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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 14 Apr 2020 23:10:08 +0200
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Subject: Re: [RFC 5/6] drm/rcar-du: fix selection of CMM driver
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Nicolas Pitre <nico@fluxnic.net>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Neil Armstrong <narmstrong@baylibre.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>
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On Tue, Apr 14, 2020 at 10:52 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Arnd,
>
> On Tue, Apr 14, 2020 at 10:38:27PM +0200, Arnd Bergmann wrote:
> > On Tue, Apr 14, 2020 at 10:17 PM Laurent Pinchart wrote:
> > > On Wed, Apr 08, 2020 at 10:27:10PM +0200, Arnd Bergmann wrote:
> > > > The 'imply' statement does not seem to have an effect, as it's
> > > > still possible to turn the CMM code into a loadable module
> > > > in a randconfig build, leading to a link error:
> > > >
> > > > arm-linux-gnueabi-ld: drivers/gpu/drm/rcar-du/rcar_du_crtc.o: in function `rcar_du_crtc_atomic_enable':
> > > > rcar_du_crtc.c:(.text+0xad4): undefined reference to `rcar_lvds_clk_enable'
> > > > arm-linux-gnueabi-ld: drivers/gpu/drm/rcar-du/rcar_du_crtc.o: in function `rcar_du_crtc_atomic_disable':
> > > > rcar_du_crtc.c:(.text+0xd7c): undefined reference to `rcar_lvds_clk_disable'
> > > > arm-linux-gnueabi-ld: drivers/gpu/drm/rcar-du/rcar_du_drv.o: in function `rcar_du_init':
> > > > rcar_du_drv.c:(.init.text+0x4): undefined reference to `rcar_du_of_init'
> > > > arm-linux-gnueabi-ld: drivers/gpu/drm/rcar-du/rcar_du_encoder.o: in function `rcar_du_encoder_init':
> > > >
> > > > Remove the 'imply', and instead use a silent symbol that defaults
> > > > to the correct setting.
> > >
> > > This will result in the CMM always being selected when DU is, increasing
> > > the kernel size even for devices that don't need it. I believe we need a
> > > better construct in Kconfig to fix this.
> >
> > I had expected this to have the same meaning that we had before the
> > Kconfig change: whenever the dependencies are available, turn it on,
> > otherwise leave it disabled.
> >
> > Can you describe what behavior you actually want instead?
>
> 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'.

> > > > --- a/drivers/gpu/drm/rcar-du/Kconfig
> > > > +++ b/drivers/gpu/drm/rcar-du/Kconfig
> > > > @@ -4,7 +4,6 @@ config DRM_RCAR_DU
> > > >       depends on DRM && OF
> > > >       depends on ARM || ARM64
> > > >       depends on ARCH_RENESAS || COMPILE_TEST
> > > > -     imply DRM_RCAR_CMM
> > > >       imply DRM_RCAR_LVDS
> > > >       select DRM_KMS_HELPER
> > > >       select DRM_KMS_CMA_HELPER
> > > > @@ -15,9 +14,8 @@ config DRM_RCAR_DU
> > > >         If M is selected the module will be called rcar-du-drm.
> > > >
> > > >  config DRM_RCAR_CMM
> > > > -     tristate "R-Car DU Color Management Module (CMM) Support"
> > > > +     def_tristate DRM_RCAR_DU
> > > >       depends on DRM && OF
> > > > -     depends on DRM_RCAR_DU
> > > >       help
> >
> > It would be easy enough to make this a visible 'bool' symbol and
> > build it into the rcu-du-drm.ko module itself. Would that help you?
>
> That could indeed simplify a few things. I wonder if it could introduce
> a few small issues though (but likely nothing we can't fix). The two
> that come to mind are the fact that the module would have two
> MODULE_DESCRIPTION and MODULE_LICENSE entries (I have no idea if that
> could cause breakages), and that it could make module unloading more
> difficult as the CMM being used by the DU would increase the refcount on
> the module. I think the latter could be worked around by manually
> unbinding the DU device through sysfs before unloading the module (and I
> can't say for sure that unloading the DU module is not broken today
> *innocent and naive look* :-)).

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.

       Arnd