Comment 39 for bug 1510344

I can partially reproduce this bug on Dell Latitude E6420.

Namely in 16.04 (4.4 kernel) backlight was always on after (re)boot and screenlock, however waking up (opening laptop) does not switch backlight on. Moreover I could control the backlight (i.e. switch it off), contrary to some reports above.

After downgrade to 14.04.4 (4.2 kernel; downgraded for plethora of more serious issues) this still occurs at least after boot and screenlock (I'm still copying my backup to hdd so I don't want to test waking up). I can still control it and switch it off (with Fn+rightarrow on my model).

Last kernel this worked for me properly was 3.19 (14.04.3 with vivid kernel I believe).