Copying here my own observations about the problem, from my previous bug report #1389215, now marked as a duplicate of this one.

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Brightness controls are not working in Unity / Xorg. When pressing the keys <fn><home> and <fn><end> on my Lenovo W510, I see the indicator on the screen showing the level of brightness changing, so key mapping is not the problem. However, the brightness is not changing.

On the kernel level, I have the single following device:

# cd /sys/class/backlight
# ls -l
total 0
lrwxrwxrwx 1 root root 0 Nov 3 17:16 acpi_video0 -> ../../devices/pci0000:00/0000:00:03.0/0000:01:00.0/backlight/acpi_video0

When playing with the <fn><home> and <fn><end>, the value of /sys/class/backlight/brightness changes accordingly. The screen brightness does not change.

Performing:

# echo 10 >brightness

...does not have any effect (regardless of the value provided, 10 being just an example).

WORKAROUND

There is an odd workaround. Simply go to the console using <CTRL><ALT><F1> and using the <fn><home> and <fn><end> keys, brightness then changes. Return to Unity / Xorg with <CTRL><ALT><F7> and the selected brightness persists.

NON WORKING WORKAROUND

On the web, I read that adding:

Option "RegistryDwords" "EnableBrightnessControl=1"'

in xorg.conf might fix the problem. I do not have an xorg and when I let the proprietary Nvidia driver generate one and add this option, X does not start anymore.

Ubuntu 14.04.1 LTS with latest updates as of this writing.

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I tried under KDE Plasma 5.1 (Neon) and still I could not change the brightness of the backlight.

However, the CLI commands that fail in a terminal running under an Xorg session work flawlessly under a console (<CTRL><ALT><F1>).

cd /sys/class/backlight/acpi_video0

echo 5 >brightness # Works under the console!
echo 10 >brightness # Works under the console!

This all suggest that somehow, Xorg is inhibiting the brightness control.

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Found out that brightness control works fine with Nouveau driver, but not with Nvidia proprietary driver.

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Discovered another behaviour. If you play with the brightness controls under Xorg, nothing changes. However, suspend the computer and resume it, and the brightness will be setup to the last setup you left when playing with the brightness controls. Thus, the value in the kernel persists.

You can also test this with the CLI:

# cd /sys/class/backlight/acpi_video0
# echo 10 >brightness # Nothing happens
# cat actual_brightness
10
# echo 5 >brightness # Nothing happens
# cat actual_brightness
5

<suspend computer>
<resume computer>

Now the brightness level is set to 5 and the screen finds itself at that level.

In the backend, the controls are changing the brightness levels, it is just that during an Xorg Session with NVIDIA proprietary drivers, the screen's real brightness is not updated. As if the hardware signal to the screen is never sent, except when in the console or upon resume.

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I replaced the official proprietary NVIDIA driver nvidia-331 with nvidia-340 from xorg-edgers and the problem persists. nvidia-340 (340.76) is the latest driver for the legacy card Quadro FX 880M of my Lenovo W510.

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I tried acpi_backlight=vendor on grub to novail. It even stopped working on the TTY. Putting grub as it was restored the functionality back in the TTY. Under X11, the problem persists.

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