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

[Mika Kuoppala]

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() ?
-Mika

> -	*dt = ktime_get_raw_fast_ns();
> +	*dt = local_clock();
>  	write_semaphore(&sema[2], 0);
>  	udelay(100);
> +	*dt = local_clock() - *dt;
>  	write_semaphore(&sema[2], 1);
> -	*dt = ktime_get_raw_fast_ns() - *dt;
>  	preempt_enable();
>  
>  	if (i915_request_wait(rq, 0, HZ / 2) < 0) {
> -- 
> 2.20.1
>
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