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From:   Jani Nikula <jani.nikula@linux.intel.com>
To:     Arnd Bergmann <arnd@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>
Cc:     Sakari Ailus <sakari.ailus@iki.fi>,
        Javier Martinez Canillas <javierm@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Documentation: kbuild: explain handling optional
 dependencies
In-Reply-To: <20230913113801.1901152-1-arnd@kernel.org>
Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo
References: <20230913113801.1901152-1-arnd@kernel.org>
Date:   Thu, 14 Sep 2023 16:42:55 +0300
Message-ID: <874jjwx44g.fsf@intel.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, 13 Sep 2023, Arnd Bergmann <arnd@kernel.org> wrote:
> From: Arnd Bergmann <arnd@arndb.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@arndb.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