drm/i915: Bail if we do not setup the RCS engine

In places, we assume that RCS exists. This has been true forever, but
let us catch this failure during bringup by adding an explicit check
that we do have an RCS engine.

v2: Make use of HAS_ENGINE (Tvrtko)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170411165658.23828-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -153,10 +153,10 @@ intel_engine_setup(struct drm_i915_private *dev_priv,
 int intel_engines_init_early(struct drm_i915_private *dev_priv)
 {
 	struct intel_device_info *device_info = mkwrite_device_info(dev_priv);
-	unsigned int ring_mask = INTEL_INFO(dev_priv)->ring_mask;
-	unsigned int mask = 0;
+	const unsigned int ring_mask = INTEL_INFO(dev_priv)->ring_mask;
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
+	unsigned int mask = 0;
 	unsigned int i;
 	int err;
 
@@ -183,6 +183,12 @@ int intel_engines_init_early(struct drm_i915_private *dev_priv)
 	if (WARN_ON(mask != ring_mask))
 		device_info->ring_mask = mask;
 
+	/* We always presume we have at least RCS available for later probing */
+	if (WARN_ON(!HAS_ENGINE(dev_priv, RCS))) {
+		err = -ENODEV;
+		goto cleanup;
+	}
+
 	device_info->num_rings = hweight32(mask);
 
 	return 0;