Subject: Re: [PATCH] drivers: depend on instead of select BACKLIGHT_CLASS_DEVICE and ACPI_VIDEO

From: Tomi Valkeinen <[email-masked]>
Date: Wed, 22 Oct 2014 11:02:26 +0300

On 18/10/14 00:13, Jani Nikula wrote:
> Documentation/kbuild/kconfig-language.txt warns to use select with care,
> and in general use select only for non-visible symbols and for symbols
> with no dependencies, because select will force a symbol to a value
> without visiting the dependencies.
> 
> Select has become particularly problematic, interdependently, with the
> BACKLIGHT_CLASS_DEVICE and ACPI_VIDEO configs. For example:
> 
> scripts/kconfig/conf --randconfig Kconfig
> KCONFIG_SEED=0x48312B00
> warning: (DRM_RADEON && DRM_NOUVEAU && DRM_I915 && DRM_GMA500 &&
> DRM_SHMOBILE && DRM_TILCDC && FB_BACKLIGHT && FB_MX3 && USB_APPLEDISPLAY
> && FB_OLPC_DCON && ASUS_LAPTOP && SONY_LAPTOP && THINKPAD_ACPI &&
> EEEPC_LAPTOP && ACPI_CMPC && SAMSUNG_Q10) selects BACKLIGHT_CLASS_DEVICE
> which has unmet direct dependencies (HAS_IOMEM && BACKLIGHT_LCD_SUPPORT)
> warning: (DRM_RADEON && DRM_NOUVEAU && DRM_I915 && DRM_GMA500 &&
> DRM_SHMOBILE && DRM_TILCDC && FB_BACKLIGHT && FB_MX3 && USB_APPLEDISPLAY
> && FB_OLPC_DCON && ASUS_LAPTOP && SONY_LAPTOP && THINKPAD_ACPI &&
> EEEPC_LAPTOP && ACPI_CMPC && SAMSUNG_Q10) selects BACKLIGHT_CLASS_DEVICE
> which has unmet direct dependencies (HAS_IOMEM && BACKLIGHT_LCD_SUPPORT)
> 
> With tristates it's possible to end up selecting FOO=y depending on
> BAR=m in the config, which gets discovered at build time, not config
> time, like reported in the thread referenced below.
> 
> Do the following to fix the dependencies:
> 
> * Depend on instead of select BACKLIGHT_CLASS_DEVICE everywhere. Drop
>   select BACKLIGHT_LCD_SUPPORT in such cases, as it's a dependency of
>   BACKLIGHT_CLASS_DEVICE.
> 
> * Remove config FB_BACKLIGHT altogether, and replace it with a
>   dependency on BACKLIGHT_CLASS_DEVICE. All configs selecting
>   FB_BACKLIGHT select or depend on FB anyway, so we can simplify.
> 
> * Depend on (ACPI && ACPI_VIDEO) || ACPI=n in several places instead of
>   selecting ACPI_VIDEO and a number of its dependencies if ACPI is
>   enabled. This is tied to backlight, as ACPI_VIDEO depends on
>   BACKLIGHT_CLASS_DEVICE.
> 
> * Replace a couple of select INPUT/VT with depends as it seemed to be
>   necessary.

While doing 'depends on' instead of 'select' is an "easy" fix for this,
I do dislike it quite a bit. It's a major pain to go around the kernel
config, trying to find all the dependencies that a particular driver
wants. If I need fb-foobar, I should just be able to enable it, instead
of first searching and selecting its minor dependencies individually.

So, not a NACK, but a "isn't there an another way to fix this?".

Looking at backlight... BACKLIGHT_LCD_SUPPORT seems to be a "meta"
option, it only enables a Kconfig submenu.

So I think we could just remove the whole BACKLIGHT_LCD_SUPPORT option.
But if we do that, all the items in drivers/video/backlight/Kconfig with
default 'y' or 'm' would get enabled by default, so I think we should
remove the 'default's from that file. That makes sense in any case, as I
don't see why "HP Jornada 700 series LCD Driver" should be "default y".

BACKLIGHT_CLASS_DEVICE doesn't depend on anything except
BACKLIGHT_LCD_SUPPORT, so after removing BACKLIGHT_LCD_SUPPORT it should
be safe to 'select' BACKLIGHT_CLASS_DEVICE.

BACKLIGHT_CLASS_DEVICE could be made a hidden option, and the drivers in
drivers/video/backlight/Kconfig which are under BACKLIGHT_CLASS_DEVICE
could be made to select BACKLIGHT_CLASS_DEVICE instead.

That doesn't exactly fix anything, but I think it makes sense as
BACKLIGHT_CLASS_DEVICE is something that's selected from all around the
kernel, so it should be a selectable "library" instead of a Kconfig menu
option.

I didn't look at the ACPI_VIDEO side, so no idea how messy that is.

 Tomi