linux-kernel - Re: [PATCH v8 0/3] drm/bridge: debugfs: show refcount and list removed bridges

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Message-ID: <20250915-celadon-dalmatian-of-defense-ca56fb@penduick>
Date: Mon, 15 Sep 2025 10:13:16 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: 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>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Hui Pu <Hui.Pu@...ealthcare.com>, 
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>, dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	Dmitry Baryshkov <lumag@...nel.org>
Subject: Re: [PATCH v8 0/3] drm/bridge: debugfs: show refcount and list
 removed bridges

Hi,

On Fri, Sep 12, 2025 at 07:03:40PM +0200, Luca Ceresoli wrote:
> This series shows removed bridges to the global <debugfs>/dri/bridges file.
> 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):
> 
> 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() (partially in drm-misc-next)
>        4. …✔ drm_for_each_bridge_in_chain() (partially in drm-misc-next)
>        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.

Once the minor issue I've reported is fixed,
Reviewed-by: Maxime Ripard <mripard@...nel.org>

Maxime

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