On Fri, Oct 03, 2025 at 12:39:23PM +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
> 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 <[email protected]>
>
> ---
>
> Changes in v2:
> - Added documentation to new APIs
> ---
> drivers/gpu/drm/drm_encoder.c | 2 ++
> include/drm/drm_encoder.h | 39 +++++++++++++++++++++++++++++++++++++++
> 2 files changed, 41 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..449281c37e39f67a0037603762f347f5086df983
> 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 +323,41 @@ static inline struct drm_encoder
> *drm_encoder_find(struct drm_device *dev,
> return mo ? obj_to_encoder(mo) : NULL;
> }
>
> +/**
> + * drm_encoder_chain_lock - lock the encoder bridge chain
> + * @encoder: encoder whose bridge chain must be locked
> + *
> + * Locks the mutex protecting the bridge chain from concurrent access.
> + * To be called by code modifying ot iterating over the bridge chain to
> + * prevent the list from changing while iterating over it.
> + * Call drm_encoder_chain_unlock() when done to unlock the mutex.
> + *
> + * Returns:
> + * Pointer to @encoder. Useful to lock the chain and then operate on the
> + * in the same statement, e.g.:
> + * list_first_entry_or_null(&drm_encoder_chain_lock(encoder)->bridge_chain)
> + */
> +static inline struct drm_encoder *drm_encoder_chain_lock(struct drm_encoder
> *encoder)
What is the use case for these wrappers? I'm asking especially since
you almost never use the return value of the _lock() one. I think with
scoped_guard you can get the same kind of code without needing extra API
or extra wrappers.
> +{
> + if (!WARN_ON_ONCE(!encoder))
> + mutex_lock(&encoder->bridge_chain_mutex);
> +
> + return encoder;
> +}
> +
> +/**
> + * drm_encoder_chain_unlock - unlock the encoder bridge chain
> + * @encoder: encoder whose bridge chain must be unlocked
> + *
> + * Unlocks the mutex protecting the bridge chain from concurrent access,
> + * matching drm_encoder_chain_lock().
> + */
> +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
>
--
With best wishes
Dmitry
