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

Luca Ceresoli
2025-09-26T16:00:44Z

This series ensures that the bridge chain of the encoder will not be
modified while some other 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
6. ➜ protect encoder bridge chain with a mutex
7. of_drm_find_bridge
8. drm_of_find_panel_or_bridge, *_of_get_bridge
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. Avoid it 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>
---
Luca Ceresoli (7):
drm/encoder: add mutex to protect the bridge chain
drm/encoder: drm_encoder_cleanup: take chain mutex while tearing down
drm/bridge: lock the encoder bridge 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 while iterating in drm_atomic_bridge_chain_post_disable/pre_enable()

drivers/gpu/drm/drm_bridge.c | 56 +++++++++++++++++++++-----------------
drivers/gpu/drm/drm_encoder.c | 4 +++
include/drm/drm_bridge.h | 62 +++++++++++++++++++++++--------------------
include/drm/drm_encoder.h | 18 +++++++++++++
4 files changed, 87 insertions(+), 53 deletions(-)
---
base-commit: a7493ff9ad96868f4c1c16813b205ba812a7573c
change-id: 20250925-drm-bridge-alloc-encoder-chain-mutex-b78d62085ee5

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