[PATCH v2 0/2] drm/bridge: handle gracefully atomic updates during bridge removal

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

This is a first attempt at gracefully handling the case of atomic updates
happening concurrently to physical removal of DRM bridges.

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 [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() [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 idea was proposed by Maxime [1] and is based on the existing
drm_dev_enter/exit() already existing for the DRM device.

This small series implements the core mechanism in drm_bridge.c and uses it
in the ti-sn65dsi83 driver. This prevents usage of device resources by
various code paths that can happen concurrently to unplug of the SN65DSI8x
bridge.

[0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/#t
[1] https://lore.kernel.org/all/20250106-vigorous-talented-viper-fa49d9@houat/

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
---
Changes in v2:
- No changes to patch 1, discussion pending
- Use devres instead of a flag in patch 2
- Link to v1: https://lore.kernel.org/r/20250808-drm-bridge-atomic-vs-remove-v1-0-a52e933b08a8@bootlin.com

---
Luca Ceresoli (2):
      drm/bridge: add drm_bridge_unplug() and drm_bridge_enter/exit()
      drm/bridge: ti-sn65dsi83: protect device resources on unplug

 drivers/gpu/drm/bridge/ti-sn65dsi83.c | 85 ++++++++++++++++++++++++++++-------
 drivers/gpu/drm/drm_bridge.c          | 58 ++++++++++++++++++++++++
 include/drm/drm_bridge.h              | 12 +++++
 3 files changed, 138 insertions(+), 17 deletions(-)
---
base-commit: 7acbe30813f04cccf7b2e8b571eb7936cfec0a87
change-id: 20250808-drm-bridge-atomic-vs-remove-1d7bb202c8ef

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