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 (johnstul@us.ibm.com) 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 48 * HZ shrinks, so values greater then 8 overflow 32bits when 49 * HZ=100. 50 */ 51 #define JIFFIES_SHIFT 8 52 53 static cycle_t jiffies_read(void) 54 { 55 return (cycle_t) jiffies; 56 } 57 58 struct clocksource clocksource_jiffies = { 59 .name = "jiffies", 60 .rating = 1, /* lowest valid rating*/ 61 .read = jiffies_read, 62 .mask = 0xffffffff, /*32bits*/ 63 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ 64 .shift = JIFFIES_SHIFT, 65 }; 66 67 static int __init init_jiffies_clocksource(void) 68 { 69 return clocksource_register(&clocksource_jiffies); 70 } 71 72 core_initcall(init_jiffies_clocksource); 73