drm/i915: Preallocate enough space for the average request

Rather than being interrupted when we run out of space halfway through
the request, and having to restart from the beginning (and returning to
userspace), flush a little more free space when we prepare the request.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-15-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -700,6 +700,12 @@ int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request
 {
 	int ret;
 
+	/* Flush enough space to reduce the likelihood of waiting after
+	 * we start building the request - in which case we will just
+	 * have to repeat work.
+	 */
+	request->reserved_space += MIN_SPACE_FOR_ADD_REQUEST;
+
 	request->ringbuf = request->ctx->engine[request->engine->id].ringbuf;
 
 	if (i915.enable_guc_submission) {
@@ -725,6 +731,7 @@ int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request
 	if (ret)
 		goto err_unpin;
 
+	request->reserved_space -= MIN_SPACE_FOR_ADD_REQUEST;
 	return 0;
 
 err_unpin:

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2339,8 +2339,22 @@ int intel_engine_idle(struct intel_engine_cs *engine)
 
 int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 {
+	int ret;
+
+	/* Flush enough space to reduce the likelihood of waiting after
+	 * we start building the request - in which case we will just
+	 * have to repeat work.
+	 */
+	request->reserved_space += MIN_SPACE_FOR_ADD_REQUEST;
+
 	request->ringbuf = request->engine->buffer;
-	return intel_ring_begin(request, 0);
+
+	ret = intel_ring_begin(request, 0);
+	if (ret)
+		return ret;
+
+	request->reserved_space -= MIN_SPACE_FOR_ADD_REQUEST;
+	return 0;
 }
 
 static int wait_for_space(struct drm_i915_gem_request *req, int bytes)