I've been using the `nv' driver for a long while now, but have now need for XRandR support, so I dediced to try nouveau once again.
I compiled a new kernel from nouveau's git tree (with KMS enabled), a new librm and a new nouveau driver, all from the latest git trees.

But when I try to start the X server it fails with:

(EE) NOUVEAU(0): [dri] unable to reference front buffer: -12
(EE) NOUVEAU(0): [dri] NVDRIFinishScreenInit failed, disbling DRI
[...]
Backtrace:
0: X(xorg_backtrace+0x26) [0x4ee056]
1: X(xf86SigHandler+0x39) [0x484199]
2: /lib/libc.so.6 [0x7fb0d0704fa0]
3: /usr/lib/xorg/modules/drivers//nouveau_drv.so(NV30_UploadFragProg+0x49) [0x7fb0cef5ff49]
4: /usr/lib/xorg/modules/drivers//nouveau_drv.so(NVAccelInitNV40TCL+0xf9a) [0x7fb0cef66fda]
5: /usr/lib/xorg/modules/drivers//nouveau_drv.so(NVAccelCommonInit+0x13d6) [0x7fb0cef48666]
6: /usr/lib/xorg/modules/drivers//nouveau_drv.so [0x7fb0cef533c6]
7: /usr/lib/xorg/modules/drivers//nouveau_drv.so [0x7fb0cef53e01]
8: X(AddScreen+0x1c6) [0x432ac6]
9: X(InitOutput+0x241) [0x46d381]
10: X(main+0x200) [0x4331d0]
11: /lib/libc.so.6(__libc_start_main+0xfd) [0x7fb0d06f1a8d]
12: X [0x432819]

Fatal server error:
Caught signal 11.  Server aborting

The dmesg also has some relevant messages:

[ 3495.415183] [drm] nouveau 0000:01:00.0: Allocating FIFO number 1
[ 3495.416652] [drm] nouveau 0000:01:00.0: nouveau_channel_alloc: initialised FIFO 1
[ 3498.416879] [TTM] Failed moving buffer. Proposed placement 0x00070004
[ 3498.416883] [TTM] Out of aperture space or DRM memory quota.
[ 3501.569024] [TTM] Failed moving buffer. Proposed placement 0x00070004
[ 3501.569028] [TTM] Out of aperture space or DRM memory quota.
[ 3506.602168] [drm] nouveau 0000:01:00.0: nouveau_channel_free: freeing fifo 1
[ 3509.601010] [drm] nouveau 0000:01:00.0: Failed to idle channel 1.