Running Fedora 21 with kernels from
https://alt.fedoraproject.org/pub/alt/rawhide-kernel-nodebug/ (tracking 3.18
development).

I have kernel-3.18.0-0.rc0.git1.2.fc22.1.x86_64 ,
kernel-3.18.0-0.rc0.git8.2.fc22.1.x86_64 and
kernel-3.18.0-0.rc0.git9.2.fc22.1.x86_64 installed .
kernel-3.18.0-0.rc0.git1.2.fc22.1.x86_64 boots fine on my system, the other two
do not.

On git1 I get boot framebuffer output on both heads, then X starts properly. On
git8 and git9, I get boot framebuffer only on one head, then both screens go
into power saving mode when X starts, or one keeps cycling power-saving /
non-power-saving but never comes up.

The logs have a bunch of stuff, but it seems to kick off with an error:

nouveau E[   PDISP][0000:01:00.0] INVALID_STATE [UNK0B] chid 1 mthd 0x0080 data
0x00000000

I'll attach drm.debug=15 logs. Hardware is:

01:00.0 VGA compatible controller [0300]: NVIDIA Corporation G94 [GeForce 9600
GT] [10de:0622] (rev a1)

Ben Skeggs has one of the same cards, somewhere in his pile. May only be
reproducible with dual displays, I didn't try with just one yet.