I've got a fresh install of trusty (xubuntu if that matters, I don't think it should) on a Asus X551MAV-EB01-B. My function keys for brightness don't work. Here's what I know so far:

If I pass acpi_osi= or acpi_osi=! to grub I can get the brightness and wifi keys to work (I know this because I can see the OSD for brightness change), BUT:

in /sys/class/backlight I have two items:

acpi_video0 -> ../../devices/pci0000:00/0000:00:02.0/backlight/acpi_video0 intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight

With cat I can see the brightness keys are adjusting acpi_video0/brightness and not intel_backlight/brightness .

I've also tried passng acpi_backlight=vendor to grub, and if I do that, then I end up with this :

asus-nb-wmi -> ../../devices/platform/asus-nb-wmi/backlight/asus-nb-wmi/ intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight

acpi_backlight=vendor also stops the brightness keys from responding at all.