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_RAW_SPINLOCK(jiffies_lock); 62 __cacheline_aligned_in_smp seqcount_t jiffies_seq; 63 64 #if (BITS_PER_LONG < 64) 65 u64 get_jiffies_64(void) 66 { 67 unsigned int seq; 68 u64 ret; 69 70 do { 71 seq = read_seqcount_begin(&jiffies_seq); 72 ret = jiffies_64; 73 } while (read_seqcount_retry(&jiffies_seq, seq)); 74 return ret; 75 } 76 EXPORT_SYMBOL(get_jiffies_64); 77 #endif 78 79 EXPORT_SYMBOL(jiffies); 80 81 static int __init init_jiffies_clocksource(void) 82 { 83 return __clocksource_register(&clocksource_jiffies); 84 } 85 86 core_initcall(init_jiffies_clocksource); 87 88 struct clocksource * __init __weak clocksource_default_clock(void) 89 { 90 return &clocksource_jiffies; 91 } 92 93 static struct clocksource refined_jiffies; 94 95 int register_refined_jiffies(long cycles_per_second) 96 { 97 u64 nsec_per_tick, shift_hz; 98 long cycles_per_tick; 99 100 101 102 refined_jiffies = clocksource_jiffies; 103 refined_jiffies.name = "refined-jiffies"; 104 refined_jiffies.rating++; 105 106 /* Calc cycles per tick */ 107 cycles_per_tick = (cycles_per_second + HZ/2)/HZ; 108 /* shift_hz stores hz<<8 for extra accuracy */ 109 shift_hz = (u64)cycles_per_second << 8; 110 shift_hz += cycles_per_tick/2; 111 do_div(shift_hz, cycles_per_tick); 112 /* Calculate nsec_per_tick using shift_hz */ 113 nsec_per_tick = (u64)NSEC_PER_SEC << 8; 114 nsec_per_tick += (u32)shift_hz/2; 115 do_div(nsec_per_tick, (u32)shift_hz); 116 117 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT; 118 119 __clocksource_register(&refined_jiffies); 120 return 0; 121 } 122