/************************************************************************ linux/kernel/time/jiffies.c** This file contains the jiffies based clocksource.** 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.*************************************************************************/#include <linux/clocksource.h>#include <linux/jiffies.h>#include <linux/module.h>#include <linux/init.h> #include "tick-internal.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 recommended * for "tick-less" systems. */#define NSEC_PER_JIFFY	((NSEC_PER_SEC+HZ/2)/HZ) /* 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;} static 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 */	.shift		= JIFFIES_SHIFT,}; __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); #if (BITS_PER_LONG < 64)u64 get_jiffies_64(void){	unsigned long 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;} 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;}