drm/i915: Clarify type evolution of uabi_node/uabi_engines

Chaining user engines happens in multiple passes during driver
initialization, mutating its type along the way. It starts off with a
simple lock-less linked list (struct llist_node/head) populated by
intel_engine_add_user() which later gets sorted and converted to an
intermediate regular list (struct list_head) just to be converted once
more to its final rb-tree structure (struct rb_node/root) in
intel_engines_driver_register().

All of these types overlay the uabi_node/uabi_engines members which is
unfortunate but safe if one takes care about using the rb-tree based
structure only after the conversion has completed. However, mistakes
happen and commit 1ec23ed7 ("drm/i915: Use uabi engines for the
default engine map") violated that assumption, as the multiple type
evolution was all to easy hidden behind casts papering over it.

Make the type evolution of uabi_node/uabi_engines more visible by
putting all members into an anonymous union and use the correctly typed
member in its various users. This allows us to drop quite some ugly
casts and, hopefully, make the evolution of the members better
recognisable to avoid future mistakes.
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230928182019.10256-3-minipli@grsecurity.net

drivers/gpu/drm/i915/gt/intel_engine_types.h

@@ -402,7 +402,15 @@ struct intel_engine_cs {
 
 	unsigned long context_tag;
 
+	/*
+	 * The type evolves during initialization, see related comment for
+	 * struct drm_i915_private's uabi_engines member.
+	 */
+	union {
+		struct llist_node uabi_llist;
+		struct list_head uabi_list;
 		struct rb_node uabi_node;
+	};
 
 	struct intel_sseu sseu;
 

drivers/gpu/drm/i915/gt/intel_engine_user.c

@@ -38,8 +38,7 @@ intel_engine_lookup_user(struct drm_i915_private *i915, u8 class, u8 instance)
 
 void intel_engine_add_user(struct intel_engine_cs *engine)
 {
-	llist_add((struct llist_node *)&engine->uabi_node,
-		  (struct llist_head *)&engine->i915->uabi_engines);
+	llist_add(&engine->uabi_llist, &engine->i915->uabi_engines_llist);
 }
 
 static const u8 uabi_classes[] = {
@@ -54,9 +53,9 @@ static int engine_cmp(void *priv, const struct list_head *A,
 		      const struct list_head *B)
 {
 	const struct intel_engine_cs *a =
-		container_of((struct rb_node *)A, typeof(*a), uabi_node);
+		container_of(A, typeof(*a), uabi_list);
 	const struct intel_engine_cs *b =
-		container_of((struct rb_node *)B, typeof(*b), uabi_node);
+		container_of(B, typeof(*b), uabi_list);
 
 	if (uabi_classes[a->class] < uabi_classes[b->class])
 		return -1;
@@ -73,7 +72,7 @@ static int engine_cmp(void *priv, const struct list_head *A,
 
 static struct llist_node *get_engines(struct drm_i915_private *i915)
 {
-	return llist_del_all((struct llist_head *)&i915->uabi_engines);
+	return llist_del_all(&i915->uabi_engines_llist);
 }
 
 static void sort_engines(struct drm_i915_private *i915,
@@ -83,9 +82,8 @@ static void sort_engines(struct drm_i915_private *i915,
 
 	llist_for_each_safe(pos, next, get_engines(i915)) {
 		struct intel_engine_cs *engine =
-			container_of((struct rb_node *)pos, typeof(*engine),
-				     uabi_node);
-		list_add((struct list_head *)&engine->uabi_node, engines);
+			container_of(pos, typeof(*engine), uabi_llist);
+		list_add(&engine->uabi_list, engines);
 	}
 	list_sort(NULL, engines, engine_cmp);
 }
@@ -213,8 +211,7 @@ void intel_engines_driver_register(struct drm_i915_private *i915)
 	p = &i915->uabi_engines.rb_node;
 	list_for_each_safe(it, next, &engines) {
 		struct intel_engine_cs *engine =
-			container_of((struct rb_node *)it, typeof(*engine),
-				     uabi_node);
+			container_of(it, typeof(*engine), uabi_list);
 
 		if (intel_gt_has_unrecoverable_error(engine->gt))
 			continue; /* ignore incomplete engines */

drivers/gpu/drm/i915/i915_drv.h

@@ -222,7 +222,22 @@ struct drm_i915_private {
 		bool mchbar_need_disable;
 	} gmch;
 
+	/*
+	 * Chaining user engines happens in multiple stages, starting with a
+	 * simple lock-less linked list created by intel_engine_add_user(),
+	 * which later gets sorted and converted to an intermediate regular
+	 * list, just to be converted once again to its final rb tree structure
+	 * in intel_engines_driver_register().
+	 *
+	 * Make sure to use the right iterator helper, depending on if the code
+	 * in question runs before or after intel_engines_driver_register() --
+	 * for_each_uabi_engine() can only be used afterwards!
+	 */
+	union {
+		struct llist_head uabi_engines_llist;
+		struct list_head uabi_engines_list;
 		struct rb_root uabi_engines;
+	};
 	unsigned int engine_uabi_class_count[I915_LAST_UABI_ENGINE_CLASS + 1];
 
 	/* protects the irq masks */