[Intel-gfx] [PATCH 16/19] drm/i915/perf: Apply Wa_18013179988

[Dixit, Ashutosh]

On Thu, 15 Sep 2022 22:16:30 -0700, Dixit, Ashutosh wrote:
>
> On Tue, 23 Aug 2022 13:41:52 -0700, Umesh Nerlige Ramappa wrote:
> >
>
> Hi Umesh,
>
> > OA reports in the OA buffer contain an OA timestamp field that helps
> > user calculate delta between 2 OA reports. The calculation relies on the
> > CS timestamp frequency to convert the timestamp value to nanoseconds.
> > The CS timestamp frequency is a function of the CTC_SHIFT value in
> > RPM_CONFIG0.
> >
> > In DG2, OA unit assumes that the CTC_SHIFT is 3, instead of using the
> > actual value from RPM_CONFIG0. At the user level, this results in an
> > error in calculating delta between 2 OA reports since the OA timestamp
> > is not shifted in the same manner as CS timestamp.
> >
> > To resolve this, return actual OA timestamp frequency to the user in
> > i915_getparam_ioctl.
>
> Rather than exposing actual OA timestamp frequency to userspace (with the
> corresponding uapi change, specially if it's only DG2 and not all future
> products) questions about a couple of other options:
>
> Option 1. Can we set CTC_SHIFT in RPM_CONFIG0 to 3, so change GT freq to be the
>           same as OA freq :-)
>
>    The HSD seems to mention this:
>    Is setting CTC SHIFT to 0b11 on driver init an acceptable W/A?
>    Note: Changing the shift setting on live driver may break apps that are
>    currently running (including desktop manager).
>
> Option 2. Is it possible to correct the timestamps in OA report headers to
>           compensate for the difference between OA and GT frequencies (say when
>           copying OA data to userspace)?
>
>	  Though not sure if this is preferable to having userspace do this.

Also do we need input from userland on this patch? UMD's might need to
assess the impact of having different GT and OA frequencies at their end
since they consume OA data?

Thanks.
--
Ashutosh