linux-kernel - [PATCH v7 0/3] drm/bridge: debugfs: show refcount and list removed bridges

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Message-Id: <20250819-drm-bridge-debugfs-removed-v7-0-970702579978@bootlin.com>
Date: Tue, 19 Aug 2025 11:42:09 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: 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>, 
 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>
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>, 
 Dmitry Baryshkov <lumag@...nel.org>
Subject: [PATCH v7 0/3] drm/bridge: debugfs: show refcount and list removed
 bridges

This series shows removed bridges to the global <debugfs>/dri/bridges file,
and shows the bridge refcount.  Removed bridges are bridges after
drm_bridges_remove() but before they are eventually freed on the last
drm_bridge_put().

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-rc1)
    C. ✔ kunit tests (v6.17-rc1)
    D. ✔ add get/put to drm_bridge_add/remove() + attach/detach()
         and warn on old allocation pattern (v6.17-rc1)
    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()
       4. … drm_for_each_bridge_in_chain()
       5. drm_bridge_connector_init
       6. of_drm_find_bridge
       7. 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
 2. … handle gracefully atomic updates during bridge removal
 3. … DSI host-device driver interaction
 4. finish the hotplug bridge work, removing the "always-disconnected"
    connector, moving code to the core and potentially removing the
    hotplug-bridge itself (this needs to be clarified as points 1-3 are
    developed)

To show the removed bridges we need to keep track of them, thus add a new
global list to store them between drm_bridge_remove() and the eventual
free. This is bit tricky in case a bridge is removed and then re-added
before being freed. This is handled in patch 2.

This series  depends on one other series:
 * [PATCH v2 0/9] drm/bridge: get/put the bridge when looping over the encoder chain
   Link: https://lore.kernel.org/r/20250808-drm-bridge-alloc-getput-for_each_bridge-v2-0-edb6ee81edf1@bootlin.com
   Reason: it uses drm_for_each_bridge_in_chain_scoped()

[0] https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/0cc6aadd7fc1e629b715ea3d1ba537ef2da95eec
[1] https://lore.kernel.org/lkml/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/t/#u

Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
---
This series was initially part of v6 of this other series:
- Link to v6: https://lore.kernel.org/dri-devel/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@bootlin.com/

---
Luca Ceresoli (3):
      drm/debugfs: bridges_show: show refcount
      drm/bridge: add list of removed refcounted bridges
      drm/debugfs: show removed bridges

 drivers/gpu/drm/drm_bridge.c | 47 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)
---
base-commit: c8cea4371e5eca30cda8660aabb337747dabc51d
change-id: 20250408-drm-bridge-debugfs-removed-2579f892e4b3

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