Re: [PATCH] fbdev: simplefb: fix Kconfig dependencies

[Rob Herring <robh+dt@kernel.org>]

On Wed, Jul 21, 2021 at 9:18 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> Configurations with both CONFIG_FB_SIMPLE=y and CONFIG_DRM_SIMPLEDRM=m
> are allowed by Kconfig because the 'depends on !DRM_SIMPLEDRM' dependency
> does not disallow FB_SIMPLE as long as SIMPLEDRM is not built-in. This

Double negative. How about:

allows FB_SIMPLE as long as SIMPLEDRM is not built-in.

> can however result in a build failure when cfb_fillrect() etc are then
> also in loadable modules:
>
> x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x1f8): undefined reference to `cfb_fillrect'
> x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x200): undefined reference to `cfb_copyarea'
> x86_64-linux-ld: drivers/video/fbdev/simplefb.o:(.rodata+0x208): undefined reference to `cfb_imageblit'
>
> To work around this, change FB_SIMPLE to be a 'tristate' symbol,
> which still allows both to be =m together, but not one of them to
> be =y if the other one is =m. If a distro kernel picks this
> configuration, it can be determined by local policy which of
> the two modules gets loaded. The 'of_chosen' export is needed
> as this is the first loadable module referencing it.
>
> Alternatively, the Kconfig dependency could be changed to
> 'depends on DRM_SIMPLEDRM=n', which would forbid the configuration
> with both drivers.
>
> Fixes: 11e8f5fd223b ("drm: Add simpledrm driver")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/of/base.c           | 1 +
>  drivers/video/fbdev/Kconfig | 5 +++--
>  2 files changed, 4 insertions(+), 2 deletions(-)

For the DT change:

Acked-by: Rob Herring <robh@kernel.org>