[PATCH] drm/fb-helper: don't call remove_conflicting_framebuffers for FB=m && DRM=y

[Arnd Bergmann <arnd@arndb.de>]

When CONFIG_DRM_KMS_FB_HELPER is disabled, we can have a configuration
in which some DRM drivers are built-in, but the framebuffer core is a
loadable module. This results in a link error, such as:

drivers/gpu/drm/radeon/radeon.o: In function `radeon_pci_probe':
radeon_kfd.c:(.text.radeon_pci_probe+0xbc): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/amd/amdgpu/amdgpu.o: In function `amdgpu_pci_probe':
amdgpu_mn.c:(.text.amdgpu_pci_probe+0xa8): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/mgag200/mgag200.o: In function `mga_vram_init':
mgag200_ttm.c:(.text.mga_vram_init+0xa8): undefined reference to `remove_conflicting_framebuffers'
drivers/gpu/drm/mgag200/mgag200.o: In function `mga_pci_probe':
mgag200_ttm.c:(.text.mga_pci_probe+0x88): undefined reference to `remove_conflicting_framebuffers'
Makefile:969: recipe for target 'vmlinux' failed

This changes the compile-time check to IS_REACHABLE, which means we end up
not calling remove_conflicting_framebuffers() in the configuration, which
seems good enough, as we know that no framebuffer driver is loaded by the
time that the built-in DRM driver calls remove_conflicting_framebuffers.

We could alternatively avoid the link error by forcing CONFIG_FB to not
be a module in this case, but that wouldn't change anything at runtime,
and just make the already convoluted set of dependencies worse here.

I could not find out what happens if the fbdev driver gets loaded as
a module after the DRM driver is already initialized, but that is a case
that can happen with or without this patch.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0a3bfe29f816 ("drm/fb-helper: Fix the dummy remove_conflicting_framebuffers")
---
 include/drm/drm_fb_helper.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index edc6cfd3aa34..797fb5f80c45 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -491,7 +491,7 @@ static inline int
 drm_fb_helper_remove_conflicting_framebuffers(struct apertures_struct *a,
 					      const char *name, bool primary)
 {
-#if IS_ENABLED(CONFIG_FB)
+#if IS_REACHABLE(CONFIG_FB)
 	return remove_conflicting_framebuffers(a, name, primary);
 #else
 	return 0;
-- 
2.9.0