drm/i915: Force the switch to the i915->kernel_context

In the next few patches, we will have a hard requirement that we emit a
context-switch to the perma-pinned i915->kernel_context (so that we can
save the HW state using that context-switch). As the first context
itself may be classed as a kernel context, we want to be explicit in our
comparison. For an extra-layer of finesse, we can check the last
unretired context on the engine; as well as the last retired context
when idle.

v2: verbose verbosity
v3: Always force the switch, even when the engine is idle, and update
the assert that this happens before suspend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> #v1
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -4693,14 +4693,16 @@ void __i915_gem_object_release_unless_active(struct drm_i915_gem_object *obj)
 		i915_gem_object_put(obj);
 }
 
-static void assert_kernel_context_is_current(struct drm_i915_private *dev_priv)
+static void assert_kernel_context_is_current(struct drm_i915_private *i915)
 {
+	struct i915_gem_context *kernel_context = i915->kernel_context;
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
 
-	for_each_engine(engine, dev_priv, id)
-		GEM_BUG_ON(engine->last_retired_context &&
-			   !i915_gem_context_is_kernel(engine->last_retired_context));
+	for_each_engine(engine, i915, id) {
+		GEM_BUG_ON(__i915_gem_active_peek(&engine->timeline->last_request));
+		GEM_BUG_ON(engine->last_retired_context != kernel_context);
+	}
 }
 
 void i915_gem_sanitize(struct drm_i915_private *i915)

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1596,10 +1596,32 @@ bool intel_engines_are_idle(struct drm_i915_private *dev_priv)
 	return true;
 }
 
+/**
+ * intel_engine_has_kernel_context:
+ * @engine: the engine
+ *
+ * Returns true if the last context to be executed on this engine, or has been
+ * executed if the engine is already idle, is the kernel context
+ * (#i915.kernel_context).
+ */
 bool intel_engine_has_kernel_context(const struct intel_engine_cs *engine)
 {
-	return (!engine->last_retired_context ||
-		i915_gem_context_is_kernel(engine->last_retired_context));
+	const struct i915_gem_context * const kernel_context =
+		engine->i915->kernel_context;
+	struct drm_i915_gem_request *rq;
+
+	lockdep_assert_held(&engine->i915->drm.struct_mutex);
+
+	/*
+	 * Check the last context seen by the engine. If active, it will be
+	 * the last request that remains in the timeline. When idle, it is
+	 * the last executed context as tracked by retirement.
+	 */
+	rq = __i915_gem_active_peek(&engine->timeline->last_request);
+	if (rq)
+		return rq->ctx == kernel_context;
+	else
+		return engine->last_retired_context == kernel_context;
 }
 
 void intel_engines_reset_default_submission(struct drm_i915_private *i915)