linux-kernel - Re: [PATCH v2] docs: dt-bindings: add DTS Coding Style document

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Date:   Tue, 21 Nov 2023 08:47:19 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Michal Simek <michal.simek@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, Andrew Davis <afd@...com>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <andersson@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Heiko Stuebner <heiko@...ech.de>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Nishanth Menon <nm@...com>, Olof Johansson <olof@...om.net>,
        linux-rockchip@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        linux-amlogic@...ts.infradead.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2] docs: dt-bindings: add DTS Coding Style document

On 21/11/2023 08:33, Michal Simek wrote:
> 
> 
> On 11/20/23 20:31, Krzysztof Kozlowski wrote:
>> On 20/11/2023 20:18, Geert Uytterhoeven wrote:
>>> Hi Krzysztof,
>>>
>>> On Mon, Nov 20, 2023 at 3:53 PM Krzysztof Kozlowski
>>> <krzysztof.kozlowski@...aro.org> wrote:
>>>> On 20/11/2023 15:01, Michal Simek wrote:> >
>>>>> On 11/20/23 09:40, Krzysztof Kozlowski wrote:
>>>>>> Document preferred coding style for Devicetree sources (DTS and DTSI),
>>>>>> to bring consistency among all (sub)architectures and ease in reviews.
>>>
>>>>>> +Organizing DTSI and DTS
>>>>>> +-----------------------
>>>>>> +
>>>>>> +The DTSI and DTS files should be organized in a way representing the common
>>>>>> +(and re-usable) parts of the hardware.  Typically this means organizing DTSI
>>>>>> +and DTS files into several files:
>>>>>> +
>>>>>> +1. DTSI with contents of the entire SoC (without nodes for hardware not present
>>>>>> +   on the SoC).
>>>>>> +2. If applicable: DTSI with common or re-usable parts of the hardware (e.g.
>>>>>> +   entire System-on-Module).
>>>>>
>>>>> DTS/DTSI - SOMs can actually run as they are that's why it is fair to say that
>>>>> there doesn't need to be DTS representing the board.
>>>>
>>>> I have never seen a SoM which can run without elaborate hardware-hacking
>>>> (e.g. connecting multiple wires to the SoM pins). The definition of the
>>>> SoM is that it is a module. Module can be re-used, just like SoC.
>>>
>>> /me looks at his board farm...
>>>
>>> The Renesas White-Hawk CPU board can be used standalone, and has a
>>> separate power input connector for this operation mode.  As it has RAM,
>>> Ethernet, serial console, eMMC, and even mini-DP, it can serve useful
>>> purposes on its own.
>>> I agree it's not a super-good example, as the board is not really a
>>> "SoM", and we currently don't have r8a779g0-white-hawk-cpu.dts, only
>>> r8a779g0-white-hawk-cpu.dtsi.
>>>
>>> The RZ/A2M CPU Board is a real SoM, which can be powered over USB.
>>> It has less standard connectors (microSD, USB, MIPI CSI-2), but still
>>> sufficient features to be usable on its own.
>>> Again, we're doing a bad job, as we only have a DTS for the full eval
>>> board (r7s9210-rza2mevb.dts).
>>>
>>> I guess there are (many) other examples...
>>
>> OK, I never had such in my hands. Anyway, the SoM which can run
>> standalone  has a meaning of a board, so how exactly you want to
>> rephrase the paragraph?
> 
> What about?
> 
> 2. If applicable: DTSI with common or re-usable parts of the hardware (e.g.
> entire System-on-Module). DTS if runs standalone.

OK, but then it's duplicating the option 3. It also suggests that SoM
should be a DTS, which is not what we want for such case. Such SoMs must
have DTSI+DTS.

Best regards,
Krzysztof