Commit | Line | Data |
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734efb46 | 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> | |
fbad1ea9 | 25 | #include <linux/module.h> |
734efb46 | 26 | #include <linux/init.h> |
27 | ||
e2830b5c TH |
28 | #include "tick-internal.h" |
29 | ||
734efb46 | 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 | |
25985edc | 37 | * requested HZ value. It is also not recommended |
734efb46 | 38 | * for "tick-less" systems. |
39 | */ | |
40 | #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ)) | |
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 | |
025dfdaf | 51 | * HZ shrinks, so values greater than 8 overflow 32bits when |
734efb46 | 52 | * HZ=100. |
53 | */ | |
54 | #define JIFFIES_SHIFT 8 | |
55 | ||
8e19608e | 56 | static cycle_t jiffies_read(struct clocksource *cs) |
734efb46 | 57 | { |
58 | return (cycle_t) jiffies; | |
59 | } | |
60 | ||
61 | struct clocksource clocksource_jiffies = { | |
62 | .name = "jiffies", | |
3f4a0b91 | 63 | .rating = 1, /* lowest valid rating*/ |
734efb46 | 64 | .read = jiffies_read, |
65 | .mask = 0xffffffff, /*32bits*/ | |
66 | .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ | |
67 | .shift = JIFFIES_SHIFT, | |
734efb46 | 68 | }; |
69 | ||
fbad1ea9 TH |
70 | #if (BITS_PER_LONG < 64) |
71 | u64 get_jiffies_64(void) | |
72 | { | |
73 | unsigned long seq; | |
74 | u64 ret; | |
75 | ||
76 | do { | |
77 | seq = read_seqbegin(&xtime_lock); | |
78 | ret = jiffies_64; | |
79 | } while (read_seqretry(&xtime_lock, seq)); | |
80 | return ret; | |
81 | } | |
82 | EXPORT_SYMBOL(get_jiffies_64); | |
83 | #endif | |
84 | ||
85 | EXPORT_SYMBOL(jiffies); | |
86 | ||
734efb46 | 87 | static int __init init_jiffies_clocksource(void) |
88 | { | |
a2752549 | 89 | return clocksource_register(&clocksource_jiffies); |
734efb46 | 90 | } |
91 | ||
98de9e3b | 92 | core_initcall(init_jiffies_clocksource); |
f1b82746 MS |
93 | |
94 | struct clocksource * __init __weak clocksource_default_clock(void) | |
95 | { | |
96 | return &clocksource_jiffies; | |
97 | } |