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/module.h>
26#include <linux/init.h>
27
28#include "timekeeping.h"
29
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
37 * requested HZ value. It is also not recommended
38 * for "tick-less" systems.
39 */
40#define NSEC_PER_JIFFY	((NSEC_PER_SEC+HZ/2)/HZ)
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
51 * HZ shrinks, so values greater than 8 overflow 32bits when
52 * HZ=100.
53 */
54#if HZ < 34
55#define JIFFIES_SHIFT	6
56#elif HZ < 67
57#define JIFFIES_SHIFT	7
58#else
59#define JIFFIES_SHIFT	8
60#endif
61
62static cycle_t jiffies_read(struct clocksource *cs)
63{
64	return (cycle_t) jiffies;
65}
66
67static struct clocksource clocksource_jiffies = {
68	.name		= "jiffies",
69	.rating		= 1, /* lowest valid rating*/
70	.read		= jiffies_read,
71	.mask		= 0xffffffff, /*32bits*/
72	.mult		= NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
73	.shift		= JIFFIES_SHIFT,
74	.max_cycles	= 10,
75};
76
77__cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock);
78
79#if (BITS_PER_LONG < 64)
80u64 get_jiffies_64(void)
81{
82	unsigned long seq;
83	u64 ret;
84
85	do {
86		seq = read_seqbegin(&jiffies_lock);
87		ret = jiffies_64;
88	} while (read_seqretry(&jiffies_lock, seq));
89	return ret;
90}
91EXPORT_SYMBOL(get_jiffies_64);
92#endif
93
94EXPORT_SYMBOL(jiffies);
95
96static int __init init_jiffies_clocksource(void)
97{
98	return __clocksource_register(&clocksource_jiffies);
99}
100
101core_initcall(init_jiffies_clocksource);
102
103struct clocksource * __init __weak clocksource_default_clock(void)
104{
105	return &clocksource_jiffies;
106}
107
108struct clocksource refined_jiffies;
109
110int register_refined_jiffies(long cycles_per_second)
111{
112	u64 nsec_per_tick, shift_hz;
113	long cycles_per_tick;
114
115
116
117	refined_jiffies = clocksource_jiffies;
118	refined_jiffies.name = "refined-jiffies";
119	refined_jiffies.rating++;
120
121	/* Calc cycles per tick */
122	cycles_per_tick = (cycles_per_second + HZ/2)/HZ;
123	/* shift_hz stores hz<<8 for extra accuracy */
124	shift_hz = (u64)cycles_per_second << 8;
125	shift_hz += cycles_per_tick/2;
126	do_div(shift_hz, cycles_per_tick);
127	/* Calculate nsec_per_tick using shift_hz */
128	nsec_per_tick = (u64)NSEC_PER_SEC << 8;
129	nsec_per_tick += (u32)shift_hz/2;
130	do_div(nsec_per_tick, (u32)shift_hz);
131
132	refined_jiffies.mult = ((u32)nsec_per_tick) << JIFFIES_SHIFT;
133
134	__clocksource_register(&refined_jiffies);
135	return 0;
136}
137