drm/i915: Refactor tests for validity of RING_TAIL

Whilst I like having the assertions clearly visible in the code, they
are quite repetitious! As we find new limits we want to incorporate into
the set of assertions, it make sense to refactor them to a common
routine.

v2: Add a guc holdout.

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/20170327131412.20293-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -529,6 +529,17 @@ intel_ring_offset(const struct drm_i915_gem_request *req, void *addr)
 	return intel_ring_wrap(req->ring, offset);
 }
 
+static inline void
+assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
+{
+	/* We could combine these into a single tail operation, but keeping
+	 * them as seperate tests will help identify the cause should one
+	 * ever fire.
+	 */
+	GEM_BUG_ON(!IS_ALIGNED(tail, 8));
+	GEM_BUG_ON(tail >= ring->size);
+}
+
 void intel_ring_update_space(struct intel_ring *ring);
 
 void intel_engine_init_global_seqno(struct intel_engine_cs *engine, u32 seqno);