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Date:   Wed, 8 Apr 2020 19:42:24 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Nicolas Pitre <nico@fluxnic.net>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Neil Armstrong <narmstrong@baylibre.com>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        Jonas Karlman <jonas@kwiboo.se>,
        Jernej Skrabec <jernej.skrabec@siol.net>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>,
        "David S. Miller" <davem@davemloft.net>,
        Saeed Mahameed <saeedm@mellanox.com>,
        Leon Romanovsky <leon@kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>,
        linux-rdma <linux-rdma@vger.kernel.org>
Subject: Re: [RFC 0/6] Regressions for "imply" behavior change
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On Wed, Apr 08, 2020 at 10:49:48PM +0200, Arnd Bergmann wrote:
> 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'

It is unfortunate kconfig doesn't have a language feature for this
idiom, as the above is confounding without a lot of kconfig knowledge

> 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 ;-)

It would be nice to have some uniform policy here

I also don't like the IS_REACHABLE solution, it makes this more
complicated, not less..

Jason