The DRM_DP_AUX_CHARDEV and DRM_DP_CEC boolean Kconfig symbols enable code
that use DP helper functions, exported by the display-helper module.

But these don't select the DRM_DISPLAY_DP_HELPER and DRM_DISPLAY_HELPER
symbols, to make sure that the functions used will be present. This leads
to the following linking errors if CONFIG_DRM_DISPLAY_HELPER is set to m:

  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@redhat.com>
---

 drivers/gpu/drm/display/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/display/Kconfig b/drivers/gpu/drm/display/Kconfig
index f84f1b0cd23f..d7a413a2c6cc 100644
--- a/drivers/gpu/drm/display/Kconfig
+++ b/drivers/gpu/drm/display/Kconfig
@@ -32,6 +32,8 @@ config DRM_DISPLAY_HDMI_HELPER
 config DRM_DP_AUX_CHARDEV
 	bool "DRM DP AUX Interface"
 	depends on DRM
+	select DRM_DISPLAY_DP_HELPER
+	select DRM_DISPLAY_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 +42,8 @@ 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 DRM_DISPLAY_HELPER
 	select CEC_CORE
 	help
 	  Choose this option if you want to enable HDMI CEC support for
-- 
2.35.1

On 4/27/22 22:25, Javier Martinez Canillas wrote:
> The DRM_DP_AUX_CHARDEV and DRM_DP_CEC boolean Kconfig symbols enable code
> that use DP helper functions, exported by the display-helper module.
> 

[snip]

> @@ -32,6 +32,8 @@ config DRM_DISPLAY_HDMI_HELPER
>  config DRM_DP_AUX_CHARDEV
>  	bool "DRM DP AUX Interface"
>  	depends on DRM
> +	select DRM_DISPLAY_DP_HELPER
> +	select DRM_DISPLAY_HELPER

Actually, this is wrong since it will prevent DRM_DISPLAY_HELPER to be set
as a module (it's tristate while this symbol is bool). I now have a better
understanding of the problem and a patch that I believe is the correct one.

I'll post a v2 soon.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat