Hi Maxime, On Mon, 29 Sep 2025 14:43:46 +0200 Maxime Ripard <[email protected]> wrote:
> On Fri, Sep 26, 2025 at 05:59:42PM +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]> > > --- > > 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); > > +} > > + > > Please provide documentation for these two functions. Ah, sure, here is a draft: /** * 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() 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(&drm_encoder_chain_lock(encoder)->bridge_chain) */ 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; } /** * 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); } > In particular, why > do we need to return the encoder pointer? It allows to lock the encoder chain and then dereference the pointer, similar to what drm_bridge_get() does. of_node_get(), which I used as a model for these functions, does the same. This one in particular is used in patch 4 to write the new scoped macros. Locking the encoder chain and then dereferencing the encoder would be very awkward in a macro if drm_encoder_chain_lock() didn't return the encoder. Luca -- Luca Ceresoli, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
