Does anyone have experience with passive pci-e risers/splitters on their motherboards? Gigabyte x399 motherboards look like some alpha workstation motherboards because of perfect pci-e and m.2 location and standard ATX size but lack of pci-e bifurcation info makes me seriously concerned. From what I see workstation wise there’s basically Asus x399 prime and Gigabyte x399 Designare if you want a lot of pci-e. And Designare seems to be overall more flexible board apart from pci-e bifurcation controversy because it doesn’t waste lanes for that weird ultra niche U.2 which doesn’t seem to be convertible to raw pci-e x4. At least I didn’t find adapters.
I got response from official Gigabyte support - their x399 boards do not support pci-e bifurcation and there are no plans to implement it.
I have the Gigabyte X399 Designare EX and with BIOS F2B there is bifurcation of 4x/4/x4/x4 I haven’t checked in a while though, because that BIOS made my system unstable for some reason so I had to downgrade again.
I will however check the new BIOS that came out a few days ago(F2D).
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Please also mention which pci-e slots support bifurcation.
I will add it when I can get around to testing the new BIOS.
After updating my BIOS and that crashing Windows + losing all my data because of encryption and having downgraded the BIOS again, I now have some data.
Gigabyte X399 Designare EX BIOS F2d has bifurcation on both x16 slots with x4/x4/x4/x4.
Windows is annoying >.<"
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I cna confirm the x16 split to x4/x4/x4/x4 works fine with e.g. the asrock quad m.2 card. I am using some break out cables for um… testing something… and those also work fine.
Interestingly I saw a bios option on the x470 CH7 to do x4x4x4x4 on the x16 slot… which would be a hell of a “server” platform for x470
Now I’m interested to know what those are o.o
Are there cables that can split a x16 slot to 4x4?
no, but you can turn the asrock m.2 ultra quad m.2 slots back into pcie x4 slots 
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I asked ASRock support whether it’d be possible to add pci-e split on first x16 slot because afaik it only works on second one. But haven’t got answer yet. Because currently ASRock only supports second x16 right?
EDIT: Now it does support on both pci-e slots
There are M.2 -> pci-e x4 adapters that can be used with those m.2 cards from ASUS and ASRock. Imho Asus is more suitable due to more conventional parallel placement of M.2. Look for powered ones and ones that indeed are x4. There’s a lot of scams that have lanes connected only for pci-e x1 but still x4 physical connectors