The HP ProBook 4540s is a good choice when you need a GNU/Linux machine, but there are some hit and miss on the details. The thing I found most tedious is the brightness controls using the keyboard: they simply don’t work at all.

My soloution

See the kernel parameters actually used:

$ cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-3.11.0-15-generic root=UUID=8850c025-676a-4e8c-ac83-4ed1147b88d4 ro quiet splash vt.handoff=7

If there’s no reference to acpi_backlight, that’s the problem. Add the parameter to /etc/default/grub changing the following line from

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

Done