No screens after (WW) Falling back to old probe method for modesetting

[Rafał Miłecki]

I'm trying to switch from fbdev to modesetting for my AMD Southern
Islands VERDE card. Of course I've KMS running just fine, radeon
module loads and sets correct resolution, dmesg looks alright

Unfortunately Xorg doesn't start after forcing "modesetting", AFAIU
the driver doesn't provide any screens/modes. It looks like this:
(II) Loading /usr/lib/xorg/modules/drivers/modesetting_drv.so
(WW) Falling back to old probe method for modesetting
(II) UnloadModule: "modesetting"
(II) Unloading modesetting
(EE) Screen(s) found, but none have a usable configuration.

Can you give me any tip on this? My X.Org X Server is 1.10.4, I'm
using xf86-video-modesetting from today's git. Any patch/trick to
debug this issue?

-- 
Rafał