After spending some more time with this, I can confirm that this is
definitely a regression. Both nouveau and the old bbswitch based method
in 16.04 were able to power off the nvidia card successfully when not in
use. The current implementation of prime-select in 18.04 doesn't do
that. It may be that this is card or ACPI specific, but my hardware is
fairly old (2012 ivybridge with kepler GPU), and it doesn't work for it.

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  SRU: PRIME Power Saving mode draws too much power

Status in HWE Next:
Status in gdm3 package in Ubuntu:
  Fix Released
Status in nvidia-graphics-drivers-390 package in Ubuntu:
  Fix Released
Status in nvidia-prime package in Ubuntu:
  Fix Released
Status in nvidia-settings package in Ubuntu:
  Fix Released
Status in sddm package in Ubuntu:
Status in ubuntu-drivers-common package in Ubuntu:
  Fix Released
Status in gdm3 source package in Bionic:
  Fix Released
Status in nvidia-graphics-drivers-390 source package in Bionic:
  Fix Released
Status in nvidia-prime source package in Bionic:
  Fix Released
Status in nvidia-settings source package in Bionic:
  Fix Released
Status in sddm source package in Bionic:
Status in ubuntu-drivers-common source package in Bionic:
  Fix Released

Bug description:
  SRU Request:

  Relying on the nouveau driver and on the vga switcheroo (to get around a 
change in systemd LP: #1777099) caused increased power consumption, and slowed 
down the switching process.

  Furthermore, if the main X/Xwayland session was started by Gdm when
  the nvidia driver was loaded, the session will keep the nvidia module
  loaded, and prevent the system from switching off the dGPU. Also, the
  nouveau driver will be loaded, if nvidia is not, and this can cause
  problems to unsupported NVIDIA GPUs.

  The solution involves the following changes:

  1) Solving the problem in systemd (LP: #1777099)

  2) Adding code in gpu-manager and in nvidia-prime to unload the nvidia
  modules, and to allow the PCI device to sleep.

  3) Making a slight change to the current patch in Gdm, used to call
  the PRIME scripts before and after a Gdm session (so that gpu-manager
  gets actually called on log out)

  4) Adding code in gpu-manager to kill the main X/Xwayland session on
  log out, if the session is preventing us from unloading the nvidia
  driver. A new X/Xwayland session will be created after unload the

  5) Removing the systemd service that loads nouveau from the nvidia

  [Test Case]
  1) Enable the -proposed repository, and install the new 
"ubuntu-drivers-common", nvidia drivers, nvidia-prime, and gdm3 *

  2) Make sure the nvidia packages are installed, and enable performance mode 
(if it is already enabled, call "sudo prime-select intel" first):
  sudo prime-select nvidia

  3) Restart your computer and attach your /var/log/gpu-manager.log. see
  if the system boots correctly. If unsure, please attach your /var/log
  /gpu-manager.log and /var/log/Xorg.0.log

  4) Select power saving mode:
  sudo prime-select intel

  5) Log out and log back in

  6) Check if the nvidia driver is still loaded:
  lsmod | grep nvidia

  [Regression Potential]
  Low, as hybrid graphics support does not work correctly, and the changes only 
affect this use case.

  * Steps to test the updates:

  1) Enable the bionic-proposed repositories

  2) Create /etc/apt/preferences.d/proposed-updates with the following

  Package: *
  Pin: release a=bionic-proposed
  Pin-Priority: 400

  3) Update the list of packages:

  sudo apt-get update

  4) Install the packages from -proposed using the following command:

  sudo apt install nvidia-driver-390/bionic-proposed gdm3/bionic-
  proposed ubuntu-drivers-common/bionic-proposed nvidia-prime/bionic-
  proposed nvidia-settings/bionic-proposed libnvidia-gl-390/bionic-
  proposed libnvidia-compute-390/bionic-proposed libnvidia-decode-390
  /bionic-proposed libnvidia-encode-390/bionic-proposed libnvidia-
  ifr1-390/bionic-proposed libnvidia-fbc1-390/bionic-proposed

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