jiffies.c (858a0d7eb5300b5f620d98ab3c4b96c9d5f19131) jiffies.c (93825f2ec736f30e034ab7c9d56b42849c5b00da)
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

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22************************************************************************/
23#include <linux/clocksource.h>
24#include <linux/jiffies.h>
25#include <linux/module.h>
26#include <linux/init.h>
27
28#include "timekeeping.h"
29
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

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22************************************************************************/
23#include <linux/clocksource.h>
24#include <linux/jiffies.h>
25#include <linux/module.h>
26#include <linux/init.h>
27
28#include "timekeeping.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
30
42/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
31/* Since jiffies uses a simple TICK_NSEC 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
32 * conversion, the .shift value could be zero. However
33 * this would make NTP adjustments impossible as they are
34 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
35 * shift both the nominator and denominator the same
36 * amount, and give ntp adjustments in units of 1/2^8
37 *
38 * 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
39 * larger can result in overflows. TICK_NSEC grows as HZ
40 * 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
62static u64 jiffies_read(struct clocksource *cs)
63{
64 return (u64) jiffies;
65}
66
41 * HZ=100.
42 */
43#if HZ < 34
44#define JIFFIES_SHIFT 6
45#elif HZ < 67
46#define JIFFIES_SHIFT 7
47#else
48#define JIFFIES_SHIFT 8
49#endif
50
51static u64 jiffies_read(struct clocksource *cs)
52{
53 return (u64) jiffies;
54}
55
56/*
57 * The Jiffies based clocksource is the lowest common
58 * denominator clock source which should function on
59 * all systems. It has the same coarse resolution as
60 * the timer interrupt frequency HZ and it suffers
61 * inaccuracies caused by missed or lost timer
62 * interrupts and the inability for the timer
63 * interrupt hardware to accuratly tick at the
64 * requested HZ value. It is also not recommended
65 * for "tick-less" systems.
66 */
67static struct clocksource clocksource_jiffies = {
68 .name = "jiffies",
69 .rating = 1, /* lowest valid rating*/
70 .read = jiffies_read,
71 .mask = CLOCKSOURCE_MASK(32),
67static struct clocksource clocksource_jiffies = {
68 .name = "jiffies",
69 .rating = 1, /* lowest valid rating*/
70 .read = jiffies_read,
71 .mask = CLOCKSOURCE_MASK(32),
72 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
72 .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
73 .shift = JIFFIES_SHIFT,
74 .max_cycles = 10,
75};
76
77__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
78
79#if (BITS_PER_LONG < 64)
80u64 get_jiffies_64(void)

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120
121 /* Calc cycles per tick */
122 cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
123 /* shift_hz stores hz<<8 for extra accuracy */
124 shift_hz = (u64)cycles_per_second << 8;
125 shift_hz += cycles_per_tick/2;
126 do_div(shift_hz, cycles_per_tick);
127 /* Calculate nsec_per_tick using shift_hz */
73 .shift = JIFFIES_SHIFT,
74 .max_cycles = 10,
75};
76
77__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
78
79#if (BITS_PER_LONG < 64)
80u64 get_jiffies_64(void)

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120
121 /* Calc cycles per tick */
122 cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
123 /* shift_hz stores hz<<8 for extra accuracy */
124 shift_hz = (u64)cycles_per_second << 8;
125 shift_hz += cycles_per_tick/2;
126 do_div(shift_hz, cycles_per_tick);
127 /* Calculate nsec_per_tick using shift_hz */
128 nsec_per_tick = (u64)NSEC_PER_SEC << 8;
128 nsec_per_tick = (u64)TICK_NSEC << 8;
129 nsec_per_tick += (u32)shift_hz/2;
130 do_div(nsec_per_tick, (u32)shift_hz);
131
132 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
133
134 __clocksource_register(&refined_jiffies);
135 return 0;
136}
129 nsec_per_tick += (u32)shift_hz/2;
130 do_div(nsec_per_tick, (u32)shift_hz);
131
132 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
133
134 __clocksource_register(&refined_jiffies);
135 return 0;
136}