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 #include "tick-internal.h"
  14
  15
  16 static u64 jiffies_read(struct clocksource *cs)
  17 {
  18         return (u64) jiffies;
  19 }
  20
  21 /*
  22  * The Jiffies based clocksource is the lowest common
  23  * denominator clock source which should function on
  24  * all systems. It has the same coarse resolution as
  25  * the timer interrupt frequency HZ and it suffers
  26  * inaccuracies caused by missed or lost timer
  27  * interrupts and the inability for the timer
  28  * interrupt hardware to accurately tick at the
  29  * requested HZ value. It is also not recommended
  30  * for "tick-less" systems.
  31  */
  32 static struct clocksource clocksource_jiffies = {
  33         .name                   = "jiffies",
  34         .rating                 = 1, /* lowest valid rating*/
  35         .uncertainty_margin     = 32 * NSEC_PER_MSEC,
  36         .read                   = jiffies_read,
  37         .mask                   = CLOCKSOURCE_MASK(32),
  38         .mult                   = TICK_NSEC << JIFFIES_SHIFT, /* details above */
  39         .shift                  = JIFFIES_SHIFT,
  40         .max_cycles             = 10,
  41 };
  42
  43 __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
  44 __cacheline_aligned_in_smp seqcount_raw_spinlock_t jiffies_seq =
  45         SEQCNT_RAW_SPINLOCK_ZERO(jiffies_seq, &jiffies_lock);
  46
  47 #if (BITS_PER_LONG < 64)
  48 u64 get_jiffies_64(void)
  49 {
  50         unsigned int seq;
  51         u64 ret;
  52
  53         do {
  54                 seq = read_seqcount_begin(&jiffies_seq);
  55                 ret = jiffies_64;
  56         } while (read_seqcount_retry(&jiffies_seq, seq));
  57         return ret;
  58 }
  59 EXPORT_SYMBOL(get_jiffies_64);
  60 #endif
  61
  62 EXPORT_SYMBOL(jiffies);
  63
  64 static int __init init_jiffies_clocksource(void)
  65 {
  66         return __clocksource_register(&clocksource_jiffies);
  67 }
  68
  69 core_initcall(init_jiffies_clocksource);
  70
  71 struct clocksource * __init __weak clocksource_default_clock(void)
  72 {
  73         return &clocksource_jiffies;
  74 }
  75
  76 static struct clocksource refined_jiffies;
  77
  78 int register_refined_jiffies(long cycles_per_second)
  79 {
  80         u64 nsec_per_tick, shift_hz;
  81         long cycles_per_tick;
  82
  83
  84
  85         refined_jiffies = clocksource_jiffies;
  86         refined_jiffies.name = "refined-jiffies";
  87         refined_jiffies.rating++;
  88
  89         /* Calc cycles per tick */
  90         cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
  91         /* shift_hz stores hz<<8 for extra accuracy */
  92         shift_hz = (u64)cycles_per_second << 8;
  93         shift_hz += cycles_per_tick/2;
  94         do_div(shift_hz, cycles_per_tick);
  95         /* Calculate nsec_per_tick using shift_hz */
  96         nsec_per_tick = (u64)NSEC_PER_SEC << 8;
  97         nsec_per_tick += (u32)shift_hz/2;
  98         do_div(nsec_per_tick, (u32)shift_hz);
  99
 100         refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
 101
 102         __clocksource_register(&refined_jiffies);
 103         return 0;
 104 }