Re: [PATCH] Documentation: kbuild: explain handling optional dependencies

[Jani Nikula]

On Wed, 13 Sep 2023, Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> 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.

Thanks for doing this.

>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  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
> +
> +  config FOO
> +	bool "Support for foo hardware"
> +	depends on BAR || !BAR

	depends on BAR || BAR=n

seems to be an alternative that's about as common:

$ git grep "depends on \([A-Z0-9_]\+\) || \!\1" | wc -l
109
$ git grep "depends on \([A-Z0-9_]\+\) || \1=n" | wc -l
107

Maybe worth mentioning both?


BR,
Jani.


> +
> +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 a more formalized approach if there are multiple drivers that have
> +the same dependency, a helper symbol can be used, like
> +
> +  config FOO
> +	bool "Support for foo hardware"
> +	depends on BAR_OPTIONAL
> +
> +  config BAR_OPTIONAL
> +	def_tristate BAR || !BAR
> +
>  Kconfig recursive dependency limitations
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-- 
Jani Nikula, Intel