linux-kernel - Re: [PATCH] drm/sun4i: link in front-end code if needed

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Message-ID: <20180226105359.k5a3rz2wow4gweba@flea.lan>
Date:   Mon, 26 Feb 2018 11:53:59 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...ux.ie>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Icenowy Zheng <icenowy@...c.io>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Liu <net147@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sun4i: link in front-end code if needed

On Mon, Feb 26, 2018 at 10:31:51AM +0100, Arnd Bergmann wrote:
> On Mon, Feb 26, 2018 at 10:21 AM, Maxime Ripard
> <maxime.ripard@...tlin.com> wrote:
> > Hi,
> >
> > On Fri, Feb 23, 2018 at 02:06:52PM +0100, Arnd Bergmann wrote:
> >> When the base sun4i DRM driver is built-in but the back-end is
> >> a loadable module, we run into a link error:
> >>
> >> drivers/gpu/drm/sun4i/sun4i_drv.o: In function `sun4i_drv_probe':
> >> sun4i_drv.c:(.text+0x60c): undefined reference to `sun4i_frontend_of_table'
> >>
> >> The dependency is a bit tricky, the best workaround I have come up
> >> with is to use a Makefile hack to to interpret both
> >> CONFIG_DRM_SUN4I_BACKEND=m and CONFIG_DRM_SUN4I_BACKEND=y
> >> as a directive to build the front-end the same way as the main module.
> >>
> >> Fixes: dd0421f47505 ("drm/sun4i: Add a driver for the display frontend")
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> ---
> >>  drivers/gpu/drm/sun4i/Makefile | 5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/sun4i/Makefile b/drivers/gpu/drm/sun4i/Makefile
> >> index 582607c0c488..db14e023256b 100644
> >> --- a/drivers/gpu/drm/sun4i/Makefile
> >> +++ b/drivers/gpu/drm/sun4i/Makefile
> >> @@ -25,6 +25,9 @@ obj-$(CONFIG_DRM_SUN4I)             += sun4i-tcon.o
> >>  obj-$(CONFIG_DRM_SUN4I)              += sun4i_tv.o
> >>  obj-$(CONFIG_DRM_SUN4I)              += sun6i_drc.o
> >>
> >> -obj-$(CONFIG_DRM_SUN4I_BACKEND)      += sun4i-backend.o sun4i-frontend.o
> >> +obj-$(CONFIG_DRM_SUN4I_BACKEND)      += sun4i-backend.o
> >> +ifdef CONFIG_DRM_SUN4I_BACKEND
> >> +obj-$(CONFIG_DRM_SUN4I)              += sun4i-frontend.o
> >> +endif
> >
> > Can we just create a Kconfig symbol, selected by DRM_SUN4I_BACKEND,
> > and we would export the symbol?
> 
> Yes, I think that would work just as well. Something like
> 
> config DRM_SUN4I_FRONTEND
>       tristate
>       default DRM_SUN4I if CONFIG_DRM_SUN4I_BACKEND != n
> 
> should do the trick.

I had more something like this in mind:
https://pastebin.com/raw/XydvzAtz

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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