Re: [Intel-gfx] [PATCH] drm/i915/selftests: Rearrange ktime_get to reduce latency against CS

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Quoting Mika Kuoppala (2021-01-12 19:19:34)
>> Chris Wilson <chris@chris-wilson.co.uk> writes:
>> 
>> > In our tests where we measure the elapsed time on both the CPU and CS
>> > using a udelay, our CS results match the udelay much more accurately
>> > than the ktime (even when using ktime_get_fast_ns). With preemption
>> > disabled, we can go one step lower than ktime and use local_clock.
>> >
>> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2919
>> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> > ---
>> >  drivers/gpu/drm/i915/gt/selftest_engine_pm.c | 4 ++--
>> >  1 file changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
>> > index ca080445695e..c3d965279fc3 100644
>> > --- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
>> > +++ b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
>> > @@ -112,11 +112,11 @@ static int __measure_timestamps(struct intel_context *ce,
>> >  
>> >       /* Run the request for a 100us, sampling timestamps before/after */
>> >       preempt_disable();
>> 
>> Do you need to promote this to local_irq_disable() ?
>
> Good suggestion. Will try to remember if we still see discrepancies...
>
> Interrupt handlers are meant to <5us, right???

With both test types, we might sometimes find out what they are :)

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> -Chris
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