linux-kernel - Re: [RFC 0/6] Regressions for "imply" behavior change

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Message-ID: <CAK8P3a2frDf4BzEpEF0uwPTV2dv6Jve+6N97z1sSuSBUAPJquA@mail.gmail.com>
Date:   Wed, 8 Apr 2020 22:49:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Leon Romanovsky <leon@...nel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [RFC 0/6] Regressions for "imply" behavior change

On Wed, Apr 8, 2020 at 10:38 PM Nicolas Pitre <nico@...xnic.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