drm/i915: Priority boost for waiting clients

Latency is in the eye of the beholder. In the case where a client stops
and waits for the gpu, give that request chain a small priority boost
(not so that it overtakes higher priority clients, to preserve the
external ordering) so that ideally the wait completes earlier.

v2: Tvrtko recommends to keep the boost-from-user-stall as small as
possible and to allow new client flows to be preferred for interactivity
over stalls.

Testcase: igt/gem_sync/switch-default
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001144755.7978-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_scheduler.h

@@ -24,13 +24,14 @@ enum {
 	I915_PRIORITY_INVALID = INT_MIN
 };
 
-#define I915_USER_PRIORITY_SHIFT 1
+#define I915_USER_PRIORITY_SHIFT 2
 #define I915_USER_PRIORITY(x) ((x) << I915_USER_PRIORITY_SHIFT)
 
 #define I915_PRIORITY_COUNT BIT(I915_USER_PRIORITY_SHIFT)
 #define I915_PRIORITY_MASK (I915_PRIORITY_COUNT - 1)
 
-#define I915_PRIORITY_NEWCLIENT	((u8)BIT(0))
+#define I915_PRIORITY_WAIT	((u8)BIT(0))
+#define I915_PRIORITY_NEWCLIENT	((u8)BIT(1))
 
 struct i915_sched_attr {
 	/**
@@ -99,6 +100,8 @@ void i915_sched_node_fini(struct drm_i915_private *i915,
 void i915_schedule(struct i915_request *request,
 		   const struct i915_sched_attr *attr);
 
+void i915_schedule_bump_priority(struct i915_request *rq, unsigned int bump);
+
 struct list_head *
 i915_sched_lookup_priolist(struct intel_engine_cs *engine, int prio);