linux-kernel - Re: [PATCH 3/6] drm/mcde: dsi: warn in case of multiple subnodes

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Message-Id: <DG2VOQLW9RYT.181HW9ZI8HHOR@bootlin.com>
Date: Sat, 31 Jan 2026 16:26:37 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Linus Walleij" <linusw@...nel.org>
Cc: "Biju Das" <biju.das.jz@...renesas.com>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "David Airlie"
 <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Anitha
 Chrisanthus" <anitha.chrisanthus@...el.com>, "Edmund Dea"
 <edmund.j.dea@...el.com>, "Laurent Pinchart"
 <laurent.pinchart+renesas@...asonboard.com>, "Tomi Valkeinen"
 <tomi.valkeinen+renesas@...asonboard.com>, "Kieran Bingham"
 <kieran.bingham+renesas@...asonboard.com>, "Geert Uytterhoeven"
 <geert+renesas@...der.be>, "Magnus Damm" <magnus.damm@...il.com>, "Tomi
 Valkeinen" <tomi.valkeinen@...asonboard.com>, "Hui Pu"
 <Hui.Pu@...ealthcare.com>, "Ian Ray" <ian.ray@...ealthcare.com>, "Thomas
 Petazzoni" <thomas.petazzoni@...tlin.com>,
 <dri-devel@...ts.freedesktop.org>, <linux-renesas-soc@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] drm/mcde: dsi: warn in case of multiple subnodes

Hello Linus,

On Mon Jan 19, 2026 at 9:54 AM CET, Linus Walleij wrote:
> On Fri, Jan 16, 2026 at 6:33 PM Luca Ceresoli <luca.ceresoli@...tlin.com> wrote:
>
>> mcde_dsi_bind() has a loop over all subnodes looking for a panel, but does
>> not exit when a match is found and only stores the last match. However this
>> will be problematic when introducing refcounting on the struct drm_device
>> pointer in a following commit, because of_drm_find_and_get_bridge() would
>> get a reference to multiple bridges.
>>
>> Assuming there is no real reason for looking for multiple panels, add a
>> warning so it gets noticed in case the assumption is wrong.
>>
>> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
>>
>> ---
>>
>> I think the correct thing to do would be adding a break statement when
>> there is a match. However I don't have knowledge of this driver and the
>> hardware, thus this patch is a prudential alternative, not changing the
>> behaviour.
>
> Go ahead and insert a break when the panel is found, there is
> no MCDE-attached device with more than one panel.
>
> There *exist* the Samsung Gavini device which has a
> LED panel, *and* the same lines attached to a mini-projector
> so these two can be used at the same time. I have no idea
> how to actually deal with that if someone one day want to
> support it. Probably by putting the projector as a side thing
> and not deal with the video stream in the device tree.

Thanks for sharing these info! I'm very happy to know I can break instead
of warning here. Change queued for v2.

Luca

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