Re: [PATCH v9 0/5] drm/bridge: add devm_drm_bridge_alloc() with bridge refcount

[Louis Chauvet]

Le 26/03/2025 à 18:47, Luca Ceresoli a écrit :
> This series improves the way DRM bridges are allocated and initialized and
> makes them reference-counted. The goal of reference counting is to avoid
> use-after-free by drivers which got a pointer to a bridge and keep it
> stored and used even after the bridge has been deallocated.
>
> The overall goal is supporting Linux devices with a DRM pipeline whose
> final components can be hot-plugged and hot-unplugged, including one or
> more bridges. For more details see the big picture [0].
>
> DRM bridge drivers will have to be adapted to the new API -- the change is
> trivial for most cases. This series converts two of them to serve as an
> example. The remaining ones will be done as a follow-up after this first
> part is merged.
>
> After that, refcounting will have to be added on the two sides: all
> functions returning a bridge pointer and all code obtaining such a
> pointer. A few examples have been sent in v7 (link below), they are OK, but
> I removed them since v8 because they must be merged only after converting
> all bridges.
>
> Here's the grand plan:
>
>   A. add new alloc API and refcounting (this series, at least patches 1-3)
>   B. after (A), convert all bridge drivers to new API
>   C. after (A), add documentation and kunit tests
>   D. after (B), add get/put to drm_bridge_add/remove() + attach/detech()
>      (patches 3-4 in the v7 series)
>   E. after (B), convert accessors (including patches 5-9 in the v7 series
>      which convert drm_bridge_chain_get_first_bridge() and its users);
>      this is a large work and can be done in chunks
>   F. after (A), debugfs: show refcount, show removed bridges
>   
> Layout of this:
>
>   1. Add the new API and refcounting:
>
>      drm/bridge: add devm_drm_bridge_alloc()
>      drm/bridge: add support for refcounting
>      drm/bridge: deprecate old-style bridge allocation
>
>   2. convert a few bridge drivers (bridge providers) to the new API:
>
>      drm/bridge: ti-sn65dsi83: use dynamic lifetime management
>      drm/bridge: samsung-dsim: use dynamic lifetime management
>
> [0] https://lore.kernel.org/dri-devel/20250206-hotplug-drm-bridge-v6-0-9d6f2c9c3058@xxxxxxxxxxx/
>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
Applied on drm-misc-next, thanks!

--
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com