Sleep mode in Surface Pro 4 and Debian 11

[nmanca <nmanca@sdf.org>]

Dear list,

I installed a debian 11 system on a Surface Pro 4 following the instruction from 
the https://github.com/linux-surface/linux-surface, I'm now thus using the 
"-surface" kernel version.

Everything work quite ok, but I'm having problems with the suspend.

If the Surface is on battery power, suspend works.
If it's on battery power, it does not wake up opening the cover or pressing the 
power button. I have to force a hard shutdown by pressing the power button.

I don't know if the problem is "real" or maybe is just the screen that does not 
recover. However, the diffrence between power plugged/unplugged is strange and I 
don't know how to debug it. I inspected the dmesg, the only difference is that 
when it is on battery, the last message after the failing wake up is:

PM: suspend entry (s2idle)

This is absent if on battery, when the wake up works. the rest is the same, 
apparently.

Any idea?

Thanks,
Nicola