This series ensures that the bridge chain of the encoder will not be
modified while some other concurrent code flows are iterating over it.
Some patches are already Reviewed-by: Maxime Ripard, some are unreviewed.
== Series description
The per-encoder bridge chain is currently assumed to be static once it is
fully initialized. Work is in progress to support 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.
Patch 2 has a detailed example to describe the problem.
This series prevents concurrency bugs by introducing a mutex to make list
insertion, removal and iterations mutually exclusive.
== Grand plan
This is part of the work to support hotplug of DRM bridges. The grand plan
was discussed in [0].
Here's the work breakdown (➜ marks the current series):
1. ➜ add refcounting to DRM bridges struct drm_bridge,
based on devm_drm_bridge_alloc()
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 cleanup action (v6.18)
2. ✔ drm_bridge_get_prev_bridge() (v6.18)
3. ✔ drm_bridge_get_next_bridge() (v6.19)
4. ✔ drm_for_each_bridge_in_chain() (v6.19)
5. ✔ drm_bridge_connector_init (v6.19)
6. ➜ protect encoder bridge chain with a mutex
7. … of_drm_find_bridge
a. ✔ add of_drm_get_bridge() (v7.0),
convert basic direct users (v7.0-v7.1)
b. ✔ convert direct of_drm_get_bridge() users, part 2 (v7.0)
c. ✔ convert direct of_drm_get_bridge() users, part 3 (v7.0)
d. ✔… convert direct of_drm_get_bridge() users, part 4
(some v7.1, some pending)
e. convert bridge-only drm_of_find_panel_or_bridge() users
8. drm_of_find_panel_or_bridge, *_of_get_bridge
9. ✔ enforce drm_bridge_add before drm_bridge_attach (v6.19)
F. ✔ debugfs improvements
1. ✔ add top-level 'bridges' file (v6.16)
2. ✔ show refcount and list lingering bridges (v6.19)
2. … handle gracefully atomic updates during bridge removal
A. ✔ Add drm_bridge_enter/exit() to protect device resources (v7.0)
B. … protect private_obj removal from list
C. ✔ Add drm_bridge_clear_and_put() (v7.1)
3. … DSI host-device driver interaction
4. ✔ removing the need for the "always-disconnected" connector
5. … Migrate i.MX LCDIF driver to bridge-connector
6. DRM bridge hotplug
A. Bridge hotplug management in the DRM core
B. Device tree description
[0]
https://lore.kernel.org/lkml/[email protected]/#t
Signed-off-by: Luca Ceresoli <[email protected]>
---
Changes in v5:
- Improved cover letter and patch 2 commit message
- Rebased and tested on current drm-misc-next (v7.1-rc3)
- Link to v4:
https://lore.kernel.org/r/20251111-drm-bridge-alloc-encoder-chain-mutex-v4-0-12b13eb8c...@bootlin.com
Changes in v4:
- No patch changes (this series was tested a lot since v3 without issues)
- Added Reviewed-by from Maxime
- Rebased on current drm-misc-next
- Small improvement to kerneldoc for the two for_each macros
- Link to v3:
https://lore.kernel.org/r/20251009-drm-bridge-alloc-encoder-chain-mutex-v3-0-c90ed744e...@bootlin.com
Changes in v3:
- Re-added the drm_bridge_put() in the for_each macros, leading to largely
rewrite them
- Removed the drm_encoder_chain_[un]lock() wrappers
- Fixed a potential ABBA deadlock in patch
- Improved some commit messages
- Link to v2:
https://lore.kernel.org/r/20251003-drm-bridge-alloc-encoder-chain-mutex-v2-0-78bf61580...@bootlin.com
Changes in v2:
- Improve commit messages and add documentation as per v1 review
- Patch 4: fixed infinite loop when encoder->bridge_chain is empty
- Link to v1:
https://lore.kernel.org/r/20250926-drm-bridge-alloc-encoder-chain-mutex-v1-0-23b62c473...@bootlin.com
---
Luca Ceresoli (7):
drm/encoder: add mutex to protect the bridge chain
drm/encoder: drm_encoder_cleanup: lock the encoder chain mutex during
removal
drm/bridge: drm_bridge_attach: lock the encoder 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 in pre_enable/post_disable
drivers/gpu/drm/drm_bridge.c | 83 ++++++++++++++++++++++---------------------
drivers/gpu/drm/drm_encoder.c | 18 ++++++++--
include/drm/drm_bridge.h | 73 +++++++++++++++++++++++--------------
include/drm/drm_encoder.h | 4 +++
4 files changed, 109 insertions(+), 69 deletions(-)
---
base-commit: f6d00f979306cb9baf5ab3d4f61dd34857e78c13
change-id: 20250925-drm-bridge-alloc-encoder-chain-mutex-b78d62085ee5
Best regards,
--
Luca Ceresoli <[email protected]>
