fbdev: simplefb: fix Kconfig dependencies

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: 11e8f5fd ("drm: Add simpledrm driver")
Acked-by: Rob Herring <robh@kernel.org> # for drivers/of/
Link: https://lore.kernel.org/all/20210721151839.2484245-1-arnd@kernel.org/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch> # fbdev support
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Borislav Petkov <bp@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: <stable@vger.kernel.org> # v5.14+
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210928145243.1098064-1-arnd@kernel.orgSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/of/base.c

@@ -36,6 +36,7 @@ LIST_HEAD(aliases_lookup);
 struct device_node *of_root;
 EXPORT_SYMBOL(of_root);
 struct device_node *of_chosen;
+EXPORT_SYMBOL(of_chosen);
 struct device_node *of_aliases;
 struct device_node *of_stdout;
 static const char *of_stdout_options;

drivers/video/fbdev/Kconfig

@@ -2193,8 +2193,9 @@ 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
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT