Re: [PATCH v2 1/3] fbdev: Fix recursive dependencies wrt BACKLIGHT_CLASS_DEVICE

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Dec 12, 2024, at 19:44, Helge Deller wrote:
> On 12/12/24 11:04, Thomas Zimmermann wrote:
>> Do not select BACKLIGHT_CLASS_DEVICE from FB_BACKLIGHT. The latter
>> only controls backlight support within fbdev core code and data
>> structures.
>>
>> Make fbdev drivers depend on BACKLIGHT_CLASS_DEVICE and let users
>> select it explicitly. Fixes warnings about recursive dependencies,
>> such as [...]
>
> I think in the fbdev drivers themselves you should do:
> 	select BACKLIGHT_CLASS_DEVICE
> instead of "depending" on it.
> This is the way as it's done in the DRM tiny and the i915/gma500 DRM drivers.
>
> So, something like:
>
> --- a/drivers/staging/fbtft/Kconfig
>         tristate "Support for small TFT LCD display modules"
>         depends on FB && SPI
>         depends on FB_DEVICE
>    +    select BACKLIGHT_DEVICE_CLASS
>         depends on GPIOLIB || COMPILE_TEST
>         select FB_BACKLIGHT
>
> config FB_BACKLIGHT
>            tristate
>            depends on FB
>    -	  select BACKLIGHT_CLASS_DEVICE
>    +       depends on BACKLIGHT_CLASS_DEVICE
>
>
> Would that fix the dependency warning?

The above is generally a mistake and the root cause of the
dependency loops. With very few exceptions, the solution in
these cases is to find the inconsistent 'select' and change
it into 'depends on'.

I actually have a few more patches like this that I've
been carrying for years now, e.g. one that changes all the
'select I2C' into appropriate dependencies.

       Arnd