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
21void 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}
31EXPORT_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 */
44static 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
65u64 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}
76EXPORT_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 */
82static 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
92u64 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}
112EXPORT_SYMBOL_GPL(timecounter_cyc2time);
113