126 lines
4.9 KiB
C
126 lines
4.9 KiB
C
/*
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* Utility code which helps transforming between two different time
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* bases, called "source" and "target" time in this code.
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*
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* Source time has to be provided via the timecounter API while target
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* time is accessed via a function callback whose prototype
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* intentionally matches ktime_get() and ktime_get_real(). These
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* interfaces where chosen like this so that the code serves its
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* initial purpose without additional glue code.
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*
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* This purpose is synchronizing a hardware clock in a NIC with system
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* time, in order to implement the Precision Time Protocol (PTP,
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* IEEE1588) with more accurate hardware assisted time stamping. In
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* that context only synchronization against system time (=
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* ktime_get_real()) is currently needed. But this utility code might
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* become useful in other situations, which is why it was written as
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* general purpose utility code.
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*
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* The source timecounter is assumed to return monotonically
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* increasing time (but this code does its best to compensate if that
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* is not the case) whereas target time may jump.
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*
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* The target time corresponding to a source time is determined by
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* reading target time, reading source time, reading target time
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* again, then assuming that average target time corresponds to source
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* time. In other words, the assumption is that reading the source
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* time is slow and involves equal time for sending the request and
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* receiving the reply, whereas reading target time is assumed to be
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* fast.
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*
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* Copyright (C) 2009 Intel Corporation.
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* Author: Patrick Ohly <patrick.ohly@intel.com>
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*
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* This program is free software; you can redistribute it and/or modify it
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* under the terms and conditions of the GNU General Public License,
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* version 2, as published by the Free Software Foundation.
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*
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* This program is distributed in the hope it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. * See the GNU General Public License for
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* more details.
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*
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* You should have received a copy of the GNU General Public License along with
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* this program; if not, write to the Free Software Foundation, Inc.,
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* 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
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*/
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#ifndef _LINUX_TIMECOMPARE_H
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#define _LINUX_TIMECOMPARE_H
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#include <linux/clocksource.h>
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#include <linux/ktime.h>
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/**
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* struct timecompare - stores state and configuration for the two clocks
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*
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* Initialize to zero, then set source/target/num_samples.
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*
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* Transformation between source time and target time is done with:
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* target_time = source_time + offset +
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* (source_time - last_update) * skew /
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* TIMECOMPARE_SKEW_RESOLUTION
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*
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* @source: used to get source time stamps via timecounter_read()
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* @target: function returning target time (for example, ktime_get
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* for monotonic time, or ktime_get_real for wall clock)
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* @num_samples: number of times that source time and target time are to
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* be compared when determining their offset
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* @offset: (target time - source time) at the time of the last update
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* @skew: average (target time - source time) / delta source time *
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* TIMECOMPARE_SKEW_RESOLUTION
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* @last_update: last source time stamp when time offset was measured
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*/
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struct timecompare {
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struct timecounter *source;
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ktime_t (*target)(void);
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int num_samples;
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s64 offset;
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s64 skew;
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u64 last_update;
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};
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/**
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* timecompare_transform - transform source time stamp into target time base
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* @sync: context for time sync
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* @source_tstamp: the result of timecounter_read() or
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* timecounter_cyc2time()
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*/
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extern ktime_t timecompare_transform(struct timecompare *sync,
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u64 source_tstamp);
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/**
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* timecompare_offset - measure current (target time - source time) offset
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* @sync: context for time sync
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* @offset: average offset during sample period returned here
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* @source_tstamp: average source time during sample period returned here
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*
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* Returns number of samples used. Might be zero (= no result) in the
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* unlikely case that target time was monotonically decreasing for all
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* samples (= broken).
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*/
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extern int timecompare_offset(struct timecompare *sync,
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s64 *offset,
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u64 *source_tstamp);
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extern void __timecompare_update(struct timecompare *sync,
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u64 source_tstamp);
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/**
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* timecompare_update - update offset and skew by measuring current offset
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* @sync: context for time sync
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* @source_tstamp: the result of timecounter_read() or
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* timecounter_cyc2time(), pass zero to force update
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*
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* Updates are only done at most once per second.
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*/
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static inline void timecompare_update(struct timecompare *sync,
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u64 source_tstamp)
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{
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if (!source_tstamp ||
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(s64)(source_tstamp - sync->last_update) >= NSEC_PER_SEC)
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__timecompare_update(sync, source_tstamp);
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}
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#endif /* _LINUX_TIMECOMPARE_H */
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