linux-kernel - Re: [PATCH 03/21] drm/tilcdc: Remove simulate_vesa

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Message-Id: <DEUQM2HNEOQU.3K4ZPL44GVZAJ@bootlin.com>
Date: Wed, 10 Dec 2025 19:10:45 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Kory Maincent (TI.com)" <kory.maincent@...tlin.com>, "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" <jonas@...boo.se>, "Jernej Skrabec"
 <jernej.skrabec@...il.com>
Cc: "Markus 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

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.

Luca

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