Comment 27 for bug 1775065

I can confirm this very same problem in my laptop, which is a Thinkpad P1 with a Xeon E-2176M and hybrid Intel / NVIDIA card working in "Discreet" mode (that is, only the NVIDIA card is enabled) using the proprietary NVIDIA driver installed via the nvidia-driver-390 package (390.87-0ubuntu1).

The NVIDIA card is a Quadro P2000 Mobile according to lspci:

  01:00.0 VGA compatible controller: NVIDIA Corporation GP107GLM [Quadro P2000 Mobile] (rev a1)

I'm currently in Ubuntu 18.10 Cosmic Cuttlefish with kernel 4.18.0-10:

$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.10 (Cosmic Cuttlefish)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.10"
VERSION_ID="18.10"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=cosmic
UBUNTU_CODENAME=cosmic

$ uname -ar
Linux rosmon 4.18.0-10-generic #11-Ubuntu SMP Thu Oct 11 15:13:55 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

And just to confirm, what I'm seeing is this:

1. Pressing the media keys for brightness cause not visible effect (no brightness change), with errors showing up in the journal:

  oct 23 11:37:47 rosmon gsd-media-keys[2532]: Failed to set new screen percentage: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gsd_5fpower_5fmanager_5ferror.Code0: Screen backlight not available
  oct 23 11:37:47 rosmon gsd-media-keys[2532]: Failed to set new screen percentage: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._gsd_5fpower_5fmanager_5ferror.Code0: Screen backlight not available

2. Installing xbacklight allows me to effectively change the brightness from the command line (e.g. `xbacklight -set 50`, `xbacklight -inc 10`...)

3. The /sys/class/backlight directory is empty, suggesting that the ACPI interface was not initialize, which I believe is why the media keys don't work and throw that error in the journal (media keys -> gsd-media-keys -> gsd-power-manager -> GUDev -> /sys/class/backlight ACPI interface):

  $ ls -l /sys/class/backlight/
  total 0

4. Booting with acpi_osi= or acpi_osi=Linux causes the /sys/class/backlight directory to be populated with a thinkpad_backlight directory, and the media keys to respond showing the OSD for the brightness, but does not change brightness at all when used:

  a. The brightness bar is always shown at "random" values between 0 and ~20%
  b. /sys/class/backlight/thinkpad_backlight/actual_brightness is always 0

Also, in this setup where /sys/class/backlight/ is not empty, xbacklight does NOT work.

I hope this is helpful, as it's kind of annoying to have to use xbacklight to change brithgness. I was myself looking into a fix for this, but I can't figure out whether this should be fixed in the NVIDIA driver, some kernel module or somewhere else, so I figured I'd comment here and hope for the best.

PS: I can do more tests if needed. Thanks!