I use BIOS f8 and the BIOS settings suggested by Vosk, and the M2 adapter i use works.
PCI numbering counting from the CPU side
M2-adapter slot: 10
1st x1: 4
1st x16: 1
2nd x1: 3
2nd x16: 9
3rd x1: 8
3rd x16: 5
But 7 is not enough, so I have played with these "splitter cards" (1 PCI-e to 4 USB connections).
These seem to take 6 PCI-e addresses and shift the numbers of the later slots by 5.
I have added two of these splitters and placed them in the 1st x16 and the 2nd x1 (the ones with the lowest initial numbers). the USB connections is compatible with the v008 risers at least.
This resulted in cards on PCI 3,4,5,6 and 10,11,12,13. And all eight cards are mining happily.
The rest of the slots got shifted from 10,4,1,3,9,8,5 to 20,14,3-6,10-13,19,18,15
I don't have risers and 6pin connectors for more cards right now but 4 more cards to test soon.
It would be nice to know if 2x splitters will make this a 13 GPU motherboard for real (and not only in theory), an alternative when more special motherboards are scarce, but it adds more sketchy components to go bad...
I'm running simpleminer if that matters when it comes to card recognition.
The PCI numbering can be important since it makes it possible to route cables so you have a clue about which card is "GPU3".
Edit:Tested with more risers and a par more GPUs.
Whatever I tried I could not get it to post with 10 GPUs, maximum seems to be 9. with/without the m2 adapter, tried to turn off all I could think off in the BIOS (but too little knowledge to really know what I was doing).
# zm 0.6
# GPU0 + GeForce GTX 1060 3GB MB: 3013 PCI: 3:0
# GPU1 + GeForce GTX 1060 3GB MB: 3013 PCI: 4:0
# GPU2 + GeForce GTX 1060 3GB MB: 3013 PCI: 5:0
# GPU3 + GeForce GTX 1060 3GB MB: 3013 PCI: 6:0
# GPU4 + GeForce GTX 1060 3GB MB: 3013 PCI: 10:0
# GPU5 + GeForce GTX 1060 3GB MB: 3013 PCI: 11:0
# GPU6 + GeForce GTX 1060 3GB MB: 3013 PCI: 12:0
# GPU7 + GeForce GTX 1060 3GB MB: 3013 PCI: 13:0
# GPU8 + GeForce GTX 1060 3GB MB: 3013 PCI: 14:0
Edit2:
After thinking a bit I thought the splitters was stealing 2 PCI-lanes as it was shifting the PCI address +5 and not +3, and by using two I lost from 13 available down to 9.
So I removed one splitter and used only the normal slots and the m2. Still only max 9 GPUs.
So I removed the m2 and settled fro one 1-4 splitter and the other 5 slots for a total of 9 GPUs (addressed _,9,3-6,8,14,13,10).
If anyone have some info on the ability to disable stuff to be able to add one extra GPU it would be nice to know. Or is it because I'm cheap and use the smallest possible Celeron with only 16 PCI-lanes?