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 #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 struct clocksource clocksource_jiffies = {
  68         .name           = "jiffies",
  69         .rating         = 1, /* lowest valid rating*/
  70         .read           = jiffies_read,
  71         .mask           = 0xffffffff, /*32bits*/
  72         .mult           = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
  73         .shift          = JIFFIES_SHIFT,
  74 };
  75
  76 #if (BITS_PER_LONG < 64)
  77 u64 get_jiffies_64(void)
  78 {
  79         unsigned long seq;
  80         u64 ret;
  81
  82         do {
  83                 seq = read_seqbegin(&xtime_lock);
  84                 ret = jiffies_64;
  85         } while (read_seqretry(&xtime_lock, seq));
  86         return ret;
  87 }
  88 EXPORT_SYMBOL(get_jiffies_64);
  89 #endif
  90
  91 EXPORT_SYMBOL(jiffies);
  92
  93 static int __init init_jiffies_clocksource(void)
  94 {
  95         return clocksource_register(&clocksource_jiffies);
  96 }
  97
  98 core_initcall(init_jiffies_clocksource);
  99
 100 struct clocksource * __init __weak clocksource_default_clock(void)
 101 {
 102         return &clocksource_jiffies;
 103 }