Description of problem:
The backlight on my Dell XPS13 cannot be dimmed. I've tried various boot flags and BIOS setting with no luck. In every case xblacklight stops with "No displays have backlight property.

Version-Release number of selected component (if applicable):
3.15.6-200 kernel

How reproducible:
100% reproducible

Steps to Reproduce:
1. Boot with your choice of BIOS settings and kernel flags
2. Attempt to change the backlight with hotkeys or by echo 15 > sys/class/backlight/dell_backlight/brightness

Actual results:
Backlight remains unchaged

Expected results:

Backlight changes.

Additional info:

With regular boot flags:
    /sys/class/backlight is empty
    /sys/module/video/parameters/use_native_backlight is -1
        booting with video.use_native_backlight=1 did not change problem
    /sys/module/video/parameters/brightness_switch_enabled is n
        echo Y > there doesn't change anything
    pressing hotkeys does nothing

Booting with acpi_backlight=vendor populates /sys/class/backlight, and adding video.use_use_native_backlight=1 makes /sys/module/video/parameters/use_native_backlight 1. In either case the XFCE animations appear and /sys/class/backlight/dell_backlight/brightness changes when using hotkeys, but the backlight does not change.

I've tried this with both "Load Legacy Option Rom" enabled and disabled in BIOS. The behaviour doesn't change. There is another option, "Boot List Option", but this gives a "no operating system found", when I try to boot with it set to "Legacy".

I've read that this worked in previous kernel versions, so this seems like a regression. Thanks in advance for any help. Let me know if I need to give more information.