drm/i915: Skip an engine reset if it recovered before our preparations

At the beginning of a reset, we disable the submission method and find
the stuck request. We expect to find a stuck request for we have
declared the engine stalled. However, if we find no active request, the
engine must have recovered from its stall before we could issue a reset,
so let the engine continue on without a reset. If the engine is truly
stuck, we will back soon enough with the next reset attempt.

v2: Remove the stale debug message.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171216002206.31737-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.c

@@ -2011,19 +2011,19 @@ int i915_reset_engine(struct intel_engine_cs *engine, unsigned int flags)
 
 	GEM_BUG_ON(!test_bit(I915_RESET_ENGINE + engine->id, &error->flags));
 
+	active_request = i915_gem_reset_prepare_engine(engine);
+	if (IS_ERR_OR_NULL(active_request)) {
+		/* Either the previous reset failed, or we pardon the reset. */
+		ret = PTR_ERR(active_request);
+		goto out;
+	}
+
 	if (!(flags & I915_RESET_QUIET)) {
 		dev_notice(engine->i915->drm.dev,
 			   "Resetting %s after gpu hang\n", engine->name);
 	}
 	error->reset_engine_count[engine->id]++;
 
-	active_request = i915_gem_reset_prepare_engine(engine);
-	if (IS_ERR(active_request)) {
-		DRM_DEBUG_DRIVER("Previous reset failed, promote to full reset\n");
-		ret = PTR_ERR(active_request);
-		goto out;
-	}
-
 	if (!engine->i915->guc.execbuf_client)
 		ret = intel_gt_reset_engine(engine->i915, engine);
 	else