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/module.h> 26 #include <linux/init.h> 27 28 /* The Jiffies based clocksource is the lowest common 29 * denominator clock source which should function on 30 * all systems. It has the same coarse resolution as 31 * the timer interrupt frequency HZ and it suffers 32 * inaccuracies caused by missed or lost timer 33 * interrupts and the inability for the timer 34 * interrupt hardware to accuratly tick at the 35 * requested HZ value. It is also not reccomended 36 * for "tick-less" systems. 37 */ 38 #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ)) 39 40 /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier 41 * conversion, the .shift value could be zero. However 42 * this would make NTP adjustments impossible as they are 43 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to 44 * shift both the nominator and denominator the same 45 * amount, and give ntp adjustments in units of 1/2^8 46 * 47 * The value 8 is somewhat carefully chosen, as anything 48 * larger can result in overflows. NSEC_PER_JIFFY grows as 49 * HZ shrinks, so values greater than 8 overflow 32bits when 50 * HZ=100. 51 */ 52 #define JIFFIES_SHIFT 8 53 54 static cycle_t jiffies_read(struct clocksource *cs) 55 { 56 return (cycle_t) jiffies; 57 } 58 59 struct clocksource clocksource_jiffies = { 60 .name = "jiffies", 61 .rating = 1, /* lowest valid rating*/ 62 .read = jiffies_read, 63 .mask = 0xffffffff, /*32bits*/ 64 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ 65 .shift = JIFFIES_SHIFT, 66 }; 67 68 #if (BITS_PER_LONG < 64) 69 u64 get_jiffies_64(void) 70 { 71 unsigned long seq; 72 u64 ret; 73 74 do { 75 seq = read_seqbegin(&xtime_lock); 76 ret = jiffies_64; 77 } while (read_seqretry(&xtime_lock, seq)); 78 return ret; 79 } 80 EXPORT_SYMBOL(get_jiffies_64); 81 #endif 82 83 EXPORT_SYMBOL(jiffies); 84 85 static int __init init_jiffies_clocksource(void) 86 { 87 return clocksource_register(&clocksource_jiffies); 88 } 89 90 core_initcall(init_jiffies_clocksource); 91 92 struct clocksource * __init __weak clocksource_default_clock(void) 93 { 94 return &clocksource_jiffies; 95 } 96