timecounter.h

  1/* SPDX-License-Identifier: GPL-2.0-or-later */
  2/*
  3 * linux/include/linux/timecounter.h
  4 *
  5 * based on code that migrated away from
  6 * linux/include/linux/clocksource.h
  7 */
  8#ifndef _LINUX_TIMECOUNTER_H
  9#define _LINUX_TIMECOUNTER_H
 10
 11#include <linux/types.h>
 12
 13/* simplify initialization of mask field */
 14#define CYCLECOUNTER_MASK(bits) (u64)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
 15
 16/**
 17 * struct cyclecounter - hardware abstraction for a free running counter
 18 *	Provides completely state-free accessors to the underlying hardware.
 19 *	Depending on which hardware it reads, the cycle counter may wrap
 20 *	around quickly. Locking rules (if necessary) have to be defined
 21 *	by the implementor and user of specific instances of this API.
 22 *
 23 * @read:		returns the current cycle value
 24 * @mask:		bitmask for two's complement
 25 *			subtraction of non-64-bit counters,
 26 *			see CYCLECOUNTER_MASK() helper macro
 27 * @mult:		cycle to nanosecond multiplier
 28 * @shift:		cycle to nanosecond divisor (power of two)
 29 */
 30struct cyclecounter {
 31	u64 (*read)(struct cyclecounter *cc);
 32	u64 mask;
 33	u32 mult;
 34	u32 shift;
 35};
 36
 37/**
 38 * struct timecounter - layer above a &struct cyclecounter which counts nanoseconds
 39 *	Contains the state needed by timecounter_read() to detect
 40 *	cycle counter wrap around. Initialize with
 41 *	timecounter_init(). Also used to convert cycle counts into the
 42 *	corresponding nanosecond counts with timecounter_cyc2time(). Users
 43 *	of this code are responsible for initializing the underlying
 44 *	cycle counter hardware, locking issues and reading the time
 45 *	more often than the cycle counter wraps around. The nanosecond
 46 *	counter will only wrap around after ~585 years.
 47 *
 48 * @cc:			the cycle counter used by this instance
 49 * @cycle_last:		most recent cycle counter value seen by
 50 *			timecounter_read()
 51 * @nsec:		continuously increasing count
 52 * @mask:		bit mask for maintaining the 'frac' field
 53 * @frac:		accumulated fractional nanoseconds
 54 */
 55struct timecounter {
 56	struct cyclecounter *cc;
 57	u64 cycle_last;
 58	u64 nsec;
 59	u64 mask;
 60	u64 frac;
 61};
 62
 63/**
 64 * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds
 65 * @cc:		Pointer to cycle counter.
 66 * @cycles:	Cycles
 67 * @mask:	bit mask for maintaining the 'frac' field
 68 * @frac:	pointer to storage for the fractional nanoseconds.
 69 *
 70 * Returns: cycle counter cycles converted to nanoseconds
 71 */
 72static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
 73				      u64 cycles, u64 mask, u64 *frac)
 74{
 75	u64 ns = (u64) cycles;
 76
 77	ns = (ns * cc->mult) + *frac;
 78	*frac = ns & mask;
 79	return ns >> cc->shift;
 80}
 81
 82/**
 83 * timecounter_adjtime - Shifts the time of the clock.
 84 * @tc:		The &struct timecounter to adjust
 85 * @delta:	Desired change in nanoseconds.
 86 */
 87static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
 88{
 89	tc->nsec += delta;
 90}
 91
 92/**
 93 * timecounter_init - initialize a time counter
 94 * @tc:			Pointer to time counter which is to be initialized/reset
 95 * @cc:			A cycle counter, ready to be used.
 96 * @start_tstamp:	Arbitrary initial time stamp.
 97 *
 98 * After this call the current cycle register (roughly) corresponds to
 99 * the initial time stamp. Every call to timecounter_read() increments
100 * the time stamp counter by the number of elapsed nanoseconds.
101 */
102extern void timecounter_init(struct timecounter *tc,
103			     struct cyclecounter *cc,
104			     u64 start_tstamp);
105
106/**
107 * timecounter_read - return nanoseconds elapsed since timecounter_init()
108 *                    plus the initial time stamp
109 * @tc:          Pointer to time counter.
110 *
111 * In other words, keeps track of time since the same epoch as
112 * the function which generated the initial time stamp.
113 *
114 * Returns: nanoseconds since the initial time stamp
115 */
116extern u64 timecounter_read(struct timecounter *tc);
117
118/**
119 * timecounter_cyc2time - convert a cycle counter to same
120 *                        time base as values returned by
121 *                        timecounter_read()
122 * @tc:		Pointer to time counter.
123 * @cycle_tstamp:	a value returned by tc->cc->read()
124 *
125 * Cycle counts that are converted correctly as long as they
126 * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
127 * with "max cycle count" == cs->mask+1.
128 *
129 * This allows conversion of cycle counter values which were generated
130 * in the past.
131 *
132 * Returns: cycle counter converted to nanoseconds since the initial time stamp
133 */
134extern u64 timecounter_cyc2time(const struct timecounter *tc,
135				u64 cycle_tstamp);
136
137#endif