drm/encoder: add mutex to protect the bridge chain

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.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-encoder-chain-mutex-v5-1-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

diff --git a/drivers/gpu/drm/drm_encoder.c b/drivers/gpu/drm/drm_encoder.c
index 8f2bc6a28482229fd0b030a1958f87753ad7885f..3261f142baea30c516499d23dbf8d0acf5952cd6 100644 (file)
--- 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 (file)
--- 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;