Suspend issues on a Dell XPS

[coypu%sdf.org@localhost]

Hi,

I have a problem with suspend. My understanding of suspend,

Write wake code to physical page 3
Set the waking vector to physical page 3
ACPI sleep preparation dance
suspend

resume (_WAK runs)
kernel starts from firmware wake code in real mode
... kernel code runs ...

I'm like 95% sure I never make it to the firmware wake code.
The laptop is stuck in some pre-resume state. The keyboard backlight
responds, but nothing else.

- To confirm this I attempted to replace the wake code with:
	.byte 0xcc
	ud2
	code to reset CMOS (from Ubuntu)

  Neither one made a difference, and I'm like 95% sure it never runs,
  but it might be "this code behaves different in real mode"

- "It must get clobbered", I tried to put it in page 17, which is where
  OpenBSD puts it (and successfully suspends). It didn't work.
- "It might be the TPM driver" - OpenBSD can suspend without its TPM
  driver.
- It might be another missing driver - tried the list of missing ones,
  no luck
- "lspci -vv" diff with OpenBSD shows mostly differences in MSI, not
  drivers. Disabling MSI/MSI-X entirely did not help.
- Trying non-multiprocessor did not help.
- Disabling SVS did not help. But it is comical to "boot -s3" to test S3 susend.
- It's dual graphics. Perhaps disabling either graphics driver helps -> No.

Currently I'm at a loss. Any suggestions are welcome!
I'm willing to try any idea 3 times, even suggestions I've already
tried.