Re: [PATCH v2 1/4] break kconfig dependency loop

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Gerd Hoffmann <kraxel@redhat.com>,
	dri-devel@lists.freedesktop.org, virtio-dev@lists.oasis-open.org
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
	mst@redhat.com, open list <linux-kernel@vger.kernel.org>,
	airlied@redhat.com,
	"open list:MEDIA INPUT INFRA..." <linux-media@vger.kernel.org>
Subject: Re: [PATCH v2 1/4] break kconfig dependency loop
Date: Wed, 01 Apr 2015 16:47:12 +0300	[thread overview]
Message-ID: <87wq1wot9b.fsf@intel.com> (raw)
In-Reply-To: <1427894130-14228-2-git-send-email-kraxel@redhat.com>

On Wed, 01 Apr 2015, Gerd Hoffmann <kraxel@redhat.com> wrote:
> After adding virtio-gpu I get this funky kconfig dependency loop.
>
> scripts/kconfig/conf --oldconfig Kconfig
> drivers/video/fbdev/Kconfig:5:error: recursive dependency detected!
> drivers/video/fbdev/Kconfig:5:  symbol FB is selected by DRM_KMS_FB_HELPER
> drivers/gpu/drm/Kconfig:34:     symbol DRM_KMS_FB_HELPER is selected by DRM_VIRTIO_GPU
> drivers/gpu/drm/virtio/Kconfig:1:       symbol DRM_VIRTIO_GPU depends on VIRTIO
> drivers/virtio/Kconfig:1:       symbol VIRTIO is selected by REMOTEPROC
> drivers/remoteproc/Kconfig:4:   symbol REMOTEPROC is selected by OMAP_REMOTEPROC
> drivers/remoteproc/Kconfig:12:  symbol OMAP_REMOTEPROC depends on OMAP_IOMMU
> drivers/iommu/Kconfig:141:      symbol OMAP_IOMMU is selected by VIDEO_OMAP3
> drivers/media/platform/Kconfig:96:      symbol VIDEO_OMAP3 depends on VIDEO_V4L2
> drivers/media/v4l2-core/Kconfig:6:      symbol VIDEO_V4L2 depends on I2C
> drivers/i2c/Kconfig:7:  symbol I2C is selected by FB_DDC
> drivers/video/fbdev/Kconfig:59: symbol FB_DDC is selected by FB_CYBER2000_DDC
> drivers/video/fbdev/Kconfig:374:        symbol FB_CYBER2000_DDC depends on FB_CYBER2000
> drivers/video/fbdev/Kconfig:362:        symbol FB_CYBER2000 depends on FB
>
> Making VIDEO_OMAP3 depend on OMAP_IOMMU instead of selecting it breaks the
> loop, which looks like the best way to handle it to me.  I'm open to better
> suggestions though.

I think part of the problem is that "select" is often used not as
documented [1] but rather as "show my config in menuconfig for
convenience even if my dependency is not met, and select the dependency
even though I know it can screw up the dependency chain".

In light of the documentation, your patch seems to DTRT. (Disclaimer: I
don't work with the drivers in question, hence no Reviewed-by.)

In the big picture, it feels like menuconfig needs a way to display
items whose dependencies are not met, and a way to recursively enable
said items and all their dependencies when told. This would reduce the
resistance to sticking with "select" when clearly "depends" is what's
meant.

BR,
Jani.

[1] Documentation/kbuild/kconfig-language.txt: "In general use select
only for non-visible symbols (no prompts anywhere) and for symbols with
no dependencies. That will limit the usefulness but on the other hand
avoid the illegal configurations all over."

>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  drivers/media/platform/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/media/platform/Kconfig b/drivers/media/platform/Kconfig
> index d9b872b..fc21734 100644
> --- a/drivers/media/platform/Kconfig
> +++ b/drivers/media/platform/Kconfig
> @@ -87,8 +87,8 @@ config VIDEO_OMAP3
>  	tristate "OMAP 3 Camera support"
>  	depends on VIDEO_V4L2 && I2C && VIDEO_V4L2_SUBDEV_API && ARCH_OMAP3
>  	depends on HAS_DMA
> +	depends on OMAP_IOMMU
>  	select ARM_DMA_USE_IOMMU
> -	select OMAP_IOMMU
>  	select VIDEOBUF2_DMA_CONTIG
>  	---help---
>  	  Driver for an OMAP 3 camera controller.
> -- 
> 1.8.3.1
>
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-- 
Jani Nikula, Intel Open Source Technology Center