1 // SPDX-License-Identifier: GPL-2.0+
3 * This file contains the jiffies based clocksource.
5 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
7 #include <linux/clocksource.h>
8 #include <linux/jiffies.h>
9 #include <linux/module.h>
10 #include <linux/init.h>
12 #include "timekeeping.h"
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
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
28 #define JIFFIES_SHIFT 6
30 #define JIFFIES_SHIFT 7
32 #define JIFFIES_SHIFT 8
35 static u64 jiffies_read(struct clocksource *cs)
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 accurately tick at the
48 * requested HZ value. It is also not recommended
49 * for "tick-less" systems.
51 static struct clocksource clocksource_jiffies = {
53 .rating = 1, /* lowest valid rating*/
54 .uncertainty_margin = 32 * NSEC_PER_MSEC,
56 .mask = CLOCKSOURCE_MASK(32),
57 .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
58 .shift = JIFFIES_SHIFT,
62 __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(jiffies_lock);
63 __cacheline_aligned_in_smp seqcount_raw_spinlock_t jiffies_seq =
64 SEQCNT_RAW_SPINLOCK_ZERO(jiffies_seq, &jiffies_lock);
66 #if (BITS_PER_LONG < 64)
67 u64 get_jiffies_64(void)
73 seq = read_seqcount_begin(&jiffies_seq);
75 } while (read_seqcount_retry(&jiffies_seq, seq));
78 EXPORT_SYMBOL(get_jiffies_64);
81 EXPORT_SYMBOL(jiffies);
83 static int __init init_jiffies_clocksource(void)
85 return __clocksource_register(&clocksource_jiffies);
88 core_initcall(init_jiffies_clocksource);
90 struct clocksource * __init __weak clocksource_default_clock(void)
92 return &clocksource_jiffies;
95 static struct clocksource refined_jiffies;
97 int register_refined_jiffies(long cycles_per_second)
99 u64 nsec_per_tick, shift_hz;
100 long cycles_per_tick;
104 refined_jiffies = clocksource_jiffies;
105 refined_jiffies.name = "refined-jiffies";
106 refined_jiffies.rating++;
108 /* Calc cycles per tick */
109 cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
110 /* shift_hz stores hz<<8 for extra accuracy */
111 shift_hz = (u64)cycles_per_second << 8;
112 shift_hz += cycles_per_tick/2;
113 do_div(shift_hz, cycles_per_tick);
114 /* Calculate nsec_per_tick using shift_hz */
115 nsec_per_tick = (u64)NSEC_PER_SEC << 8;
116 nsec_per_tick += (u32)shift_hz/2;
117 do_div(nsec_per_tick, (u32)shift_hz);
119 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
121 __clocksource_register(&refined_jiffies);