2 * Utility code which helps transforming between two different time
3 * bases, called "source" and "target" time in this code.
5 * Source time has to be provided via the timecounter API while target
6 * time is accessed via a function callback whose prototype
7 * intentionally matches ktime_get() and ktime_get_real(). These
8 * interfaces where chosen like this so that the code serves its
9 * initial purpose without additional glue code.
11 * This purpose is synchronizing a hardware clock in a NIC with system
12 * time, in order to implement the Precision Time Protocol (PTP,
13 * IEEE1588) with more accurate hardware assisted time stamping. In
14 * that context only synchronization against system time (=
15 * ktime_get_real()) is currently needed. But this utility code might
16 * become useful in other situations, which is why it was written as
17 * general purpose utility code.
19 * The source timecounter is assumed to return monotonically
20 * increasing time (but this code does its best to compensate if that
21 * is not the case) whereas target time may jump.
23 * The target time corresponding to a source time is determined by
24 * reading target time, reading source time, reading target time
25 * again, then assuming that average target time corresponds to source
26 * time. In other words, the assumption is that reading the source
27 * time is slow and involves equal time for sending the request and
28 * receiving the reply, whereas reading target time is assumed to be
31 * Copyright (C) 2009 Intel Corporation.
32 * Author: Patrick Ohly <patrick.ohly@intel.com>
34 * This program is free software; you can redistribute it and/or modify it
35 * under the terms and conditions of the GNU General Public License,
36 * version 2, as published by the Free Software Foundation.
38 * This program is distributed in the hope it will be useful, but WITHOUT
39 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
40 * FITNESS FOR A PARTICULAR PURPOSE. * See the GNU General Public License for
43 * You should have received a copy of the GNU General Public License along with
44 * this program; if not, write to the Free Software Foundation, Inc.,
45 * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
47 #ifndef _LINUX_TIMECOMPARE_H
48 #define _LINUX_TIMECOMPARE_H
50 #include <linux/clocksource.h>
51 #include <linux/ktime.h>
54 * struct timecompare - stores state and configuration for the two clocks
56 * Initialize to zero, then set source/target/num_samples.
58 * Transformation between source time and target time is done with:
59 * target_time = source_time + offset +
60 * (source_time - last_update) * skew /
61 * TIMECOMPARE_SKEW_RESOLUTION
63 * @source: used to get source time stamps via timecounter_read()
64 * @target: function returning target time (for example, ktime_get
65 * for monotonic time, or ktime_get_real for wall clock)
66 * @num_samples: number of times that source time and target time are to
67 * be compared when determining their offset
68 * @offset: (target time - source time) at the time of the last update
69 * @skew: average (target time - source time) / delta source time *
70 * TIMECOMPARE_SKEW_RESOLUTION
71 * @last_update: last source time stamp when time offset was measured
74 struct timecounter
*source
;
75 ktime_t (*target
)(void);
84 * timecompare_transform - transform source time stamp into target time base
85 * @sync: context for time sync
86 * @source_tstamp: the result of timecounter_read() or
87 * timecounter_cyc2time()
89 extern ktime_t
timecompare_transform(struct timecompare
*sync
,
93 * timecompare_offset - measure current (target time - source time) offset
94 * @sync: context for time sync
95 * @offset: average offset during sample period returned here
96 * @source_tstamp: average source time during sample period returned here
98 * Returns number of samples used. Might be zero (= no result) in the
99 * unlikely case that target time was monotonically decreasing for all
100 * samples (= broken).
102 extern int timecompare_offset(struct timecompare
*sync
,
106 extern void __timecompare_update(struct timecompare
*sync
,
110 * timecompare_update - update offset and skew by measuring current offset
111 * @sync: context for time sync
112 * @source_tstamp: the result of timecounter_read() or
113 * timecounter_cyc2time(), pass zero to force update
115 * Updates are only done at most once per second.
117 static inline void timecompare_update(struct timecompare
*sync
,
120 if (!source_tstamp
||
121 (s64
)(source_tstamp
- sync
->last_update
) >= NSEC_PER_SEC
)
122 __timecompare_update(sync
, source_tstamp
);
125 #endif /* _LINUX_TIMECOMPARE_H */
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