linux-kernel - Re: [PATCH v3 3/3] ARM: dts: stm32: fix several DT warnings on stm32mp15

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Date:   Wed, 17 May 2023 19:04:39 +0200
From:   Raphael Gallais-Pou <raphael.gallais-pou@...s.st.com>
To:     Marek Vasut <marex@...x.de>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Philippe Cornu <philippe.cornu@...s.st.com>,
        Yannick Fertre <yannick.fertre@...s.st.com>
CC:     <dri-devel@...ts.freedesktop.org>, <devicetree@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kernel@...electronics.com>
Subject: Re: [PATCH v3 3/3] ARM: dts: stm32: fix several DT warnings on
 stm32mp15

Hi Marek

On 5/17/23 17:41, Marek Vasut wrote:
> On 5/17/23 16:35, Raphael Gallais-Pou wrote:
>
> Hi,
>
>> diff --git a/arch/arm/boot/dts/stm32mp15xx-dkx.dtsi
>> b/arch/arm/boot/dts/stm32mp15xx-dkx.dtsi
>> index 0f1110e42c93..a6e2e20f12fa 100644
>> --- a/arch/arm/boot/dts/stm32mp15xx-dkx.dtsi
>> +++ b/arch/arm/boot/dts/stm32mp15xx-dkx.dtsi
>> @@ -457,8 +457,7 @@ &ltdc {
>>       status = "okay";
>>         port {
>> -        ltdc_ep0_out: endpoint@0 {
>> -            reg = <0>;
>> +        ltdc_ep0_out: endpoint {
>>               remote-endpoint = <&sii9022_in>;
>>           };
>>       };
>
> This LTDC port/endpoint stuff always scares me, because I always feel I get it
> wrong.
>
> I believe the LTDC does have one "port" , correct.
>
> But I think (?) that the LTDC has two endpoints, endpoint@0 for DPI (parallel
> output out of the SoC) and endpoint@1 for DSI (internal connection into the
> DSI serializer) ?

You are correct indeed, I rushed the patch and did not thought about this. I
agree that this can be confusing, as I also take some time to think through it.

>
> Only one of the endpoints can be connected at a time, but there are actually
> two endpoints in the LTDC port {} node, aren't there ?
Yes, they are mutually exclusive.
>
> So the original description should be OK I think , maybe #address/#size-cells
> are missing instead ?

Thing is: this file is only included in two device-trees : stm32mp157c-dk1.dts
and stm32mp157c-dk2.dts.

Among those two files there is only one which adds a second endpoint. Thus if
the fields are set higher in the hierarchy, a warning yields.

One way to do it would be to make the endpoint@0 go down in the device-tree with
its dependencies, so that both endpoints are the same level without generating
noise.


Raphaël