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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/init.h> |
26 | |
27 | /* The Jiffies based clocksource is the lowest common |
28 | * denominator clock source which should function on |
29 | * all systems. It has the same coarse resolution as |
30 | * the timer interrupt frequency HZ and it suffers |
31 | * inaccuracies caused by missed or lost timer |
32 | * interrupts and the inability for the timer |
33 | * interrupt hardware to accuratly tick at the |
34 | * requested HZ value. It is also not reccomended |
35 | * for "tick-less" systems. |
36 | */ |
37 | #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ)) |
38 | |
39 | /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier |
40 | * conversion, the .shift value could be zero. However |
41 | * this would make NTP adjustments impossible as they are |
42 | * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to |
43 | * shift both the nominator and denominator the same |
44 | * amount, and give ntp adjustments in units of 1/2^8 |
45 | * |
46 | * The value 8 is somewhat carefully chosen, as anything |
47 | * larger can result in overflows. NSEC_PER_JIFFY grows as |
48 | * HZ shrinks, so values greater then 8 overflow 32bits when |
49 | * HZ=100. |
50 | */ |
51 | #define JIFFIES_SHIFT 8 |
52 | |
53 | static cycle_t jiffies_read(void) |
54 | { |
55 | return (cycle_t) jiffies; |
56 | } |
57 | |
58 | struct clocksource clocksource_jiffies = { |
59 | .name = "jiffies", |
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60 | .rating = 1, /* lowest valid rating*/ |
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61 | .read = jiffies_read, |
62 | .mask = 0xffffffff, /*32bits*/ |
63 | .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ |
64 | .shift = JIFFIES_SHIFT, |
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65 | }; |
66 | |
67 | static int __init init_jiffies_clocksource(void) |
68 | { |
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69 | return clocksource_register(&clocksource_jiffies); |
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70 | } |
71 | |
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72 | core_initcall(init_jiffies_clocksource); |