LXR linux/kernel/time/jiffies.c

   1/***********************************************************************
   2* linux/kernel/time/jiffies.c
   3*
   4* This file contains the jiffies based clocksource.
   5*
   6* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
   7*
   8* This program is free software; you can redistribute it and/or modify
   9* it under the terms of the GNU General Public License as published by
  10* the Free Software Foundation; either version 2 of the License, or
  11* (at your option) any later version.
  12*
  13* This program is distributed in the hope that it will be useful,
  14* but WITHOUT ANY WARRANTY; without even the implied warranty of
  15* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16* GNU General Public License for more details.
  17*
  18* You should have received a copy of the GNU General Public License
  19* along with this program; if not, write to the Free Software
  20* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  21*
  22************************************************************************/
  23#include <linux/clocksource.h>
  24#include <linux/jiffies.h>
  25#include <linux/module.h>
  26#include <linux/init.h>
  27
  28#include "timekeeping.h"
  29
  30
  31/* Since jiffies uses a simple TICK_NSEC multiplier
  32 * conversion, the .shift value could be zero. However
  33 * this would make NTP adjustments impossible as they are
  34 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
  35 * shift both the nominator and denominator the same
  36 * amount, and give ntp adjustments in units of 1/2^8
  37 *
  38 * The value 8 is somewhat carefully chosen, as anything
  39 * larger can result in overflows. TICK_NSEC grows as HZ
  40 * shrinks, so values greater than 8 overflow 32bits when
  41 * HZ=100.
  42 */
  43#if HZ < 34
  44#define JIFFIES_SHIFT   6
  45#elif HZ < 67
  46#define JIFFIES_SHIFT   7
  47#else
  48#define JIFFIES_SHIFT   8
  49#endif
  50
  51static u64 jiffies_read(struct clocksource *cs)
  52{
  53        return (u64) jiffies;
  54}
  55
  56/*
  57 * The Jiffies based clocksource is the lowest common
  58 * denominator clock source which should function on
  59 * all systems. It has the same coarse resolution as
  60 * the timer interrupt frequency HZ and it suffers
  61 * inaccuracies caused by missed or lost timer
  62 * interrupts and the inability for the timer
  63 * interrupt hardware to accuratly tick at the
  64 * requested HZ value. It is also not recommended
  65 * for "tick-less" systems.
  66 */
  67static struct clocksource clocksource_jiffies = {
  68        .name           = "jiffies",
  69        .rating         = 1, /* lowest valid rating*/
  70        .read           = jiffies_read,
  71        .mask           = CLOCKSOURCE_MASK(32),
  72        .mult           = TICK_NSEC << JIFFIES_SHIFT, /* details above */
  73        .shift          = JIFFIES_SHIFT,
  74        .max_cycles     = 10,
  75};
  76
  77__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
  78
  79#if (BITS_PER_LONG < 64)
  80u64 get_jiffies_64(void)
  81{
  82        unsigned long seq;
  83        u64 ret;
  84
  85        do {
  86                seq = read_seqbegin(&jiffies_lock);
  87                ret = jiffies_64;
  88        } while (read_seqretry(&jiffies_lock, seq));
  89        return ret;
  90}
  91EXPORT_SYMBOL(get_jiffies_64);
  92#endif
  93
  94EXPORT_SYMBOL(jiffies);
  95
  96static int __init init_jiffies_clocksource(void)
  97{
  98        return __clocksource_register(&clocksource_jiffies);
  99}
 100
 101core_initcall(init_jiffies_clocksource);
 102
 103struct clocksource * __init __weak clocksource_default_clock(void)
 104{
 105        return &clocksource_jiffies;
 106}
 107
 108struct clocksource refined_jiffies;
 109
 110int register_refined_jiffies(long cycles_per_second)
 111{
 112        u64 nsec_per_tick, shift_hz;
 113        long cycles_per_tick;
 114
 115
 116
 117        refined_jiffies = clocksource_jiffies;
 118        refined_jiffies.name = "refined-jiffies";
 119        refined_jiffies.rating++;
 120
 121        /* Calc cycles per tick */
 122        cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
 123        /* shift_hz stores hz<<8 for extra accuracy */
 124        shift_hz = (u64)cycles_per_second << 8;
 125        shift_hz += cycles_per_tick/2;
 126        do_div(shift_hz, cycles_per_tick);
 127        /* Calculate nsec_per_tick using shift_hz */
 128        nsec_per_tick = (u64)NSEC_PER_SEC << 8;
 129        nsec_per_tick += (u32)shift_hz/2;
 130        do_div(nsec_per_tick, (u32)shift_hz);
 131
 132        refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
 133
 134        __clocksource_register(&refined_jiffies);
 135        return 0;
 136}
 137