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
  53 static cycle_t jiffies_read(struct clocksource *cs)
  54 {
  55         return (cycle_t) jiffies;
  56 }
  57
  58 struct 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
  67 static int __init init_jiffies_clocksource(void)
  68 {
  69         return clocksource_register(&clocksource_jiffies);
  70 }
  71
  72 core_initcall(init_jiffies_clocksource);
  73
  74 struct clocksource * __init __weak clocksource_default_clock(void)
  75 {
  76         return &clocksource_jiffies;
  77 }