Re: [PATCH] [RESEND] drm: fb_helper: fix CONFIG_FB dependency

[Javier Martinez Canillas <javierm@redhat.com>]

On 10/27/21 14:18, Arnd Bergmann wrote:
> On Wed, Oct 27, 2021 at 1:47 PM Jani Nikula <jani.nikula@linux.intel.com> wrote:

[snip]

>> drm_fb_helper.o is not part of drm.ko, it's part of
>> drm_kms_helper.ko. This adds some sort of indirect dependency via DRM
>> which might work, maybe by coincidence, maybe not - but it's certainly
>> not obvious.

Indeed, you are correct that's not semantically correct.

> 
> Right, how about this change on top?
> 
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -117,9 +117,8 @@ config DRM_DEBUG_MODESET_LOCK
> 
>  config DRM_FBDEV_EMULATION
>         bool "Enable legacy fbdev support for your modesetting driver"
> -       depends on DRM
> -       depends on FB=y || FB=DRM
> -       select DRM_KMS_HELPER
> +       depends on DRM_KMS_HELPER
> +       depends on FB=y || FB=DRM_KMS_HELPER
>         select FB_CFB_FILLRECT
>         select FB_CFB_COPYAREA
>         select FB_CFB_IMAGEBLIT
> 
> That would probably make it work for DRM=y, FB=m, DRM_KMS_HELPER=m,
> but it needs more randconfig testing, which I can help with.
>
>> The likely culprit is, again, the overuse of select, and in this case
>> select DRM_KMS_HELPER. And DRM_KMS_HELPER should depend on FB if
>> DRM_FBDEV_EMULATION=y. That's the problem.
> 
> This is something we can't easily express in Kconfig, as we can't add the
> dependency to a symbol that only gets selected by other drivers, which
> is why the dependency has to be in the user-visible symbol,
> in this case DRM_FBDEV_EMULATION.
> 

Why the dependency has to be in a user-visible symbol? What could be the
problem with having something like:

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index cea777ae7fb9..f80b404946ca 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -82,6 +82,7 @@ config DRM_DEBUG_SELFTEST
 config DRM_KMS_HELPER
        tristate
        depends on DRM
+       depends on (DRM_FBDEV_EMULATION && FB) || !DRM_FBDEV_EMULATION
        help
          CRTC helpers for KMS drivers.
 
@@ -104,7 +105,6 @@ config DRM_FBDEV_EMULATION
        bool "Enable legacy fbdev support for your modesetting driver"
        depends on DRM
        depends on FB
-       select DRM_KMS_HELPER
        select FB_CFB_FILLRECT
        select FB_CFB_COPYAREA
        select FB_CFB_IMAGEBLIT

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat