1 /***********************************************************************
2 * linux/kernel/time/jiffies.c
4 * This file contains the jiffies based clocksource.
6 * Copyright (C) 2004, 2005 IBM, John Stultz (johnstul@us.ibm.com)
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.
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.
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.
22 ************************************************************************/
23 #include <linux/clocksource.h>
24 #include <linux/jiffies.h>
25 #include <linux/module.h>
26 #include <linux/init.h>
28 /* The Jiffies based clocksource is the lowest common
29 * denominator clock source which should function on
30 * all systems. It has the same coarse resolution as
31 * the timer interrupt frequency HZ and it suffers
32 * inaccuracies caused by missed or lost timer
33 * interrupts and the inability for the timer
34 * interrupt hardware to accuratly tick at the
35 * requested HZ value. It is also not reccomended
36 * for "tick-less" systems.
38 #define NSEC_PER_JIFFY ((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
40 /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
41 * conversion, the .shift value could be zero. However
42 * this would make NTP adjustments impossible as they are
43 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
44 * shift both the nominator and denominator the same
45 * amount, and give ntp adjustments in units of 1/2^8
47 * The value 8 is somewhat carefully chosen, as anything
48 * larger can result in overflows. NSEC_PER_JIFFY grows as
49 * HZ shrinks, so values greater than 8 overflow 32bits when
52 #define JIFFIES_SHIFT 8
54 static cycle_t jiffies_read(struct clocksource *cs)
56 return (cycle_t) jiffies;
59 struct clocksource clocksource_jiffies = {
61 .rating = 1, /* lowest valid rating*/
63 .mask = 0xffffffff, /*32bits*/
64 .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
65 .shift = JIFFIES_SHIFT,
68 #if (BITS_PER_LONG < 64)
69 u64 get_jiffies_64(void)
75 seq = read_seqbegin(&xtime_lock);
77 } while (read_seqretry(&xtime_lock, seq));
80 EXPORT_SYMBOL(get_jiffies_64);
83 EXPORT_SYMBOL(jiffies);
85 static int __init init_jiffies_clocksource(void)
87 return clocksource_register(&clocksource_jiffies);
90 core_initcall(init_jiffies_clocksource);
92 struct clocksource * __init __weak clocksource_default_clock(void)
94 return &clocksource_jiffies;