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Diffstat (limited to 'include')
| -rw-r--r-- | include/linux/timecompare.h | 125 |
1 files changed, 125 insertions, 0 deletions
diff --git a/include/linux/timecompare.h b/include/linux/timecompare.h new file mode 100644 index 00000000000..546e2234e4b --- /dev/null +++ b/include/linux/timecompare.h @@ -0,0 +1,125 @@ +/* + * 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 */ |