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

[Krzysztof Kozlowski]

On 22/11/2023 09:09, Chen-Yu Tsai wrote:
> On Wed, Nov 22, 2023 at 4:05 PM Krzysztof Kozlowski
> <krzysztof.kozlowski at linaro.org> wrote:
>>
>> On 21/11/2023 14:50, Rafał Miłecki wrote:
>>>> +Order of Nodes
>>>> +--------------
>>>> +
>>>> +1. Nodes within any bus, thus using unit addresses for children, shall be
>>>> +   ordered incrementally by unit address.
>>>> +   Alternatively for some sub-architectures, nodes of the same type can be
>>>> +   grouped together (e.g. all I2C controllers one after another even if this
>>>> +   breaks unit address ordering).
>>>> +
>>>> +2. Nodes without unit addresses should be ordered alpha-numerically by the node
>>>> +   name.  For a few types of nodes, they can be ordered by the main property
>>>> +   (e.g. pin configuration states ordered by value of "pins" property).
>>>> +
>>>> +3. When extending nodes in the board DTS via &label, the entries should be
>>>> +   ordered alpha-numerically.
>>>
>>> Just an idea. Would that make (more) sense to make &label-like entries
>>> match order of nodes in included .dts(i)?
>>>
>>> Adventages:
>>> 1. We keep unit address incremental order that is unlikely to change
>>>
>>> Disadventages:
>>> 1. More difficult to verify
>>
>> Rob also proposed this and I believe above disadvantage here is crucial.
>> If you add new SoC with board DTS you are fine. But if you add only new
>> board, the order of entries look random in the diff hunk. Reviewer must
>> open SoC DTSI to be able to review the patch with board DTS.
>>
>> If review is tricky and we do not have tool to perform it automatically,
>> I am sure submissions will have disordered board DTS.
>>
>>>
>>>
>>>> +Example::
>>>> +
>>>> +    // SoC DTSI
>>>> +
>>>> +    / {
>>>> +            cpus {
>>>> +                    // ...
>>>> +            };
>>>> +
>>>> +            psci {
>>>> +                    // ...
>>>> +            };
>>>> +
>>>> +            soc@ {
>>>> +                    dma: dma-controller at 10000 {
>>>> +                            // ...
>>>> +                    };
>>>> +
>>>> +                    clk: clock-controller at 80000 {
>>>> +                            // ...
>>>> +                    };
>>>> +            };
>>>> +    };
>>>> +
>>>> +    // Board DTS
>>>> +
>>>> +    &clk {
>>>> +            // ...
>>>> +    };
>>>> +
>>>> +    &dma {
>>>> +            // ...
>>>> +    };
>>>> +
>>>> +
>>>> +Order of Properties in Device Node
>>>> +----------------------------------
>>>> +
>>>> +Following order of properties in device nodes is preferred:
>>>> +
>>>> +1. compatible
>>>> +2. reg
>>>> +3. ranges
>>>> +4. Standard/common properties (defined by common bindings, e.g. without
>>>> +   vendor-prefixes)
>>>> +5. Vendor-specific properties
>>>> +6. status (if applicable)
>>>> +7. Child nodes, where each node is preceded with a blank line
>>>> +
>>>> +The "status" property is by default "okay", thus it can be omitted.
>>>
>>> I think it would really help to include position of #address-cells and
>>> #size-cells here. In some files I saw them above "compatible" that seems
>>> unintuitive. Some prefer putting them at end which I think makes sense
>>> as they affect children nodes.
>>>
>>> Whatever you choose it'd be just nice to have things consistent.
>>
>> This is a standard/common property, thus it goes to (4) above.
> 
> It's probably a mix, but AFAIK a lot of the device trees in tree have
> #*-cells after "status". In some cases they are added in the board
> .dts files, not the chip/module .dtsi files.

Existing DTS is not a good example :)

> 
> +1 that it makes sense at the end as they affect child nodes.

I still insist that status must be the last, because:
1. Many SoC nodes have address/size cells but do not have any children
(I2C, SPI), so we put useless information at the end.
2. Status should be the final information to say whether the node is
ready or is not. I read the node, check properties and then look at the end:
a. Lack of status means it is ready.
b. status=disabled means device still needs board resources/customization


Best regards,
Krzysztof