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Linux/kernel/time/jiffies.c

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  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  ((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 #define JIFFIES_SHIFT   8
 55 
 56 static cycle_t jiffies_read(struct clocksource *cs)
 57 {
 58         return (cycle_t) jiffies;
 59 }
 60 
 61 static struct 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 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
 71 
 72 #if (BITS_PER_LONG < 64)
 73 u64 get_jiffies_64(void)
 74 {
 75         unsigned long seq;
 76         u64 ret;
 77 
 78         do {
 79                 seq = read_seqbegin(&jiffies_lock);
 80                 ret = jiffies_64;
 81         } while (read_seqretry(&jiffies_lock, seq));
 82         return ret;
 83 }
 84 EXPORT_SYMBOL(get_jiffies_64);
 85 #endif
 86 
 87 EXPORT_SYMBOL(jiffies);
 88 
 89 static int __init init_jiffies_clocksource(void)
 90 {
 91         return clocksource_register(&clocksource_jiffies);
 92 }
 93 
 94 core_initcall(init_jiffies_clocksource);
 95 
 96 struct clocksource * __init __weak clocksource_default_clock(void)
 97 {
 98         return &clocksource_jiffies;
 99 }
100 
101 struct clocksource refined_jiffies;
102 
103 int register_refined_jiffies(long cycles_per_second)
104 {
105         u64 nsec_per_tick, shift_hz;
106         long cycles_per_tick;
107 
108 
109 
110         refined_jiffies = clocksource_jiffies;
111         refined_jiffies.name = "refined-jiffies";
112         refined_jiffies.rating++;
113 
114         /* Calc cycles per tick */
115         cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
116         /* shift_hz stores hz<<8 for extra accuracy */
117         shift_hz = (u64)cycles_per_second << 8;
118         shift_hz += cycles_per_tick/2;
119         do_div(shift_hz, cycles_per_tick);
120         /* Calculate nsec_per_tick using shift_hz */
121         nsec_per_tick = (u64)NSEC_PER_SEC << 8;
122         nsec_per_tick += (u32)shift_hz/2;
123         do_div(nsec_per_tick, (u32)shift_hz);
124 
125         refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
126 
127         clocksource_register(&refined_jiffies);
128         return 0;
129 }
130 

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