Adjust backlight via DDC

backlightbrightnessdisplayhdmiscripting

I'm looking for a simple way of adjusting an external monitor's backlight
using DDC. Something like good-ddc-tool --monitor 0 --backlight 100 that just works without complicating things when a single monitor is connected (the common case).

I've tried the answers to this question without success:

First answer (ddccontrol)

$ sudo ddccontrol -p

[...]

Probing for available monitors........
Detected monitors :
No monitor supporting DDC/CI available.
If your graphics card need it, please check all the required kernel modules are loaded (i2c-dev, and your framebuffer driver).

$ sudo modprobe i2c-dev
$ sudo lsmod | grep i2c
i2c_nforce2            20480  0

…fail?

Second answer (ddcutil)

Quick test:

$ sudo ddcutil capabilities
Display not found

…fail.

Guess:

$ sudo ddcutil detect
Invalid display
   I2C bus:             /dev/i2c-4
   EDID synopsis:
      Mfg id:           DEL
      Model:            DELL 2209WA
      Serial number:    [redacted]
      Manufacture year: 2009
      EDID version:     1.3
   DDC communication failed

Invalid display? Well it's the right manufacturer, model and manufacture year — not sure how it managed to display this information if "DDC communication failed".

I know the display and cable are fully capable of adjusting backlight on other OSes on other systems, and it feels far-fetched to think a Geforce GTX 550 lacks basic DDC support. Obviously it doesn't since it successfully read the display model.

Third answer (xrandr)

Quick (guess) tests:

$ xrandr --output HDMI2 --brightness 0.6
warning: output HDMI2 not found; ignoring
xrandr: Need crtc to set gamma on.

$ xrandr --output HDMI1 --brightness 0.6
warning: output HDMI1 not found; ignoring
xrandr: Need crtc to set gamma on.

…fail. Same for HDMI0 and just HDMI.

Guess:

$ xrandr --listproviders
Providers: number : 1
Provider 0: id: 0x279 cap: 0x1, Source Output crtcs: 2 outputs: 4 associated providers: 0 name:NVIDIA-0

I don't really know what this tells me, and I couldn't find a way to list "outputs" which just leaves me guessing.

Update

I found special instructions for ddcutil for nvidia cards, but I couldn't find the file that was mentioned in the first of the two suggestions:

A file for making this change is 90-nvidia_i2c.conf located in distribution directory data/etc/X11/xorg.conf.d

I'm using Xubuntu. Where is it suggesting I look?

The results of the second suggestion in the link can be found in an answer below.

Update 2

I tried ddcutil on a MacBook/Intel HD Graphics/Ubuntu with the same monitor and it works without a problem.

This suggests to me that the GeForce GTX 550 or its driver is most likely the problem.

Best Answer

I am the developer of ddcutil.

It looks like you have a monitor with a marginal I2C implementation. The Nvidia options are forcing the driver to use the lowest bus speed defined in the I2C spec, 100 kbps. Unfortunately, there's no way to tell the driver to use an even lower bus speed.

Use the --ddc option to see DDC data errors, and the --stats option for cumulative error counts and the number of times ddcutil had to retry operations.

There are a couple ddcutil options that may help. If you see lots of failures in the --stats output showing maximum retries exceeded, you can try increasing the number of retries using the --maxtries option. Second, the DDC/CI protocol mandates that the application wait a certain amount of time (commonly 50 ms) between the time it sends a request to the monitor and the time it reads the reply. This is to allow the monitor time to perform the operation and/or assemble a reply. The --sleep-multiplier option increases the time that ddcutil waits. For example, --sleep-multiplier 2 makes ddcutil wait 100 ms when the spec mandates a 50 ms wait. (Conversely, I've seen some well behaved video card/monitor implementations that work with a sleep-multiplier value as low as .1, i.e., waiting only 5 ms when the spec requires 50.)

Finally, I apologize for the difficulty of finding file 90-nvidia_i2c.conf. The documentation for the file on the web site had not been updated for the current location.

Related Solutions

Ubuntu – How does the screen backlight work

At the heart of backlighting is this Linux Kernel parameter that's exposed to you through here under /sys. You can manipulate it by setting the value to something between 1 and 15. For example:

$ echo 5 | sudo tee /sys/class/backlight/acpi_video0/brightness

Set's the brightness to 5. Manipulating this Kernel parameter is abstracted away so that when you're changing the value with your keyboard or a desktop applet you're manipulating it through D-Bus and HAL.

D-Bus is allowing you to manipulate this structure, org.freedesktop.Hal.Device.KeyboardBacklight, and HAL is allowing the privilege to do so. You can see this on my Fedora 14 system like this:

$ grep -i backlight /etc/dbus-1/system.d/*
/etc/dbus-1/system.d/hal.conf:         send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>
/etc/dbus-1/system.d/hal.conf:         send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>

In the file hal.conf:

  <!-- Only allow users at the local console to manipulate devices -->
  <policy at_console="true">
  ...
      <allow send_destination="org.freedesktop.Hal"
           send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>

You can query the current value, through D-Bus like so:

$ dbus-send \
     --print-reply \
     --system \
     --dest=org.freedesktop.Hal  \
     /org/freedesktop/Hal/devices/computer_backlight \
     org.freedesktop.Hal.Device.LaptopPanel.GetBrightness | \
     tail -1 | \
     awk '{print $2}'

Which returns the value:

You can also manipulate it from the command line like so, (the bit int32:10 below is setting brightness to "10"):

$ dbus-send \
     --print-reply \
     --system \
     --dest=org.freedesktop.Hal  \
     /org/freedesktop/Hal/devices/computer_backlight \
     org.freedesktop.Hal.Device.LaptopPanel.SetBrightness \
     int32:10 #2&>1 > /dev/null

You can see that we changed the brightness:

$ cat /sys/class/backlight/acpi_video0/brightness
10

So how do I fix this?

One idea would be to save the current brightness out to a file prior to either a shutdown and/or reboot and then add to your startup (perhaps ~/.xinitrc) the dbus-send ... command above adding in the brightness value you previously saved out to the file.

Why do I have multiple files under /sys/class/backlight?

I came across this Q&A on askubuntu.com titled: Why there are two brightness control file (/sys/class/) in my system. In the answer to this there was this comment:

If the system starts with the kernel parameter acpi_backlight=vendor, the item acpi_video0 is replaced by the item intel, but then the Fn-Keys can not change the value of this item.

I also came across this documentation for the Kernel, titled: Kernel Parameters. In this doc the following aCPI options are mentioned:

acpi_backlight= [HW,ACPI]
        acpi_backlight=vendor
        acpi_backlight=video
        If set to vendor, prefer vendor specific driver
        (e.g. thinkpad_acpi, sony_acpi, etc.) instead
        of the ACPI video.ko driver.

I think the intel_backlight referenced in /sys/class/backlight is part of the backlighting for the video card drivers provided for Intel graphics cards.

References

Linux – How to turn off the backlight but leave the LCD on

From X Windows

From X Windows you can use the application xbacklight to get and set the percent brightness for your screen.

Example

current level

$ xbacklight -get
100.000000

set to 75%

$ xbacklight -set 75
$ xbacklight -get
73.333333

set back to 100%

$ xbacklight -set 100
$ xbacklight -get
100.000000

From the console

To achieve something similar for a virtual terminal you'll likely need to interact with the ACPI settings via the /sys filesystem.

Example

dims it

$ echo "10" | sudo tee /sys/class/backlight/acpi_video0/brightness 
10

full brightness

$ echo "15" | sudo tee /sys/class/backlight/acpi_video0/brightness 
15

You'll have to play with this one, the range of brightnesses can be from 0-9 or 0-15, I believe it ultimately depends on your laptop.

None of this works?

If neither of these 2 options suites your needs take a look at the ArchLinux Wiki's topic titled: Backlight. That article contains every method I've ever seen employed to achieve this!

First answer (ddccontrol)

Second answer (ddcutil)

Third answer (xrandr)

Update

Update 2

Best Answer

Related Solutions

Ubuntu – How does the screen backlight work

So how do I fix this?

Why do I have multiple files under /sys/class/backlight?

References

Linux – How to turn off the backlight but leave the LCD on

From X Windows

Example

From the console

Example

None of this works?

References

Related Question