ia64/linux-2.6.18-xen.hg

view kernel/time/jiffies.c @ 562:66faefe721eb

pvSCSI backend driver

Signed-off-by: Tomonari Horikoshi <t.horikoshi@jp.fujitsu.com>
Signed-off-by: Jun Kamada <kama@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Jun 02 09:58:27 2008 +0100 (2008-06-02)
parents 831230e53067
children
line source
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>
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))
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
53 static cycle_t jiffies_read(void)
54 {
55 return (cycle_t) jiffies;
56 }
58 struct clocksource clocksource_jiffies = {
59 .name = "jiffies",
60 .rating = 0, /* lowest rating*/
61 .read = jiffies_read,
62 .mask = 0xffffffff, /*32bits*/
63 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
64 .shift = JIFFIES_SHIFT,
65 .is_continuous = 0, /* tick based, not free running */
66 };
68 static int __init init_jiffies_clocksource(void)
69 {
70 return clocksource_register(&clocksource_jiffies);
71 }
73 module_init(init_jiffies_clocksource);