[PATCH v5 0/7] drm/bridge: protect encoder bridge chain with a mutex

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

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/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/#t

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
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-v4-0-12b13eb8c0f8@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-c90ed744efec@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-v2-0-78bf61580a06@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-v1-0-23b62c47356a@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 <luca.ceresoli@bootlin.com>