[Intel-gfx] [PATCH] drm/i915: fix SFC reset flow

[Chris Wilson]

Quoting Daniele Ceraolo Spurio (2019-09-16 22:41:04)
> Our assumption that the we can ask the HW to lock the SFC even if not
> currently in use does not match the HW commitment. The expectation from
> the HW is that SW will not try to lock the SFC if the engine is not
> using it and if we do that the behavior is undefined; on ICL the HW
> ends up to returning the ack and ignoring our lock request, but this is
> not guaranteed and we shouldn't expect it going forward.
> 
> Reported-by: Owen Zhang <owen.zhang at intel.com>
> Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> ---
> @@ -366,10 +368,13 @@ static u32 gen11_lock_sfc(struct intel_engine_cs *engine)
>                                          sfc_forced_lock_ack_bit,
>                                          sfc_forced_lock_ack_bit,
>                                          1000, 0, NULL)) {
> -               DRM_DEBUG_DRIVER("Wait for SFC forced lock ack failed\n");
> +               /* did we race the unlock? */
> +               if (intel_uncore_read_fw(uncore, sfc_usage) & sfc_usage_bit)
> +                       DRM_ERROR("Wait for SFC forced lock ack failed\n");

What's our plan if this *ERROR* is ever triggered?

If it remains do nothing and check the logs on death, then it remains
just a debug splat. If there is a plan to actually do something to
handle the error, do it!
-Chris