nouveau regression: read fault PAGE_NOT_PRESENT with new fence interface

[Ted Percival <ted@tedp.id.au>]

Hi, I noticed a regression in the next-20140903 kernel that was not
present in next-20140902. When Xorg starts up, the display is garbled
(or contains old image bits) and I see a page fault in the kernel log. X
is not usable in this state - there is no pointer and gdm doesn't come
up. It works fine after Xorg (gdm3) is restarted. The fault is still
present in next-20140918.

[drm] Initialized nouveau 1.2.0 20120801 for 0000:01:00.0 on minor 0
nouveau E[   PFIFO][0000:01:00.0] read fault at 0x0001260000
[PAGE_NOT_PRESENT] from PGRAPH/GPC0/TEX on channel 0x001fcd1000 [Xorg[3874]]
nouveau E[   PFIFO][0000:01:00.0] PGRAPH engine fault on channel 2,
recovering...
nouveau E[  PGRAPH][0000:01:00.0] TRAP ch 2 [0x001fcd1000 Xorg[3874]]
nouveau E[  PGRAPH][0000:01:00.0] GPC0/TPC0/TEX: 0x80000041

My hardware is: NVIDIA Corporation GF119 [Quadro NVS 4200M] (rev a1)

I bisected it down to this commit:

  commit 29ba89b2371d466ca68973525816cf10debc2655
  Author: Maarten Lankhorst <maarten.lankhorst@ubuntu.com>
  Date:   Thu Jan 9 11:03:11 2014 +0100

      drm/nouveau: rework to new fence interface

      Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
      Acked-by: Ben Skeggs <bskeggs@redhat.com>

I tried tracing the DEVICE, PGRAPH, PFIFO & PCE0 engines but nothing
obvious stood out before the read fault shown above.

Is there any other information I can provide to help track this down?
paranoia or spam level logging, for example?