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

Luca Ceresoli
2025-09-26T16:00:47Z

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.

Also add two APIs to lock/unlock the mutex without the need to manipulate
internal struct drm_encoder fields.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
drivers/gpu/drm/drm_encoder.c | 2 ++
include/drm/drm_encoder.h | 18 ++++++++++++++++++
2 files changed, 20 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..6c962de640a345bfbb18308c83076628547c9ab9 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;

@@ -319,6 +323,20 @@ static inline struct drm_encoder *drm_encoder_find(struct drm_device *dev,
return mo ? obj_to_encoder(mo) : NULL;
}

+static inline struct drm_encoder *drm_encoder_chain_lock(struct drm_encoder *encoder)
+{
+ if (!WARN_ON_ONCE(!encoder))
+ mutex_lock(&encoder->bridge_chain_mutex);
+
+ return encoder;
+}
+
+static inline void drm_encoder_chain_unlock(struct drm_encoder *encoder)
+{
+ if (!WARN_ON_ONCE(!encoder))
+ mutex_unlock(&encoder->bridge_chain_mutex);
+}
+
void drm_encoder_cleanup(struct drm_encoder *encoder);

/**

--
2.51.0