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Message-ID: <874jjwx44g.fsf@intel.com>
Date: Thu, 14 Sep 2023 16:42:55 +0300
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Arnd Bergmann <arnd@...nel.org>,
Masahiro Yamada <masahiroy@...nel.org>,
Jonathan Corbet <corbet@....net>
Cc: 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>,
Nicolas Schier <nicolas@...sle.eu>,
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, 13 Sep 2023, Arnd Bergmann <arnd@...nel.org> 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.
Thanks for doing this.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> 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
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