xref: /openbmc/linux/kernel/time/timecounter.c (revision 07ff4aed)
1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3  * Based on clocksource code. See commit 74d23cc704d1
4  */
5 #include <linux/export.h>
6 #include <linux/timecounter.h>
7 
timecounter_init(struct timecounter * tc,const struct cyclecounter * cc,u64 start_tstamp)8 void timecounter_init(struct timecounter *tc,
9 		      const struct cyclecounter *cc,
10 		      u64 start_tstamp)
11 {
12 	tc->cc = cc;
13 	tc->cycle_last = cc->read(cc);
14 	tc->nsec = start_tstamp;
15 	tc->mask = (1ULL << cc->shift) - 1;
16 	tc->frac = 0;
17 }
18 EXPORT_SYMBOL_GPL(timecounter_init);
19 
20 /**
21  * timecounter_read_delta - get nanoseconds since last call of this function
22  * @tc:         Pointer to time counter
23  *
24  * When the underlying cycle counter runs over, this will be handled
25  * correctly as long as it does not run over more than once between
26  * calls.
27  *
28  * The first call to this function for a new time counter initializes
29  * the time tracking and returns an undefined result.
30  */
timecounter_read_delta(struct timecounter * tc)31 static u64 timecounter_read_delta(struct timecounter *tc)
32 {
33 	u64 cycle_now, cycle_delta;
34 	u64 ns_offset;
35 
36 	/* read cycle counter: */
37 	cycle_now = tc->cc->read(tc->cc);
38 
39 	/* calculate the delta since the last timecounter_read_delta(): */
40 	cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
41 
42 	/* convert to nanoseconds: */
43 	ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
44 					tc->mask, &tc->frac);
45 
46 	/* update time stamp of timecounter_read_delta() call: */
47 	tc->cycle_last = cycle_now;
48 
49 	return ns_offset;
50 }
51 
timecounter_read(struct timecounter * tc)52 u64 timecounter_read(struct timecounter *tc)
53 {
54 	u64 nsec;
55 
56 	/* increment time by nanoseconds since last call */
57 	nsec = timecounter_read_delta(tc);
58 	nsec += tc->nsec;
59 	tc->nsec = nsec;
60 
61 	return nsec;
62 }
63 EXPORT_SYMBOL_GPL(timecounter_read);
64 
65 /*
66  * This is like cyclecounter_cyc2ns(), but it is used for computing a
67  * time previous to the time stored in the cycle counter.
68  */
cc_cyc2ns_backwards(const struct cyclecounter * cc,u64 cycles,u64 mask,u64 frac)69 static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
70 			       u64 cycles, u64 mask, u64 frac)
71 {
72 	u64 ns = (u64) cycles;
73 
74 	ns = ((ns * cc->mult) - frac) >> cc->shift;
75 
76 	return ns;
77 }
78 
timecounter_cyc2time(const struct timecounter * tc,u64 cycle_tstamp)79 u64 timecounter_cyc2time(const struct timecounter *tc,
80 			 u64 cycle_tstamp)
81 {
82 	u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
83 	u64 nsec = tc->nsec, frac = tc->frac;
84 
85 	/*
86 	 * Instead of always treating cycle_tstamp as more recent
87 	 * than tc->cycle_last, detect when it is too far in the
88 	 * future and treat it as old time stamp instead.
89 	 */
90 	if (delta > tc->cc->mask / 2) {
91 		delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
92 		nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);
93 	} else {
94 		nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);
95 	}
96 
97 	return nsec;
98 }
99 EXPORT_SYMBOL_GPL(timecounter_cyc2time);
100