kernel/time/timecounter.c

/*
 * linux/kernel/time/timecounter.c
 *
 * based on code that migrated away from
 * linux/kernel/time/clocksource.c
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that 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.
 */

#include <linux/export.h>
#include <linux/timecounter.h>

void timecounter_init(struct timecounter *tc,
		      const struct cyclecounter *cc,
		      u64 start_tstamp)
{
	tc->cc = cc;
	tc->cycle_last = cc->read(cc);
	tc->nsec = start_tstamp;
}
EXPORT_SYMBOL_GPL(timecounter_init);

/**
 * timecounter_read_delta - get nanoseconds since last call of this function
 * @tc:         Pointer to time counter
 *
 * When the underlying cycle counter runs over, this will be handled
 * correctly as long as it does not run over more than once between
 * calls.
 *
 * The first call to this function for a new time counter initializes
 * the time tracking and returns an undefined result.
 */
static u64 timecounter_read_delta(struct timecounter *tc)
{
	cycle_t cycle_now, cycle_delta;
	u64 ns_offset;

	/* read cycle counter: */
	cycle_now = tc->cc->read(tc->cc);

	/* calculate the delta since the last timecounter_read_delta(): */
	cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;

	/* convert to nanoseconds: */
	ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta);

	/* update time stamp of timecounter_read_delta() call: */
	tc->cycle_last = cycle_now;

	return ns_offset;
}

u64 timecounter_read(struct timecounter *tc)
{
	u64 nsec;

	/* increment time by nanoseconds since last call */
	nsec = timecounter_read_delta(tc);
	nsec += tc->nsec;
	tc->nsec = nsec;

	return nsec;
}
EXPORT_SYMBOL_GPL(timecounter_read);

u64 timecounter_cyc2time(struct timecounter *tc,
			 cycle_t cycle_tstamp)
{
	u64 cycle_delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;
	u64 nsec;

	/*
	 * Instead of always treating cycle_tstamp as more recent
	 * than tc->cycle_last, detect when it is too far in the
	 * future and treat it as old time stamp instead.
	 */
	if (cycle_delta > tc->cc->mask / 2) {
		cycle_delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;
		nsec = tc->nsec - cyclecounter_cyc2ns(tc->cc, cycle_delta);
	} else {
		nsec = cyclecounter_cyc2ns(tc->cc, cycle_delta) + tc->nsec;
	}

	return nsec;
}
EXPORT_SYMBOL_GPL(timecounter_cyc2time);