linux-kernel - Re: [RFC PATCH 1/2] Kconfig: Introduce "uses" keyword

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Date:   Mon, 20 Apr 2020 11:42:18 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...pe.ca>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Leon Romanovsky <leon@...nel.org>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        jonas@...boo.se, David Airlie <airlied@...ux.ie>,
        jernej.skrabec@...l.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, linux-rdma@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] Kconfig: Introduce "uses" keyword

On Mon, 20 Apr 2020 11:43:13 +0300 Jani Nikula wrote:
> On Sun, 19 Apr 2020, Masahiro Yamada <masahiroy@...nel.org> wrote:
> > On Sun, Apr 19, 2020 at 4:11 AM Nicolas Pitre <nico@...xnic.net> wrote:  
> >>
> >> On Sun, 19 Apr 2020, Masahiro Yamada wrote:
> >>  
> >> > (FOO || !FOO) is difficult to understand, but
> >> > the behavior of "uses FOO" is as difficult to grasp.  
> >>
> >> Can't this be expressed as the following instead:
> >>
> >>         depends on FOO if FOO
> >>
> >> That would be a little clearer.
> >>
> >>
> >> Nicolas  
> >
> > 'depends on' does not take the 'if <expr>'
> >
> > 'depends on A if B' is the syntax sugar of
> > 'depends on (A || !B), right ?
> >
> > I do not know how clearer it would make things.
> >
> > depends on (m || FOO != m)
> > is another equivalent, but we are always
> > talking about a matter of expression.
> >
> >
> > How important is it to stick to
> > depends on (FOO || !FOO)
> > or its equivalents?
> >
> >
> > If a driver wants to use the feature FOO
> > in most usecases, 'depends on FOO' is sensible.
> >
> > If FOO is just optional, you can get rid of the dependency,
> > and IS_REACHABLE() will do logically correct things.  
> 
> If by logically correct you mean the kernel builds, you're
> right. However the proliferation of IS_REACHABLE() is making the kernel
> config *harder* to understand. User enables FOO=m and expects BAR to use
> it, however if BAR=y it silently gets ignored. I have and I will oppose
> adding IS_REACHABLE() usage to i915 because it's just silently accepting
> configurations that should be flagged and forbidden at kconfig stage.

+1

I wholeheartedly agree. In case of Ethernet drivers having higher
layers of the stack not able to communicate with drivers is just 
broken IMHO.