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

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Date:   Tue, 24 Jan 2023 22:19:09 +0000
From:   Rayyan Ansari <rayyan@...ari.sh>
To:     Rob Herring <robh@...nel.org>
Cc:     dri-devel@...ts.freedesktop.org,
        ~postmarketos/upstreaming@...ts.sr.ht, asahi@...ts.linux.dev,
        janne@...nau.net, Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>, devicetree@...r.kernel.org,
        Hans de Goede <hdegoede@...hat.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [PATCH v2 2/2] dt-bindings: display: simple-framebuffer: Document
 physical width and height properties

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).

> 
> So why add a temporary DT property that's tied to your *current* kernel? > The DT should not be tightly coupled to the kernel.

I'm not sure what you mean by it being "tightly coupled" to the kernel.

> 
> Rob

-- 
Rayyan Ansari
https://ansari.sh