Commit 2eebdde6 authored by Richard Cochran's avatar Richard Cochran Committed by David S. Miller
Browse files

timecounter: keep track of accumulated fractional nanoseconds 



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: default avatarJanusz Użycki <j.uzycki@elproma.com.pl>
Signed-off-by: default avatarRichard Cochran <richardcochran@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent f25a30be

drivers/net/ethernet/mellanox/mlx4/en_clock.c

+2 −2
Original line number Diff line number Diff line
@@ -240,7 +240,7 @@ void mlx4_en_init_timestamp(struct mlx4_en_dev *mdev) 
{

	struct mlx4_dev *dev = mdev->dev;

	unsigned long flags;

	u64 ns;

	u64 ns, zero = 0;



	rwlock_init(&mdev->clock_lock);
@@ -265,7 +265,7 @@ void mlx4_en_init_timestamp(struct mlx4_en_dev *mdev) 
	/* Calculate period in seconds to call the overflow watchdog - to make

	 * sure counter is checked at least once every wrap around.

	 */

	ns = cyclecounter_cyc2ns(&mdev->cycles, mdev->cycles.mask);

	ns = cyclecounter_cyc2ns(&mdev->cycles, mdev->cycles.mask, zero, &zero);

	do_div(ns, NSEC_PER_SEC / 2 / HZ);

	mdev->overflow_period = ns;

include/linux/timecounter.h

+12 −7
Original line number Diff line number Diff line
@@ -55,27 +55,32 @@ struct cyclecounter { 
 * @cycle_last:		most recent cycle counter value seen by

 *			timecounter_read()

 * @nsec:		continuously increasing count

 * @mask:		bit mask for maintaining the 'frac' field

 * @frac:		accumulated fractional nanoseconds

 */

struct timecounter {

	const struct cyclecounter *cc;

	cycle_t cycle_last;

	u64 nsec;

	u64 mask;

	u64 frac;

};



/**

 * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds

 * @cc:		Pointer to cycle counter.

 * @cycles:	Cycles

 *

 * XXX - This could use some mult_lxl_ll() asm optimization. Same code

 * as in cyc2ns, but with unsigned result.

 * @mask:	bit mask for maintaining the 'frac' field

 * @frac:	pointer to storage for the fractional nanoseconds.

 */

static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,

				      cycle_t cycles)

				      cycle_t cycles, u64 mask, u64 *frac)

{

	u64 ret = (u64)cycles;

	ret = (ret * cc->mult) >> cc->shift;

	return ret;

	u64 ns = (u64) cycles;



	ns = (ns * cc->mult) + *frac;

	*frac = ns & mask;

	return ns >> cc->shift;

}



/**

kernel/time/timecounter.c

+24 −7
Original line number Diff line number Diff line
@@ -25,6 +25,8 @@ void timecounter_init(struct timecounter *tc, 
	tc->cc = cc;

	tc->cycle_last = cc->read(cc);

	tc->nsec = start_tstamp;

	tc->mask = (1ULL << cc->shift) - 1;

	tc->frac = 0;

}

EXPORT_SYMBOL_GPL(timecounter_init);
@@ -51,7 +53,8 @@ static u64 timecounter_read_delta(struct timecounter *tc) 
	cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;



	/* convert to nanoseconds: */

	ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta);

	ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,

					tc->mask, &tc->frac);



	/* update time stamp of timecounter_read_delta() call: */

	tc->cycle_last = cycle_now;
@@ -72,22 +75,36 @@ u64 timecounter_read(struct timecounter *tc) 
}

EXPORT_SYMBOL_GPL(timecounter_read);



/*

 * This is like cyclecounter_cyc2ns(), but it is used for computing a

 * time previous to the time stored in the cycle counter.

 */

static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc,

			       cycle_t cycles, u64 mask, u64 frac)

{

	u64 ns = (u64) cycles;



	ns = ((ns * cc->mult) - frac) >> cc->shift;



	return ns;

}



u64 timecounter_cyc2time(struct timecounter *tc,

			 cycle_t cycle_tstamp)

{

	u64 cycle_delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;

	u64 nsec;

	u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask;

	u64 nsec = tc->nsec, frac = tc->frac;



	/*

	 * 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);

	if (delta > tc->cc->mask / 2) {

		delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask;

		nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac);

	} else {

		nsec = cyclecounter_cyc2ns(tc->cc, cycle_delta) + tc->nsec;

		nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac);

	}



	return nsec;

virt/kvm/arm/arch_timer.c

+2 −1
Original line number Diff line number Diff line
@@ -152,7 +152,8 @@ void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu) 
		return;

	}



	ns = cyclecounter_cyc2ns(timecounter->cc, cval - now);

	ns = cyclecounter_cyc2ns(timecounter->cc, cval - now, timecounter->mask,

				 &timecounter->frac);

	timer_arm(timer, ns);

}

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