linux-kernel - [PATCH v8 1/3] drm/bridge: add list of removed refcounted bridges

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Message-Id: <20250912-drm-bridge-debugfs-removed-v8-1-5c33d87ccb55@bootlin.com>
Date: Fri, 12 Sep 2025 19:03:41 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>, 
 Dmitry Baryshkov <lumag@...nel.org>
Subject: [PATCH v8 1/3] drm/bridge: add list of removed refcounted bridges

Between drm_bridge_add() and drm_bridge_remove() bridges are registered to
the DRM core via the global bridge_list and visible in
/sys/kernel/debug/dri/bridges. However between drm_bridge_remove() and the
last drm_bridge_put() memory is still allocated even though the bridge is
not registered, i.e. not in bridges_list, and also not visible in
debugfs. This prevents debugging refcounted bridges lifetime, especially
leaks due to a missing drm_bridge_put().

In order to allow debugfs to also show the removed bridges, move such
bridges into a new ad-hoc list until they are eventually freed.

Note this requires adding INIT_LIST_HEAD(&bridge->list) in the bridge
initialization code. The lack of such init was not exposing any bug so far,
but it would with the new code, for example when a bridge is allocated and
then freed without calling drm_bridge_add(), which is common on probe
errors.

drm_bridge_add() needs special care for bridges being added after having
been previously added and then removed.  This happens for example for many
non-DCS DSI host bridge drivers like samsung-dsim which
drm_bridge_add/remove() themselves every time the DSI device does a DSI
attaches/detach. When the DSI device is hot-pluggable this happens multiple
times in the lifetime of the DSI host bridge.  On every attach after the
first one, drm_bridge_add() finds bridge->list in the removed list, not at
the initialized state as drm_bridge_add() currently expects. Add a
list_del_init() to remove the bridge from the lingering list and bring
bridge->list back to the initialized state.

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

---

Changes in v8:
- split the documentation changes to a separate patch
- rename "removed" to "lingering"
- improve commits message about the special care needed for "un-removed"
  bridges

Changes in v7:
- rebase on current drm-misc-next
- remove if (drm_bridge_is_refcounted(bridge)), refcounting is now
  mandatory
- add check to detect when re-adding a bridge that is in the removed list
- improve commit message
- fix typo

This patch was added in v6.
---
 drivers/gpu/drm/drm_bridge.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 5ec2ff77da79e51f614f98ebe58b6171c611867a..9491ae7c884d355be4a82fb02a43a42d17fa8e0c 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -197,15 +197,22 @@
  * driver.
  */

+/* Protect bridge_list and bridge_lingering_list */
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
+static LIST_HEAD(bridge_lingering_list);

 static void __drm_bridge_free(struct kref *kref)
 {
 	struct drm_bridge *bridge = container_of(kref, struct drm_bridge, refcount);

+	mutex_lock(&bridge_lock);
+	list_del(&bridge->list);
+	mutex_unlock(&bridge_lock);
+
 	if (bridge->funcs->destroy)
 		bridge->funcs->destroy(bridge);
+
 	kfree(bridge->container);
 }

@@ -275,6 +282,7 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 		return ERR_PTR(-ENOMEM);

 	bridge = container + offset;
+	INIT_LIST_HEAD(&bridge->list);
 	bridge->container = container;
 	bridge->funcs = funcs;
 	kref_init(&bridge->refcount);
@@ -304,6 +312,14 @@ void drm_bridge_add(struct drm_bridge *bridge)

 	drm_bridge_get(bridge);

+	/*
+	 * If the bridge was previously added and then removed, it is now
+	 * in bridge_lingering_list. Remove it or bridge_lingering_list will be
+	 * corrupted when adding this bridge to bridge_list below.
+	 */
+	if (!list_empty(&bridge->list))
+		list_del_init(&bridge->list);
+
 	mutex_init(&bridge->hpd_mutex);

 	if (bridge->ops & DRM_BRIDGE_OP_HDMI)
@@ -357,7 +373,7 @@ void drm_bridge_remove(struct drm_bridge *bridge)
 			br->funcs->bridge_event_notify(br, DRM_EVENT_BRIDGE_REMOVING, bridge);

 	mutex_lock(&bridge_lock);
-	list_del_init(&bridge->list);
+	list_move_tail(&bridge->list, &bridge_lingering_list);
 	mutex_unlock(&bridge_lock);

 	mutex_destroy(&bridge->hpd_mutex);

-- 
2.51.0