linux-kernel - Re: [PATCH] drm/bridge: ite-it66121: Add drm

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Message-ID: <e6af5c37-d18a-423e-b822-367441a48f86@bootlin.com>
Date: Thu, 11 Sep 2025 17:40:16 +0200
From: Miguel Gazquez <miguel.gazquez@...tlin.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Maxime Ripard <mripard@...nel.org>, Phong LE <ple@...libre.com>,
 Neil Armstrong <neil.armstrong@...aro.org>,
 Andrzej Hajda <andrzej.hajda@...el.com>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, thomas.petazzoni@...tlin.com,
 miquel.raynal@...tlin.com, kory.maincent@...tlin.com,
 romain.gantois@...tlin.com, praneeth@...com,
 Aradhya Bhatia <a-bhatia1@...com>
Subject: Re: [PATCH] drm/bridge: ite-it66121: Add drm_connector support



Le 11/09/2025 à 15:09, Dmitry Baryshkov a écrit :
> On Thu, Sep 11, 2025 at 02:49:59PM +0200, Miguel Gazquez wrote:
>>
>>
>> Le 11/09/2025 à 11:50, Maxime Ripard a écrit :
>>> On Thu, Sep 11, 2025 at 10:51:06AM +0200, Miguel Gazquez wrote:
>>>>
>>>>
>>>> Le 10/09/2025 à 04:28, Dmitry Baryshkov a écrit :
>>>>> On Tue, Sep 09, 2025 at 06:16:43PM +0200, Miguel Gazquez wrote:
>>>>>> From: Aradhya Bhatia <a-bhatia1@...com>
>>>>>>
>>>>>> Add support for DRM connector and make the driver support the older
>>>>>> format of attaching connectors onto the encoder->bridge->connector
>>>>>> chain.
>>>>>> This makes the driver compatible with display controller that only
>>>>>> supports the old format.
>>>>>>
>>>>>> [Miguel Gazquez: Rebased + made driver work with or without
>>>>>> DRM_BRIDGE_ATTACH_NO_CONNECTOR]
>>>>>
>>>>> What is the use case for not using DRM_BRIDGE_ATTACH_NO_CONNECTOR?
>>>>
>>>> Some display controller drivers (like the tilcdc) call drm_bridge_attach
>>>> without DRM_BRIDGE_ATTACH_NO_CONNECTOR, so the bridge must support both with
>>>> and without DRM_BRIDGE_ATTACH_NO_CONNECTOR to be compatible with all display
>>>> controllers.
>>>
>>> I'd rather convert tilcdc to use DRM_BRIDGE_ATTACH_NO_CONNECTOR then.
>>
>> The problem is that doing that break devicetrees using the tilcdc and a
>> bridge who doesn't support DRM_BRIDGE_ATTACH_NO_CONNECTOR (there are
>> multiple bridges that don't support DRM_BRIDGE_ATTACH_NO_CONNECTOR), and if
>> my understanding is correct breaking devicetrees is not allowed.
> 
> How does it break devicetree? The drm_bridge_connector isn't a part of
> DT.


In the current situation, a board could have the tilcdc linked with a 
bridge that does not support DRM_BRIDGE_ATTACH_NO_CONNECTOR (for 
example, the analogix-anx6345) , and everything will work fine.
If we convert the tilcdc to always use DRM_BRIDGE_ATTACH_NO_CONNECTOR, 
that same configuration will stop working.

When I said "breaking devicetree" I meant that a devicetree describing 
this setup would no longer produce a working system, not that the DT 
files or bindings themselves are incorrect.
I didn't find any upstream dts with this configuration, but maybe there 
is some out-of-tree dts which would be affected.
As far as I understand, we should avoid that.


On top of that, having the it66121 handle both cases makes it compatible 
with display controllers that don’t use DRM_BRIDGE_ATTACH_NO_CONNECTOR, 
which are still fairly common.

-- 
Miguel Gazquez, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com