linux-kernel - Re: [PATCH v2 2/2] dt-bindings: display: simple-framebuffer: Document physical width and height properties

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Message-Id: <87r0vjmu9i.fsf@bloch.sibelius.xs4all.nl>
Date:   Tue, 24 Jan 2023 23:40:09 +0100
From:   Mark Kettenis <mark.kettenis@...all.nl>
To:     Rayyan Ansari <rayyan@...ari.sh>
Cc:     robh@...nel.org, dri-devel@...ts.freedesktop.org,
        ~postmarketos/upstreaming@...ts.sr.ht, asahi@...ts.linux.dev,
        janne@...nau.net, daniel@...ll.ch, airlied@...il.com,
        devicetree@...r.kernel.org, hdegoede@...hat.com,
        javierm@...hat.com, krzysztof.kozlowski+dt@...aro.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        tzimmermann@...e.de
Subject: Re: [PATCH v2 2/2] dt-bindings: display: simple-framebuffer: Document
 physical width and height properties

> Date: Tue, 24 Jan 2023 22:19:09 +0000
> From: Rayyan Ansari <rayyan@...ari.sh>
> 
> On 23/01/2023 17:53, Rob Herring wrote:
> > On Sun, Jan 22, 2023 at 05:25:38PM +0000, Rayyan Ansari wrote:
> >> On 22/01/2023 15:36, Rob Herring wrote:
> >>> On Sat, Jan 21, 2023 at 9:36 AM Rayyan Ansari <rayyan@...ari.sh> wrote:
> >>>>
> >>>
> >>> Why do you need this change?
> >>>
> >>> The 'simple-framebuffer' contains data on how the bootloader
> >>> configured the display. The bootloader doesn't configure the display
> >>> size, so this information doesn't belong here. The information should
> >>> already be in the panel node, so also no point in duplicating it here.
> >>>
> >>>> Signed-off-by: Rayyan Ansari <rayyan@...ari.sh>
> >>>> ---
> >>>>    .../devicetree/bindings/display/simple-framebuffer.yaml   | 8 ++++++++
> >>>>    1 file changed, 8 insertions(+)
> >>
> >> Hi Rob,
> >>
> >> There is the usecase that Hans has mentioned, but I have also mentioned
> >> another usecase previously.
> >>
> >> Adding the width-mm and height-mm properties allows user interfaces such as
> >> Phosh (https://puri.sm/posts/phosh-overview/) to scale correctly to the
> >> screen. In my case, a panel node is not available and the aforementioned
> >> interface is in fact running on the SimpleDRM driver (which binds to the
> >> simple-framebuffer device).
> > 
> > Why is the panel node not available? Why not add it? Presumably it is
> > not there because you aren't (yet) using the simple-panel driver (and
> > others that would need). But presumably you will eventually as I'd
> > imagine turning the screen off and back on might be a desired feature.
> 
> It requires more than using the simple-panel driver: first the SoC side 
> display hardware needs to be brought up, then a panel driver that 
> implements the proper DCS initialisation sequence needs to be written 
> (which is currently not fully known).

You don't really need a driver.  You can just lookup the panel node
from your simple-framebuffer driver and get the values of the
properties there.