linux-kernel - Re: [PATCH] Documentation: kbuild: explain handling optional dependencies

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Date:   Wed, 13 Sep 2023 21:48:42 +0200
From:   Nicolas Schier <nicolas@...sle.eu>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Sakari Ailus <sakari.ailus@....fi>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation: kbuild: explain handling optional
 dependencies

On Wed, Sep 13, 2023 at 01:37:52PM +0200 Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> This problem frequently comes up in randconfig testing, with
> drivers failing to link because of a dependency on an optional
> feature.
> 
> The Kconfig language for this is very confusing, so try to
> document it in "Kconfig hints" section.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---

Hi Arnd,

thanks for documenting this!  Three questions below:

>  Documentation/kbuild/kconfig-language.rst | 26 +++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
> index 858ed5d80defe..89dea587a469a 100644
> --- a/Documentation/kbuild/kconfig-language.rst
> +++ b/Documentation/kbuild/kconfig-language.rst
> @@ -573,6 +573,32 @@ above, leading to:
>  	bool "Support for foo hardware"
>  	depends on ARCH_FOO_VENDOR || COMPILE_TEST
>  
> +Optional dependencies
> +~~~~~~~~~~~~~~~~~~~~~
> +
> +Some drivers are able to optionally use a feature from another module
> +or build cleanly with that module disabled, but cause a link failure
> +when trying to use that loadable module from a built-in driver.
> +
> +The most common way to express this optional dependency in Kconfig logic
> +uses the slighly counterintuitive

slighly -> slightly

For better RST compliance: could you explicitly start the code block e.g. by
appending '::' as in "... counterintuitive::"?

> +
> +  config FOO
> +	bool "Support for foo hardware"
> +	depends on BAR || !BAR

are you sure that this is enough?  While testing, I needed to explicitly use
=y|=n:

    depends on BAR=y || BAR=n

to prevent FOO to be selectable iff BAR=m.

> +
> +This means that there is either a dependency on BAR that disallows
> +the combination of FOO=y with BAR=m, or BAR is completely disabled.

For me, this sentence is hard to parse (but I am not a native speaker); what
about something like this:

This means that FOO can only be enabled, iff BAR is either built-in or
completely disabled.  If BAR is built as a module, FOO cannot be enabled.

Kind regards,
Nicolas

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