/sys/class/backlight/acpi_video0/brightness is broken

[Jason Gerard DeRose]

With nvidia-352 (and for that matter nvidia-346), the /sys/class/backlight interface is broken.

Writing to /sys/class/backlight/acpi_video0/brightness has no effect, and reading from it returns a value that does not necessarily reflect the current brightness.

I don't know when exactly this broke in the Nvidia driver, but I do know that not that long ago it worked because the system76-driver uses this to restore the previous backlight brightness level during boot. I believe this still worked with nvidia-343 and perhaps also worked in early nvidia-346 versions.

This problem is somewhat masked because with Ubuntu (Unity), unity-settings-daemon has two code paths for setting the brightness:

1) Using /sys/class/backlight

2) Using xrandr, which as far as I can tell takes (roughly) the same approach as the `xbacklight` command line tool

However, approach (2) does not work with Ubuntu GNOME because gnome-settings-daemon has dropped this code (it dropped gnome-settings-daemon/gsd-rr*).

I've started digging into the dkms module source code, but haven't come up with anything yet. Of course, there is a chance the problem is in the closed proprietary bits.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: nvidia-352 352.63-0ubuntu0.15.10.1
ProcVersionSignature: Ubuntu 4.2.0-18.22-generic 4.2.3
Uname: Linux 4.2.0-18-generic x86_64
NonfreeKernelModules: nvidia
.proc.driver.nvidia.registry: Binary: ""
.proc.driver.nvidia.version:
 NVRM version: NVIDIA UNIX x86_64 Kernel Module 352.63 Sat Nov 7 21:25:42 PST 2015
 GCC version: gcc version 5.2.1 20151010 (Ubuntu 5.2.1-22ubuntu2)
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Nov 20 15:01:42 2015
DistUpgraded: Fresh install
DistroCodename: wily
DistroVariant: ubuntu
DkmsStatus:
 bbswitch, 0.7, 4.2.0-18-generic, x86_64: installed
 fwts-efi-runtime-dkms, 15.09.00, 4.2.0-18-generic, x86_64: installed
 nvidia-352, 352.63, 4.2.0-18-generic, x86_64: installed
GraphicsCard:
 NVIDIA Corporation GM204M [GeForce GTX 970M] [10de:1618] (rev a1) (prog-if 00 [VGA controller])
   Subsystem: CLEVO/KAPOK Computer Device [1558:7701]
MachineType: System76, Inc. Serval WS
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.2.0-18-generic.efi.signed root=UUID=d68b8062-f67c-495c-8c16-f2e11d9a9916 ro quiet splash
SourcePackage: nvidia-graphics-drivers-352
UdevLog: Error: [Errno 2] No such file or directory: '/var/log/udev'
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/09/2015
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1.03.03RSY2
dmi.board.asset.tag: Tag 12345
dmi.board.name: Serval WS
dmi.board.vendor: System76, Inc.
dmi.board.version: serw8-17g
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 9
dmi.chassis.vendor: System76, Inc.
dmi.chassis.version: Serval WS
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1.03.03RSY2:bd06/09/2015:svnSystem76,Inc.:pnServalWS:pvrserw8-17g:rvnSystem76,Inc.:rnServalWS:rvrserw8-17g:cvnSystem76,Inc.:ct9:cvrServalWS:
dmi.product.name: Serval WS
dmi.product.version: serw8-17g
dmi.sys.vendor: System76, Inc.
modified.conffile..etc.modprobe.d.nvidia.352.hybrid.conf: [deleted]
version.compiz: compiz 1:0.9.12.2+15.10.20151015-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.64-1
version.libgl1-mesa-dri: libgl1-mesa-dri 11.0.2-1ubuntu4
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 11.0.2-1ubuntu4
version.nvidia-graphics-drivers: nvidia-graphics-drivers-* N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.17.2-1ubuntu9
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.9.2-1ubuntu1
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.5.0+git20150819-0ubuntu1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20150808-0ubuntu4
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.11-1ubuntu3
xserver.bootTime: Fri Nov 20 08:41:35 2015
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:

xserver.version: 2:1.17.2-1ubuntu9

[Link Dupont (link-dupont)]

I encountered the same issue when installing Fedora 23 onto my Oryx Pro (I know Fedora isn't supported, etc.). I found a solution on the Archlinux wiki that talks about using inotify to watch for changes to the brightness file and run xbacklight to adjust the backlight levels. For my own sake, I documented it, but this might help for you on Ubuntu. It really sounds like a regression with the proprietary NVIDIA drivers. Maybe a similar file-monitoring could be incorporated into system76-daemon?

http://www.moxbant.net/fedora-23-on-system76-oryx-pro/

[Jason Gerard DeRose (jderose)]

Link,

Your write-up for getting this working on Fedora is great, thanks!

We're considering adding something like this to the system76-daemon assuming Nvidia isn't going to support this mechanism any more.

Thanks again!

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[dhenry (tfc-duke)]

Same issue with GTX 980M on Ubuntu 16.04, even with latest NVIDIA drivers (367.27).

I described this on NVIDIA forums:
https://devtalk.nvidia.com/default/topic/942337/linux/backlight-brightness-not-hooked-to-acpi-events-ubuntu-16-04-/

I manually fixed it by created an acpid event handler, however in my solution, it only works when changing the brightness with the Fn-keys, but not by changing /sys/class/backlight/acpi_video0/brightness value.

Link Dupont's fix works better (but I didn't use video.use_native_backlight=1 on GRUB_CMDLINE_LINUX as he said). By the way, the link to his solution is dead. This one works: http://sub-pop.net/post/fedora-23-on-system76-oryx-pro/

[Alejandro Palacios (pamanes7)]

link-dupont really nailed it! I am having the same issue on an HP Elitebook 8770w with ONLY an NVIDIA Quadro K3000M, using the proprietary driver version 387.34 on Fedora 27, Kernel 4.14.3-300

I tried all:

acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native

to no avail, so removed all of that and installed inotify-tools, I do have /sys/class/backlight/acpi_video0 so I ended up with the following script in /home/YOURUSERHERE/Scripts/backlight_inotify.sh

#!/bin/sh

path=/sys/class/backlight/acpi_video0

luminance() {
    read -r level < "$path"/actual_brightness
    factor=$((100 / max))
    printf '%d\n' "$((level * factor))"
}

read -r max < "$path"/max_brightness

nvidia-settings -n -a BacklightBrightness="$(luminance)"

inotifywait -me modify --format '' "$path"/actual_brightness | while read; do
    nvidia-settings -n -a BacklightBrightness="$(luminance)"
done

And then created file brightnessfix.desktop under /home/YOURUSERHERE/.config/autostart

[Desktop Entry]
Name=Brightness Fix
Exec=/home/YOURUSERHERE/Scripts/backlight_inotify.sh
Terminal=false
Type=Application
Name[en_US]=backlightfix.desktop

It is a very effective workaround, thanks.

[Vaclav Cermak (disnel)]

I have also this problem with Clevo P775DM3. With one (big) difference, neither xbacklight nor nvidia-settings works in my case :(