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/init.h>
  26
  27/* The Jiffies based clocksource is the lowest common
  28 * denominator clock source which should function on
  29 * all systems. It has the same coarse resolution as
  30 * the timer interrupt frequency HZ and it suffers
  31 * inaccuracies caused by missed or lost timer
  32 * interrupts and the inability for the timer
  33 * interrupt hardware to accuratly tick at the
  34 * requested HZ value. It is also not reccomended
  35 * for "tick-less" systems.
  36 */
  37#define NSEC_PER_JIFFY  ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
  38
  39/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
  40 * conversion, the .shift value could be zero. However
  41 * this would make NTP adjustments impossible as they are
  42 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
  43 * shift both the nominator and denominator the same
  44 * amount, and give ntp adjustments in units of 1/2^8
  45 *
  46 * The value 8 is somewhat carefully chosen, as anything
  47 * larger can result in overflows. NSEC_PER_JIFFY grows as
  48 * HZ shrinks, so values greater than 8 overflow 32bits when
  49 * HZ=100.
  50 */
  51#define JIFFIES_SHIFT   8
  52
  53static cycle_t jiffies_read(struct clocksource *cs)
  54{
  55        return (cycle_t) jiffies;
  56}
  57
  58struct clocksource clocksource_jiffies = {
  59        .name           = "jiffies",
  60        .rating         = 1, /* lowest valid rating*/
  61        .read           = jiffies_read,
  62        .mask           = 0xffffffff, /*32bits*/
  63        .mult           = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
  64        .shift          = JIFFIES_SHIFT,
  65};
  66
  67static int __init init_jiffies_clocksource(void)
  68{
  69        return clocksource_register(&clocksource_jiffies);
  70}
  71
  72core_initcall(init_jiffies_clocksource);
  73
  74struct clocksource * __init __weak clocksource_default_clock(void)
  75{
  76        return &clocksource_jiffies;
  77}
  78