Created attachment 138431 [details]
Collected backlight debug data

3.14.5 breaks the backlight on a DELL XPS 13 (L322X).
The screen goes dark the moment the Kernel initializes.
The brightness FN keys don't have any effect.
A 'cat max_brightness > brightness' either.

The reason is very likely:
https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/commit/?h=linux-next&id=1811fcb029fa3eca5bd1d25eab386b0c4b80fb93

As outlined in http://hansdegoede.livejournal.com/13889.html some debug information:

DMI data:
bios_date: 08/28/2013
bios_vendor: Dell Inc.
bios_version: A10
board_asset_tag:
board_name:
board_serial: .7CKFVW1.              .
board_vendor: Dell Inc.
board_version:
chassis_asset_tag:
chassis_serial: 7CKFVW1
chassis_type: 8
chassis_vendor: Dell Inc.
chassis_version: 0.1
product_name: XPS L322X
product_serial: 7CKFVW1
product_uuid: 44454C4C-4300-104B-8046-B7C04F565731
product_version:
sys_vendor: Dell Inc.

System is booted in UEFI-mode.

I booted with each of those options:
video.use_native_backlight=0
video.use_native_backlight=1
acpi_backlight=vendor
acpi_osi="!Windows 2012"
acpi_osi="!Windows 2009"

For each boot, I ran this hacked up script (resulting output is attached as XZ):
#!/bin/sh
DIR="$1"
mkdir ${DIR}
ls -l /sys/class/backlight/* > ${DIR}/backlights
for backlight in /sys/class/backlight/*
do
  cat ${backlight}/brightness > ${DIR}/$(basename ${backlight})_brightness.default
  cat ${backlight}/max_brightness > ${DIR}/$(basename ${backlight})_max_brightness.default
done
echo "Please lower brightness via keyboard now, then press <ENTER>"
read something
for backlight in /sys/class/backlight/*
do
  cat ${backlight}/brightness > ${DIR}/$(basename ${backlight})_brightness.low
  cat ${backlight}/max_brightness > ${DIR}/$(basename ${backlight})_max_brightness.low
done
echo "Please increase brightness via keyboard now, then press <ENTER>"
read something
for backlight in /sys/class/backlight/*
do
  cat ${backlight}/brightness > ${DIR}/$(basename ${backlight})_brightness.high
  cat ${backlight}/max_brightness > ${DIR}/$(basename ${backlight})_max_brightness.high
done
cat /proc/cmdline > ${DIR}/cmdline
dmesg > ${DIR}/dmesg.log
grep '.*' /sys/class/dmi/id/*_* 2> /dev/null > ${DIR}/dmi.log