drm/i915: Mark the GT as busy before idling the previous request

In a synchronous setup, we may retire the last request before we
complete allocating the next request. As the last request is retired, we
queue a timer to mark the device as idle, and promptly have to execute
ad cancel that timer once we complete allocating the request and need to
keep the device awake. If we rearrange the mark_busy() to occur before
we retire the previous request, we can skip this ping-pong.

v2: Joonas pointed out that unreserve_seqno() was now doing more than
doing seqno handling and should be renamed to reflect its wider purpose.
That also highlighted the new asymmetry with reserve_seqno(), so fixup
that and rename both to [un]reserve_engine().
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817144719.10968-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_request.c

@@ -244,27 +244,60 @@ int i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno)
 	return reset_all_global_seqno(dev_priv, seqno - 1);
 }
 
-static int reserve_seqno(struct intel_engine_cs *engine)
+static void mark_busy(struct drm_i915_private *i915)
 {
+	if (i915->gt.awake)
+		return;
+
+	GEM_BUG_ON(!i915->gt.active_requests);
+
+	intel_runtime_pm_get_noresume(i915);
+	i915->gt.awake = true;
+
+	intel_enable_gt_powersave(i915);
+	i915_update_gfx_val(i915);
+	if (INTEL_GEN(i915) >= 6)
+		gen6_rps_busy(i915);
+
+	queue_delayed_work(i915->wq,
+			   &i915->gt.retire_work,
+			   round_jiffies_up_relative(HZ));
+}
+
+static int reserve_engine(struct intel_engine_cs *engine)
+{
+	struct drm_i915_private *i915 = engine->i915;
 	u32 active = ++engine->timeline->inflight_seqnos;
 	u32 seqno = engine->timeline->seqno;
 	int ret;
 
 	/* Reservation is fine until we need to wrap around */
-	if (likely(!add_overflows(seqno, active)))
-		return 0;
-
-	ret = reset_all_global_seqno(engine->i915, 0);
-	if (ret) {
-		engine->timeline->inflight_seqnos--;
-		return ret;
+	if (unlikely(add_overflows(seqno, active))) {
+		ret = reset_all_global_seqno(i915, 0);
+		if (ret) {
+			engine->timeline->inflight_seqnos--;
+			return ret;
+		}
 	}
 
+	if (!i915->gt.active_requests++)
+		mark_busy(i915);
+
 	return 0;
 }
 
-static void unreserve_seqno(struct intel_engine_cs *engine)
+static void unreserve_engine(struct intel_engine_cs *engine)
 {
+	struct drm_i915_private *i915 = engine->i915;
+
+	if (!--i915->gt.active_requests) {
+		/* Cancel the mark_busy() from our reserve_engine() */
+		GEM_BUG_ON(!i915->gt.awake);
+		mod_delayed_work(i915->wq,
+				 &i915->gt.idle_work,
+				 msecs_to_jiffies(100));
+	}
+
 	GEM_BUG_ON(!engine->timeline->inflight_seqnos);
 	engine->timeline->inflight_seqnos--;
 }
@@ -333,13 +366,7 @@ static void i915_gem_request_retire(struct drm_i915_gem_request *request)
 	list_del_init(&request->link);
 	spin_unlock_irq(&engine->timeline->lock);
 
-	if (!--request->i915->gt.active_requests) {
-		GEM_BUG_ON(!request->i915->gt.awake);
-		mod_delayed_work(request->i915->wq,
-				 &request->i915->gt.idle_work,
-				 msecs_to_jiffies(100));
-	}
-	unreserve_seqno(request->engine);
+	unreserve_engine(request->engine);
 	advance_ring(request);
 
 	free_capture_list(request);
@@ -575,7 +602,7 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
 		return ERR_CAST(ring);
 	GEM_BUG_ON(!ring);
 
-	ret = reserve_seqno(engine);
+	ret = reserve_engine(engine);
 	if (ret)
 		goto err_unpin;
 
@@ -681,7 +708,7 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
 
 	kmem_cache_free(dev_priv->requests, req);
 err_unreserve:
-	unreserve_seqno(engine);
+	unreserve_engine(engine);
 err_unpin:
 	engine->context_unpin(engine, ctx);
 	return ERR_PTR(ret);
@@ -863,28 +890,6 @@ i915_gem_request_await_object(struct drm_i915_gem_request *to,
 	return ret;
 }
 
-static void i915_gem_mark_busy(const struct intel_engine_cs *engine)
-{
-	struct drm_i915_private *dev_priv = engine->i915;
-
-	if (dev_priv->gt.awake)
-		return;
-
-	GEM_BUG_ON(!dev_priv->gt.active_requests);
-
-	intel_runtime_pm_get_noresume(dev_priv);
-	dev_priv->gt.awake = true;
-
-	intel_enable_gt_powersave(dev_priv);
-	i915_update_gfx_val(dev_priv);
-	if (INTEL_GEN(dev_priv) >= 6)
-		gen6_rps_busy(dev_priv);
-
-	queue_delayed_work(dev_priv->wq,
-			   &dev_priv->gt.retire_work,
-			   round_jiffies_up_relative(HZ));
-}
-
 /*
  * NB: This function is not allowed to fail. Doing so would mean the the
  * request is not being tracked for completion but the work itself is
@@ -966,9 +971,6 @@ void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 	list_add_tail(&request->ring_link, &ring->request_list);
 	request->emitted_jiffies = jiffies;
 
-	if (!request->i915->gt.active_requests++)
-		i915_gem_mark_busy(engine);
-
 	/* Let the backend know a new request has arrived that may need
 	 * to adjust the existing execution schedule due to a high priority
 	 * request - i.e. we may want to preempt the current request in order