Message-Id: <DEYUNHVYCKYJ.2HU878WBYCJMV@bootlin.com>
Date: Mon, 15 Dec 2025 15:11:21 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Maxime Ripard" <mripard@...nel.org>
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Subject: Re: [PATCH 06/26] drm/bridge: add devm_drm_of_find_bridge

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)
 * not add devm_drm_of_find_bridge()
 * add next_bridge pointer to struct drm_bridge and call
   drm_bridge_put(bridge->next_bridge) in __drm_bridge_free, document it
 * convert patches 7-26 to use bridge->next_bridge where applicable,
   or to do something different when needed
 * 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?

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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