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