[SOLVED] Brightness control broken on Ubuntu

OS: Ubuntu 12.04 LTS
Hardware:  HP Envy dv7 7390eb laptop
Problem: brightness control broken on kernel upgrade from "precise" to "trusty"

Explanation:

I received a notification in Update Manager that my "current Hardware Enablement Stack (HWE) is going out of support" so I did the proposed upgrade.

I had some problems with dependencies which I solved according to these instructions:

"Command:

amd64:
apt-get install -V libglapi-mesa-lts-trusty libgl1-mesa-glx-lts-trusty xserver-xorg-lts-trusty xserver-xorg-input-all-lts-trusty xserver-xorg-video-all-lts-trusty libgl1-mesa-dri-lts-trusty x11-xserver-utils-lts-trusty libglapi-mesa-lts-trusty:i386 libgl1-mesa-dri-lts-trusty:i386 libgl1-mesa-glx-lts-trusty:i386 libgles2-mesa-lts-trusty libglapi-mesa-lts-trusty mesa-vdpau-drivers-lts-trusty

i386:
apt-get install -V libglapi-mesa-lts-trusty libgl1-mesa-glx-lts-trusty xserver-xorg-lts-trusty xserver-xorg-input-all-lts-trusty xserver-xorg-video-all-lts-trusty libgl1-mesa-dri-lts-trusty x11-xserver-utils-lts-trusty mesa-vdpau-drivers-lts-trusty libgles2-mesa-lts-trusty libglapi-mesa-lts-trusty"

found in the comments on this bug report.

Everything seemed to be OK after reboot, apart from the fact that the brightness control on my HP Envy dv7 laptop was broken. The function keys for brightness did move the brightness slider that appears on screen, but brightness itself was not affected and stayed at 100%.

After testing some different workarounds that didn't work, I found a solution on this blog post (I copy it here for completeness):

"Open terminal ( Ctrl+Alt+T ) and type:

sudo gedit /etc/default/grub

You will find this line in the new opened window:

1	GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Change it to:

1	GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

Save and close the window and type this in the terminal:

sudo update-grub

This will update your grub and while rebooting your PC, it will set an extra parameter on the grub menu during boot."

Happy Ubuntuing!