xref: /linux/kernel/time/jiffies.c (revision 762f99f4f3cb41a775b5157dd761217beba65873)
1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3  * This file contains the jiffies based clocksource.
4  *
5  * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
6  */
7 #include <linux/clocksource.h>
8 #include <linux/jiffies.h>
9 #include <linux/module.h>
10 #include <linux/init.h>
11 
12 #include "timekeeping.h"
13 #include "tick-internal.h"
14 
15 
jiffies_read(struct clocksource * cs)16 static u64 jiffies_read(struct clocksource *cs)
17 {
18 	return (u64) jiffies;
19 }
20 
21 /*
22  * The Jiffies based clocksource is the lowest common
23  * denominator clock source which should function on
24  * all systems. It has the same coarse resolution as
25  * the timer interrupt frequency HZ and it suffers
26  * inaccuracies caused by missed or lost timer
27  * interrupts and the inability for the timer
28  * interrupt hardware to accurately tick at the
29  * requested HZ value. It is also not recommended
30  * for "tick-less" systems.
31  */
32 static struct clocksource clocksource_jiffies = {
33 	.name			= "jiffies",
34 	.rating			= 1, /* lowest valid rating*/
35 	.uncertainty_margin	= 32 * NSEC_PER_MSEC,
36 	.read			= jiffies_read,
37 	.mask			= CLOCKSOURCE_MASK(32),
38 	.mult			= TICK_NSEC << JIFFIES_SHIFT, /* details above */
39 	.shift			= JIFFIES_SHIFT,
40 	.max_cycles		= 10,
41 };
42 
43 __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
44 __cacheline_aligned_in_smp seqcount_raw_spinlock_t jiffies_seq =
45 	SEQCNT_RAW_SPINLOCK_ZERO(jiffies_seq, &jiffies_lock);
46 
47 #if (BITS_PER_LONG < 64)
get_jiffies_64(void)48 u64 get_jiffies_64(void)
49 {
50 	unsigned int seq;
51 	u64 ret;
52 
53 	do {
54 		seq = read_seqcount_begin(&jiffies_seq);
55 		ret = jiffies_64;
56 	} while (read_seqcount_retry(&jiffies_seq, seq));
57 	return ret;
58 }
59 EXPORT_SYMBOL(get_jiffies_64);
60 #endif
61 
62 EXPORT_SYMBOL(jiffies);
63 
init_jiffies_clocksource(void)64 static int __init init_jiffies_clocksource(void)
65 {
66 	return __clocksource_register(&clocksource_jiffies);
67 }
68 
69 core_initcall(init_jiffies_clocksource);
70 
clocksource_default_clock(void)71 struct clocksource * __init __weak clocksource_default_clock(void)
72 {
73 	return &clocksource_jiffies;
74 }
75 
76 static struct clocksource refined_jiffies;
77 
register_refined_jiffies(long cycles_per_second)78 int register_refined_jiffies(long cycles_per_second)
79 {
80 	u64 nsec_per_tick, shift_hz;
81 	long cycles_per_tick;
82 
83 
84 
85 	refined_jiffies = clocksource_jiffies;
86 	refined_jiffies.name = "refined-jiffies";
87 	refined_jiffies.rating++;
88 
89 	/* Calc cycles per tick */
90 	cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
91 	/* shift_hz stores hz<<8 for extra accuracy */
92 	shift_hz = (u64)cycles_per_second << 8;
93 	shift_hz += cycles_per_tick/2;
94 	do_div(shift_hz, cycles_per_tick);
95 	/* Calculate nsec_per_tick using shift_hz */
96 	nsec_per_tick = (u64)NSEC_PER_SEC << 8;
97 	nsec_per_tick += (u32)shift_hz/2;
98 	do_div(nsec_per_tick, (u32)shift_hz);
99 
100 	refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
101 
102 	__clocksource_register(&refined_jiffies);
103 	return 0;
104 }
105