Re: [PATCH 5/8] drm/i915/perf: lock powergating configuration to default when active

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On 16/08/18 08:31, Tvrtko Ursulin wrote:

On 15/08/2018 12:58, Tvrtko Ursulin wrote:

On 14/08/2018 15:59, Lionel Landwerlin wrote:
Hey Tvrtko,

Thanks for taking over this series.

I've been talking to developers using the i915/perf interface and from their point of view, they expect the system to be in a stable configuration when doing measurements.

One issue with this patch on Gen11 is that it will lock the system in a configuration that isn't workable for media workloads (all subslices enabled). So I think we should set the value for the locked configuration per generation (gen8-10: all slices/subslices, gen11: only subslices that contain VME samplers) so that we always get a functional configurations for all workloads.
Could we want to select that configuration when opening perf?

This would be via i915_perf.c/gen8_configure_all_contexts?

Sounds like an unfortunate but workable compromise. As long as there doesn't appear another asymmetric slice feature in the future, which doesn't align with VME. Like one workloads wants slices 0 & 2, another wants 1 & 3, or whatever. If that happens then I don't know what we do apart from locking out perf/OA.

Another question is how do we expose the selected value to the user. But that can be solved in a different series.

SSEU get_param would cut it? Although it would be perhaps be unexpected to get different results depending on whether perf/OA is active or not.. Hm... export device and available bitmasks via get param? Device bitmask would be fixed and available would change depending on whether perf/OA is active or not.

I would avoid changing the get_param. This is supposed to report the physical device's properties.
Maybe add an ioctl entry to report that configuration onto the i915_perf fd?


Big downside to this is that observing something via OA would cause slow down of the whole system - since the SSEU config would be locked to a subset of slices on Gen11. :( Regardless or not of the details.. say even if we choose to lock only if there is one non-default SSEU context, that still locks all whilst perf/OA is active.

That's quite bad I think. But the alternative is to lock out perf/OA while non-default SSEU contexts are present. Which is also very bad. :(

Overall I don't see a solution. Latter is maybe better by being more explicit? Who are the perf/OA users and what would they prefer?

The other solution I could think of would be to prevent schedule contexts that are not compatible with the OA configuration.
That doesn't sound great either...


Regards,

Tvrtko


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