Re: [PATCH] i915/oa: Simplify updating contexts

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2018-09-12 16:29:30)
>         /*
>          * The OA register config is setup through the context image. This image
>          * might be written to by the GPU on context switch (in particular on
> @@ -1833,7 +1727,7 @@ static int gen8_configure_all_contexts(struct drm_i915_private *dev_priv,
>          * the GPU from any submitted work.
>          */
>         ret = i915_gem_wait_for_idle(dev_priv,
> -                                    wait_flags,
> +                                    I915_WAIT_LOCKED,
>                                      MAX_SCHEDULE_TIMEOUT);

Wait until idle includes a wait for the gpu to switch off. At least it
does for execlists, not so clear for ringbuffer as there is no explicit
idle-event. However, that shouldn't matter as the kernel context doesn't
exist for legacy ringbuffer anyway ;) But the reload will be forced on
the next actual use.

>         if (ret)
>                 return ret;
> @@ -1859,7 +1753,17 @@ static int gen8_configure_all_contexts(struct drm_i915_private *dev_priv,
>                 i915_gem_object_unpin_map(ce->state->obj);
>         }
>  
> -       return ret;
> +       /*
> +        * Apply the configuration by doing one context restore of the edited
> +        * context image.
> +        */
> +       rq = i915_request_alloc(engine, dev_priv->kernel_context);

By feeding a request, you ensure the reconfig is loaded again. +1 for
having it turn off when idle (and not instrument the kernel context at
all)!

Still I follow your logic that this should leave the oa config in
exactly the same state as before the patch, so
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris
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