On Wed, Jul 21, 2021 at 9:18 AM Arnd Bergmann [off-list ref] 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>