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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 8 Apr 2020 22:49:48 +0200
Message-ID: <CAK8P3a2frDf4BzEpEF0uwPTV2dv6Jve+6N97z1sSuSBUAPJquA@mail.gmail.com>
Subject: Re: [RFC 0/6] Regressions for "imply" behavior change
To:     Nicolas Pitre <nico@fluxnic.net>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
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On Wed, Apr 8, 2020 at 10:38 PM Nicolas Pitre <nico@fluxnic.net> wrote:
> On Wed, 8 Apr 2020, Arnd Bergmann wrote:
> > I have created workarounds for the Kconfig files, which now stop using
> > imply and do something else in each case. I don't know whether there was
> > a bug in the kconfig changes that has led to allowing configurations that
> > were not meant to be legal even with the new semantics, or if the Kconfig
> > files have simply become incorrect now and the tool works as expected.
>
> In most cases it is the code that has to be fixed. It typically does:
>
>         if (IS_ENABLED(CONFIG_FOO))
>                 foo_init();
>
> Where it should rather do:
>
>         if (IS_REACHABLE(CONFIG_FOO))
>                 foo_init();
>
> A couple of such patches have been produced and queued in their
> respective trees already.

I try to use IS_REACHABLE() only as a last resort, as it tends to
confuse users when a subsystem is built as a module and already
loaded but something relying on that subsystem does not use it.

In the six patches I made, I had to use IS_REACHABLE() once,
for the others I tended to use a Kconfig dependency like

'depends on FOO || FOO=n'

which avoids the case that IS_REACHABLE() works around badly.

I did come up with the IS_REACHABLE() macro originally, but that
doesn't mean I think it's a good idea to use it liberally ;-)

      Arnd