linux-kernel - Re: [PATCH] drm/i915: fix build error without CONFIG_BACKLIGHT_CLASS

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Date:   Fri, 31 Mar 2017 08:35:09 +0200
From:   Tobias Regnery <tobias.regnery@...il.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     daniel.vetter@...el.com, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915: fix build error without
 CONFIG_BACKLIGHT_CLASS_DEVICE

On 30.03.17, Jani Nikula wrote:
> On Wed, 29 Mar 2017, Tobias Regnery <tobias.regnery@...il.com> wrote:
> > With CONFIG_ACPI=n and CONFIG_BACKLIGHT_CLASS_DEVICE=n we see the following
> > link error in the i915 driver:
> >
> > drivers/built-in.o: In function 'intel_backlight_device_register':
> > (.text+0x2a921d): undefined reference to 'backlight_device_register'
> >
> > Fix this by removing the condition on ACPI from the appropriate select
> > statement.
> 
> The right fix for the build problem is to add empty stub functions for
> BACKLIGHT_CLASS_DEVICE=n in include/linux/backlight.h. I'm frankly
> surprised nobody's done that yet.

Thanks for the advice, I will try to come up with a patch.

--
Tobias

> 
> It's another question whether we should support and select backlight for
> ACPI=n, and yet another question whether we should support ACPI=n.
> 
> Also, selecting BACKLIGHT_CLASS_DEVICE is fundamentally broken, but
> people aren't interested [1].
> 
> 
> BR,
> Jani.
> 
> [1] http://mid.mail-archive.com/1413580403-16225-1-git-send-email-jani.nikula@intel.com
> 
> >
> > Signed-off-by: Tobias Regnery <tobias.regnery@...il.com>
> > ---
> >  drivers/gpu/drm/i915/Kconfig | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
> > index a5cd5dacf055..532df4bb9283 100644
> > --- a/drivers/gpu/drm/i915/Kconfig
> > +++ b/drivers/gpu/drm/i915/Kconfig
> > @@ -15,7 +15,7 @@ config DRM_I915
> >  	# i915 depends on ACPI_VIDEO when ACPI is enabled
> >  	# but for select to work, need to select ACPI_VIDEO's dependencies, ick
> >  	select BACKLIGHT_LCD_SUPPORT if ACPI
> > -	select BACKLIGHT_CLASS_DEVICE if ACPI
> > +	select BACKLIGHT_CLASS_DEVICE
> >  	select INPUT if ACPI
> >  	select ACPI_VIDEO if ACPI
> >  	select ACPI_BUTTON if ACPI
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center