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  */
30 struct cyclecounter {
31 	u64 (*read)(const 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  */
55 struct timecounter {
56 	const 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  */
cyclecounter_cyc2ns(const struct cyclecounter * cc,u64 cycles,u64 mask,u64 * frac)70 static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
71 				      u64 cycles, u64 mask, u64 *frac)
72 {
73 	u64 ns = (u64) cycles;
74 
75 	ns = (ns * cc->mult) + *frac;
76 	*frac = ns & mask;
77 	return ns >> cc->shift;
78 }
79 
80 /**
81  * timecounter_adjtime - Shifts the time of the clock.
82  * @delta:	Desired change in nanoseconds.
83  */
timecounter_adjtime(struct timecounter * tc,s64 delta)84 static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
85 {
86 	tc->nsec += delta;
87 }
88 
89 /**
90  * timecounter_init - initialize a time counter
91  * @tc:			Pointer to time counter which is to be initialized/reset
92  * @cc:			A cycle counter, ready to be used.
93  * @start_tstamp:	Arbitrary initial time stamp.
94  *
95  * After this call the current cycle register (roughly) corresponds to
96  * the initial time stamp. Every call to timecounter_read() increments
97  * the time stamp counter by the number of elapsed nanoseconds.
98  */
99 extern void timecounter_init(struct timecounter *tc,
100 			     const struct cyclecounter *cc,
101 			     u64 start_tstamp);
102 
103 /**
104  * timecounter_read - return nanoseconds elapsed since timecounter_init()
105  *                    plus the initial time stamp
106  * @tc:          Pointer to time counter.
107  *
108  * In other words, keeps track of time since the same epoch as
109  * the function which generated the initial time stamp.
110  */
111 extern u64 timecounter_read(struct timecounter *tc);
112 
113 /**
114  * timecounter_cyc2time - convert a cycle counter to same
115  *                        time base as values returned by
116  *                        timecounter_read()
117  * @tc:		Pointer to time counter.
118  * @cycle_tstamp:	a value returned by tc->cc->read()
119  *
120  * Cycle counts that are converted correctly as long as they
121  * fall into the interval [-1/2 max cycle count, +1/2 max cycle count],
122  * with "max cycle count" == cs->mask+1.
123  *
124  * This allows conversion of cycle counter values which were generated
125  * in the past.
126  */
127 extern u64 timecounter_cyc2time(const struct timecounter *tc,
128 				u64 cycle_tstamp);
129 
130 #endif
131