drm/i915/gt: Be paranoid and reset the GPU before release

Just in the very unlikely case we have not stopped the GPU before we
return the pages being used by the GPU to the system, force a reset.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117180309.3249427-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -35,6 +35,7 @@
 #include "intel_engine_user.h"
 #include "intel_gt.h"
 #include "intel_gt_requests.h"
+#include "intel_gt_pm.h"
 #include "intel_lrc.h"
 #include "intel_reset.h"
 #include "intel_ring.h"
@@ -392,8 +393,23 @@ void intel_engines_release(struct intel_gt *gt)
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
 
+	/*
+	 * Before we release the resources held by engine, we must be certain
+	 * that the HW is no longer accessing them -- having the GPU scribble
+	 * to or read from a page being used for something else causes no end
+	 * of fun.
+	 *
+	 * The GPU should be reset by this point, but assume the worst just
+	 * in case we aborted before completely initialising the engines.
+	 */
+	GEM_BUG_ON(intel_gt_pm_is_awake(gt));
+	if (!INTEL_INFO(gt->i915)->gpu_reset_clobbers_display)
+		__intel_gt_reset(gt, ALL_ENGINES);
+
 	/* Decouple the backend; but keep the layout for late GPU resets */
 	for_each_engine(engine, gt, id) {
+		GEM_BUG_ON(intel_engine_pm_is_awake(engine));
+
 		if (!engine->release)
 			continue;