[PATCH v2] drm/display: Select DP helper for DRM_DP_AUX_CHARDEV and DRM_DP_CEC

[Javier Martinez Canillas]

The DRM_DP_AUX_CHARDEV and DRM_DP_CEC Kconfig symbols enable code that use
DP helper functions, that are only present if CONFIG_DRM_DISPLAY_DP_HELPER
is also enabled.

But these don't select the DRM_DISPLAY_DP_HELPER symbol, meaning that it
is possible to enable any of them without CONFIG_DRM_DISPLAY_DP_HELPER.

That will lead to the following linking errors with the mentioned config:

  LD      vmlinux.o
  MODPOST vmlinux.symvers
  MODINFO modules.builtin.modinfo
  GEN     modules.builtin
  LD      .tmp_vmlinux.kallsyms1
  KSYMS   .tmp_vmlinux.kallsyms1.S
  AS      .tmp_vmlinux.kallsyms1.S
  LD      .tmp_vmlinux.kallsyms2
  KSYMS   .tmp_vmlinux.kallsyms2.S
  AS      .tmp_vmlinux.kallsyms2.S
  LD      vmlinux
  SYSMAP  System.map
  SORTTAB vmlinux
  OBJCOPY arch/arm64/boot/Image
  MODPOST modules-only.symvers
ERROR: modpost: "drm_dp_dpcd_write" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
ERROR: modpost: "drm_dp_read_desc" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
ERROR: modpost: "drm_dp_dpcd_read" [drivers/gpu/drm/display/drm_display_helper.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:134: modules-only.symvers] Error 1
make[1]: *** Deleting file 'modules-only.symvers'
make: *** [Makefile:1749: modules] Error 2

Note: It seems this has been an issue for a long time but was made easier
to reproduce after the commit 1e0f66420b13 ("drm/display: Introduce a DRM
display-helper module"). Adding a Fixes: tag just to make sure that this
fix will be picked for stable once the mentioned change also lands there.

Fixes: 1e0f66420b13 ("drm/display: Introduce a DRM display-helper module")
Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
---

Changes in v2:
- Explain better the issue in the change description.
- Only select DRM_DISPLAY_DP_HELPER and not DRM_DISPLAY_HELPER.

 drivers/gpu/drm/display/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/display/Kconfig b/drivers/gpu/drm/display/Kconfig
index f84f1b0cd23f..9fe80c4e555c 100644
--- a/drivers/gpu/drm/display/Kconfig
+++ b/drivers/gpu/drm/display/Kconfig
@@ -32,6 +32,7 @@ config DRM_DISPLAY_HDMI_HELPER
 config DRM_DP_AUX_CHARDEV
 	bool "DRM DP AUX Interface"
 	depends on DRM
+	select DRM_DISPLAY_DP_HELPER
 	help
 	  Choose this option to enable a /dev/drm_dp_auxN node that allows to
 	  read and write values to arbitrary DPCD registers on the DP aux
@@ -40,6 +41,7 @@ config DRM_DP_AUX_CHARDEV
 config DRM_DP_CEC
 	bool "Enable DisplayPort CEC-Tunneling-over-AUX HDMI support"
 	depends on DRM
+	select DRM_DISPLAY_DP_HELPER
 	select CEC_CORE
 	help
 	  Choose this option if you want to enable HDMI CEC support for
-- 
2.35.1