[Intel-gfx] [PATCH 3/7] drm/i915/perf: lock powergating configuration to default when active

[Joonas Lahtinen]

Quoting Tvrtko Ursulin (2019-01-08 13:22:48)
> From: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> 
> If some of the contexts submitting workloads to the GPU have been
> configured to shutdown slices/subslices, we might loose the NOA
> configurations written in the NOA muxes.
> 
> One possible solution to this problem is to reprogram the NOA muxes
> when we switch to a new context. We initially tried this in the
> workaround batchbuffer but some concerns where raised about the cost
> of reprogramming at every context switch. This solution is also not
> without consequences from the userspace point of view. Reprogramming
> of the muxes can only happen once the powergating configuration has
> changed (which happens after context switch). This means for a window
> of time during the recording, counters recorded by the OA unit might
> be invalid. This requires userspace dealing with OA reports to discard
> the invalid values.
> 
> Minimizing the reprogramming could be implemented by tracking of the
> last programmed configuration somewhere in GGTT and use MI_PREDICATE
> to discard some of the programming commands, but the command streamer
> would still have to parse all the MI_LRI instructions in the
> workaround batchbuffer.
> 
> Another solution, which this change implements, is to simply disregard
> the user requested configuration for the period of time when i915/perf
> is active. There is no known issue with this apart from a performance
> penality for some media workloads that benefit from running on a
> partially powergated GPU. We already prevent RC6 from affecting the
> programming so it doesn't sound completely unreasonable to hold on
> powergating for the same reason.

Lets squash patch 6 here and update the commit message/comments to
reflect. With that done, the resulting patch is:

Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

Regards, Joonas