[PATCH v7 0/3] drm/bridge: debugfs: show refcount and list removed bridges

[PATCH v7 0/3] drm/bridge: debugfs: show refcount and list removed bridges

[Luca Ceresoli]

This series shows removed bridges to the global <debugfs>/dri/bridges file,
and shows the bridge refcount.  Removed bridges are bridges after
drm_bridges_remove() but before they are eventually freed on the last
drm_bridge_put().

This is part of the work towards removal of bridges from a still existing
DRM pipeline without use-after-free. The grand plan was discussed in [1].
Here's the work breakdown (➜ marks the current series):

 1. ➜ add refcounting to DRM bridges (struct drm_bridge)
    (based on devm_drm_bridge_alloc() [0])
    A. ✔ add new alloc API and refcounting (v6.16)
    B. ✔ convert all bridge drivers to new API (v6.17-rc1)
    C. ✔ kunit tests (v6.17-rc1)
    D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
         and warn on old allocation pattern (v6.17-rc1)
    E. … add get/put on drm_bridge accessors
       1. ✔ drm_bridge_chain_get_first_bridge() + add a cleanup action
            (drm-misc-next)
       2. ✔ drm_bridge_get_prev_bridge() (drm-misc-next)
       3. … drm_bridge_get_next_bridge()
       4. … drm_for_each_bridge_in_chain()
       5. drm_bridge_connector_init
       6. of_drm_find_bridge
       7. drm_of_find_panel_or_bridge, *_of_get_bridge
    F. ➜ debugfs improvements
       1. ✔ add top-level 'bridges' file (v6.16)
       2. ➜ show refcount and list removed bridges
 2. … handle gracefully atomic updates during bridge removal
 3. … DSI host-device driver interaction
 4. finish the hotplug bridge work, removing the "always-disconnected"
    connector, moving code to the core and potentially removing the
    hotplug-bridge itself (this needs to be clarified as points 1-3 are
    developed)

To show the removed bridges we need to keep track of them, thus add a new
global list to store them between drm_bridge_remove() and the eventual
free. This is bit tricky in case a bridge is removed and then re-added
before being freed. This is handled in patch 2.

This series  depends on one other series:
 * [PATCH v2 0/9] drm/bridge: get/put the bridge when looping over the encoder chain
   Link: https://lore.kernel.org/r/20250808-drm-bridge-alloc-getput-for_each_bridge-v2-0-edb6ee81edf1@bootlin.com
   Reason: it uses drm_for_each_bridge_in_chain_scoped()

[0] https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/0cc6aadd7fc1e629b715ea3d1ba537ef2da95eec
[1] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/t/#u

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
This series was initially part of v6 of this other series:
- Link to v6: https://lore.kernel.org/dri-devel/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/

---
Luca Ceresoli (3):
      drm/debugfs: bridges_show: show refcount
      drm/bridge: add list of removed refcounted bridges
      drm/debugfs: show removed bridges

 drivers/gpu/drm/drm_bridge.c | 47 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)
---
base-commit: c8cea4371e5eca30cda8660aabb337747dabc51d
change-id: 20250408-drm-bridge-debugfs-removed-2579f892e4b3

Best regards,
-- 
Luca Ceresoli <luca.ceresoli@bootlin.com>

[PATCH v7 1/3] drm/debugfs: bridges_show: show refcount

[Luca Ceresoli]

Now that bridges are refcounted, exposing the refcount in debugfs can be
useful.

Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

---

Changes in v7:
- rebased on current code:
  - code is in drm_bridge.c now
  - removed if (drm_bridge_is_refcounted(bridge)), refcounting is not
    optional

This patch was added in v6.
---
 drivers/gpu/drm/drm_bridge.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 0688f936eeb69d9f5655f2b00de4a0843dc76088..36e0829d25c29457cff5da5fec99646c74b6ad5a 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -1477,6 +1477,9 @@ static void drm_bridge_debugfs_show_bridge(struct drm_printer *p,
 					   unsigned int idx)
 {
 	drm_printf(p, "bridge[%u]: %ps\n", idx, bridge->funcs);
+
+	drm_printf(p, "\trefcount: %u\n", kref_read(&bridge->refcount));
+
 	drm_printf(p, "\ttype: [%d] %s\n",
 		   bridge->type,
 		   drm_get_connector_type_name(bridge->type));

-- 
2.50.1

[PATCH v7 2/3] drm/bridge: add list of removed refcounted bridges

[Luca Ceresoli]

Between drm_bridge_add() and drm_bridge_remove() bridges are "published" 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 "published", i.e. not in bridges_list, and also not visible in
debugfs. This prevents debugging refcounted bridges lifetime, especially
leaks due to any 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.

Document the new behaviour of drm_bridge_remove() and update the
drm_bridge_add() documentation to stay consistent.

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.  When this happens, the
bridge->list is found in the removed list, not at the initialized state as
drm_bridge_add() currently expects.

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

---

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 | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 36e0829d25c29457cff5da5fec99646c74b6ad5a..2e688ee14b9efbc810bcdb0ab7ecd4b688be8299 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_removed_list */
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
+static LIST_HEAD(bridge_removed_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);
@@ -288,10 +296,13 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
 EXPORT_SYMBOL(__devm_drm_bridge_alloc);
 
 /**
- * drm_bridge_add - add the given bridge to the global bridge list
+ * drm_bridge_add - publish a bridge
  *
  * @bridge: bridge control structure
  *
+ * Add the given bridge to the global list of "published" bridges, where
+ * they can be found by users via of_drm_find_bridge().
+ *
  * The bridge to be added must have been allocated by
  * devm_drm_bridge_alloc().
  */
@@ -304,6 +315,14 @@ 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_removed_list. Remove it or bridge_removed_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)
@@ -344,9 +363,14 @@ int devm_drm_bridge_add(struct device *dev, struct drm_bridge *bridge)
 EXPORT_SYMBOL(devm_drm_bridge_add);
 
 /**
- * drm_bridge_remove - remove the given bridge from the global bridge list
+ * drm_bridge_remove - unpublish a bridge
  *
  * @bridge: bridge control structure
+ *
+ * Remove the given bridge from the global list of "published" bridges,
+ * so it won't be found by users via of_drm_find_bridge(), and add it to
+ * the removed bridge list, to keep track of removed bridges until their
+ * allocated memory is actually freed.
  */
 void drm_bridge_remove(struct drm_bridge *bridge)
 {
@@ -357,7 +381,7 @@ 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_removed_list);
 	mutex_unlock(&bridge_lock);
 
 	mutex_destroy(&bridge->hpd_mutex);

-- 
2.50.1

[PATCH v7 3/3] drm/debugfs: show removed bridges

[Luca Ceresoli]

The usefulness of /sys/kernel/debug/dri/bridges is limited as it only shows
bridges between drm_bridge_add() and drm_bridge_remove(). However
refcounted bridges can stay allocated for a long time after
drm_bridge_remove(), and a memory leak due to a missing drm_bridge_put()
would not be visible in this debugfs file.

Add removed bridges to the /sys/kernel/debug/dri/bridges output.

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

---

Changes in v7:
- rebased on current code which is in drm_bridge.c now
- removed if (drm_bridge_is_refcounted(bridge)), refcounting is not
  optional
- don't show bridge address
- improve commit message

This patch was added in v6.
---
 drivers/gpu/drm/drm_bridge.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 2e688ee14b9efbc810bcdb0ab7ecd4b688be8299..4e9ad8a198e357c448b82a1ff5e07e558a6168d6 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -1498,17 +1498,20 @@ EXPORT_SYMBOL(devm_drm_put_bridge);
 
 static void drm_bridge_debugfs_show_bridge(struct drm_printer *p,
 					   struct drm_bridge *bridge,
-					   unsigned int idx)
+					   unsigned int idx,
+					   bool removed)
 {
 	drm_printf(p, "bridge[%u]: %ps\n", idx, bridge->funcs);
 
-	drm_printf(p, "\trefcount: %u\n", kref_read(&bridge->refcount));
+	drm_printf(p, "\trefcount: %u%s\n", kref_read(&bridge->refcount),
+		   removed ? " [removed]" : "");
 
 	drm_printf(p, "\ttype: [%d] %s\n",
 		   bridge->type,
 		   drm_get_connector_type_name(bridge->type));
 
-	if (bridge->of_node)
+	/* The OF node could be freed after drm_bridge_remove() */
+	if (bridge->of_node && !removed)
 		drm_printf(p, "\tOF: %pOFfc\n", bridge->of_node);
 
 	drm_printf(p, "\tops: [0x%x]", bridge->ops);
@@ -1534,7 +1537,10 @@ static int allbridges_show(struct seq_file *m, void *data)
 	mutex_lock(&bridge_lock);
 
 	list_for_each_entry(bridge, &bridge_list, list)
-		drm_bridge_debugfs_show_bridge(&p, bridge, idx++);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false);
+
+	list_for_each_entry(bridge, &bridge_removed_list, list)
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, true);
 
 	mutex_unlock(&bridge_lock);
 
@@ -1549,7 +1555,7 @@ static int encoder_bridges_show(struct seq_file *m, void *data)
 	unsigned int idx = 0;
 
 	drm_for_each_bridge_in_chain_scoped(encoder, bridge)
-		drm_bridge_debugfs_show_bridge(&p, bridge, idx++);
+		drm_bridge_debugfs_show_bridge(&p, bridge, idx++, false);
 
 	return 0;
 }

-- 
2.50.1

Re: [PATCH v7 1/3] drm/debugfs: bridges_show: show refcount

[Maxime Ripard]

On Tue, 19 Aug 2025 11:42:10 +0200, Luca Ceresoli wrote:
> Now that bridges are refcounted, exposing the refcount in debugfs can be
> useful.
> 
> Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> [ ... ]

Reviewed-by: Maxime Ripard <mripard@kernel.org>

Thanks!
Maxime

Re: [PATCH v7 2/3] drm/bridge: add list of removed refcounted bridges

[Maxime Ripard]

[-- Attachment #1: Type: text/plain, Size: 4386 bytes --]

On Tue, Aug 19, 2025 at 11:42:11AM +0200, Luca Ceresoli wrote:
> Between drm_bridge_add() and drm_bridge_remove() bridges are "published" 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 "published", i.e. not in bridges_list, and also not visible in
> debugfs. This prevents debugging refcounted bridges lifetime, especially
> leaks due to any 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.
> 
> Document the new behaviour of drm_bridge_remove() and update the
> drm_bridge_add() documentation to stay consistent.
> 
> 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.  When this happens, the
> bridge->list is found in the removed list, not at the initialized state as
> drm_bridge_add() currently expects.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> ---
> 
> 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 | 30 +++++++++++++++++++++++++++---
>  1 file changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index 36e0829d25c29457cff5da5fec99646c74b6ad5a..2e688ee14b9efbc810bcdb0ab7ecd4b688be8299 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_removed_list */
>  static DEFINE_MUTEX(bridge_lock);
>  static LIST_HEAD(bridge_list);
> +static LIST_HEAD(bridge_removed_list);

I'm not super fond of "removed" here, it's ambiguous, especially since
the bridge wouldn't be considered as removed after the last put.

lingering maybe?

>  
>  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);
> @@ -288,10 +296,13 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
>  EXPORT_SYMBOL(__devm_drm_bridge_alloc);
>  
>  /**
> - * drm_bridge_add - add the given bridge to the global bridge list
> + * drm_bridge_add - publish a bridge
>   *
>   * @bridge: bridge control structure
>   *
> + * Add the given bridge to the global list of "published" bridges, where
> + * they can be found by users via of_drm_find_bridge().

It's quite a change in semantics, at least in the doc. I believe it
should be a separate patch, since it's really more about updating the
drm_bridge_add / drm_bridge_remove doc than collecting the
removed-but-not-freed bridges.

Also, I'm not sure if it's more obvious here. The quotes around publish
kind of it to that too. Maybe using register / registration would make
it more obvious?

Maxime

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Re: [PATCH v7 2/3] drm/bridge: add list of removed refcounted bridges

[Luca Ceresoli]

Hi Maxime,

On Tue, 19 Aug 2025 13:15:30 +0200
Maxime Ripard <mripard@kernel.org> wrote:

> > @@ -197,15 +197,22 @@
> >   * driver.
> >   */
> >  
> > +/* Protect bridge_list and bridge_removed_list */
> >  static DEFINE_MUTEX(bridge_lock);
> >  static LIST_HEAD(bridge_list);
> > +static LIST_HEAD(bridge_removed_list);  
> 
> I'm not super fond of "removed" here, it's ambiguous, especially since
> the bridge wouldn't be considered as removed after the last put.
> 
> lingering maybe?

Sure, will rename.

> > @@ -288,10 +296,13 @@ void *__devm_drm_bridge_alloc(struct device *dev, size_t size, size_t offset,
> >  EXPORT_SYMBOL(__devm_drm_bridge_alloc);
> >  
> >  /**
> > - * drm_bridge_add - add the given bridge to the global bridge list
> > + * drm_bridge_add - publish a bridge
> >   *
> >   * @bridge: bridge control structure
> >   *
> > + * Add the given bridge to the global list of "published" bridges, where
> > + * they can be found by users via of_drm_find_bridge().  
> 
> It's quite a change in semantics, at least in the doc. I believe it
> should be a separate patch, since it's really more about updating the
> drm_bridge_add / drm_bridge_remove doc than collecting the
> removed-but-not-freed bridges.
> 
> Also, I'm not sure if it's more obvious here. The quotes around publish
> kind of it to that too. Maybe using register / registration would make
> it more obvious?

OK, I'll reword using register/registration and definitely move to a
separate patch.

Thanks for reviewing.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com