drm/bridge: add support for refcounting

DRM bridges are currently considered as a fixed element of a DRM card, and
thus their lifetime is assumed to extend for as long as the card
exists. New use cases, such as hot-pluggable hardware with video bridges,
require DRM bridges to be added to and removed from a DRM card without
tearing the card down. This is possible for connectors already (used by DP
MST), it is now needed for DRM bridges as well.

As a first preliminary step, make bridges reference-counted to allow a
struct drm_bridge (along with the private driver structure embedding it) to
stay allocated even after the driver has been removed, until the last
reference is put.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326-drm-bridge-refcount-v9-2-5e0661fe1f84@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>

drivers/gpu/drm/drm_bridge.c

@@ -199,23 +199,84 @@
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
 
+static void __drm_bridge_free(struct kref *kref)
+{
+	struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);
+
+	kfree(bridge->container);
+}
+
+/**
+ * drm_bridge_get - Acquire a bridge reference
+ * @bridge: DRM bridge
+ *
+ * This function increments the bridge's refcount.
+ *
+ * Returns:
+ * Pointer to @bridge.
+ */
+struct drm_bridge *drm_bridge_get(struct drm_bridge *bridge)
+{
+	if (bridge)
+		kref_get(&bridge->refcount);
+
+	return bridge;
+}
+EXPORT_SYMBOL(drm_bridge_get);
+
+/**
+ * drm_bridge_put - Release a bridge reference
+ * @bridge: DRM bridge
+ *
+ * This function decrements the bridge's reference count and frees the
+ * object if the reference count drops to zero.
+ */
+void drm_bridge_put(struct drm_bridge *bridge)
+{
+	if (bridge)
+		kref_put(&bridge->refcount, __drm_bridge_free);
+}
+EXPORT_SYMBOL(drm_bridge_put);
+
+/**
+ * drm_bridge_put_void - wrapper to drm_bridge_put() taking a void pointer
+ *
+ * @data: pointer to @struct drm_bridge, cast to a void pointer
+ *
+ * Wrapper of drm_bridge_put() to be used when a function taking a void
+ * pointer is needed, for example as a devm action.
+ */
+static void drm_bridge_put_void(void *data)
+{
+	struct drm_bridge *bridge = (struct drm_bridge *)data;
+
+	drm_bridge_put(bridge);
+}
+
 void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 			      const struct drm_bridge_funcs *funcs)
 {
 	void *container;
 	struct drm_bridge *bridge;
+	int err;
 
 	if (!funcs) {
 		dev_warn(dev, "Missing funcs pointer\n");
 		return ERR_PTR(-EINVAL);
 	}
 
-	container = devm_kzalloc(dev, size, GFP_KERNEL);
+	container = kzalloc(size, GFP_KERNEL);
 	if (!container)
 		return ERR_PTR(-ENOMEM);
 
 	bridge = container + offset;
+	bridge->container = container;
 	bridge->funcs = funcs;
+	kref_init(&bridge->refcount);
+
+	err = devm_add_action_or_reset(dev, drm_bridge_put_void, bridge);
+	if (err)
+		return ERR_PTR(err);
 
 	return container;
 }

include/drm/drm_bridge.h

@@ -840,6 +840,18 @@ struct drm_bridge {
 	const struct drm_bridge_timings *timings;
 	/** @funcs: control functions */
 	const struct drm_bridge_funcs *funcs;
+
+	/**
+	 * @container: Pointer to the private driver struct embedding this
+	 * @struct drm_bridge.
+	 */
+	void *container;
+
+	/**
+	 * @refcount: reference count of users referencing this bridge.
+	 */
+	struct kref refcount;
+
 	/** @driver_private: pointer to the bridge driver's internal context */
 	void *driver_private;
 	/** @ops: bitmask of operations supported by the bridge */
@@ -941,6 +953,9 @@ drm_priv_to_bridge(struct drm_private_obj *priv)
 	return container_of(priv, struct drm_bridge, base);
 }
 
+struct drm_bridge *drm_bridge_get(struct drm_bridge *bridge);
+void drm_bridge_put(struct drm_bridge *bridge);
+
 void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 			      const struct drm_bridge_funcs *funcs);
 
@@ -951,6 +966,10 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
  * @member: the name of the &drm_bridge within @type
  * @funcs: callbacks for this bridge
  *
+ * The reference count of the returned bridge is initialized to 1. This
+ * reference will be automatically dropped via devm (by calling
+ * drm_bridge_put()) when @dev is removed.
+ *
  * Returns:
  * Pointer to new bridge, or ERR_PTR on failure.
  */