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 "tick-internal.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 #define JIFFIES_SHIFT 8 55 56 static cycle_t jiffies_read(struct clocksource *cs) 57 { 58 return (cycle_t) jiffies; 59 } 60 61 static struct clocksource clocksource_jiffies = { 62 .name = "jiffies", 63 .rating = 1, /* lowest valid rating*/ 64 .read = jiffies_read, 65 .mask = 0xffffffff, /*32bits*/ 66 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ 67 .shift = JIFFIES_SHIFT, 68 }; 69 70 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); 71 72 #if (BITS_PER_LONG < 64) 73 u64 get_jiffies_64(void) 74 { 75 unsigned long seq; 76 u64 ret; 77 78 do { 79 seq = read_seqbegin(&jiffies_lock); 80 ret = jiffies_64; 81 } while (read_seqretry(&jiffies_lock, seq)); 82 return ret; 83 } 84 EXPORT_SYMBOL(get_jiffies_64); 85 #endif 86 87 EXPORT_SYMBOL(jiffies); 88 89 static int __init init_jiffies_clocksource(void) 90 { 91 return clocksource_register(&clocksource_jiffies); 92 } 93 94 core_initcall(init_jiffies_clocksource); 95 96 struct clocksource * __init __weak clocksource_default_clock(void) 97 { 98 return &clocksource_jiffies; 99 } 100 101 struct clocksource refined_jiffies; 102 103 int register_refined_jiffies(long cycles_per_second) 104 { 105 u64 nsec_per_tick, shift_hz; 106 long cycles_per_tick; 107 108 109 110 refined_jiffies = clocksource_jiffies; 111 refined_jiffies.name = "refined-jiffies"; 112 refined_jiffies.rating++; 113 114 /* Calc cycles per tick */ 115 cycles_per_tick = (cycles_per_second + HZ/2)/HZ; 116 /* shift_hz stores hz<<8 for extra accuracy */ 117 shift_hz = (u64)cycles_per_second << 8; 118 shift_hz += cycles_per_tick/2; 119 do_div(shift_hz, cycles_per_tick); 120 /* Calculate nsec_per_tick using shift_hz */ 121 nsec_per_tick = (u64)NSEC_PER_SEC << 8; 122 nsec_per_tick += (u32)shift_hz/2; 123 do_div(nsec_per_tick, (u32)shift_hz); 124 125 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT; 126 127 clocksource_register(&refined_jiffies); 128 return 0; 129 } 130