XPS 13 7390 power management (c-states) borked

Hi,

on my new XPS 13 7390 Developer Edition, there are significant power management issues.

Normally, when the system idles, the SoC package c-state gets as low as PC8 (or PC10 with the screen off). Power usage is as low as 1.6W with the screen on, and just 500 mW with the screen off. That's great!

However, often when I plug something into a USB/TB port and then remove it later again, something breaks and the SoC will only idle at up to PC3. That means more like 4-5W best case idle power usage. Ouch. It doesn't even matter what is plugged into the ports and which port I use - the power supply suffices to trigger this.

I can "fix" this by suspending the laptop and resuming again.

Since these kinds of things are typically at least in part managed by various bits of firmware, I assume there's something going wrong there and an update is needed. Do you have any suggestions how to proceed?