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 /* The Jiffies based clocksource is the lowest common
  31  * denominator clock source which should function on
  32  * all systems. It has the same coarse resolution as
  33  * the timer interrupt frequency HZ and it suffers
  34  * inaccuracies caused by missed or lost timer
  35  * interrupts and the inability for the timer
  36  * interrupt hardware to accuratly tick at the
  37  * requested HZ value. It is also not recommended
  38  * for "tick-less" systems.
  39  */
  40 #define NSEC_PER_JIFFY  ((NSEC_PER_SEC+HZ/2)/HZ)
  41
  42 /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
  43  * conversion, the .shift value could be zero. However
  44  * this would make NTP adjustments impossible as they are
  45  * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
  46  * shift both the nominator and denominator the same
  47  * amount, and give ntp adjustments in units of 1/2^8
  48  *
  49  * The value 8 is somewhat carefully chosen, as anything
  50  * larger can result in overflows. NSEC_PER_JIFFY grows as
  51  * HZ shrinks, so values greater than 8 overflow 32bits when
  52  * HZ=100.
  53  */
  54 #if HZ < 34
  55 #define JIFFIES_SHIFT   6
  56 #elif HZ < 67
  57 #define JIFFIES_SHIFT   7
  58 #else
  59 #define JIFFIES_SHIFT   8
  60 #endif
  61
  62 static cycle_t jiffies_read(struct clocksource *cs)
  63 {
  64         return (cycle_t) jiffies;
  65 }
  66
  67 static struct clocksource clocksource_jiffies = {
  68         .name           = "jiffies",
  69         .rating         = 1, /* lowest valid rating*/
  70         .read           = jiffies_read,
  71         .mask           = CLOCKSOURCE_MASK(32),
  72         .mult           = NSEC_PER_JIFFY << 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)
  80 u64 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 }
  91 EXPORT_SYMBOL(get_jiffies_64);
  92 #endif
  93
  94 EXPORT_SYMBOL(jiffies);
  95
  96 static int __init init_jiffies_clocksource(void)
  97 {
  98         return __clocksource_register(&clocksource_jiffies);
  99 }
 100
 101 core_initcall(init_jiffies_clocksource);
 102
 103 struct clocksource * __init __weak clocksource_default_clock(void)
 104 {
 105         return &clocksource_jiffies;
 106 }
 107
 108 struct clocksource refined_jiffies;
 109
 110 int 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 }