/*********************************************************************** * 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/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 then 8 overflow 32bits when * HZ=100. */ #define JIFFIES_SHIFT 8 static cycle_t jiffies_read(void) { 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, }; static int __init init_jiffies_clocksource(void) { return clocksource_register(&clocksource_jiffies); } core_initcall(init_jiffies_clocksource);