Commit 1be39679 authored Aug 14, 2009 by Martin Schwidefsky Committed by Thomas Gleixner Aug 15, 2009

timekeeping: Move reset of cycle_last for tsc clocksource to tsc



change_clocksource resets the cycle_last value to zero then sets it to
a value read from the clocksource. The reset to zero is required only
for the TSC clocksource to make the read_tsc function work after a
resume. The reason is that the TSC read function uses cycle_last to
detect backwards going TSCs. In the resume case cycle_last contains
the TSC value from the last update before the suspend. On resume the
TSC starts counting from 0 again and would trip over the cycle_last
comparison.

This is subtle and surprising. Move the reset to a resume function in
the tsc code.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
LKML-Reference: <20090814134808.142191175@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent a0f7d48b

arch/x86/kernel/tsc.c

+6 −0

Original line number	Original line	Diff line number	Diff line
	@@ -744,10 +744,16 @@ static cycle_t __vsyscall_fn vread_tsc(void)
	}		}
	#endif		#endif

			static void resume_tsc(void)
			{
			clocksource_tsc.cycle_last = 0;
			}

	static struct clocksource clocksource_tsc = {		static struct clocksource clocksource_tsc = {
	.name = "tsc",		.name = "tsc",
	.rating = 300,		.rating = 300,
	.read = read_tsc,		.read = read_tsc,
			.resume = resume_tsc,
	.mask = CLOCKSOURCE_MASK(64),		.mask = CLOCKSOURCE_MASK(64),
	.shift = 22,		.shift = 22,
	.flags = CLOCK_SOURCE_IS_CONTINUOUS \|		.flags = CLOCK_SOURCE_IS_CONTINUOUS \|

kernel/time/timekeeping.c

+0 −1

Original line number	Original line	Diff line number	Diff line
	@@ -295,7 +295,6 @@ static void change_clocksource(void)
	if (old->disable)		if (old->disable)
	old->disable(old);		old->disable(old);

	clock->cycle_last = 0;
	clock->cycle_last = clock->read(clock);		clock->cycle_last = clock->read(clock);
	clock->error = 0;		clock->error = 0;
	clock->xtime_nsec = 0;		clock->xtime_nsec = 0;

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