I have a lenove thinkpad T61p with a NVIDIA Corporation G84GLM [Quadro FX
570M].
When connecting my external VGA monitor my monitor is no more detected
properly:
> cat /sys/class/drm/card0-VGA-1/status
gives now "disconnected"

I bisected the kernel sources from
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
and filed a bug report at https://bugzilla.kernel.org
(https://bugzilla.kernel.org/show_bug.cgi?id=58021).

Last good commit is
ef22c8bb7b3fac45919b7fde412d36d1a8367d51
drm/nv50/disp: call into core to handle dac/sor power state changes

First bad commit is
7ebb38b556485449bfaa506a196439f6a6fd6ebd
drm/nv50/disp: call into core for dac load detection

The responsible function is
nv50_dac_detect in drivers/gpu/drm/nouveau/nv50_dac.c (moved to
drivers/gpu/drm/nouveau/nv50_display.c in more recent kernels).

Also tried a 3.9.1 kernel. The problem still remains.

However, replacing nv50_dac_detect with the corresponding version from the last
good commit fixed the problem (tried with a 3.8.10 and a 3.8.13 kernel).