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
14
15 /* Since jiffies uses a simple TICK_NSEC multiplier
16 * conversion, the .shift value could be zero. However
17 * this would make NTP adjustments impossible as they are
18 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
19 * shift both the nominator and denominator the same
20 * amount, and give ntp adjustments in units of 1/2^8
21 *
22 * The value 8 is somewhat carefully chosen, as anything
23 * larger can result in overflows. TICK_NSEC grows as HZ
24 * shrinks, so values greater than 8 overflow 32bits when
25 * HZ=100.
26 */
27 #if HZ < 34
28 #define JIFFIES_SHIFT 6
29 #elif HZ < 67
30 #define JIFFIES_SHIFT 7
31 #else
32 #define JIFFIES_SHIFT 8
33 #endif
34
jiffies_read(struct clocksource * cs)35 static u64 jiffies_read(struct clocksource *cs)
36 {
37 return (u64) jiffies;
38 }
39
40 /*
41 * The Jiffies based clocksource is the lowest common
42 * denominator clock source which should function on
43 * all systems. It has the same coarse resolution as
44 * the timer interrupt frequency HZ and it suffers
45 * inaccuracies caused by missed or lost timer
46 * interrupts and the inability for the timer
47 * interrupt hardware to accuratly tick at the
48 * requested HZ value. It is also not recommended
49 * for "tick-less" systems.
50 */
51 static struct clocksource clocksource_jiffies = {
52 .name = "jiffies",
53 .rating = 1, /* lowest valid rating*/
54 .read = jiffies_read,
55 .mask = CLOCKSOURCE_MASK(32),
56 .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
57 .shift = JIFFIES_SHIFT,
58 .max_cycles = 10,
59 };
60
61 __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
62 __cacheline_aligned_in_smp seqcount_t jiffies_seq;
63
64 #if (BITS_PER_LONG < 64)
get_jiffies_64(void)65 u64 get_jiffies_64(void)
66 {
67 unsigned int seq;
68 u64 ret;
69
70 do {
71 seq = read_seqcount_begin(&jiffies_seq);
72 ret = jiffies_64;
73 } while (read_seqcount_retry(&jiffies_seq, seq));
74 return ret;
75 }
76 EXPORT_SYMBOL(get_jiffies_64);
77 #endif
78
79 EXPORT_SYMBOL(jiffies);
80
init_jiffies_clocksource(void)81 static int __init init_jiffies_clocksource(void)
82 {
83 return __clocksource_register(&clocksource_jiffies);
84 }
85
86 core_initcall(init_jiffies_clocksource);
87
clocksource_default_clock(void)88 struct clocksource * __init __weak clocksource_default_clock(void)
89 {
90 return &clocksource_jiffies;
91 }
92
93 static struct clocksource refined_jiffies;
94
register_refined_jiffies(long cycles_per_second)95 int register_refined_jiffies(long cycles_per_second)
96 {
97 u64 nsec_per_tick, shift_hz;
98 long cycles_per_tick;
99
100
101
102 refined_jiffies = clocksource_jiffies;
103 refined_jiffies.name = "refined-jiffies";
104 refined_jiffies.rating++;
105
106 /* Calc cycles per tick */
107 cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
108 /* shift_hz stores hz<<8 for extra accuracy */
109 shift_hz = (u64)cycles_per_second << 8;
110 shift_hz += cycles_per_tick/2;
111 do_div(shift_hz, cycles_per_tick);
112 /* Calculate nsec_per_tick using shift_hz */
113 nsec_per_tick = (u64)NSEC_PER_SEC << 8;
114 nsec_per_tick += (u32)shift_hz/2;
115 do_div(nsec_per_tick, (u32)shift_hz);
116
117 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
118
119 __clocksource_register(&refined_jiffies);
120 return 0;
121 }
122
