xref: /openbmc/linux/kernel/time/timecounter.c (revision d7a3d85e)
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 
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  */
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 
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  */
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 
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