/************************************************************************ 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	((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 */	.shift		= JIFFIES_SHIFT,}; #if (BITS_PER_LONG < 64)u64 get_jiffies_64(void){	unsigned long seq;	u64 ret; 	do {		seq = read_seqbegin(&xtime_lock);		ret = jiffies_64;	} while (read_seqretry(&xtime_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;}