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 "tick-internal.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  ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
  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#define JIFFIES_SHIFT   8
  55
  56static cycle_t jiffies_read(struct clocksource *cs)
  57{
  58        return (cycle_t) jiffies;
  59}
  60
  61struct clocksource clocksource_jiffies = {
  62        .name           = "jiffies",
  63        .rating         = 1, /* lowest valid rating*/
  64        .read           = jiffies_read,
  65        .mask           = 0xffffffff, /*32bits*/
  66        .mult           = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
  67        .shift          = JIFFIES_SHIFT,
  68};
  69
  70#if (BITS_PER_LONG < 64)
  71u64 get_jiffies_64(void)
  72{
  73        unsigned long seq;
  74        u64 ret;
  75
  76        do {
  77                seq = read_seqbegin(&xtime_lock);
  78                ret = jiffies_64;
  79        } while (read_seqretry(&xtime_lock, seq));
  80        return ret;
  81}
  82EXPORT_SYMBOL(get_jiffies_64);
  83#endif
  84
  85EXPORT_SYMBOL(jiffies);
  86
  87static int __init init_jiffies_clocksource(void)
  88{
  89        return clocksource_register(&clocksource_jiffies);
  90}
  91
  92core_initcall(init_jiffies_clocksource);
  93
  94struct clocksource * __init __weak clocksource_default_clock(void)
  95{
  96        return &clocksource_jiffies;
  97}
  98