drm/i915: Trim the flush for the legacy request emission

At the start of request emission, we flush some space for the request,
estimating the typical size for the request body. The tail is now much
larger than the typical body, so we can shrink the flush slightly.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461917226-9132-2-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -34,6 +34,11 @@
 #include "i915_trace.h"
 #include "intel_drv.h"
 
+/* Rough estimate of the typical request size, performing a flush,
+ * set-context and then emitting the batch.
+ */
+#define LEGACY_REQUEST_SIZE 200
+
 int __intel_ring_space(int head, int tail, int size)
 {
 	int space = head - tail;
@@ -2345,7 +2350,7 @@ int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 	 * we start building the request - in which case we will just
 	 * have to repeat work.
 	 */
-	request->reserved_space += MIN_SPACE_FOR_ADD_REQUEST;
+	request->reserved_space += LEGACY_REQUEST_SIZE;
 
 	request->ringbuf = request->engine->buffer;
 
@@ -2353,7 +2358,7 @@ int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 	if (ret)
 		return ret;
 
-	request->reserved_space -= MIN_SPACE_FOR_ADD_REQUEST;
+	request->reserved_space -= LEGACY_REQUEST_SIZE;
 	return 0;
 }