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Date:   Fri, 15 Sep 2023 07:26:59 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Masahiro Yamada" <masahiroy@kernel.org>
Cc:     "Jani Nikula" <jani.nikula@linux.intel.com>,
        "Arnd Bergmann" <arnd@kernel.org>,
        "Jonathan Corbet" <corbet@lwn.net>,
        "Sakari Ailus" <sakari.ailus@iki.fi>,
        "Javier Martinez Canillas" <javierm@redhat.com>,
        "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
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On Thu, Sep 14, 2023, at 19:23, Masahiro Yamada wrote:
> On Thu, Sep 14, 2023 at 11:57=E2=80=AFPM Arnd Bergmann <arnd@arndb.de>=
 wrote:
>> On Thu, Sep 14, 2023, at 15:42, Jani Nikula wrote:
>
> It is unclear why WIREGUARD must be entirely disabled
> just because of the optional feature being modular.

I don't think anyone is asking for that, and the current
"depends on IPV6 || !IPV6" seems fine here, and is consistent
with dozens of other symbols.

> My preference is to use IS_REACHABLE(CONFIG_IPV6)
> instead of IS_ENABLED(CONFIG_IPV6)
> under drivers/net/wireguard, then
> get rid of "depends on IPV6 || !IPV6)

My feeling is that this would be significantly worse from a
usability point of view even if it made it a little easier
for maintainers:

When a user selects both IPV6 and WIREGUARD, they expect
to be able to use them together, and a normal user setting
WIREGUARD=3Dy would have a hard time figuring out why that
leads it becoming IPv4-only.

> If you want to make it clearer on the Kconfig level,
> perhaps the following is also possible.
>
>
> config WIREGUARD
>        tristate "WireGuard"
>
> config WIREGUARD_IPV6
>        def_bool y
>        depends on WIREGUARD
>        depends on IPV6 >=3D WIREGUARD
>
> config IPV6
>        tristate "IPV6"

That has the same downside, with the added problem
of also confusing kernel developers with the '>=3D'
Kconfig syntax, which IMHO makes no sense unless one
knows way too much about Kconfig internals.

      Arnd