linux-kernel - Re: [PATCH] docs: kconfig: Mention IS_REACHABLE as way for optional dependency

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Message-ID: <CAK7LNATqSPBnGfVXCfJq7oCtE1ge4-L5QY6gVx8_chpmKDQusg@mail.gmail.com>
Date: Sat, 15 Feb 2025 21:54:29 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
	Jonathan Corbet <corbet@....net>, linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] docs: kconfig: Mention IS_REACHABLE as way for optional dependency

On Sat, Feb 15, 2025 at 8:42 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> Several drivers express optional Kconfig dependency with FOO || !FOO,
> but for many choices this is neither suitable (lack of stubs for !FOO
> like in HWMON) nor really needed and driver can be built in even if FOO
> is the module.  This is achieved with IS_REACHABLE, so provide cross
> reference to it.
>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> ---
>  Documentation/kbuild/kconfig-language.rst | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
> index 2619fdf56e68..66248294a552 100644
> --- a/Documentation/kbuild/kconfig-language.rst
> +++ b/Documentation/kbuild/kconfig-language.rst
> @@ -194,6 +194,8 @@ applicable everywhere (see syntax).
>    ability to hook into a secondary subsystem while allowing the user to
>    configure that subsystem out without also having to unset these drivers.
>
> +.. _is_reachable:

Instead of this, could you move this hunk below ?





>    Note: If the combination of FOO=y and BAZ=m causes a link error,
>    you can guard the function call with IS_REACHABLE()::
>
> @@ -580,10 +582,15 @@ 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 slightly counterintuitive::
> +There are two ways to express this optional dependency:
>
> -  config FOO
> +1. If pre-processor can discard entire optional code or module FOO does not
> +   provide !FOO stubs then in the C code :ref:`IS_REACHABLE<is_reachable>`

Instead of the link, please move the code example at line 200 to here.

The note at line 197 is not strongly related to the 'imply' keyword.


One more thing, please document the drawback of IS_REACHABLE.

It is true that IS_REACHABLE() resolves the link error, but we
will end up with run-time debugging.

foo_init()
{
        if (IS_REACHABLE(CONFIG_BAZ))
                baz_register(&foo);
        ...
}

Even if CONFIG_BAZ is enabled, baz_register() may get discarded.
Users may scratch their head why baz_register() does not work.
Due to this reason, IS_REACHABLE() tends to be avoided.


"depends on BAR || !BAR" is configuration-time debugging.


> +
> +2. Otherwise (and module FOO must provide all !FOO stubs) use the slightly
> +   counterintuitive Kconfig syntax::
> +
> +     config FOO
>         tristate "Support for foo hardware"
>         depends on BAR || !BAR
>
> --
> 2.43.0
>


-- 
Best Regards
Masahiro Yamada