewhat carefully chosen, as anything
	47	* larger can result in overflows. NSEC_PER_JIFFY grows as
025dfdaf	48	* HZ shrinks, so values greater than 8 overflow 32bits when
734efb46 JS	49	* HZ=100.
	50	*/
	51	#define JIFFIES_SHIFT 8
	52
8e19608e	53	static cycle_t jiffies_read(struct clocksource *cs)
734efb46 JS	54	{
	55	return (cycle_t) jiffies;
	56	}
	57
	58	struct clocksource clocksource_jiffies = {
	59	.name = "jiffies",
3f4a0b91	60	.rating = 1, /* lowest valid rating*/
734efb46 JS	61	.read = jiffies_read,
	62	.mask = 0xffffffff, /32bits/
	63	.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
1aa5dfb7	64	.mult_orig = NSEC_PER_JIFFY << JIFFIES_SHIFT,
734efb46	65	.shift = JIFFIES_SHIFT,
734efb46 JS	66	};
	67
	68	static int __init init_jiffies_clocksource(void)
	69	{
a2752549	70	return clocksource_register(&clocksource_jiffies);
734efb46 JS	71	}
734efb46 JS	72
98de9e3b	73	core_initcall(init_jiffies_clocksource);