summaryrefslogtreecommitdiffstats
path: root/include/linux/timecounter.h
blob: 4382035a75bbcdc65d8099a9b3d2d384e31815ff (plain)
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
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
/*
 * linux/include/linux/timecounter.h
 *
 * based on code that migrated away from
 * linux/include/linux/clocksource.h
 *
 * 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.
 */
#ifndef _LINUX_TIMECOUNTER_H
#define _LINUX_TIMECOUNTER_H

#include <linux/types.h>

/* simplify initialization of mask field */
#define CYCLECOUNTER_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)

/**
 * struct cyclecounter - hardware abstraction for a free running counter
 *	Provides completely state-free accessors to the underlying hardware.
 *	Depending on which hardware it reads, the cycle counter may wrap
 *	around quickly. Locking rules (if necessary) have to be defined
 *	by the implementor and user of specific instances of this API.
 *
 * @read:		returns the current cycle value
 * @mask:		bitmask for two's complement
 *			subtraction of non 64 bit counters,
 *			see CYCLECOUNTER_MASK() helper macro
 * @mult:		cycle to nanosecond multiplier
 * @shift:		cycle to nanosecond divisor (power of two)
 */
struct cyclecounter {
	cycle_t (*read)(const struct cyclecounter *cc);
	cycle_t mask;
	u32 mult;
	u32 shift;
};

/**
 * struct timecounter - layer above a %struct cyclecounter which counts nanoseconds
 *	Contains the state needed by timecounter_read() to detect
 *	cycle counter wrap around. Initialize with
 *	timecounter_init(). Also used to convert cycle counts into the
 *	corresponding nanosecond counts with timecounter_cyc2time(). Users
 *	of this code are responsible for initializing the underlying
 *	cycle counter hardware, locking issues and reading the time
 *	more often than the cycle counter wraps around. The nanosecond
 *	counter will only wrap around after ~585 years.
 *
 * @cc:			the cycle counter used by this instance
 * @cycle_last:		most recent cycle counter value seen by
 *			timecounter_read()
 * @nsec:		continuously increasing count
 * @mask:		bit mask for maintaining the 'frac' field
 * @frac:		accumulated fractional nanoseconds
 */
struct timecounter {
	const struct cyclecounter *cc;
	cycle_t cycle_last;
	u64 nsec;
	u64 mask;
	u64 frac;
};

/**
 * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
 * @cc:		Pointer to cycle counter.
 * @cycles:	Cycles
 * @mask:	bit mask for maintaining the 'frac' field
 * @frac:	pointer to storage for the fractional nanoseconds.
 */
static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
				      cycle_t cycles, u64 mask, u64 *frac)
{
	u64 ns = (u64) cycles;

	ns = (ns * cc->mult) + *frac;
	*frac = ns & mask;
	return ns >> cc->shift;
}

/**
 * timecounter_adjtime - Shifts the time of the clock.
 * @delta:	Desired change in nanoseconds.
 */
static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
{
	tc->nsec += delta;
}

/**
 * timecounter_init - initialize a time counter
 * @tc:			Pointer to time counter which is to be initialized/reset
 * @cc:			A cycle counter, ready to be used.
 * @start_tstamp:	Arbitrary initial time stamp.
 *
 * After this call the current cycle register (roughly) corresponds to
 * the initial time stamp. Every call to timecounter_read() increments
 * the time stamp counter by the number of elapsed nanoseconds.
 */
extern void timecounter_init(struct timecounter *tc,
			     const struct cyclecounter *cc,
			     u64 start_tstamp);

/**
 * timecounter_read - return nanoseconds elapsed since timecounter_init()
 *                    plus the initial time stamp
 * @tc:          Pointer to time counter.
 *
 * In other words, keeps track of time since the same epoch as
 * the function which generated the initial time stamp.
 */
extern u64 timecounter_read(struct timecounter *tc);

/**
 * timecounter_cyc2time - convert a cycle counter to same
 *                        time base as values returned by
 *                        timecounter_read()
 * @tc:		Pointer to time counter.
 * @cycle_tstamp:	a value returned by tc->cc->read()
 *
 * Cycle counts that are converted correctly as long as they
 * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
 * with "max cycle count" == cs->mask+1.
 *
 * This allows conversion of cycle counter values which were generated
 * in the past.
 */
extern u64 timecounter_cyc2time(struct timecounter *tc,
				cycle_t cycle_tstamp);

#endif