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

From: Jani Nikula <jani.nikula@linux.intel.com>
To: Daniel Vetter <daniel@ffwll.ch>, Kees Cook <keescook@chromium.org>
Cc: "Arnd Bergmann" <arnd@kernel.org>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Maxime Ripard" <mripard@kernel.org>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"David Airlie" <airlied@linux.ie>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Christian König" <christian.koenig@amd.com>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	"Javier Martinez Canillas" <javierm@redhat.com>
Subject: Re: [PATCH] [RESEND] drm: fb_helper: fix CONFIG_FB dependency
Date: Wed, 27 Oct 2021 14:47:37 +0300	[thread overview]
Message-ID: <878ryeit9i.fsf@intel.com> (raw)
In-Reply-To: <YVXJLE8UqgcUNIKl@phenom.ffwll.local>

On Thu, 30 Sep 2021, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Mon, Sep 27, 2021 at 09:23:45AM -0700, Kees Cook wrote:
>> On Mon, Sep 27, 2021 at 04:28:02PM +0200, Arnd Bergmann wrote:
>> > From: Arnd Bergmann <arnd@arndb.de>
>> > 
>> > With CONFIG_FB=m and CONFIG_DRM=y, we get a link error in the fb helper:
>> > 
>> > aarch64-linux-ld: drivers/gpu/drm/drm_fb_helper.o: in function `drm_fb_helper_alloc_fbi':
>> > (.text+0x10cc): undefined reference to `framebuffer_alloc'
>> > 
>> > Tighten the dependency so it is only allowed in the case that DRM can
>> > link against FB.
>> > 
>> > Fixes: f611b1e7624c ("drm: Avoid circular dependencies for CONFIG_FB")
>> > Link: https://lore.kernel.org/all/20210721152211.2706171-1-arnd@kernel.org/
>> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>> 
>> Thanks for fixing this!
>> 
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> Stuffed into drm-misc-next.

The problem is, I don't think the patch is semantically correct.

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.

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.

All of the drm Kconfigs could use an overhaul to be semantically
correct, but that's a hill nobody wants to die on. Instead we keep
piling up tweaks to paper over the issues, ad infinitum.

(And this ties to a previous comment I had about the organization of
files under drm/, a hundred files in one big lump that belong to
different modules, and it's not helping people figure out the
dependencies.)

BR,
Jani.

PS. I was brought here via [1] which is another complicated "fix" to the
same problem.

[1] https://lore.kernel.org/r/20211027072044.4105113-1-javierm@redhat.com

-- 
Jani Nikula, Intel Open Source Graphics Center