drm/i915: Keep one request in our ring_list

Don't pre-emptively retire the oldest request in our ring's list if it
is the only request. We keep various bits of state alive using the
active reference from the request and would rather transfer that state
over to a new request rather than the more involved process of retiring
and reacquiring it.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503195115.22309-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_request.c

@@ -695,9 +695,9 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
 		goto err_unreserve;
 
 	/* Move our oldest request to the slab-cache (if not in use!) */
-	rq = list_first_entry_or_null(&ring->request_list,
-				      typeof(*rq), ring_link);
-	if (rq && i915_request_completed(rq))
+	rq = list_first_entry(&ring->request_list, typeof(*rq), ring_link);
+	if (!list_is_last(&rq->ring_link, &ring->request_list) &&
+	    i915_request_completed(rq))
 		i915_request_retire(rq);
 
 	/*