1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * This file contains the jiffies based clocksource. 4 * 5 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com) 6 */ 7 #include <linux/clocksource.h> 8 #include <linux/jiffies.h> 9 #include <linux/module.h> 10 #include <linux/init.h> 11 12 #include "timekeeping.h" 13 14 15 /* Since jiffies uses a simple TICK_NSEC multiplier 16 * conversion, the .shift value could be zero. However 17 * this would make NTP adjustments impossible as they are 18 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to 19 * shift both the nominator and denominator the same 20 * amount, and give ntp adjustments in units of 1/2^8 21 * 22 * The value 8 is somewhat carefully chosen, as anything 23 * larger can result in overflows. TICK_NSEC grows as HZ 24 * shrinks, so values greater than 8 overflow 32bits when 25 * HZ=100. 26 */ 27 #if HZ < 34 28 #define JIFFIES_SHIFT 6 29 #elif HZ < 67 30 #define JIFFIES_SHIFT 7 31 #else 32 #define JIFFIES_SHIFT 8 33 #endif 34 35 static u64 jiffies_read(struct clocksource *cs) 36 { 37 return (u64) jiffies; 38 } 39 40 /* 41 * The Jiffies based clocksource is the lowest common 42 * denominator clock source which should function on 43 * all systems. It has the same coarse resolution as 44 * the timer interrupt frequency HZ and it suffers 45 * inaccuracies caused by missed or lost timer 46 * interrupts and the inability for the timer 47 * interrupt hardware to accuratly tick at the 48 * requested HZ value. It is also not recommended 49 * for "tick-less" systems. 50 */ 51 static struct clocksource clocksource_jiffies = { 52 .name = "jiffies", 53 .rating = 1, /* lowest valid rating*/ 54 .read = jiffies_read, 55 .mask = CLOCKSOURCE_MASK(32), 56 .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */ 57 .shift = JIFFIES_SHIFT, 58 .max_cycles = 10, 59 }; 60 61 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); 62 63 #if (BITS_PER_LONG < 64) 64 u64 get_jiffies_64(void) 65 { 66 unsigned long seq; 67 u64 ret; 68 69 do { 70 seq = read_seqbegin(&jiffies_lock); 71 ret = jiffies_64; 72 } while (read_seqretry(&jiffies_lock, seq)); 73 return ret; 74 } 75 EXPORT_SYMBOL(get_jiffies_64); 76 #endif 77 78 EXPORT_SYMBOL(jiffies); 79 80 static int __init init_jiffies_clocksource(void) 81 { 82 return __clocksource_register(&clocksource_jiffies); 83 } 84 85 core_initcall(init_jiffies_clocksource); 86 87 struct clocksource * __init __weak clocksource_default_clock(void) 88 { 89 return &clocksource_jiffies; 90 } 91 92 static struct clocksource refined_jiffies; 93 94 int register_refined_jiffies(long cycles_per_second) 95 { 96 u64 nsec_per_tick, shift_hz; 97 long cycles_per_tick; 98 99 100 101 refined_jiffies = clocksource_jiffies; 102 refined_jiffies.name = "refined-jiffies"; 103 refined_jiffies.rating++; 104 105 /* Calc cycles per tick */ 106 cycles_per_tick = (cycles_per_second + HZ/2)/HZ; 107 /* shift_hz stores hz<<8 for extra accuracy */ 108 shift_hz = (u64)cycles_per_second << 8; 109 shift_hz += cycles_per_tick/2; 110 do_div(shift_hz, cycles_per_tick); 111 /* Calculate nsec_per_tick using shift_hz */ 112 nsec_per_tick = (u64)NSEC_PER_SEC << 8; 113 nsec_per_tick += (u32)shift_hz/2; 114 do_div(nsec_per_tick, (u32)shift_hz); 115 116 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT; 117 118 __clocksource_register(&refined_jiffies); 119 return 0; 120 } 121