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

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

On Tue, Dec 10, 2024, at 15:09, 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
>
> error: recursive dependency detected!
> 	symbol BACKLIGHT_CLASS_DEVICE is selected by FB_BACKLIGHT
> 	symbol FB_BACKLIGHT is selected by FB_SH_MOBILE_LCDC
> 	symbol FB_SH_MOBILE_LCDC depends on FB_DEVICE
> 	symbol FB_DEVICE depends on FB_CORE
> 	symbol FB_CORE is selected by DRM_GEM_DMA_HELPER
> 	symbol DRM_GEM_DMA_HELPER is selected by DRM_PANEL_ILITEK_ILI9341
> 	symbol DRM_PANEL_ILITEK_ILI9341 depends on BACKLIGHT_CLASS_DEVICE
>
> BACKLIGHT_CLASS_DEVICE is user-selectable, so making drivers adapt to
> it is the correct approach in any case. For most drivers, backlight
> support is also configurable separately.
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>

Thanks for revisiting this!

My patch that failed to work correctly happened to work on my
randconfig tree because I still have an old variant of this
change, see

https://lore.kernel.org/linux-fbdev/20200417155553.675905-8-arnd@arndb.de/

This is almost the same as your version, except for the
optional fbdev Kconfig bits
PERS
> @@ -660,7 +661,6 @@ config FB_ATMEL
>  config FB_NVIDIA
>  	tristate "nVidia Framebuffer Support"
>  	depends on FB && PCI
> -	select FB_BACKLIGHT if FB_NVIDIA_BACKLIGHT
>  	select FB_CFB_FILLRECT
>  	select FB_CFB_COPYAREA
>  	select FB_CFB_IMAGEBLIT
> @@ -700,6 +700,8 @@ config FB_NVIDIA_DEBUG
>  config FB_NVIDIA_BACKLIGHT
>  	bool "Support for backlight control"
>  	depends on FB_NVIDIA
> +	depends on BACKLIGHT_CLASS_DEVICE
> +	select FB_BACKLIGHT
>  	default y
>  	help
>  	  Say Y here if you want to control the backlight of your display.

For instance here I used

@@ -702,6 +703,7 @@ config FB_NVIDIA_DEBUG
 config FB_NVIDIA_BACKLIGHT
        bool "Support for backlight control"
        depends on FB_NVIDIA
+       depends on BACKLIGHT_CLASS_DEVICE=y || BACKLIGHT_CLASS_DEVICE=FB_NVIDIA
        default y
        help
          Say Y here if you want to control the backlight of your display.

while your patch causes a link failure with

CONFIG_FB_NVIDIA=y
CONFIG_FB_NVIDIA_BACKLIGHT=y
CONFIG_BACKLIGHT_CLASS_DEVICE=m

       Arnd