LXR linux/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
  35static 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 */
  51static 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)
  65u64 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}
  76EXPORT_SYMBOL(get_jiffies_64);
  77#endif
  78
  79EXPORT_SYMBOL(jiffies);
  80
  81static int __init init_jiffies_clocksource(void)
  82{
  83        return __clocksource_register(&clocksource_jiffies);
  84}
  85
  86core_initcall(init_jiffies_clocksource);
  87
  88struct clocksource * __init __weak clocksource_default_clock(void)
  89{
  90        return &clocksource_jiffies;
  91}
  92
  93static struct clocksource refined_jiffies;
  94
  95int 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