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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 : : #if HZ < 34
55 : : #define JIFFIES_SHIFT 6
56 : : #elif HZ < 67
57 : : #define JIFFIES_SHIFT 7
58 : : #else
59 : : #define JIFFIES_SHIFT 8
60 : : #endif
61 : :
62 : 0 : static cycle_t jiffies_read(struct clocksource *cs)
63 : : {
64 : 0 : return (cycle_t) jiffies;
65 : : }
66 : :
67 : : static struct clocksource clocksource_jiffies = {
68 : : .name = "jiffies",
69 : : .rating = 1, /* lowest valid rating*/
70 : : .read = jiffies_read,
71 : : .mask = 0xffffffff, /*32bits*/
72 : : .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
73 : : .shift = JIFFIES_SHIFT,
74 : : };
75 : :
76 : : __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
77 : :
78 : : #if (BITS_PER_LONG < 64)
79 : 5595537 : u64 get_jiffies_64(void)
80 : : {
81 : : unsigned long seq;
82 : : u64 ret;
83 : :
84 : : do {
85 : : seq = read_seqbegin(&jiffies_lock);
86 : 5595537 : ret = jiffies_64;
87 [ + + ]: 5595537 : } while (read_seqretry(&jiffies_lock, seq));
88 : 5595463 : return ret;
89 : : }
90 : : EXPORT_SYMBOL(get_jiffies_64);
91 : : #endif
92 : :
93 : : EXPORT_SYMBOL(jiffies);
94 : :
95 : 0 : static int __init init_jiffies_clocksource(void)
96 : : {
97 : 0 : return clocksource_register(&clocksource_jiffies);
98 : : }
99 : :
100 : : core_initcall(init_jiffies_clocksource);
101 : :
102 : 0 : struct clocksource * __init __weak clocksource_default_clock(void)
103 : : {
104 : 0 : return &clocksource_jiffies;
105 : : }
106 : :
107 : : struct clocksource refined_jiffies;
108 : :
109 : 0 : int register_refined_jiffies(long cycles_per_second)
110 : : {
111 : : u64 nsec_per_tick, shift_hz;
112 : : long cycles_per_tick;
113 : :
114 : :
115 : :
116 : 0 : refined_jiffies = clocksource_jiffies;
117 : 0 : refined_jiffies.name = "refined-jiffies";
118 : 0 : refined_jiffies.rating++;
119 : :
120 : : /* Calc cycles per tick */
121 : 0 : cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
122 : : /* shift_hz stores hz<<8 for extra accuracy */
123 : 0 : shift_hz = (u64)cycles_per_second << 8;
124 : 0 : shift_hz += cycles_per_tick/2;
125 [ # # ][ # # ]: 0 : do_div(shift_hz, cycles_per_tick);
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126 : : /* Calculate nsec_per_tick using shift_hz */
127 : : nsec_per_tick = (u64)NSEC_PER_SEC << 8;
128 : 0 : nsec_per_tick += (u32)shift_hz/2;
129 [ # # ][ # # ]: 0 : do_div(nsec_per_tick, (u32)shift_hz);
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130 : :
131 : 0 : refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
132 : :
133 : 0 : clocksource_register(&refined_jiffies);
134 : 0 : return 0;
135 : : }
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