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Commit a386b5af authored by Kasper Pedersen's avatar Kasper Pedersen Committed by Thomas Gleixner
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time: Compensate for rounding on odd-frequency clocksources 

When the clocksource is not a multiple of HZ, the clock will be off.  For
acpi_pm, HZ=1000 the error is 127.111 ppm:

The rounding of cycle_interval ends up generating a false error term in
ntp_error accumulation since xtime_interval is not exactly 1/HZ.  So, we
subtract out the error caused by the rounding.

This has been visible since 2.6.32-rc2
	commit a092ff0f
	time: Implement logarithmic time accumulation
That commit raised NTP_INTERVAL_FREQ and exposed the rounding error.

testing tool: http://n1.taur.dk/permanent/testpmt.c


Also tested with ntpd and a frequency counter.

Signed-off-by: default avatarKasper Pedersen <kkp2010@kasperkp.dk>
Acked-by: default avatarjohn stultz <johnstul@us.ibm.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 8af3c153

kernel/time/timekeeping.c

+7 −2
Original line number Diff line number Diff line
@@ -32,6 +32,8 @@ struct timekeeper { 
	cycle_t cycle_interval;

	/* Number of clock shifted nano seconds in one NTP interval. */

	u64	xtime_interval;

	/* shifted nano seconds left over when rounding cycle_interval */

	s64	xtime_remainder;

	/* Raw nano seconds accumulated per NTP interval. */

	u32	raw_interval;
@@ -62,7 +64,7 @@ struct timekeeper timekeeper; 
static void timekeeper_setup_internals(struct clocksource *clock)

{

	cycle_t interval;

	u64 tmp;

	u64 tmp, ntpinterval;



	timekeeper.clock = clock;

	clock->cycle_last = clock->read(clock);
@@ -70,6 +72,7 @@ static void timekeeper_setup_internals(struct clocksource *clock) 
	/* Do the ns -> cycle conversion first, using original mult */

	tmp = NTP_INTERVAL_LENGTH;

	tmp <<= clock->shift;

	ntpinterval = tmp;

	tmp += clock->mult/2;

	do_div(tmp, clock->mult);

	if (tmp == 0)
@@ -80,6 +83,7 @@ static void timekeeper_setup_internals(struct clocksource *clock) 


	/* Go back from cycles -> shifted ns */

	timekeeper.xtime_interval = (u64) interval * clock->mult;

	timekeeper.xtime_remainder = ntpinterval - timekeeper.xtime_interval;

	timekeeper.raw_interval =

		((u64) interval * clock->mult) >> clock->shift;
@@ -719,7 +723,8 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift) 


	/* Accumulate error between NTP and clock interval */

	timekeeper.ntp_error += tick_length << shift;

	timekeeper.ntp_error -= timekeeper.xtime_interval <<

	timekeeper.ntp_error -=

	    (timekeeper.xtime_interval + timekeeper.xtime_remainder) <<

				(timekeeper.ntp_error_shift + shift);



	return offset;