1 /*
2 * linux/kernel/time/timecounter.c
3 *
4 * based on code that migrated away from
5 * linux/kernel/time/clocksource.c
6 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License as published by
9 * the Free Software Foundation; either version 2 of the License, or
10 * (at your option) any later version.
11 *
12 * This program is distributed in the hope that it will be useful,
13 * but WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 * GNU General Public License for more details.
16 */
17
18 #include <linux/export.h>
19 #include <linux/timecounter.h>
20
timecounter_init(struct timecounter * tc,const struct cyclecounter * cc,u64 start_tstamp)21 void timecounter_init(struct timecounter *tc,
22 const struct cyclecounter *cc,
23 u64 start_tstamp)
24 {
25 tc->cc = cc;
26 tc->cycle_last = cc->read(cc);
27 tc->nsec = start_tstamp;
28 tc->mask = (1ULL << cc->shift) - 1;
29 tc->frac = 0;
30 }
31 EXPORT_SYMBOL_GPL(timecounter_init);
32
33 /**
34 * timecounter_read_delta - get nanoseconds since last call of this function
35 * @tc: Pointer to time counter
36 *
37 * When the underlying cycle counter runs over, this will be handled
38 * correctly as long as it does not run over more than once between
39 * calls.
40 *
41 * The first call to this function for a new time counter initializes
42 * the time tracking and returns an undefined result.
43 */
timecounter_read_delta(struct timecounter * tc)44 static u64 timecounter_read_delta(struct timecounter *tc)
45 {
46 cycle_t cycle_now, cycle_delta;
47 u64 ns_offset;
48
49 /* read cycle counter: */
50 cycle_now = tc->cc->read(tc->cc);
51
52 /* calculate the delta since the last timecounter_read_delta(): */
53 cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
54
55 /* convert to nanoseconds: */
56 ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
57 tc->mask, &tc->frac);
58
59 /* update time stamp of timecounter_read_delta() call: */
60 tc->cycle_last = cycle_now;
61
62 return ns_offset;
63 }
64
timecounter_read(struct timecounter * tc)65 u64 timecounter_read(struct timecounter *tc)
66 {
67 u64 nsec;
68
69 /* increment time by nanoseconds since last call */
70 nsec = timecounter_read_delta(tc);
71 nsec += tc->nsec;
72 tc->nsec = nsec;
73
74 return nsec;
75 }
76 EXPORT_SYMBOL_GPL(timecounter_read);
77
78 /*
79 * This is like cyclecounter_cyc2ns(), but it is used for computing a
80 * time previous to the time stored in the cycle counter.
81 */
cc_cyc2ns_backwards(const struct cyclecounter * cc,cycle_t cycles,u64 mask,u64 frac)82 static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
83 cycle_t cycles, u64 mask, u64 frac)
84 {
85 u64 ns = (u64) cycles;
86
87 ns = ((ns * cc->mult) - frac) >> cc->shift;
88
89 return ns;
90 }
91
timecounter_cyc2time(struct timecounter * tc,cycle_t cycle_tstamp)92 u64 timecounter_cyc2time(struct timecounter *tc,
93 cycle_t cycle_tstamp)
94 {
95 u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
96 u64 nsec = tc->nsec, frac = tc->frac;
97
98 /*
99 * Instead of always treating cycle_tstamp as more recent
100 * than tc->cycle_last, detect when it is too far in the
101 * future and treat it as old time stamp instead.
102 */
103 if (delta > tc->cc->mask / 2) {
104 delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
105 nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);
106 } else {
107 nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);
108 }
109
110 return nsec;
111 }
112 EXPORT_SYMBOL_GPL(timecounter_cyc2time);
113