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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
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.
12 : : *
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.
17 : : *
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.
21 : : *
22 : : ************************************************************************/
23 : : #include <linux/clocksource.h>
24 : : #include <linux/jiffies.h>
25 : : #include <linux/module.h>
26 : : #include <linux/init.h>
27 : :
28 : : #include "tick-internal.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 : :
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
48 : : *
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
52 : : * HZ=100.
53 : : */
54 : : #define JIFFIES_SHIFT 8
55 : :
56 : 0 : static cycle_t jiffies_read(struct clocksource *cs)
57 : : {
58 : 0 : return (cycle_t) jiffies;
59 : : }
60 : :
61 : : static struct clocksource clocksource_jiffies = {
62 : : .name = "jiffies",
63 : : .rating = 1, /* lowest valid rating*/
64 : : .read = jiffies_read,
65 : : .mask = 0xffffffff, /*32bits*/
66 : : .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
67 : : .shift = JIFFIES_SHIFT,
68 : : };
69 : :
70 : : __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
71 : :
72 : : #if (BITS_PER_LONG < 64)
73 : 5279007 : u64 get_jiffies_64(void)
74 : : {
75 : : unsigned long seq;
76 : : u64 ret;
77 : :
78 : : do {
79 : : seq = read_seqbegin(&jiffies_lock);
80 : 5279006 : ret = jiffies_64;
81 [ + + ]: 5279006 : } while (read_seqretry(&jiffies_lock, seq));
82 : 5278922 : return ret;
83 : : }
84 : : EXPORT_SYMBOL(get_jiffies_64);
85 : : #endif
86 : :
87 : : EXPORT_SYMBOL(jiffies);
88 : :
89 : 0 : static int __init init_jiffies_clocksource(void)
90 : : {
91 : 0 : return clocksource_register(&clocksource_jiffies);
92 : : }
93 : :
94 : : core_initcall(init_jiffies_clocksource);
95 : :
96 : 0 : struct clocksource * __init __weak clocksource_default_clock(void)
97 : : {
98 : 0 : return &clocksource_jiffies;
99 : : }
100 : :
101 : : struct clocksource refined_jiffies;
102 : :
103 : 0 : int register_refined_jiffies(long cycles_per_second)
104 : : {
105 : : u64 nsec_per_tick, shift_hz;
106 : : long cycles_per_tick;
107 : :
108 : :
109 : :
110 : 0 : refined_jiffies = clocksource_jiffies;
111 : 0 : refined_jiffies.name = "refined-jiffies";
112 : 0 : refined_jiffies.rating++;
113 : :
114 : : /* Calc cycles per tick */
115 : 0 : cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
116 : : /* shift_hz stores hz<<8 for extra accuracy */
117 : 0 : shift_hz = (u64)cycles_per_second << 8;
118 : 0 : shift_hz += cycles_per_tick/2;
119 [ # # ][ # # ]: 0 : do_div(shift_hz, cycles_per_tick);
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120 : : /* Calculate nsec_per_tick using shift_hz */
121 : : nsec_per_tick = (u64)NSEC_PER_SEC << 8;
122 : 0 : nsec_per_tick += (u32)shift_hz/2;
123 [ # # ][ # # ]: 0 : do_div(nsec_per_tick, (u32)shift_hz);
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124 : :
125 : 0 : refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
126 : :
127 : 0 : clocksource_register(&refined_jiffies);
128 : 0 : return 0;
129 : : }
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