include/linux/timecompare.h

   1 /*
   2  * Utility code which helps transforming between two different time
   3  * bases, called "source" and "target" time in this code.
   4  *
   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.
  10  *
  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.
  18  *
  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.
  22  *
  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
  29  * fast.
  30  *
  31  * Copyright (C) 2009 Intel Corporation.
  32  * Author: Patrick Ohly <patrick.ohly@intel.com>
  33  *
  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.
  37  *
  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
  41  * more details.
  42  *
  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.
  46  */
  47 #ifndef _LINUX_TIMECOMPARE_H
  48 #define _LINUX_TIMECOMPARE_H
  49
  50 #include <linux/clocksource.h>
  51 #include <linux/ktime.h>
  52
  53 /**
  54  * struct timecompare - stores state and configuration for the two clocks
  55  *
  56  * Initialize to zero, then set source/target/num_samples.
  57  *
  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
  62  *
  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
  72  */
  73 struct timecompare {
  74         struct timecounter *source;
  75         ktime_t (*target)(void);
  76         int num_samples;
  77
  78         s64 offset;
  79         s64 skew;
  80         u64 last_update;
  81 };
  82
  83 /**
  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()
  88  */
  89 extern ktime_t timecompare_transform(struct timecompare *sync,
  90                                      u64 source_tstamp);
  91
  92 /**
  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
  97  *
  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).
 101  */
 102 extern int timecompare_offset(struct timecompare *sync,
 103                               s64 *offset,
 104                               u64 *source_tstamp);
 105
 106 extern void __timecompare_update(struct timecompare *sync,
 107                                  u64 source_tstamp);
 108
 109 /**
 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
 114  *
 115  * Updates are only done at most once per second.
 116  */
 117 static inline void timecompare_update(struct timecompare *sync,
 118                                       u64 source_tstamp)
 119 {
 120         if (!source_tstamp ||
 121             (s64)(source_tstamp - sync->last_update) >= NSEC_PER_SEC)
 122                 __timecompare_update(sync, source_tstamp);
 123 }
 124
 125 #endif /* _LINUX_TIMECOMPARE_H */