Message-ID: <20250702-dicing-yo-yo-ddccd5ae7d5a@spud>
Date: Wed, 2 Jul 2025 17:38:06 +0100
From: Conor Dooley <conor@...nel.org>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: dt: writing-bindings: Express better expectation
 of "specific"

On Wed, Jul 02, 2025 at 06:17:01PM +0200, Krzysztof Kozlowski wrote:
> Devicetree bindings are supposed to be specific in terms of compatibles
> and other properties.  Short "specific" has many implications, so extend
> the description to cover them: no family names, avoid generic SoC
> fallbacks, avoid versioned compatibles, avoid properties implied by
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
IDK if it is intentional, but while you mention this here it doesn't
appear in the text below. Probably as simple as "DON'T use versioned
compatibles, unless documenting a HDL IP core"?

> compatible.
> 
> Also document desired lack of ABI impact and acceptable solution if such
> needs arises: clearly marking it in commit msg.
> 
> All above follows established Devicetree bindings maintainers review
> practice, so no new rules are introduced here.
> 
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> 
> ---
> 
> I have vast (~10-page) document describing DT bindings and DTS rules,
> based on Rob's and other people's reviews (with references...). Let me
> try to funnel it here gradually. Why gradually? The intention of
> writing-bindings document is to be concise, so rephrasing my 10 pages
> covering each little detail into generic, concise rule is not that easy,
> especially for non-native speaker.
> 
> Optionally I could also post my 10-page guide somewhere, but then it
> would be one more document to ignore. I think we have enough of such.
> Ah, and I would have one less topic for conference. :)
> ---
>  .../devicetree/bindings/writing-bindings.rst  | 25 +++++++++++++++----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/writing-bindings.rst b/Documentation/devicetree/bindings/writing-bindings.rst
> index 1ad081de2dd0..dc0e4b63984c 100644
> --- a/Documentation/devicetree/bindings/writing-bindings.rst
> +++ b/Documentation/devicetree/bindings/writing-bindings.rst
> @@ -39,10 +39,15 @@ Overall design
>  Properties
>  ==========
>  
> -- DO make 'compatible' properties specific. DON'T use wildcards in compatible
> -  strings. DO use fallback compatibles when devices are the same as or a subset
> -  of prior implementations. DO add new compatibles in case there are new
> -  features or bugs.
> +- DO make 'compatible' properties specific. DON'T use wildcards or
> +  device-family names in compatible strings.

> DO use fallback compatibles when
> +  devices are the same as or a subset of prior implementations.

Isn't this wrong? If you're a subset of a prior implementation, falling
back to the prior implementation's compatible would enable features that
are not present in the new device. That's not something you introduced,
it's the existing wording, but I don't think that's correct. It would have
to be a /superset/ of a prior implementation for this to be correct, no?

> DO add new
> +  compatibles in case there are new features or bugs.

I think it might be best to break each "DO" or "DON'T" into a bullet
point of its own. At least it'd make it more clear what the specific dos
and don'ts are.

> +
> +   - Use SoC-specific compatible for all SoC devices, followed by a fallback if

"Use a".

> +     appropriate.
> +
> +   - Specific SoC compatible is also preferred for the fallbacks.

I hate to nitpick language to a non-native speaker, but when you have a
plural "fallbacks", it would be "Specific SoC compatibles are also
preferred". Probably also "SoC-specific" too, to match the wording used
in the bullet prior?

>  
>  - DO use a vendor prefix on device-specific property names. Consider if
>    properties could be common among devices of the same class. Check other
> @@ -51,12 +56,22 @@ Properties
>  - DON'T redefine common properties. Just reference the definition and define
>    constraints specific to the device.
>  
> +- DON'T add properties to avoid a specific compatible.  DON'T add properties if
> +  they are implied by (deducible from) the compatible.
> +
>  - DO use common property unit suffixes for properties with scientific units.
>    Recommended suffixes are listed at
>    https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/property-units.yaml
>  
>  - DO define properties in terms of constraints. How many entries? What are
> -  possible values? What is the order?
> +  possible values? What is the order? All these constraints represent the ABI
> +  as well.
> +
> +- DON't change the ABI, but if ever needed to change, then DO explicitly

"DON'T", not "DON't". But this immediately contradicts itself, so maybe
a different wording could be used here. Maybe "DON'T make changes that
break the ABI without explicit and detailed rationale for why the
changes have to be made and their impact."?

> +  document that in the commit msg with rationale WHY ABI has to be broken and
> +  what is the impact. ABI impact is beyond Linux kernel, because it covers also

I think this should be "ABI impact /goes/ beyond /the/ Linux kernel,
because it /also/ covers other open-source upstream projects.".

Cheers,
Conor.

> +  other open-source upstream projects.
> +
>  
>  Typical cases and caveats
>  =========================
> -- 
> 2.43.0
> 

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