[Nouveau] [Bug 86503] Discrete card seems to be powered on even if reported as Off

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=86503

--- Comment #1 from fredgib at free.fr ---
I have the same problem but found a workaround that may give a hint to
developers.

My system is an up-to-date Kali Linux (kernel 4.16, nouveau package:
'xserver-xorg-video-nouveau/kali-rolling,now 1:1.0.15-2 amd64'). I don't have
bumblebee or prime installed.

If I don't blacklist nouveau in /etc/modprobe.d/, there are 2 possibilities:
- either I disable runtime management in the kernel options of
/etc/default/grub, then the discrete card will be on (vgaswitcheroo would say
'Pwr') and I can turn it off by writing in vgaswitcheroo/switch.
- or I don't disable runtime management, then vgaswitcheroo indicates "DynPwr"
and whatever I do, I cannot change that state.

In both cases, the power consumption remains the same, typical of when the
discrete card is on.

Now my preferred scenario, explaining the workaround:

If I blacklist nouveau in /etc/modprobe.d/ and reboot, I still have a high
consumption, typical of when the discrete card is on. I have only one provider
indicated by xrandr (the integrated i915 in modesetting) and
/sys/kernel/debug/vgaswitcheroo/switch does not exist. THEN I launch nouveau
("sudo modprobe nouveau") and that's it: my consumption is 5 Watts less
(typical of what I had with bumblebee and bbswitch before), vgaswitcheroo
indicates "DynOff".

There is however a limitation: if I connect an HDMI screen (knowing the HDMI
port of my laptop is fostered by the discrete card) and use it with 'xrandr
--setprovideroutputsource 1 0', nouveau will automatically turn on the discrete
card (and switcheroo will indicate "DynPwr"), but there is no return; the
discrete card will remain ON, whatever I do (I even tried to 'sudo modprobe
bbswitch' at that point and write "OFF" in /proc/acpi/bbswitch).

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