[Intel-gfx] [PATCH 2/2] drm/i915: Skip switch to kernel context if already done

[Chris Wilson]

Some engines are never user or already sitting idle in the kernel
context and for those we can skip flushing the current context for
i915_gem_switch_to_kernel_context(). We used to perform this
optimisation but that was removed for convenience of converting over to
multiple timelines and handling the pending request queues.

>From the perspective of writing selftests, reducing the number of
background operations on the engines makes defining assertions easier.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c         |  3 ++-
 drivers/gpu/drm/i915/i915_gem_context.c | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d4c59b53532e..83cd2eff37af 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4140,7 +4140,8 @@ static void assert_kernel_context_is_current(struct drm_i915_private *dev_priv)
 	enum intel_engine_id id;
 
 	for_each_engine(engine, dev_priv, id)
-		GEM_BUG_ON(!i915_gem_context_is_kernel(engine->last_retired_context));
+		GEM_BUG_ON(engine->last_retired_context &&
+			   !i915_gem_context_is_kernel(engine->last_retired_context));
 }
 
 int i915_gem_suspend(struct drm_i915_private *dev_priv)
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index ae99c25397ca..0a4728fdecdc 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -877,6 +877,26 @@ int i915_switch_context(struct drm_i915_gem_request *req)
 	return do_rcs_switch(req);
 }
 
+static bool engine_has_kernel_context(struct intel_engine_cs *engine)
+{
+	struct i915_gem_timeline *timeline;
+
+	list_for_each_entry(timeline, &engine->i915->gt.timelines, link) {
+		struct intel_timeline *tl;
+
+		if (timeline == &engine->i915->gt.global_timeline)
+			continue;
+
+		tl = &timeline->engine[engine->id];
+		if (i915_gem_active_peek(&tl->last_request,
+					 &engine->i915->drm.struct_mutex))
+			return false;
+	}
+
+	return (!engine->last_retired_context ||
+		i915_gem_context_is_kernel(engine->last_retired_context));
+}
+
 int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv)
 {
 	struct intel_engine_cs *engine;
@@ -885,10 +905,15 @@ int i915_gem_switch_to_kernel_context(struct drm_i915_private *dev_priv)
 
 	lockdep_assert_held(&dev_priv->drm.struct_mutex);
 
+	i915_gem_retire_requests(dev_priv);
+
 	for_each_engine(engine, dev_priv, id) {
 		struct drm_i915_gem_request *req;
 		int ret;
 
+		if (engine_has_kernel_context(engine))
+			continue;
+
 		req = i915_gem_request_alloc(engine, dev_priv->kernel_context);
 		if (IS_ERR(req))
 			return PTR_ERR(req);
-- 
2.11.0