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

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Message-ID: <cqtjavx2aljj34d6rdxysrl3fsmi6i45o6w7gmcul7v3xkws5v@d4sensdcyrfp>
Date: Fri, 12 Sep 2025 14:20:17 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Miguel Gazquez <miguel.gazquez@...tlin.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

On Thu, Sep 11, 2025 at 05:47:57PM +0200, Miguel Gazquez wrote:
> 
> 
> Le 11/09/2025 à 17:40, Miguel Gazquez a écrit :
> > 
> > 
> > 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.
> > 
> 
> If I can rephrase myself, is my understanding correct ? Do we care about
> breaking out-of-tree dts ?

>From my practice: only in a very limited way, if there are well-known
out-of-tree DTS (e.g. we kept some bits and pieces of panel code because
of out-of-tree ChromeBook devices).

But you can easility keep compatibility: inside the ticldc driver first
try attaching the bridge with the flag set, then, if it fails, try
attaching without the flag. Add dev_warn() and some grace period.
his was the path that we used to migrate the drm/msm/dsi: enable
DRM_BRIDGE_ATTACH_NO_CONNECTOR by default, keeping legacy support in
place, then drop legacy after some time after converting all users.

-- 
With best wishes
Dmitry