mirror of
https://github.com/torvalds/linux
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2eebdde652
The current timecounter implementation will drop a variable amount of resolution, depending on the magnitude of the time delta. In other words, reading the clock too often or too close to a time stamp conversion will introduce errors into the time values. This patch fixes the issue by introducing a fractional nanosecond field that accumulates the low order bits. Reported-by: Janusz Użycki <j.uzycki@elproma.com.pl> Signed-off-by: Richard Cochran <richardcochran@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
112 lines
3 KiB
C
112 lines
3 KiB
C
/*
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* linux/kernel/time/timecounter.c
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*
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* based on code that migrated away from
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* linux/kernel/time/clocksource.c
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*/
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#include <linux/export.h>
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#include <linux/timecounter.h>
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void timecounter_init(struct timecounter *tc,
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const struct cyclecounter *cc,
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u64 start_tstamp)
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{
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tc->cc = cc;
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tc->cycle_last = cc->read(cc);
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tc->nsec = start_tstamp;
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tc->mask = (1ULL << cc->shift) - 1;
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tc->frac = 0;
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}
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EXPORT_SYMBOL_GPL(timecounter_init);
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/**
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* timecounter_read_delta - get nanoseconds since last call of this function
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* @tc: Pointer to time counter
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*
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* When the underlying cycle counter runs over, this will be handled
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* correctly as long as it does not run over more than once between
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* calls.
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*
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* The first call to this function for a new time counter initializes
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* the time tracking and returns an undefined result.
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*/
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static u64 timecounter_read_delta(struct timecounter *tc)
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{
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cycle_t cycle_now, cycle_delta;
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u64 ns_offset;
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/* read cycle counter: */
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cycle_now = tc->cc->read(tc->cc);
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/* calculate the delta since the last timecounter_read_delta(): */
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cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;
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/* convert to nanoseconds: */
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ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
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tc->mask, &tc->frac);
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/* update time stamp of timecounter_read_delta() call: */
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tc->cycle_last = cycle_now;
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return ns_offset;
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}
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u64 timecounter_read(struct timecounter *tc)
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{
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u64 nsec;
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/* increment time by nanoseconds since last call */
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nsec = timecounter_read_delta(tc);
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nsec += tc->nsec;
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tc->nsec = nsec;
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return nsec;
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}
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EXPORT_SYMBOL_GPL(timecounter_read);
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/*
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* This is like cyclecounter_cyc2ns(), but it is used for computing a
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* time previous to the time stored in the cycle counter.
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*/
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static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,
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cycle_t cycles, u64 mask, u64 frac)
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{
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u64 ns = (u64) cycles;
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ns = ((ns * cc->mult) - frac) >> cc->shift;
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return ns;
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}
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u64 timecounter_cyc2time(struct timecounter *tc,
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cycle_t cycle_tstamp)
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{
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u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
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u64 nsec = tc->nsec, frac = tc->frac;
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/*
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* Instead of always treating cycle_tstamp as more recent
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* than tc->cycle_last, detect when it is too far in the
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* future and treat it as old time stamp instead.
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*/
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if (delta > tc->cc->mask / 2) {
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delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
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nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);
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} else {
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nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);
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}
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return nsec;
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}
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EXPORT_SYMBOL_GPL(timecounter_cyc2time);
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