Dell has released A06 BIOS version for the XPS 13 (9333) model. The release notes (available here) are extremely light on detail, however I can tell you it does seem to resolve (at least so far for me, and others) the issue with unreliable suspend and resume!
It doesn’t seem to resolve the bizarre problem with delayed brightness key controls though.
For those of you tux lovers out there, the upgrade is simple, grab a good USB stick (the cheap ones tend to not boot correctly), install FreeDOS via unetbootin, download the BIOS image .exe file, copy it to your USB stick, boot the USB stick, and the execute the .exe, the BIOS installer will start, walk through the prompts and wait for it to reboot your system.
Well, sadly after a few days of testing, it would seem the bug still persists within the BIOS which results in lock up on shutdown. It took considerably longer to get there, however it’s still present.
So over the last few weeks I’ve been trying to sort out the final problems with my XPS 13. The last, and most annoying problem I’ve seen is the frequent and seemingly random lockups on suspend.
This bothers me a lot because it effectively means I can’t just close the lid on the laptop and toss it in a bag and go.
Based on what I’ve observed the issue seems to be solidly within the A05 BIOS. From my understanding, speaking with Dell Pro Support, this BIOS revision is beta, and only available to those users who have either received new systems after October 15th 2014, or had their motherboards replaced to address the coil whine problem.
The issue it self appears to relate closely to the power state the laptop is in, and the fact that the OS may change the power state when plugged in, to something not regonized or supported by the BIOS at the time.
So effectively you can suspend and resume reliably, so long as you don’t have the power plug, plugged in while the system is running. Of that, it seems to take some time (i.e. secondary power state change) for it to reliably trigger the hang when suspending into memory.
I can’t seem to determine much past that as the OS has handed off duties to the BIOS and it’s the BIOS which fails to bring the system down to sleep.
If you want to comment with Dell on this, I have an active thread here: