[PATCH] fbdev: wait for references go away

[Gerd Hoffmann]

  Hi,

> > open.  Which can result in drm driver not being able to grab resources
> > (and fail initialization) because the firmware framebuffer still holds
> > them.  Reportedly plymouth can trigger this.
> 
> Could you please describe issue some more?
> 
> I guess that a problem is happening during DRM driver load while fbdev
> driver is loaded? I assume do_unregister_framebuffer() is called inside
> do_remove_conflicting_framebuffers()?

Yes.  Specifically bochs-drm.ko and efifb in virtual machines.

> At first glance it seems to be an user-space issue as it should not be
> holding references on /dev/fb0 while DRM driver is being loaded.

Well, the drm driver is loaded by udev like everything else.

Dunno what plymouth (graphical boot screen tool) does to handle the
situation.  I guess listening to udev events.  So it should notice efifb
going away and drop the /dev/fb0 reference, but this races against
bochs-drm initializing.

> > Fix this by trying to wait until all references are gone.  Don't wait
> > forever though given that userspace might keep the file handle open.
> 
> Where does the 1s maximum delay come from?

Pulled out something out of thin air which I expect being on the safe
side.  plymouth responding on the udev event should need only a small
fraction of that.

cheers,
  Gerd