Backlight Function Keys Do Not Work with Latitude E7240 and Linux

Using a linux-4.8.17 kernel with a Latitude E7240, I cannot get the backlight function keys to work.

The relevant dell kernel modules seem to be loaded, but nothing happens when I try to use the keys.

Any suggestions would be much appreciated

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Note that I can change the screen backlight brightness with:

# echo 500 > /sys/class/backlight/intel_backlight/brightness

..but not with:

# echo 500 > /sys/class/backlight/dell_backlight/brightness

$ lsmod | grep dell
dell_laptop            16384  0
dell_wmi               12288  0
dell_smbios            12288  2 dell_laptop,dell_wmi
dcdbas                 12288  1 dell_smbios
sparse_keymap          12288  1 dell_wmi
video                  28672  2 dell_laptop,dell_wmi
backlight              12288  2 dell_laptop,video
wmi                    12288  1 dell_wmi
dell_rbtn              12288  1