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

[Chris Wilson]

Quoting Mika Kuoppala (2021-01-12 19:19:34)
> Chris Wilson <chris at 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 at 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???
-Chris