Re: [PATCH] drm: Make DRM_FBDEV_EMULATION deps more robust to fix linker errors

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 27 Oct 2021, Javier Martinez Canillas <javierm@redhat.com> wrote:
> Enabling the CONFIG_DRM_FBDEV_EMULATION Kconfig symbol can lead to linker
> errors, if CONFIG_DRM_KMS_HELPER is built-in but CONFIG_FB as a module.
>
> Because in that case the drm_kms_helper.o object will have references to
> symbols that are defined in the fb.ko module, i.e:
>
>   $ make bzImage modules
>     DESCEND objtool
>     CALL    scripts/atomic/check-atomics.sh
>     CALL    scripts/checksyscalls.sh
>     CHK     include/generated/compile.h
>     GEN     .version
>     CHK     include/generated/compile.h
>     UPD     include/generated/compile.h
>     CC      init/version.o
>     AR      init/built-in.a
>     LD      vmlinux.o
>     MODPOST vmlinux.symvers
>     MODINFO modules.builtin.modinfo
>     GEN     modules.builtin
>     LD      .tmp_vmlinux.kallsyms1
>   ld: drivers/gpu/drm/drm_fb_helper.o: in function `drm_fb_helper_resume_worker':
>   drm_fb_helper.c:(.text+0x2a0): undefined reference to `fb_set_suspend'
>   ...
>
> To prevent this, make following changes to the config option dependencies:
>
>   * Depend on FB && !(DRM_KMS_HELPER=y && FB=m), to avoid invalid configs.
>   * Depend on DRM_KMS_HELPER instead selecting it, to avoid circular deps.
>
> Reported-by: Justin M. Forbes <jforbes@fedoraproject.org>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

Please see [1]. I think it's a big mess. I don't think this is the fix
either.

BR,
Jani.

[1] https://lore.kernel.org/r/878ryeit9i.fsf@intel.com

> ---
>
> This fixes linker errors found when building the Fedora kernel package
> for the s390x architecture:
>
> https://kojipkgs.fedoraproject.org//work/tasks/9849/77859849/build.log
>
>  drivers/gpu/drm/Kconfig | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index cea777ae7fb9..9a21e57b4a0d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -103,8 +103,8 @@ config DRM_DEBUG_DP_MST_TOPOLOGY_REFS
>  config DRM_FBDEV_EMULATION
>  	bool "Enable legacy fbdev support for your modesetting driver"
>  	depends on DRM
> -	depends on FB
> -	select DRM_KMS_HELPER
> +	depends on FB && !(DRM_KMS_HELPER=y && FB=m)
> +	depends on DRM_KMS_HELPER
>  	select FB_CFB_FILLRECT
>  	select FB_CFB_COPYAREA
>  	select FB_CFB_IMAGEBLIT

-- 
Jani Nikula, Intel Open Source Graphics Center