Message-Id: <20251003-b4-drm-bridge-alloc-add-before-attach-v1-0-92fb40d27704@bootlin.com>
Date: Fri, 03 Oct 2025 10:59:54 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Alain Volmat <alain.volmat@...s.st.com>, 
 Raphael Gallais-Pou <rgallaispou@...il.com>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Andrzej Hajda <andrzej.hajda@...el.com>, 
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>
Cc: Hui Pu <Hui.Pu@...ealthcare.com>, 
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 Luca Ceresoli <luca.ceresoli@...tlin.com>
Subject: [PATCH 0/4] drm/bridge: enforce drm_bridge_add() before
 drm_bridge_attach()

This small series enforces that DRM bridges must be added before they are
attached as discussed in [1].

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 (drm-misc-next)
       6. … protect encoder bridge chain with a mutex
       7. of_drm_find_bridge
       8. drm_of_find_panel_or_bridge, *_of_get_bridge
       9. ➜ enforce drm_bridge_add before drm_bridge_attach
    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)

Series layout:
 - patches 1-2: add missing drm_bridge_add() to the 2 drivers known for not
                calling it
 - patch 3: document that drm_bridge_add() is required before attach
 - patch 4: add a warning in drm_bridge_attach() is drm_bridge_add() was
            not called

The added warning might reveal more non-compliant drivers, in that case
they will be fixed as they are found.

[0] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/#t
[1] https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
Luca Ceresoli (4):
      drm/sti: hda: add bridge before attaching
      drm/sti: hdmi: add bridge before attaching
      drm/bridge: document that adding a bridge is mandatory before attach
      drm/bridge: add warning for bridges attached without being added

 drivers/gpu/drm/drm_bridge.c   | 6 ++++++
 drivers/gpu/drm/sti/sti_hda.c  | 5 +++++
 drivers/gpu/drm/sti/sti_hdmi.c | 2 ++
 3 files changed, 13 insertions(+)
---
base-commit: 0f2efbe6d8305b91c9b2c92ebaf8c24a614bc305
change-id: 20251003-b4-drm-bridge-alloc-add-before-attach-ef312c0b8a8c

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

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