[PATCH v3 0/7] drm/bridge: protect encoder bridge chain with a mutex

[PATCH v3 0/7] drm/bridge: protect encoder bridge chain with a mutex

[Luca Ceresoli]

This series ensures that the bridge chain of the encoder will not be
modified while some other concurrent code flows are iterating over it.

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)
    C. ✔ kunit tests (v6.17)
    D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
         and warn on old allocation pattern (v6.17)
    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() (drm-misc-next)
       4. ✔ drm_for_each_bridge_in_chain() (drm-misc-next)
       5. ✔ drm_bridge_connector_init (drm-misc-next)
       6. ➜ protect encoder bridge chain with a mutex
       7. of_drm_find_bridge
       8. drm_of_find_panel_or_bridge, *_of_get_bridge
       9. … enforce drm_bridge_add before drm_bridge_attach
    F. ✔ debugfs improvements
       1. ✔ add top-level 'bridges' file (v6.16)
       2. ✔ show refcount and list removed bridges (drm-misc-next)
 2. … handle gracefully atomic updates during bridge removal
 3. … DSI host-device driver interaction
 4. ✔ removing the need for the "always-disconnected" connector
 5. finish the hotplug bridge work, moving code to the core and potentially
    removing the hotplug-bridge itself (this needs to be clarified as
    points 1-3 are developed)

The per-encoder bridge chain is currently assumed to be static once it is
fully initialized. Work is in progress to add hot-pluggable bridges,
breaking that assumption.

With hotplug and especially hot-unplug, bridges will be added and removed
without notice, and thus be added/removed to/from the encoder chain in
drm_bridge_attach/detach(), concurrently to the code iterating on the
chain. This can result in disruption of the code iterating over the
chain. The rationale is explained by a detailed example in patch 2.

Avoid bugs by introducing a mutex to make list insertion, removal and
iterations mutually exclusive.

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

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
Changes in v3:
- Re-added the drm_bridge_put() in the for_each macros, leading to largely
  rewrite them
- Removed the drm_encoder_chain_[un]lock() wrappers
- Fixed a potential ABBA deadlock in patch
- Improved some commit messages
- Link to v2: https://lore.kernel.org/r/20251003-drm-bridge-alloc-encoder-chain-mutex-v2-0-78bf61580a06@bootlin.com

Changes in v2:
- Improve commit messages and add documentation as per v1 review
- Patch 4: fixed infinite loop when encoder->bridge_chain is empty
- Link to v1: https://lore.kernel.org/r/20250926-drm-bridge-alloc-encoder-chain-mutex-v1-0-23b62c47356a@bootlin.com

---
Luca Ceresoli (7):
      drm/encoder: add mutex to protect the bridge chain
      drm/encoder: drm_encoder_cleanup: lock the encoder chain mutex during removal
      drm/bridge: drm_bridge_attach: lock the encoder chain mutex during insertion
      drm/bridge: lock the encoder chain in scoped for_each loops
      drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_from()
      drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_reverse()
      drm/bridge: prevent encoder chain changes in pre_enable/post_disable

 drivers/gpu/drm/drm_bridge.c  | 83 ++++++++++++++++++++++---------------------
 drivers/gpu/drm/drm_encoder.c | 18 ++++++++--
 include/drm/drm_bridge.h      | 73 +++++++++++++++++++++++--------------
 include/drm/drm_encoder.h     |  4 +++
 4 files changed, 109 insertions(+), 69 deletions(-)
---
base-commit: a036f5fceedb9fbd715565fef7b824a121503de7
change-id: 20250925-drm-bridge-alloc-encoder-chain-mutex-b78d62085ee5

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

[PATCH v3 1/7] drm/encoder: add mutex to protect the bridge chain

[Luca Ceresoli]

The per-encoder bridge chain is currently assumed to be static once it is
fully initialized. Work is in progress to add hot-pluggable bridges,
breaking that assumption.

With bridge removal, the encoder chain can change without notice, removing
tail bridges. This can be problematic while iterating over the chain.

Add a mutex to be taken whenever looping or changing the encoder chain.

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

---

Changes in v3:
- Removed the drm_encoder_chain_[un]lock() wrappers

Changes in v2:
- Added documentation to new APIs
---
 drivers/gpu/drm/drm_encoder.c | 2 ++
 include/drm/drm_encoder.h     | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/drm_encoder.c b/drivers/gpu/drm/drm_encoder.c
index 8f2bc6a28482229fd0b030a1958f87753ad7885f..3261f142baea30c516499d23dbf8d0acf5952cd6 100644
--- a/drivers/gpu/drm/drm_encoder.c
+++ b/drivers/gpu/drm/drm_encoder.c
@@ -129,6 +129,7 @@ static int __drm_encoder_init(struct drm_device *dev,
 	}
 
 	INIT_LIST_HEAD(&encoder->bridge_chain);
+	mutex_init(&encoder->bridge_chain_mutex);
 	list_add_tail(&encoder->head, &dev->mode_config.encoder_list);
 	encoder->index = dev->mode_config.num_encoder++;
 
@@ -202,6 +203,7 @@ void drm_encoder_cleanup(struct drm_encoder *encoder)
 	kfree(encoder->name);
 	list_del(&encoder->head);
 	dev->mode_config.num_encoder--;
+	mutex_destroy(&encoder->bridge_chain_mutex);
 
 	memset(encoder, 0, sizeof(*encoder));
 }
diff --git a/include/drm/drm_encoder.h b/include/drm/drm_encoder.h
index 977a9381c8ba943b4d3e021635ea14856df8a17d..eded7c34481a2e9b4a1273004627a03f4035b24f 100644
--- a/include/drm/drm_encoder.h
+++ b/include/drm/drm_encoder.h
@@ -25,6 +25,7 @@
 
 #include <linux/list.h>
 #include <linux/ctype.h>
+#include <linux/mutex.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_mode.h>
 #include <drm/drm_mode_object.h>
@@ -189,6 +190,9 @@ struct drm_encoder {
 	 */
 	struct list_head bridge_chain;
 
+	/** @bridge_chain_mutex: protect bridge_chain from changes while iterating */
+	struct mutex bridge_chain_mutex;
+
 	const struct drm_encoder_funcs *funcs;
 	const struct drm_encoder_helper_funcs *helper_private;
 

-- 
2.51.0

Re: [PATCH v3 1/7] drm/encoder: add mutex to protect the bridge chain

[Maxime Ripard]

On Thu, 9 Oct 2025 13:38:56 +0200, Luca Ceresoli wrote:
> The per-encoder bridge chain is currently assumed to be static once it is
> fully initialized. Work is in progress to add hot-pluggable bridges,
> breaking that assumption.
> 
> With bridge removal, the encoder chain can change without notice, removing
> 
> [ ... ]

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

Thanks!
Maxime

[PATCH v3 2/7] drm/encoder: drm_encoder_cleanup: lock the encoder chain mutex during removal

[Luca Ceresoli]

drm_encoder_cleanup() modifies the encoder chain by removing bridges via
drm_bridge_detach(). Protect this whole operation by taking the mutex, so
that:

 * any users iterating over the chain will not access it during the change
 * other code willing to modify the list (drm_bridge_attach()) will wait
   until drm_encoder_cleanup() is done

Note that the _safe macro in use here is providing a different and
orthogonal kind of protection than the mutex:

 1. list_for_each_entry_safe() allows removing the current entry from the
    list it is iterating on, synchronously; the non-safe version would be
    unable to find the next entry after the current entry has been removed
 2. the mutex being added allows to ensure that the list is not used
    asynchronously by other code while it is being modified; this prevents
    such other concurrent code to derail because it is iterating over an
    element while it is removed

The _safe macro, which works by taking the "next" pointer in addition to
the "current" one, is insufficient to provide the protection at item 2
above. This is visible e.g. when the "next" element is removed by other
concurrent code. This is what would happen without the added mutex:

 1. start loop: list_for_each_entry_safe(pos, n, ...) sets:
    pos = list_first_entry()   = (bridge 1)
    n   = list_next_entry(pos) = (bridge 2)
 2. enter the loop 1st time, do something with *pos (bridge 1)
 3. in the meanwhile bridge 2 is hot-unplugged
    -> another thread removes bridge 2
       -> drm_bridge_detach()
          -> list_del() sets (bridge 2)->next = LIST_POISON1
 4. loop iteration 1 finishes, list_for_each_entry_safe() sets:
    pos = n (previously set to bridge 2)
    n   = (bridge 2)->next = LIST_POISON1
 5. enter the loop 2nd time, do something with *pos (bridge 2)
 6. loop iteration 2 finishes, list_for_each_entry_safe() sets:
    pos = n = LIST_POISON1 ==> bug!

However, simply adding mutex_[un]lock(&encoder->bridge_chain_mutex)
before/after the list_for_each_entry_safe() seems a simple and good
solution, but it is introducing a possible ABBA deadlock (found by
PROVE_LOCKING). The two code paths involved are:

 * drm_encoder_cleanup():
   - takes the bridge_chain_mutex (A)
   - calls drm_bridge_detach -> drm_atomic_private_obj_fini ->
     DRM_MODESET_LOCK_ALL_BEGIN() which takes all locks in the
     acquisition context (B)
 * drm_mode_getconnector() (and other code paths):
   - calls drm_helper_probe_single_connector_modes() which:
     - takes a drm_modeset_lock in the acquisition context (B)
     - calls __drm_helper_update_and_validate ->
       drm_bridge_chain_mode_valid -> drm_for_each_bridge_in_chain_from()
       which takes the bridge_chain_mutex (A)

To avoid this potential ABBA deadlock, move all list items to a temporary
list while holding the bridge_chain_mutex, then detach all elements from
the temporary list without the mutex.

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

---

Changes in v3:
- Prevent ABBA deadlock by using a temporary list
- Improve commit message

Changes in v2:
- Expanded commit messge with rationale, as discussed
---
 drivers/gpu/drm/drm_encoder.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_encoder.c b/drivers/gpu/drm/drm_encoder.c
index 3261f142baea30c516499d23dbf8d0acf5952cd6..0d5dbed06db4461263d28b47e152dc55a7a88bb4 100644
--- a/drivers/gpu/drm/drm_encoder.c
+++ b/drivers/gpu/drm/drm_encoder.c
@@ -189,14 +189,26 @@ void drm_encoder_cleanup(struct drm_encoder *encoder)
 {
 	struct drm_device *dev = encoder->dev;
 	struct drm_bridge *bridge, *next;
+	LIST_HEAD(tmplist);
 
 	/* Note that the encoder_list is considered to be static; should we
 	 * remove the drm_encoder at runtime we would have to decrement all
 	 * the indices on the drm_encoder after us in the encoder_list.
 	 */
 
-	list_for_each_entry_safe(bridge, next, &encoder->bridge_chain,
-				 chain_node)
+	/*
+	 * We need the bridge_chain_mutex to modify the chain, but
+	 * drm_bridge_detach() will call DRM_MODESET_LOCK_ALL_BEGIN() (in
+	 * drm_modeset_lock_fini()), resulting in a possible ABBA circular
+	 * deadlock. Avoid it by first moving all the bridges to a
+	 * temporary list holding the lock, and then calling
+	 * drm_bridge_detach() without the lock.
+	 */
+	mutex_lock(&encoder->bridge_chain_mutex);
+	list_cut_before(&tmplist, &encoder->bridge_chain, &encoder->bridge_chain);
+	mutex_unlock(&encoder->bridge_chain_mutex);
+
+	list_for_each_entry_safe(bridge, next, &tmplist, chain_node)
 		drm_bridge_detach(bridge);
 
 	drm_mode_object_unregister(dev, &encoder->base);

-- 
2.51.0

[PATCH v3 3/7] drm/bridge: drm_bridge_attach: lock the encoder chain mutex during insertion

[Luca Ceresoli]

drm_bridge_attach() modifies the encoder bridge chain, so take a mutex
around such operations to allow users of the chain to protect themselves
from chain modifications while iterating.

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

---

Changes in v3:
- Lock encoder->bridge_chain_mutex directly, no wrappers

Changes in v2:
- Removed comment before on drm_bridge_detach()
---
 drivers/gpu/drm/drm_bridge.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 53e7ece36dd940aabd1c0880f296fce7224a12ac..a25e379237f95c037eddc7602d405f958a4efa44 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -453,10 +453,12 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 	bridge->dev = encoder->dev;
 	bridge->encoder = encoder;
 
+	mutex_lock(&encoder->bridge_chain_mutex);
 	if (previous)
 		list_add(&bridge->chain_node, &previous->chain_node);
 	else
 		list_add(&bridge->chain_node, &encoder->bridge_chain);
+	mutex_unlock(&encoder->bridge_chain_mutex);
 
 	if (bridge->funcs->attach) {
 		ret = bridge->funcs->attach(bridge, encoder, flags);
@@ -487,7 +489,9 @@ int drm_bridge_attach(struct drm_encoder *encoder, struct drm_bridge *bridge,
 err_reset_bridge:
 	bridge->dev = NULL;
 	bridge->encoder = NULL;
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_del(&bridge->chain_node);
+	mutex_unlock(&encoder->bridge_chain_mutex);
 
 	if (ret != -EPROBE_DEFER)
 		DRM_ERROR("failed to attach bridge %pOF to encoder %s: %d\n",

-- 
2.51.0

Re: [PATCH v3 3/7] drm/bridge: drm_bridge_attach: lock the encoder chain mutex during insertion

[Maxime Ripard]

On Thu, 9 Oct 2025 13:38:58 +0200, Luca Ceresoli wrote:
> drm_bridge_attach() modifies the encoder bridge chain, so take a mutex
> around such operations to allow users of the chain to protect themselves
> from chain modifications while iterating.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> [ ... ]

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

Thanks!
Maxime

[PATCH v3 4/7] drm/bridge: lock the encoder chain in scoped for_each loops

[Luca Ceresoli]

drm_for_each_bridge_in_chain_scoped() and
drm_for_each_bridge_in_chain_from() currently get/put the bridge at each
iteration. But they don't protect the encoder chain, so it could change
(bridges added/removed) while some code is iterating over the list
itself. Such code can then derail on incorrect pointers.

To make iterations safe, augment these for_each macros to lock the encoder
chain mutex at the beginning and unlock it at the end of the loop (be it at
the end of the list, or earlier due to a 'break' or 'return' statement).

This change requires more operations when starting and ending the loop. To
avoid making the macros even more complex, move these operations to helper
functions. Also remname some of the existing helper functions for
consistency.

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

---

Changed in v3:
- Re-add drm_bridge_get/put()

Changed in v2:
- Fixed infinite loop in drm_for_each_bridge_in_chain_scoped() when
  encoder->bridge_chain is empty, reported here:
  https://lore.kernel.org/lkml/202509301358.38036b85-lkp@intel.com/
- Slightly improved commit message
---
 include/drm/drm_bridge.h | 73 +++++++++++++++++++++++++++++++-----------------
 1 file changed, 47 insertions(+), 26 deletions(-)

diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
index 0ff7ab4aa8689a022458f935a7ffb23a2b715802..d9eefe903208c0bd2464b9d4991456c93dc285aa 100644
--- a/include/drm/drm_bridge.h
+++ b/include/drm/drm_bridge.h
@@ -1440,26 +1440,37 @@ drm_bridge_chain_get_last_bridge(struct drm_encoder *encoder)
 						      struct drm_bridge, chain_node));
 }
 
-/**
- * drm_bridge_get_next_bridge_and_put - Get the next bridge in the chain
- *                                      and put the previous
- * @bridge: bridge object
- *
- * Same as drm_bridge_get_next_bridge() but additionally puts the @bridge.
- *
- * RETURNS:
- * the next bridge in the chain after @bridge, or NULL if @bridge is the last.
- */
-static inline struct drm_bridge *
-drm_bridge_get_next_bridge_and_put(struct drm_bridge *bridge)
+/* Internal to drm_for_each_bridge_in_chain*() */
+static inline struct drm_bridge *__drm_for_each_bridge_in_chain_next(struct drm_bridge *bridge)
 {
 	struct drm_bridge *next = drm_bridge_get_next_bridge(bridge);
 
+	if (!next)
+		mutex_unlock(&bridge->encoder->bridge_chain_mutex);
+
 	drm_bridge_put(bridge);
 
 	return next;
 }
 
+/* Internal to drm_for_each_bridge_in_chain*() */
+DEFINE_FREE(__drm_for_each_bridge_in_chain_cleanup, struct drm_bridge *,
+	if (_T) { mutex_unlock(&_T->encoder->bridge_chain_mutex); drm_bridge_put(_T); })
+
+/* Internal to drm_for_each_bridge_in_chain_scoped() */
+static inline struct drm_bridge *
+__drm_for_each_bridge_in_chain_scoped_start(struct drm_encoder *encoder)
+{
+	mutex_lock(&encoder->bridge_chain_mutex);
+
+	struct drm_bridge *bridge = drm_bridge_chain_get_first_bridge(encoder);
+
+	if (!bridge)
+		mutex_unlock(&encoder->bridge_chain_mutex);
+
+	return bridge;
+}
+
 /**
  * drm_for_each_bridge_in_chain_scoped - iterate over all bridges attached
  *                                       to an encoder
@@ -1469,14 +1480,24 @@ drm_bridge_get_next_bridge_and_put(struct drm_bridge *bridge)
  *
  * Iterate over all bridges present in the bridge chain attached to @encoder.
  *
- * Automatically gets/puts the bridge reference while iterating, and puts
- * the reference even if returning or breaking in the middle of the loop.
+ * Automatically locks the encoder chain mutex to prevent chain
+ * modifications while iterating.
  */
-#define drm_for_each_bridge_in_chain_scoped(encoder, bridge)		\
-	for (struct drm_bridge *bridge __free(drm_bridge_put) =		\
-	     drm_bridge_chain_get_first_bridge(encoder);		\
-	     bridge;							\
-	     bridge = drm_bridge_get_next_bridge_and_put(bridge))
+#define drm_for_each_bridge_in_chain_scoped(encoder, bridge)				\
+	for (struct drm_bridge *bridge __free(__drm_for_each_bridge_in_chain_cleanup) =	\
+		__drm_for_each_bridge_in_chain_scoped_start((encoder));			\
+	     bridge;									\
+	     bridge = __drm_for_each_bridge_in_chain_next(bridge))			\
+
+/* Internal to drm_for_each_bridge_in_chain_from() */
+static inline struct drm_bridge *
+__drm_for_each_bridge_in_chain_from_start(struct drm_bridge *bridge)
+{
+	drm_bridge_get(bridge);
+	mutex_lock(&bridge->encoder->bridge_chain_mutex);
+
+	return bridge;
+}
 
 /**
  * drm_for_each_bridge_in_chain_from - iterate over all bridges starting
@@ -1488,14 +1509,14 @@ drm_bridge_get_next_bridge_and_put(struct drm_bridge *bridge)
  * Iterate over all bridges in the encoder chain starting from
  * @first_bridge, included.
  *
- * Automatically gets/puts the bridge reference while iterating, and puts
- * the reference even if returning or breaking in the middle of the loop.
+ * Automatically locks the encoder chain mutex to prevent chain
+ * modifications while iterating.
  */
-#define drm_for_each_bridge_in_chain_from(first_bridge, bridge)		\
-	for (struct drm_bridge *bridge __free(drm_bridge_put) =		\
-		     drm_bridge_get(first_bridge);			\
-	     bridge;							\
-	     bridge = drm_bridge_get_next_bridge_and_put(bridge))
+#define drm_for_each_bridge_in_chain_from(first_bridge, bridge)				\
+	for (struct drm_bridge *bridge __free(__drm_for_each_bridge_in_chain_cleanup) =	\
+		__drm_for_each_bridge_in_chain_from_start(first_bridge);		\
+	     bridge;									\
+	     bridge = __drm_for_each_bridge_in_chain_next(bridge))			\
 
 enum drm_mode_status
 drm_bridge_chain_mode_valid(struct drm_bridge *bridge,

-- 
2.51.0

[PATCH v3 5/7] drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_from()

[Luca Ceresoli]

These loops in drm_bridge.c iterate over the encoder chain using
list_for_each_entry_from(), which does not prevent changes to the bridge
chain while iterating over it.

Convert most of those loops to instead use
drm_for_each_bridge_in_chain_from(), which locks the chain.

This also simplifies code.

All the "simple" loops are converted here. The only ones not touched are
those in drm_atomic_bridge_chain_pre_enable() and
drm_atomic_bridge_chain_post_disable(), because they have nested loops
which are not well handled by drm_for_each_bridge_in_chain_from(). These
two functions are handled by a separate commit.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
 drivers/gpu/drm/drm_bridge.c | 35 ++++++++++++-----------------------
 1 file changed, 12 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index a25e379237f95c037eddc7602d405f958a4efa44..a56547a7bcfd05b17c07a03bd4ce52b0994e7cd8 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -618,7 +618,7 @@ void drm_bridge_detach(struct drm_bridge *bridge)
 /**
  * drm_bridge_chain_mode_valid - validate the mode against all bridges in the
  *				 encoder chain.
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @info: display info against which the mode shall be validated
  * @mode: desired mode to be validated
  *
@@ -632,17 +632,14 @@ void drm_bridge_detach(struct drm_bridge *bridge)
  * MODE_OK on success, drm_mode_status Enum error code on failure
  */
 enum drm_mode_status
-drm_bridge_chain_mode_valid(struct drm_bridge *bridge,
+drm_bridge_chain_mode_valid(struct drm_bridge *first_bridge,
 			    const struct drm_display_info *info,
 			    const struct drm_display_mode *mode)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return MODE_OK;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge) {
 		enum drm_mode_status ret;
 
 		if (!bridge->funcs->mode_valid)
@@ -660,7 +657,7 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_valid);
 /**
  * drm_bridge_chain_mode_set - set proposed mode for all bridges in the
  *			       encoder chain
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @mode: desired mode to be set for the encoder chain
  * @adjusted_mode: updated mode that works for this encoder chain
  *
@@ -669,20 +666,16 @@ EXPORT_SYMBOL(drm_bridge_chain_mode_valid);
  *
  * Note: the bridge passed should be the one closest to the encoder
  */
-void drm_bridge_chain_mode_set(struct drm_bridge *bridge,
+void drm_bridge_chain_mode_set(struct drm_bridge *first_bridge,
 			       const struct drm_display_mode *mode,
 			       const struct drm_display_mode *adjusted_mode)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge)
 		if (bridge->funcs->mode_set)
 			bridge->funcs->mode_set(bridge, mode, adjusted_mode);
-	}
 }
 EXPORT_SYMBOL(drm_bridge_chain_mode_set);
 
@@ -906,7 +899,7 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
 
 /**
  * drm_atomic_bridge_chain_enable - enables all bridges in the encoder chain
- * @bridge: bridge control structure
+ * @first_bridge: bridge control structure
  * @state: atomic state being committed
  *
  * Calls &drm_bridge_funcs.atomic_enable (falls back on
@@ -916,22 +909,18 @@ EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
  *
  * Note: the bridge passed should be the one closest to the encoder
  */
-void drm_atomic_bridge_chain_enable(struct drm_bridge *bridge,
+void drm_atomic_bridge_chain_enable(struct drm_bridge *first_bridge,
 				    struct drm_atomic_state *state)
 {
-	struct drm_encoder *encoder;
-
-	if (!bridge)
+	if (!first_bridge)
 		return;
 
-	encoder = bridge->encoder;
-	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
+	drm_for_each_bridge_in_chain_from(first_bridge, bridge)
 		if (bridge->funcs->atomic_enable) {
 			bridge->funcs->atomic_enable(bridge, state);
 		} else if (bridge->funcs->enable) {
 			bridge->funcs->enable(bridge);
 		}
-	}
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_enable);
 

-- 
2.51.0

[PATCH v3 6/7] drm/bridge: prevent encoder chain changes while iterating with list_for_each_entry_reverse()

[Luca Ceresoli]

These loops in drm_bridge.c iterate over the encoder chain using
list_for_each_entry_reverse(), which does not prevent changes to the bridge
chain while iterating over it.

Take the encoder chain mutex while iterating to avoid chain changes while
iterating.

All the "simple" loops are converted. drm_atomic_bridge_chain_pre_enable()
and drm_atomic_bridge_chain_post_disable() are handled by a separate
commit.

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

---

Changes in v3:
- Lock encoder->bridge_chain_mutex directly, no wrappers

Changes in v2: none
---
 drivers/gpu/drm/drm_bridge.c | 40 ++++++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index a56547a7bcfd05b17c07a03bd4ce52b0994e7cd8..6de9b296c0be247f861815b7f558a80fd7938fae 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -701,6 +701,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge,
 		return;
 
 	encoder = bridge->encoder;
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		if (iter->funcs->atomic_disable) {
 			iter->funcs->atomic_disable(iter, state);
@@ -711,6 +712,7 @@ void drm_atomic_bridge_chain_disable(struct drm_bridge *bridge,
 		if (iter == bridge)
 			break;
 	}
+	mutex_unlock(&encoder->bridge_chain_mutex);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_disable);
 
@@ -1215,25 +1217,27 @@ int drm_atomic_bridge_chain_check(struct drm_bridge *bridge,
 		return ret;
 
 	encoder = bridge->encoder;
-	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
-		int ret;
-
-		/*
-		 * Bus flags are propagated by default. If a bridge needs to
-		 * tweak the input bus flags for any reason, it should happen
-		 * in its &drm_bridge_funcs.atomic_check() implementation such
-		 * that preceding bridges in the chain can propagate the new
-		 * bus flags.
-		 */
-		drm_atomic_bridge_propagate_bus_flags(iter, conn,
-						      crtc_state->state);
-
-		ret = drm_atomic_bridge_check(iter, crtc_state, conn_state);
-		if (ret)
-			return ret;
+	scoped_guard(mutex, &encoder->bridge_chain_mutex) {
+		list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
+			int ret;
+
+			/*
+			 * Bus flags are propagated by default. If a bridge needs to
+			 * tweak the input bus flags for any reason, it should happen
+			 * in its &drm_bridge_funcs.atomic_check() implementation such
+			 * that preceding bridges in the chain can propagate the new
+			 * bus flags.
+			 */
+			drm_atomic_bridge_propagate_bus_flags(iter, conn,
+							      crtc_state->state);
+
+			ret = drm_atomic_bridge_check(iter, crtc_state, conn_state);
+			if (ret)
+				return ret;
 
-		if (iter == bridge)
-			break;
+			if (iter == bridge)
+				break;
+		}
 	}
 
 	return 0;

-- 
2.51.0

[PATCH v3 7/7] drm/bridge: prevent encoder chain changes in pre_enable/post_disable

[Luca Ceresoli]

Take the encoder chain mutex while iterating over the encoder chain in
drm_atomic_bridge_chain_pre_enable() and
drm_atomic_bridge_chain_post_disable() to ensure the lists won't change
while being inspected.

These functions have nested list_for_each_*() loops, which makes them
complicated. list_for_each_entry_from() loops could be replaced by
drm_for_each_bridge_in_chain_from(), but it would not work in a nested way
in its current implementation. Besides, there is no "_reverse" variant of
drm_for_each_bridge_in_chain_from().

Keep code simple and readable by explicitly locking around the outer
loop. Thankfully there are no break or return points inside the loops, so
the change is trivial and readable.

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

---

Changes in v3:
- Lock encoder->bridge_chain_mutex directly, no wrappers
- Improved commit message

Changes in v2:
- Improved commit message
---
 drivers/gpu/drm/drm_bridge.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index 6de9b296c0be247f861815b7f558a80fd7938fae..a84aa68064513c654d6cc908718c79179f776ee7 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -760,6 +760,7 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
 
 	encoder = bridge->encoder;
 
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_for_each_entry_from(bridge, &encoder->bridge_chain, chain_node) {
 		limit = NULL;
 
@@ -808,6 +809,7 @@ void drm_atomic_bridge_chain_post_disable(struct drm_bridge *bridge,
 			/* Jump all bridges that we have already post_disabled */
 			bridge = limit;
 	}
+	mutex_unlock(&encoder->bridge_chain_mutex);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_post_disable);
 
@@ -854,6 +856,7 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
 
 	encoder = bridge->encoder;
 
+	mutex_lock(&encoder->bridge_chain_mutex);
 	list_for_each_entry_reverse(iter, &encoder->bridge_chain, chain_node) {
 		if (iter->pre_enable_prev_first) {
 			next = iter;
@@ -896,6 +899,7 @@ void drm_atomic_bridge_chain_pre_enable(struct drm_bridge *bridge,
 		if (iter == bridge)
 			break;
 	}
+	mutex_unlock(&encoder->bridge_chain_mutex);
 }
 EXPORT_SYMBOL(drm_atomic_bridge_chain_pre_enable);
 

-- 
2.51.0

Re: [PATCH v3 7/7] drm/bridge: prevent encoder chain changes in pre_enable/post_disable

[Maxime Ripard]

On Thu, 9 Oct 2025 13:39:02 +0200, Luca Ceresoli wrote:
> Take the encoder chain mutex while iterating over the encoder chain in
> drm_atomic_bridge_chain_pre_enable() and
> drm_atomic_bridge_chain_post_disable() to ensure the lists won't change
> while being inspected.
> 
> 
> [ ... ]

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

Thanks!
Maxime