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SubjectRe: [PATCH] [RESEND] drm: fb_helper: fix CONFIG_FB dependency
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On Wed, 27 Oct 2021, Javier Martinez Canillas <javierm@redhat.com> wrote:
> 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

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

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