netdev - Re: [RFC 5/6] drm/rcar-du: fix selection of CMM driver

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Date:   Wed, 15 Apr 2020 17:18:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.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 5/6] drm/rcar-du: fix selection of CMM driver

On Wed, Apr 15, 2020 at 4:13 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Wed, Apr 15, 2020 at 3:47 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Tue, Apr 14, 2020 at 10:52 PM Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:
> > > Doesn't "imply" mean it gets selected by default but can be manually
> > > disabled ?
> >
> > That may be what it means now (I still don't understand how it's defined
> > as of v5.7-rc1), but traditionally it was more like a 'select if all
> > dependencies are met'.
>
> That's still what it is supposed to mean right now ;-)
> Except that now it should correctly handle the modular case, too.

Then there is a bug. If I run 'make menuconfig' now on a mainline kernel
and enable CONFIG_DRM_RCAR_DU, I can set
DRM_RCAR_CMM and DRM_RCAR_LVDS to 'y', 'n' or 'm' regardless
of whether CONFIG_DRM_RCAR_DU is 'm' or 'y'. The 'implies'
statement seems to be ignored entirely, except as reverse 'default'
setting.

> >
> > In that case, a Makefile trick could also work, doing
> >
> > ifdef CONFIG_DRM_RCAR_CMM
> > obj-$(CONFIG_DRM_RCAR_DU) += rcar-cmm.o
> > endif
> >
> > Thereby making the cmm module have the same state (y or m) as
> > the du module whenever the option is enabled.
>
> What about dropping the "imply DRM_RCAR_CMM", but defaulting to
> enable CMM if DU is enabled?
>
>     config DRM_RCAR_CMM
>             tristate "R-Car DU Color Management Module (CMM) Support"
>             depends on DRM_RCAR_DU && OF
>             default DRM_RCAR_DU

That doesn't work because it allows DRM_RCAR_DU=y with
DRM_RCAR_CMM=m, which causes a link failure.

         Arnd