When the base sun4i DRM driver is built-in but the back-end is
a loadable module, we run into a link error:

drivers/gpu/drm/sun4i/sun4i_drv.o: In function `sun4i_drv_probe':
sun4i_drv.c:(.text+0x60c): undefined reference to `sun4i_frontend_of_table'

The dependency is a bit tricky, the best workaround I have come up
with is to use a Makefile hack to to interpret both
CONFIG_DRM_SUN4I_BACKEND=m and CONFIG_DRM_SUN4I_BACKEND=y
as a directive to build the front-end the same way as the main module.

Fixes: dd0421f47505 ("drm/sun4i: Add a driver for the display frontend")
Signed-off-by: Arnd Bergmann <a...@arndb.de>
---
 drivers/gpu/drm/sun4i/Makefile | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sun4i/Makefile b/drivers/gpu/drm/sun4i/Makefile
index 582607c0c488..db14e023256b 100644
--- a/drivers/gpu/drm/sun4i/Makefile
+++ b/drivers/gpu/drm/sun4i/Makefile
@@ -25,6 +25,9 @@ obj-$(CONFIG_DRM_SUN4I)               += sun4i-tcon.o
 obj-$(CONFIG_DRM_SUN4I)                += sun4i_tv.o
 obj-$(CONFIG_DRM_SUN4I)                += sun6i_drc.o
 
-obj-$(CONFIG_DRM_SUN4I_BACKEND)        += sun4i-backend.o sun4i-frontend.o
+obj-$(CONFIG_DRM_SUN4I_BACKEND)        += sun4i-backend.o
+ifdef CONFIG_DRM_SUN4I_BACKEND
+obj-$(CONFIG_DRM_SUN4I)                += sun4i-frontend.o
+endif
 obj-$(CONFIG_DRM_SUN4I_HDMI)   += sun4i-drm-hdmi.o
 obj-$(CONFIG_DRM_SUN8I_MIXER)  += sun8i-mixer.o
-- 
2.9.0

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