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timekeeping: Move reset of cycle_last for tsc clocksource to tsc

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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>
This commit is contained in:
Martin Schwidefsky 2009-08-14 15:47:20 +02:00 committed by Thomas Gleixner
parent a0f7d48bfb
commit 1be3967948
2 changed files with 6 additions and 1 deletions
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6
arch/x86/kernel/tsc.c
View file

@ -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 |

1
kernel/time/timekeeping.c
View file

@ -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;