I added acpi_backlight=vendor and with the following boot line:
I got the dell backlight interface instead of the acpi interface - in addition to nvidiabl:
aa@aa-Latitude-E6530:~$ cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-3.11.0-14-generic root=UUID=c86aca8f-c879-4a03-a76a-7effda7f037d ro quiet splash acpi_backlight=vendor
The Fn keys do not change brightness as before and in the system settings the brightness can not be changed as well. The only way I can now change the brightness changed is by (100 - for maximum brightness)
aa@aa-Latitude-E6530:~$ for i in /sys/class/backlight/*; do echo $i; cat $i/brightness; cat $i/actual_brightness; cat $i/max_brightness; done
another VERY STRANGE thing: sometimes I get the brightness changed when I open a new window to firefox: the brightness goes to the default level set by the BIOS (in this case the value for /sys/class/backlight/nvidia_backlight/brightness was not changed).
aa@aa-Latitude-E6530:~$ echo 100 | sudo tee /sys/class/backlight/nvidia_backlight/brightness
However, this did not happen always, so it might be just another error.
I would really welcome any comments!