[fixed-upstream] Kernel Input: xpad - correctly handle concurrent LED and FF requests

[Matt Sturgeon]

There is a bug in the kernel's xpad driver that causes a kernel panic when playing games that use force feedback (rumble/vibration) or LED signals.

This bug has already been fixed upstream and should hopefully feature in Linux 4.5: https://github.com/torvalds/linux/commit/7fc595f4c02636eadaeeecfe7bbc45b57c173004

This fix (and a predecessor) have been tested by several members of the Steam community:
http://steamcommunity.com/app/233130/discussions/0/611702631210587760
http://steamcommunity.com/app/255220/discussions/0/458604254425520518

I feel this fix should be included in the Ubuntu spin of the kernel for as far back as practical.

This can be reproduced with the following shell script if you have a controller plugged in:

#!/bin/bash

echo "This will flash your Controller's LEDs to try and crash the system"
echo "If you have a fixed xpad installed it will keep going forever"
echo "If the bug is present, your system should freeze up soon"
echo ""
echo "Press Ctrl+C on your keyboard at any time to end this test."

while true; do
        for i in $(seq 0 5); do
                echo $i > /sys/class/leds/xpad0/subsystem/xpad0/brightness
        done
done