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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 19 Feb 2020 09:42:10 +0100
Message-ID: <CAMuHMdU7=jRNCWvtiFhJwUM6P4xNmqwXdPX14qvE=6by1V7APA@mail.gmail.com>
Subject: Re: [PATCH] kconfig: make 'imply' obey the direct dependency
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kbuild <linux-kbuild@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Nicolas Pitre <nico@fluxnic.net>,
        Richard Cochran <richardcochran@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mark Brown <broonie@kernel.org>,
        Ulf Magnusson <ulfalizer@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Michal Marek <michal.lkml@markovi.net>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi Yamada-san,

On Wed, Feb 19, 2020 at 8:51 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> The 'imply' statement may create unmet direct dependency when the
> implied symbol depends on m.
>
> [Test Code]
>
>   config FOO
>           tristate "foo"
>           imply BAZ
>
>   config BAZ
>           tristate "baz"
>           depends on BAR
>
>   config BAR
>           def_tristate m
>
>   config MODULES
>           def_bool y
>           option modules
>
> If you set FOO=y, BAZ is also promoted to y, which results in the
> following .config file:
>
>   CONFIG_FOO=y
>   CONFIG_BAZ=y
>   CONFIG_BAR=m
>   CONFIG_MODULES=y
>
> This ignores the dependency "BAZ depends on BAR".
>
> Unlike 'select', what is worse, Kconfig never shows the
> "WARNING: unmet direct dependencies detected for ..." for this case.
>
> Because 'imply' should be weaker than 'depends on', Kconfig should
> take the direct dependency into account.
>
> Describe this case in Documentation/kbuild/kconfig-language.rst for
> clarification.
>
> Commit 237e3ad0f195 ("Kconfig: Introduce the "imply" keyword") says that
> a symbol implied by y is restricted to y or n, excluding m.
>
> As for the combination of FOO=y and BAR=m, the case of BAZ=m is excluded
> by the 'imply', and BAZ=y is also excluded by 'depends on'. So, only the
> possible value is BAZ=n.
>
> Having said that, this case was probably "We don't care" at that time
> because Kconfig did not handle 'depends on m' correctly until
> commit f622f8279581 ("kconfig: warn unmet direct dependency of tristate
> symbols selected by y") fixed it.
>
> Backporting this to 4.19+ will probably be fine. If you care this
> problem on 4.14.x, you need to backport f622f8279581 as well.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Thanks a lot! This fixes the build issues in
https://lore.kernel.org/alsa-devel/CAMuHMdW8SvDgQJyenTtEm4Xn2Ma6PK9pfwKR2_gn60t2AqNWXg@mail.gmail.com/

Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds