kernel/time/jiffies.c

   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 int 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 }