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 #include "timekeeping.h" 29 30 /* The Jiffies based clocksource is the lowest common 31 * denominator clock source which should function on 32 * all systems. It has the same coarse resolution as 33 * the timer interrupt frequency HZ and it suffers 34 * inaccuracies caused by missed or lost timer 35 * interrupts and the inability for the timer 36 * interrupt hardware to accuratly tick at the 37 * requested HZ value. It is also not recommended 38 * for "tick-less" systems. 39 */ 40 #define NSEC_PER_JIFFY ((NSEC_PER_SEC+HZ/2)/HZ) 41 42 /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier 43 * conversion, the .shift value could be zero. However 44 * this would make NTP adjustments impossible as they are 45 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to 46 * shift both the nominator and denominator the same 47 * amount, and give ntp adjustments in units of 1/2^8 48 * 49 * The value 8 is somewhat carefully chosen, as anything 50 * larger can result in overflows. NSEC_PER_JIFFY grows as 51 * HZ shrinks, so values greater than 8 overflow 32bits when 52 * HZ=100. 53 */ 54 #if HZ < 34 55 #define JIFFIES_SHIFT 6 56 #elif HZ < 67 57 #define JIFFIES_SHIFT 7 58 #else 59 #define JIFFIES_SHIFT 8 60 #endif 61 62 static cycle_t jiffies_read(struct clocksource *cs) 63 { 64 return (cycle_t) jiffies; 65 } 66 67 static struct clocksource clocksource_jiffies = { 68 .name = "jiffies", 69 .rating = 1, /* lowest valid rating*/ 70 .read = jiffies_read, 71 .mask = 0xffffffff, /*32bits*/ 72 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ 73 .shift = JIFFIES_SHIFT, 74 .max_cycles = 10, 75 }; 76 77 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); 78 79 #if (BITS_PER_LONG < 64) 80 u64 get_jiffies_64(void) 81 { 82 unsigned long seq; 83 u64 ret; 84 85 do { 86 seq = read_seqbegin(&jiffies_lock); 87 ret = jiffies_64; 88 } while (read_seqretry(&jiffies_lock, seq)); 89 return ret; 90 } 91 EXPORT_SYMBOL(get_jiffies_64); 92 #endif 93 94 EXPORT_SYMBOL(jiffies); 95 96 static int __init init_jiffies_clocksource(void) 97 { 98 return __clocksource_register(&clocksource_jiffies); 99 } 100 101 core_initcall(init_jiffies_clocksource); 102 103 struct clocksource * __init __weak clocksource_default_clock(void) 104 { 105 return &clocksource_jiffies; 106 } 107 108 struct clocksource refined_jiffies; 109 110 int register_refined_jiffies(long cycles_per_second) 111 { 112 u64 nsec_per_tick, shift_hz; 113 long cycles_per_tick; 114 115 116 117 refined_jiffies = clocksource_jiffies; 118 refined_jiffies.name = "refined-jiffies"; 119 refined_jiffies.rating++; 120 121 /* Calc cycles per tick */ 122 cycles_per_tick = (cycles_per_second + HZ/2)/HZ; 123 /* shift_hz stores hz<<8 for extra accuracy */ 124 shift_hz = (u64)cycles_per_second << 8; 125 shift_hz += cycles_per_tick/2; 126 do_div(shift_hz, cycles_per_tick); 127 /* Calculate nsec_per_tick using shift_hz */ 128 nsec_per_tick = (u64)NSEC_PER_SEC << 8; 129 nsec_per_tick += (u32)shift_hz/2; 130 do_div(nsec_per_tick, (u32)shift_hz); 131 132 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT; 133 134 __clocksource_register(&refined_jiffies); 135 return 0; 136 } 137