[Nouveau] [REGRESSION] nouveau: 30 second boot hang after commit 2b700825e

[Eric Biggers]

Hello,

I am encountering a 30 second hang during boot, in the Xorg process just before
the display manager comes up.  I have bisected the problem to the following
commit:

	commit 2b700825e7a7702fb862edba1262c98040dc1bf6
	Author: Ben Skeggs <bskeggs at redhat.com>
	Date:   Thu Aug 20 14:54:22 2015 +1000

	    drm/nouveau/mc: move device irq handling to platform-specific code

The hang only occurs with CONFIG_DEBUG_SHIRQ=y, which is why I didn't notice it
sooner (I don't usually compile my kernels with that option).

Based on the log output with drm_debug=0x3f, the hang is occurring in
nouveau_gem_ioctl_cpu_prep(), where there is a 30 second timeout.

This is on an "Optimus" laptop which has both an Intel and an Nvidia card.  I
understand that the Nvidia card is being disabled automatically, and only the
Intel one is used; but the nouveau driver obviously still does a few things
either way.  The Nvidia hardware is: NVIDIA Corporation GK107GLM [Quadro K1100M]
(rev a1).

I don't have the expertise to fully assess the suspect commit.  Any thoughts as
to the root cause of this problem?  Is there any other information I can
provide?

Eric