linux-kernel - Re: [PATCH 06/12] riscv: dts: allwinner: Add the D1 SoC base devicetree

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Date:   Tue, 16 Aug 2022 12:42:39 +0300
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Jernej Škrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...ts.linux.dev,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org,
        Heiko Stübner <heiko@...ech.de>
Cc:     Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Subject: Re: [PATCH 06/12] riscv: dts: allwinner: Add the D1 SoC base
 devicetree

On 16/08/2022 12:25, Jernej Škrabec wrote:
> Dne torek, 16. avgust 2022 ob 11:12:05 CEST je Heiko Stübner napisal(a):
>> Am Dienstag, 16. August 2022, 09:49:58 CEST schrieb Jernej Škrabec:
>>> Dne torek, 16. avgust 2022 ob 09:41:45 CEST je Krzysztof Kozlowski 
> napisal(a):
>>>> On 15/08/2022 08:08, Samuel Holland wrote:
>>>>> +
>>>>> +	de: display-engine {
>>>>> +		compatible = "allwinner,sun20i-d1-display-engine";
>>>>> +		allwinner,pipelines = <&mixer0>, <&mixer1>;
>>>>> +		status = "disabled";
>>>>> +	};
>>>>> +
>>>>> +	osc24M: osc24M-clk {
>>>>
>>>> lowercase
>>>>
>>>>> +		compatible = "fixed-clock";
>>>>> +		clock-frequency = <24000000>;
>>>>
>>>> This is a property of the board, not SoC.
>>>
>>> SoC needs 24 MHz oscillator for correct operation, so each and every board
>>> has it. Having it here simplifies board DT files.
>>
>> I guess the oscillator is a separate component on each board, right?
> 
> Correct.
> 
>> And DT obvious is meant to describe the hardware - independently from
>> implementation-specific choices.
> 
> There is no choice in this case. 24 MHz crystal has to be present.
> 
> FWIW, including crystal node in SoC specific DTSI is already common pattern in 
> Allwinner ARM SoC DTSI files.
> 
>>
>> Starting to discuss which exceptions to allow then might lead to even more
>> exceptions.
>>
>> Also having to look for a board-component in the soc dtsi also is surprising
>> if one gets to the party later on :-) .
> 
> As I said, if one is accustomed to Allwinner ARM DT development, it would be 
> more surprising to include 24 MHz crystal node in each and every board DT.

It's same everywhere. Allwinner, Exynos, iMX, Qualcomm. Everywhere this
is a part of the board, so even if oscillator frequency is fixed (as in
99% of cases although some SoCs I think might just allow to implement
one of few), still this is a property of the board. Because:
1. DTSI describes the SoC part, not board.
2. So the DTS developer is a bit more conscious about his design.

Keeping things in SoC DTSI just because it simplifies DTS is not correct
IMHO. So again - like in several other cases - minimum the frequency is
property of the board, not the SoC DTSI.

Everywhere. Allwinner is not special to receive exceptions.

Best regards,
Krzysztof