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clocksource: Move cycle_last validation to core code

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The only user of the cycle_last validation is the x86 TSC. In order to
provide NMI safe accessor functions for clock monotonic and
monotonic_raw we need to do that in the core.

We can't do the TSC specific

    if (now < cycle_last)
       	    now = cycle_last;

for the other wrapping around clocksources, but TSC has
CLOCKSOURCE_MASK(64) which actually does not mask out anything so if
now is less than cycle_last the subtraction will give a negative
result. So we can check for that in clocksource_delta() and return 0
for that case.

Implement and enable it for x86

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
Thomas Gleixner
2014-07-16 21:05:12 +00:00
committed by John Stultz
parent 3a97837784
commit 09ec54429c
4 changed files with 24 additions and 12 deletions
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21
arch/x86/kernel/tsc.c
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@@ -951,7 +951,7 @@ core_initcall(cpufreq_tsc);
static struct clocksource clocksource_tsc;
/*
* We compare the TSC to the cycle_last value in the clocksource
* We used to compare the TSC to the cycle_last value in the clocksource
* structure to avoid a nasty time-warp. This can be observed in a
* very small window right after one CPU updated cycle_last under
* xtime/vsyscall_gtod lock and the other CPU reads a TSC value which
@@ -961,26 +961,23 @@ static struct clocksource clocksource_tsc;
* due to the unsigned delta calculation of the time keeping core
* code, which is necessary to support wrapping clocksources like pm
* timer.
*
* This sanity check is now done in the core timekeeping code.
* checking the result of read_tsc() - cycle_last for being negative.
* That works because CLOCKSOURCE_MASK(64) does not mask out any bit.
*/
static cycle_t read_tsc(struct clocksource *cs)
{
cycle_t ret = (cycle_t)get_cycles();
return ret >= clocksource_tsc.cycle_last ?
ret : clocksource_tsc.cycle_last;
}
static void resume_tsc(struct clocksource *cs)
{
if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
clocksource_tsc.cycle_last = 0;
return (cycle_t)get_cycles();
}
/*
* .mask MUST be CLOCKSOURCE_MASK(64). See comment above read_tsc()
*/
static struct clocksource clocksource_tsc = {
.name = "tsc",
.rating = 300,
.read = read_tsc,
.resume = resume_tsc,
.mask = CLOCKSOURCE_MASK(64),
.flags = CLOCK_SOURCE_IS_CONTINUOUS |
CLOCK_SOURCE_MUST_VERIFY,