linux/include/linux/timecompare.h
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   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 */
  73struct 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 */
  89extern 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 */
 102extern int timecompare_offset(struct timecompare *sync,
 103                              s64 *offset,
 104                              u64 *source_tstamp);
 105
 106extern 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 */
 117static 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 */
 126
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