[Intel-gfx] [PATCH] drm/i915/selftest: Bump up sample period for busy stats selftest

[Tvrtko Ursulin]

On 03/11/2022 00:11, Umesh Nerlige Ramappa wrote:
> Engine busyness samples around a 10ms period is failing with busyness
> ranging approx. from 87% to 115%. The expected range is +/- 5% of the
> sample period.
> 
> When determining busyness of active engine, the GuC based engine
> busyness implementation relies on a 64 bit timestamp register read. The
> latency incurred by this register read causes the failure.
> 
> On DG1, when the test fails, the observed latencies range from 900us -
> 1.5ms.

Do I read this right - that the latency of a 64 bit timestamp register 
read is 0.9 - 1.5ms? That would be the read in guc_update_pm_timestamp?

Regards,

Tvrtko

> One solution tried was to reduce the latency between reg read and
> CPU timestamp capture, but such optimization does not add value to user
> since the CPU timestamp obtained here is only used for (1) selftest and
> (2) i915 rps implementation specific to execlist scheduler. Also, this
> solution only reduces the frequency of failure and does not eliminate
> it.
> 
> In order to make the selftest more robust and account for such
> latencies, increase the sample period to 100 ms.
> 
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/selftest_engine_pm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> index 0dcb3ed44a73..87c94314cf67 100644
> --- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> +++ b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
> @@ -317,7 +317,7 @@ static int live_engine_busy_stats(void *arg)
>   		ENGINE_TRACE(engine, "measuring busy time\n");
>   		preempt_disable();
>   		de = intel_engine_get_busy_time(engine, &t[0]);
> -		mdelay(10);
> +		mdelay(100);
>   		de = ktime_sub(intel_engine_get_busy_time(engine, &t[1]), de);
>   		preempt_enable();
>   		dt = ktime_sub(t[1], t[0]);