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

[Jani Nikula]

On Wed, 27 Oct 2021, Javier Martinez Canillas <javierm@xxxxxxxxxx> wrote:
> On 10/27/21 14:18, Arnd Bergmann wrote:
>> On Wed, Oct 27, 2021 at 1:47 PM Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> 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

To me, this seems like the right solution. Depend on FB if
DRM_FBDEV_EMULATION is enabled. That's exactly what the relationship is.

BR,
Jani.



>         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,

-- 
Jani Nikula, Intel Open Source Graphics Center