Nouveau and constant power-state cycling/loop in (X)Ubuntu 16.04/XFCE with Optimus

You've got yourself an Optimus machine, you've installed nouveau, and one thing that caught your eye is this:

entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynOff:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynOff:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynOff:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
0:DIS: :DynPwr:0000:02:00.0
1:IGD:+:Pwr:0000:00:02.0
entropy@symplex:~$

Your card is cycling between the Dynamic power-on (DynPwr) and Dynamic power-off (DynOff) state. It's wasting your battery and you want it to stay off when it's not being used.

You'll also notice that dmesg shows you this:

[ 1096.319606] nouveau 0000:02:00.0: DRM: resuming kernel object tree...
[ 1096.362341] nouveau 0000:02:00.0: priv: HUB0: 10ecc0 ffffffff (1f70822c)
[ 1096.362355] nouveau 0000:02:00.0: priv: GPC0: 4188ac 00000001 (1a70822e)
[ 1096.434465] nouveau 0000:02:00.0: DRM: resuming client object trees...
[ 1101.536496] ACPI Warning: \_SB_.PCI0.PEG_.VID_._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150930/nsarguments-95)
[ 1101.537262] ACPI Warning: \_SB_.PCI0.PEG_.VID_._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150930/nsarguments-95)
[ 1101.537955] nouveau 0000:02:00.0: DRM: evicting buffers...
[ 1101.537962] nouveau 0000:02:00.0: DRM: waiting for kernel channels to go idle...
[ 1101.537988] nouveau 0000:02:00.0: DRM: suspending client object trees...
[ 1101.539366] nouveau 0000:02:00.0: DRM: suspending kernel object tree...
[ 1106.758460] nouveau 0000:02:00.0: DRM: resuming kernel object tree...
[ 1106.801176] nouveau 0000:02:00.0: priv: HUB0: 10ecc0 ffffffff (1b70822c)
[ 1106.801187] nouveau 0000:02:00.0: priv: GPC0: 4188ac 00000001 (1c70822e)
[ 1106.860253] nouveau 0000:02:00.0: DRM: resuming client object trees...
[ 1112.528066] ACPI Warning: \_SB_.PCI0.PEG_.VID_._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150930/nsarguments-95)
[ 1112.528321] ACPI Warning: \_SB_.PCI0.PEG_.VID_._DSM: Argument #4 type mismatch - Found [Buffer], ACPI requires [Package] (20150930/nsarguments-95)
[ 1112.528545] nouveau 0000:02:00.0: DRM: evicting buffers...
[ 1112.528548] nouveau 0000:02:00.0: DRM: waiting for kernel channels to go idle...
[ 1112.528566] nouveau 0000:02:00.0: DRM: suspending client object trees...
[ 1112.529810] nouveau 0000:02:00.0: DRM: suspending kernel object tree...
[ 1117.593058] nouveau 0000:02:00.0: DRM: resuming kernel object tree...
[ 1117.635657] nouveau 0000:02:00.0: priv: HUB0: 10ecc0 ffffffff (1870822c)
[ 1117.635664] nouveau 0000:02:00.0: priv: GPC0: 4188ac 00000001 (1e70822e)
[ 1117.680262] nouveau 0000:02:00.0: DRM: resuming client object trees...

Over and over again. Suspending, resuming, suspending, resuming ...

Well, it turns out that it's a Window Manager tweak in XFCE(xfwm4) . This is what you're looking for:

Untick/unselect/disable the "Synchronise drawing to the vertical blank" and you will find that the problem disappears. The card will now stay in DynOff mode while it's not being used.