summaryrefslogblamecommitdiff
path: root/kernel/time/jiffies.c
blob: 4c256fdb8875b54f19e1c8062371c4b362cc5773 (plain) (tree) 
734efb467b31 ^



























































3f4a0b917ce7 ^

734efb467b31 ^




734efb467b31 ^




a275254975a2 ^

734efb467b31 ^


98de9e3ba234 ^

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59

60

61
62
63
64

65
66
67
68

69

70
71

72



























































                                                                         
                                                     



                                                                              



                                                
                                                          

 
                                        
/***********************************************************************
* linux/kernel/time/jiffies.c
*
* This file contains the jiffies based clocksource.
*
* Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*
************************************************************************/
#include <linux/clocksource.h>
#include <linux/jiffies.h>
#include <linux/init.h>

/* The Jiffies based clocksource is the lowest common
 * denominator clock source which should function on
 * all systems. It has the same coarse resolution as
 * the timer interrupt frequency HZ and it suffers
 * inaccuracies caused by missed or lost timer
 * interrupts and the inability for the timer
 * interrupt hardware to accuratly tick at the
 * requested HZ value. It is also not reccomended
 * for "tick-less" systems.
 */
#define NSEC_PER_JIFFY	((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))

/* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
 * conversion, the .shift value could be zero. However
 * this would make NTP adjustments impossible as they are
 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
 * shift both the nominator and denominator the same
 * amount, and give ntp adjustments in units of 1/2^8
 *
 * The value 8 is somewhat carefully chosen, as anything
 * larger can result in overflows. NSEC_PER_JIFFY grows as
 * HZ shrinks, so values greater then 8 overflow 32bits when
 * HZ=100.
 */
#define JIFFIES_SHIFT	8

static cycle_t jiffies_read(void)
{
	return (cycle_t) jiffies;
}

struct clocksource clocksource_jiffies = {
	.name		= "jiffies",
	.rating		= 1, /* lowest valid rating*/
	.read		= jiffies_read,
	.mask		= 0xffffffff, /*32bits*/
	.mult		= NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
	.shift		= JIFFIES_SHIFT,
};

static int __init init_jiffies_clocksource(void)
{
	return clocksource_register(&clocksource_jiffies);
}

core_initcall(init_jiffies_clocksource);