MP 1,1-5,1 cMP 5,1 - diagnose power-on-self-test OR no POST with Nvidia M40 Tesla

[dizco]

good morning y'all,

as the title suggests, I try to run an Nvidia Tesla M40 GPU accelerator in my cMP 5,1. The card is not supposed to work in macOS, but should work in Linux, after the boot issue is fixed.

The main issue: With the M40 card installed, the MacPro does not POST at all. No "chime".

This is what I have and what I tried so far:

• cMP 5,1 with 144.0.0.0.0 vanilla firmware
  • minimal setup, 1 RAM stick, 1 HDD, boots High Sierra, OC installed on main disk in DEBUG mode
• PCIe slot 1: GT120 GPU (alternative: Radeon HD 6770)
• PCIe slot 2: Nvidia Tesla M40, Pixlas-Mod applied with 8-pin CPU power connector
  • M40 with vanilla vBIOS
  • M40 with updated GOP part in vBIOS via GOPupd
  • M40 with w/o the GOP part (see this German thread)
  • PCIe pins 5&6 for SMBus "disabled" as shown in this video

Any combination of the variants above will result in the same behavior:

• MacPro turns on, diagnose LED "SYS PG" comes on, stays on all the time
• diagnose LEDs "GPU OK" and "EFI DONE" light up shortly
• 10 seconds, disk seeking, quiet
• diagnose LEDs "GPU OK" and "EFI DONE" light up shortly

After this sequence, nothing happens. No boot loop, no chime. Without the M40 card, the chime will go off and system starts booting the EFI part from disk.
Unfortunately, even if the failing POST gets recorded, I can not access the result. The next time I boot the system w/o the M40, the POST result is "passed" in system information diagnostic.

I dumped the MacPro bootrom and also had a look at the bootrom templates from this thread: Guide: How to rebuild/update Mac Pro 4.1/5.1 bootrom with template files.

Is there a way to diagnose the power-on-self-test part of the internal bootrom, so I get an idea what's blocking the boot process? Any custom DXE driver, like EnableGop, which will record the POST failure, or ignore the M40 card during POST?