I bought a Dell Inspiron 14 5000 series and dual booted Windows 8.1 and Ubuntu GNOME 14.04. The brightness would not change, so I booted with kernel parameter
acpi_backlight=vendor and it worked. I later had to return that specific unit because of a hardware defect for another one.
With the second (current) unit I installed just regular Ubuntu 14.04. I tried to use the kernel parameter
acpi_backlight=vendor with it, but with
quiet splash acpi_backlight=vendor all it gave me was a black screen. I booted without quiet splash and it froze at allocating swap space.
Why is it refusing to boot?
I also tried adding
acpi_backlight=vendor. Still wouldn't boot.
acpi_osi=Linux on its own does not have any effect.