1 /***********************************************************************
2 * linux/kernel/time/jiffies.c
4 * This file contains the jiffies based clocksource.
6 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
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.
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.
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.
22 ************************************************************************/
23 #include <linux/clocksource.h>
24 #include <linux/jiffies.h>
25 #include <linux/module.h>
26 #include <linux/init.h>
28 #include "timekeeping.h"
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.
40 #define NSEC_PER_JIFFY ((NSEC_PER_SEC+HZ/2)/HZ)
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
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
55 #define JIFFIES_SHIFT 6
57 #define JIFFIES_SHIFT 7
59 #define JIFFIES_SHIFT 8
62 static cycle_t jiffies_read(struct clocksource *cs)
64 return (cycle_t) jiffies;
67 static struct clocksource clocksource_jiffies = {
69 .rating = 1, /* lowest valid rating*/
71 .mask = CLOCKSOURCE_MASK(32),
72 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
73 .shift = JIFFIES_SHIFT,
77 __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
79 #if (BITS_PER_LONG < 64)
80 u64 get_jiffies_64(void)
86 seq = read_seqbegin(&jiffies_lock);
88 } while (read_seqretry(&jiffies_lock, seq));
91 EXPORT_SYMBOL(get_jiffies_64);
94 EXPORT_SYMBOL(jiffies);
96 static int __init init_jiffies_clocksource(void)
98 return __clocksource_register(&clocksource_jiffies);
101 core_initcall(init_jiffies_clocksource);
103 struct clocksource * __init __weak clocksource_default_clock(void)
105 return &clocksource_jiffies;
108 struct clocksource refined_jiffies;
110 int register_refined_jiffies(long cycles_per_second)
112 u64 nsec_per_tick, shift_hz;
113 long cycles_per_tick;
117 refined_jiffies = clocksource_jiffies;
118 refined_jiffies.name = "refined-jiffies";
119 refined_jiffies.rating++;
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;
129 nsec_per_tick += (u32)shift_hz/2;
130 do_div(nsec_per_tick, (u32)shift_hz);
132 refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
134 __clocksource_register(&refined_jiffies);