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
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