Message-ID: <20251216-cute-bandicoot-of-authority-3c492a@penduick>
Date: Tue, 16 Dec 2025 14:49:39 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
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Subject: Re: [PATCH 06/26] drm/bridge: add devm_drm_of_find_bridge

On Mon, Dec 15, 2025 at 03:11:21PM +0100, Luca Ceresoli wrote:
> Hi Maxime,
> 
> On Mon Dec 15, 2025 at 11:35 AM CET, Maxime Ripard wrote:
> [...]
> >> > Additionally, as a matter of fact there are currently drivers storing
> >> > bridge pointers. The next_bridge is the most common case. Code using
> >> > drm_bridge_connector_init() for example can store up to eight of them, but
> >> > individual drivers are the hardest to hunt for.
> >> >
> >> > I can see these (potential) tools to handle this (not mutually exclusive):
> >> >
> >> >  1. remove drm_bridge pointers pointing to other bridges
> >> >  2. check whether a bridge (say B) still exists before any dereference
> >> >     to B->another_bridge: that's drm_bridge_enter/exit()
> >> >  3. let owners of bridge pointers be notified when a bridge is unplugged,
> >> >     so they can actively put their reference and clear their pointer
> >> >
> >> > For item 1, I think the drm_of_bridge_attach() idea quoted above would
> >> > work, at least for the simple cases where bridge drivers use the
> >> > next_bridge only for attach. A next_bridge pointer in struct drm_bridge is
> >> > not even needed in that case, the pointer would be computed from OF when
> >> > needed and not stored. I can do an experiment and send a first series, do
> >> > you think it would be useful?
> >>
> >> I had a look and, while the implementation should be simple, only a few
> >> drivers could benefit right now. The majority fall into one of these
> >> categories:
> >>
> >>  * drivers using drm_of_find_panel_or_bridge() or *_of_get_bridge()
> >>    (maybe 60-80% of all drivers, those will have to wait for the panel
> >>    improvements)
> >>  * drivers using the next_bridge pointer for more than just attach
> >>  * drivers doing more complicated stuff
> >>
> >> I think your "put next_bridge in __drm_bridge_free" idea would fit well the
> >> 2nd category and perhaps also the 1st one. For the 3rd category we'd need
> >> something different, e.g. a per-driver .destroy callback.
> >
> > Yep, that's fine. We should optimize for the common case, with an escape
> > hatch. That's exactly what we are talking about here.
> 
> Not sure why, but it's taking a while before I grasp your ideas about this
> series and meld them with mine. I hopefully got a clear POV now, so based
> on it my plan is to rework this series to:
> 
>  * keep drm_of_find_bridge() but renamed to of_drm_get_bridge(), and keep
>    patches 1-5 (with the changes suggested by you and Louis, nothing big
>    and all already sent in v2)

I don't think we should use that name, but on principle, yes.

>  * not add devm_drm_of_find_bridge()

Yep.

>  * add next_bridge pointer to struct drm_bridge and call
>    drm_bridge_put(bridge->next_bridge) in __drm_bridge_free, document it

We don't have to at this point, you can add next_bridge to the
drm_bridge pointer if you want, but I don't consider it a prerequisite.
If we don't we would have to create drm_bridge_funcs->destroy
implemnentations to give up the driver pointer reference though. Your
call.

>  * convert patches 7-26 to use bridge->next_bridge where applicable,
>    or to do something different when needed

Depending on your decision above, we'd need to either convert them to
use drm_bridge->next_bridge or create a destroy hook for them. Either
way is fine for me.

>  * maybe remove part of patches 7-26 just to reduce spam and rework effort
>    in case of further iterations, to send them separately once the approach
>    is accepted
> 
> Does it look OK?

Yep

Maxime

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