[PATCH] fbdev: simplefb: fix Kconfig dependencies

From: Arnd Bergmann
Date: Wed Jul 21 2021 - 11:18:48 EST

From: Arnd Bergmann <arnd@xxxxxxxx>

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
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@xxxxxxxx>
drivers/of/base.c | 1 +
drivers/video/fbdev/Kconfig | 5 +++--
2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 48e941f99558..073ea7cd007b 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -36,6 +36,7 @@ LIST_HEAD(aliases_lookup);
struct device_node *of_root;
struct device_node *of_chosen;
struct device_node *of_aliases;
struct device_node *of_stdout;
static const char *of_stdout_options;
diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
index 8a98539ec490..5fbcb3cc2e8e 100644
--- a/drivers/video/fbdev/Kconfig
+++ b/drivers/video/fbdev/Kconfig
@@ -2207,8 +2207,9 @@ config FB_HYPERV
This framebuffer driver supports Microsoft Hyper-V Synthetic Video.

config FB_SIMPLE
- bool "Simple framebuffer support"
- depends on (FB = y) && !DRM_SIMPLEDRM
+ tristate "Simple framebuffer support"
+ depends on FB
+ depends on !DRM_SIMPLEDRM