Message-Id: <DEVAJEYEL7SW.2F8RT465LAL5U@bootlin.com>
Date: Thu, 11 Dec 2025 10:47:39 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Kory Maincent" <kory.maincent@...tlin.com>
Cc: "Jyri Sarha" <jyri.sarha@....fi>, "Tomi Valkeinen"
 <tomi.valkeinen@...asonboard.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>, "Rob Herring"
 <robh@...nel.org>, "Krzysztof Kozlowski" <krzk+dt@...nel.org>, "Conor
 Dooley" <conor+dt@...nel.org>, "Russell King" <linux@...linux.org.uk>,
 "Bartosz Golaszewski" <brgl@...ev.pl>, "Tony Lindgren" <tony@...mide.com>,
 "Andrzej Hajda" <andrzej.hajda@...el.com>, "Neil Armstrong"
 <neil.armstrong@...aro.org>, "Robert Foss" <rfoss@...nel.org>, "Laurent
 Pinchart" <Laurent.pinchart@...asonboard.com>, "Jonas Karlman"
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 Schneider-Pargmann" <msp@...libre.com>, "Louis Chauvet"
 <louis.chauvet@...tlin.com>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, "Miguel Gazquez"
 <miguel.gazquez@...tlin.com>, <dri-devel@...ts.freedesktop.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-arm-kernel@...ts.infradead.org>, <linux-omap@...r.kernel.org>
Subject: Re: [PATCH 03/21] drm/tilcdc: Remove simulate_vesa_sync flag

On Thu Dec 11, 2025 at 10:40 AM CET, Kory Maincent wrote:
> On Wed, 10 Dec 2025 19:10:45 +0100
> "Luca Ceresoli" <luca.ceresoli@...tlin.com> wrote:
>
>> Hi Köry,
>>
>> On Wed Nov 26, 2025 at 6:35 PM CET, Kory Maincent (TI.com) wrote:
>> > The tilcdc hardware does not generate VESA-compliant sync signals. It
>> > aligns the vertical sync (VS) on the second edge of the horizontal sync
>> > (HS) instead of the first edge. To compensate for this hardware
>> > behavior, the driver applies a timing adjustment in mode_fixup().
>> >
>> > Previously, this adjustment was conditional based on the simulate_vesa_sync
>> > flag, which was only set when using external encoders. This appears
>> > problematic because:
>> >
>> > 1. The timing adjustment seems needed for the hardware behavior regardless
>> >    of whether an external encoder is used
>> > 2. The external encoder infrastructure is driver-specific and being
>> >    removed due to design issues
>> > 3. Boards using tilcdc without bridges (e.g., am335x-evm, am335x-evmsk)
>> >    may not be getting the necessary timing adjustments
>> >
>> > Remove the simulate_vesa_sync flag and apply the VESA sync timing
>> > adjustment unconditionally, ensuring consistent behavior across all
>> > configurations. While it's unclear if the previous conditional behavior
>> > was causing actual issues, the unconditional adjustment better reflects
>> > the hardware's characteristics.
>> >
>> > Signed-off-by: Kory Maincent (TI.com) <kory.maincent@...tlin.com>
>> > ---
>> >
>> > Only few board currently use tilcdc not associated to a bridge like the
>> > am335x_evm or the am335x-evmsk.
>>
>> Have you tested this change on any affected board?
>>
>> The change looks good to me but without some testing it would be risky.
>
> I have tested it on few boards but not these mainline devicetree as I don't
> have them.
>
> I have tested a tilcdc with tda998x bridge (BeagleBone Black), a tilcdc with
> ti,tilcdc,panel panel (BeagleBone with LCD cape), a tilcdc with it66121 bridge
> (new Beagle Bone Green Eco board with HDMI cape).
> That's all the boards I have.

OK, thanks for the info. For future iterations, I think it's useful to
mention, for all patches that may potentially break existing hardware, such
info on which hardware you have tested.

Luca

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

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