linux-kernel - Re: [PATCH 7/7] dt-bindings: display: add panel-timing property to sitronix,st7789v

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Message-ID: <20230329091636.mu6ml3gvw5mvkhm4@penduick>
Date:   Wed, 29 Mar 2023 11:16:36 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Michael Riesch <michael.riesch@...fvision.net>
Cc:     Rob Herring <robh@...nel.org>,
        Gerald Loacker <gerald.loacker@...fvision.net>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Subject: Re: [PATCH 7/7] dt-bindings: display: add panel-timing property to
 sitronix,st7789v

On Thu, Mar 16, 2023 at 11:29:53PM +0100, Michael Riesch wrote:
> Hi Rob,
> 
> On 3/16/23 22:57, Rob Herring wrote:
> > On Tue, Mar 14, 2023 at 12:56:44PM +0100, Gerald Loacker wrote:
> >> The sitronix-st7789v driver now considers the panel-timing property.
> > 
> > I read the patch for that and still don't know 'why'. Commit messages 
> > should answer why.
> > 
> >> Add the property to the documentation.
> > 
> > We generally don't put timings in DT for panels. Why is this one 
> > special?
> 
> For now, having the timings in the device tree allows for setting the
> hsync/vsync/de polarity.
> 
> As a next step, we aim to implement the partial mode feature of this
> panel. It is possible to use only a certain region of the panel, which
> is helpful e.g., when a part of the panel is occluded and should not be
> considered by DRM. We thought that this could be specified as timing in DT.
>
> (The hactive and vactive properties serve as dimensions of this certain
> region, of course. We still need to specify somehow the position of the
> region. Maybe with additional properties hactive-start and vactive-start?)
> 
> What do you think about that?

I don't see why we would need the device tree to support that. What you
described is essentially what overscan is for HDMI/analog output, and we
already have everything to deal with overscan properly in KMS.

Maxime