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