linux-kernel - Re: [PATCH] drm/panel: rm68200: add backlight dependency

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Date:   Wed, 14 Mar 2018 11:52:30 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     David Airlie <airlied@...ux.ie>,
        Philippe CORNU <philippe.cornu@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/panel: rm68200: add backlight dependency

On Wed, Mar 14, 2018 at 09:49:54AM +0100, Arnd Bergmann wrote:
> On Wed, Mar 14, 2018 at 12:01 AM, Thierry Reding
> <thierry.reding@...il.com> wrote:
> > On Tue, Mar 13, 2018 at 09:59:54PM +0100, Arnd Bergmann wrote:
> >> Like many other panel drivers, this one fails to build
> >> when backlight support is disabled:
> >>
> >> drivers/gpu/drm/panel/panel-raydium-rm68200.o: In function `rm68200_probe':
> >> panel-raydium-rm68200.c:(.text+0x14a): undefined reference to `devm_of_find_backlight'
> >>
> >> This adds the appropriate dependency.
> >>
> >> Fixes: 2b7ed18bed1a ("drm/panel: Add support for Raydium RM68200 panel driver")
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> ---
> >>  drivers/gpu/drm/panel/Kconfig | 1 +
> >>  1 file changed, 1 insertion(+)
> >
> > This shouldn't be necessary. include/linux/backlight.h defines a stub if
> > the backlight class is not enabled.
> >
> > What tree are you seeing this on?
> 
> This is on linux-next.
> 
> It must be with BACKLIGHT_CLASS_DEVICE=m and
> DRM_PANEL_RAYDIUM_RM68200=y, meaning that it should
> be sufficient to do
> 
>     depends on BACKLIGHT_CLASS_DEVICE || BACKLIGHT_CLASS_DEVICE=n
> 
> to force DRM_PANEL_RAYDIUM_RM68200 to be a loadable module
> whenever BACKLIGHT_CLASS_DEVICE=m. For the patch, I looked at
> what the other drivers in the same directory do and followed their
> example.
> 
> I see three options here:
> 
> 1. update my patch changelog with the explanation I wrote here but leave it
>     untouched
> 2. use the more elaborate dependency (after testing) but not change the
>     others
> 3. change all panel drivers with a backlight dependency the same way,
>     possibly with a helper symbol like
> 
> config BACKLIGHT_CLASS_DEVICE_OPTIONAL
>          tristate
>          default m if BACKLIGHT_CLASS_DEVICE=m
>          default y
> 
>       Arnd

I went with option 1) and added some text that hopefully clarifies the
issue.

	https://cgit.freedesktop.org/drm/drm-misc/commit/?id=a8efe516316472ac771ba3f591295c7515e46172

Thierry

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