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Commit 1be39679 authored by Martin Schwidefsky's avatar Martin Schwidefsky Committed by Thomas Gleixner
Browse files

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: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarJohn Stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
LKML-Reference: <20090814134808.142191175@de.ibm.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent a0f7d48b

arch/x86/kernel/tsc.c

+6 −0
Original line number Diff line number Diff line
@@ -744,10 +744,16 @@ static cycle_t __vsyscall_fn vread_tsc(void) 
}

#endif



static void resume_tsc(void)

{

	clocksource_tsc.cycle_last = 0;

}



static struct clocksource clocksource_tsc = {

	.name                   = "tsc",

	.rating                 = 300,

	.read                   = read_tsc,

	.resume			= resume_tsc,

	.mask                   = CLOCKSOURCE_MASK(64),

	.shift                  = 22,

	.flags                  = CLOCK_SOURCE_IS_CONTINUOUS |

kernel/time/timekeeping.c

+0 −1
Original line number Diff line number Diff line
@@ -295,7 +295,6 @@ static void change_clocksource(void) 
	if (old->disable)

		old->disable(old);



	clock->cycle_last = 0;

	clock->cycle_last = clock->read(clock);

	clock->error = 0;

	clock->xtime_nsec = 0;