Re: [PATCH v6 14/26] drm/bridge: add support for refcounted DRM bridges

Luca Ceresoli
2025-03-13T11:57:11Z

Hello Maxime,

On Fri, 7 Feb 2025 12:47:51 +0100
Maxime Ripard <mripard@kernel.org> wrote:

> Hi,
>
> On Thu, Feb 06, 2025 at 07:14:29PM +0100, Luca Ceresoli wrote:
> > DRM bridges are currently considered as a fixed element of a DRM card, and
> > thus their lifetime is assumed to extend for as long as the card
> > exists. New use cases, such as hot-pluggable hardware with video bridges,
> > require DRM bridges to be added and removed to a DRM card without tearing
> > the card down. This is possible for connectors already (used by DP MST), so
> > add this possibility to DRM bridges as well.
> >
> > Implementation is based on drm_connector_init() as far as it makes sense,
> > and differs when it doesn't. A difference is that bridges are not exposed
> > to userspace, hence struct drm_bridge does not embed a struct
> > drm_mode_object which would provide the refcount. Instead we add to struct
> > drm_bridge a refcount field (we don't need other struct drm_mode_object
> > fields here) and instead of using the drm_mode_object_*() functions we
> > reimplement from those functions the few lines that drm_bridge needs for
> > refcounting.
> >
> > Also add a new devm_drm_bridge_alloc() macro to allocate a new refcounted
> > bridge.
> >
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
>
> So, a couple of general comments:
>
> - I've said it a couple of times already, but I really think you're
> making it harder than necessary for you here. This (and only this!)
> should be the very first series you should be pushing. The rest can
> only ever work if that work goes through, and it's already hard enough
> as it is. So, split that patch into a series of its own, get that
> merged, and then we will be able to deal with panels conversion and
> whatever. That's even more true with panels since there's ongoing work
> that will make it easier for you too. So the best thing here is
> probably to wait.
>
> - This patch really needs to be split into several patches, something
> along the lines of:
>
> + Creating devm_drm_bridge_alloc()
> + Adding refcounting
> + Taking the references in all the needed places
> + Converting a bunch of drivers

After reading Anusha's "[PATCH RFC 0/2] drm/panel: Refcounted panel
allocation" [0] I think I need a clarification about the 4 steps you had
outlined in the above quoted text. Are you suggesting those are four
_series_, and you'd want to see a series only creating
devm_drm_bridge_alloc() as a first step, similarly to Anusha's work?

That was not my understanding so far, and so I've been working on a
series containing all 4 items, and it's growing very long due to item 3
needing to touch many dozen drivers which need to put a bridge (many
are identical oneliner patches though).

Luca

[0] https://lore.kernel.org/all/20250312-drm-panel-v1-0-e99cd69f6136@redhat.com/

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering