drm/i915: Stop engines when declaring the machine wedged

If we fail to reset the GPU, we declare the machine wedged. However, the
GPU may well still be running in the background with an in-flight
request. So despite our efforts in cleaning up the request queue and
faking the breadcrumb in the HWSP, the GPU may eventually write the
in-flght seqno there breaking all of our assumptions and throwing the
driver into a deep turmoil, wedging beyond wedged.

To avoid this we ideally want to reset the GPU. Since that has already
failed, make sure the rings have the stop bit set instead. This is part
of the normal GPU reset sequence, but that is actually disabled by
igt/gem_eio to force the wedged state. If we assume the worst, we must
poke at the bit again before we give up.

v2: Move the intel_gpu_reset() from set-wedged in the reset error path
into i915_gem_set_wedged() itself. Even if the reset fails (e.g. if it is
disabled by gem_eio), it still tries to make sure the engines are
stopped. For i915_gem_set_wedged() callers from outside of i915_reset(),
this should make sure the GPU is disabled while the driver is marked as
being wedged.

Testcase: igt/gem_eio
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180315151015.22741-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.c

@@ -1995,7 +1995,6 @@ void i915_reset(struct drm_i915_private *i915, unsigned int flags)
 error:
 	i915_gem_set_wedged(i915);
 	i915_retire_requests(i915);
-	intel_gpu_reset(i915, ALL_ENGINES);
 	goto finish;
 }
 

drivers/gpu/drm/i915/i915_gem.c

@@ -3246,6 +3246,9 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 	}
 	i915->caps.scheduler = 0;
 
+	/* Even if the GPU reset fails, it should still stop the engines */
+	intel_gpu_reset(i915, ALL_ENGINES);
+
 	/*
 	 * Make sure no one is running the old callback before we proceed with
 	 * cancelling requests and resetting the completion tracking. Otherwise