Re: [PATCH v7 00/11] drm/bridge: add devm_drm_bridge_alloc() with bridge refcount

[Maxime Ripard <mripard@kernel.org>]

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Hi,

On Fri, Mar 14, 2025 at 11:31:13AM +0100, Luca Ceresoli wrote:
> 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, which is pretty
> simple for most cases. Refcounting will have to be adopted on the two
> sides: all functions returning a bridge pointer and all code obtaining such
> a pointer. This series has just an overview of some of those conversions,
> because for now the main goal is to agree on the API.
> 
> Series layout:
> 
>  1. Add the new API and refcounting:
> 
>     drm/bridge: add devm_drm_bridge_alloc()
>     drm/bridge: add support for refcounting
> 
>  2. get/put the reference in basic operations in the bridge core:
> 
>     drm/bridge: get/put the bridge reference in drm_bridge_add/remove()
>     drm/bridge: get/put the bridge reference in drm_bridge_attach/detach()
> 
>  3. as an example of changes for bridge consumers, get a reference for the
>     bridge returned by drm_bridge_chain_get_first_bridge(), have it put by
>     all callers (all users will be covered later on separately):
> 
>     drm/bridge: add a cleanup action for scope-based drm_bridge_put() invocation
>     drm/bridge: get the bridge returned by drm_bridge_chain_get_first_bridge()
>     drm/mxsfb: put the bridge returned by drm_bridge_chain_get_first_bridge()
>     drm/atomic-helper: put the bridge returned by drm_bridge_chain_get_first_bridge()
>     drm/probe-helper: put the bridge returned by drm_bridge_chain_get_first_bridge()
> 
>  4. 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
> 
> This work was formerly a part of my v6 DRM bridge hotplug series[0], now
> split as a standalone series with many improvements, hence the "v7" version
> number.

Except for one patch where I had comments, I think the series is in
excellent shape. We're still missing a couple of things to close this
topic though:

  - Converting the other bridge iterators/accessors to take / put the references
  - Mass converting the drivers to devm_drm_bridge_alloc
  - Documenting somewhere (possibly in drm_bridge_init?) that it really
    shouldn't be used anymore

Maxime

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