exynos-drm registration: infinite loop

[Thierry Reding]

On Thu, Nov 06, 2014 at 03:23:27PM +0900, Inki Dae wrote:
> On 2014년 11월 05일 23:38, Thierry Reding wrote:
> > On Tue, Nov 04, 2014 at 09:18:46PM +0400, Matwey V. Kornilov wrote:
> >> Hi,
> >>
> >> I run 3.18-rc3 kernel on BeagleBone Black. It doesn't have Exynos DRM
> >> of course, but I run multi-platform kernel where CONFIG_DRM_EXYNOS is
> >> set to 'y'.
> >> The issue here is that the platform probe/init goes to infinite loop
> >> as the following:
> >>
> >> [    5.717343] platform exynos-drm: Driver exynos-drm requests probe deferral
> >> [    5.726848] exynos-drm-ipp exynos-drm-ipp: drm ipp registered successfully.
> >> [    5.734700] platform exynos-drm: Driver exynos-drm requests probe deferral
> >> [    5.744044] exynos-drm-ipp exynos-drm-ipp: drm ipp registered successfully.
> >> [    5.752010] platform exynos-drm: Driver exynos-drm requests probe deferral
> >> [    5.761377] exynos-drm-ipp exynos-drm-ipp: drm ipp registered successfully.
> >> [    5.769291] platform exynos-drm: Driver exynos-drm requests probe deferral
> >>
> >> It is quite unexpectable behavior. I would expect that the exynos-drm
> >> failed to initialize with 'no device' error.
> >>
> >> See also for the reference: https://bugzilla.kernel.org/show_bug.cgi?id=87691
> > 
> > The reason for this seems to be that Exynos DRM instantiates a dummy
> > device that it can bind to (see exynos_drm_init()). Now this code is
> > executed unconditonally, so the device will be created whether or not
> > the kernel/module runs on an Exynos SoC. The driver should really bind
> > to some real device rather than instantiating a dummy. Or if it must
> > bind to a dummy then the code needs to at least check that it's running
> > on an Exynos SoC before instantiating the dummy.
> > 
> 
> Right, I also think the problem is because we use a dummay device for
> Exynos drm. As you mentioned, this device can always be bound as long as
> Exynos drm driver is enabled even without real device. So this issue
> gives us that we need super device node again. Actually, I had already
> tried the use  of the super device node like below for Exynos drm but
> other Samsung guys didn't want to use it.
> 	http://lists.freedesktop.org/archives/dri-devel/2014-April/056618.html
> 
> I think now we have the reason that we should use super device node so I
> will try to support the super device for Exynos drm like other ARM drm
> drivers did. However, for this, I'd like to listen to other opinions
> for a while before that.

Irrespective of any long-term solution you need to fix this as soon as
possible because this currently causes all sorts of weirdness when
booting a multi-platform kernel on non-Exynos boards. Probably the
easiest for now would be to add some soc_is_exynos*() check at the very
beginning of exynos_drm_init().

Thierry
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