PCI lane sharing on X370 Gaming K4

Nobody? Something "official" would be great :D 

Thanks a lot! You don't happen to remember how that option was called?

Radical_53 wrote: Hi! I started using this board some days ago, first with bios 1.50, now with 2.10. Right now, my issue is that my graphics card only runs at PCIe 8x instead of 16x. It's sitting in the second PCIe slot, the first PCIe 16x slot, with no other cards installed apart from a SSD in M2-1. I'm also using 4 SATA devices. Now, from what I get out of the manual, only the second PCIe 16x slot should lower its speed to 8x if it's being used with some "smaller" CPU. I'm using a Ryzen 1700X though. I tried various things like re-seating the card, clearing/defaulting the bios settings, making sure Windows had all the drivers it needs. No change. Any ideas? The only thing left would be either a hardware defect of the board or the graphics card (which worked fine on my previous X99 board). Thanks in advance!

What is telling you that your video card is running in x8 mode?

The first PCIe 3.0 x16 physical slot (PCIE2) is x16 electrically, meaning 16 PCIe lanes connected to it.

The second PCIe 3.0 x16 physical slot (PCIE4) is x8 electrically at all times, and will only have 8 PCIe lanes connected to it. It cannot work at x16 electrically.

My X370 Killer SLI/ac board does not have an option to configure the number of electrical lanes connected to the PCIE2 or PCIE4 slots. The PCIe lane allocation is the same as your board's.

There really is not any reason to have a UEFI/BIOS option for configuring the number of PCIe lanes active on the PCIE2 slot on our boards, since they are either x16 with one card, or x8 with two. Also, why would the default setting of such an option be x8 for the PCIE2 slot? The PCIE4 slot being x8 electrically by design further makes such an option useless.

The X370 Taichi board also has the same PCIe 3.0 electrical lane allocation for the PCIE2 and PCIE3 slots (different numbering on the Taichi), but includes a PCIe 2.0 x16 physical, x4 electrical slot. I'm skeptical that the X370 Taichi board's UEFI has one or more options to configure the electrical lane allocation, as again the hard wiring of the electrical lanes to each slot makes those options worthless. The manuals for these boards are already out of date compared to the latest UEFI versions, so checking them is no help.

There have been in the past options for setting the PCIe generation support, such as 3.0 and 2.0, but that is independent of lane allocation.

If you feel certain that your video card is running in x8 mode, then given the things you've done to fix it, it's either the card or the board. Testing with a different video card would be good if possible.

ho hum
so in responce (again) please find below exactly what i described in my answer
the taichi DOES IN FACT HAVE THE BIOS SWITCH
2nd set of bios pictures  down the linked page
2nd picture in the set
clearly showing the switch for 1x16 or 2x8 and below it auto
and what i thought i quite clearly said was a suggestion to check this is not causing the problem by being set to 2x8
why it may be set to that we dont know
we do know that a lot of am4 board were being sent back to dealers, maybe the board was a returned one resold by a dealer who knows ?
what i do know is that before writing off the board as defective, is it that much of an issue to just go and check ?
skeptical you may be parsec but once again i have showed that i dont just invent things !
heres the link of my imaginary bios setting

http://www.tomshardware.co.uk/asrock-x370-taichi-amd-ryzen-am4-atx-motherboard,review-33826-2.html

Radical_53 wrote: Well, so far any program I tried reported 8x to me: GPU-Z 1.20, HWInfo64 5.51-3141 for example. What you're saying was how I understood it, so either the board is defective, thinks that there's a second card installed in PCIe 4 or the card itself is faulty.  I already ordered a different graphics card to test this.  @datonyb: If you happen to find it, please let me know. I'll check my bios again, thanks.

I can tell you that my X370 Killer SLI/ac board with a GTX 1060 is shown as PCIe 3.0 x16 in HWiNFO64. So something is clearly not right.

Please try the various settings of the PCIe x16 switch option. We can't tell what the other settings besides Auto are yet, so I cannot suggest which one should be used. I have not seen another post about this issue with the X370 Taichi board yet. Apparently datonyb does not have this problem with his RX 480, but he did not say either way.

In general we have occasionally seen issues with x8 instead of x16 connections with some video cards with different boards. Sometimes a UEFI/BIOS clear is the fix, but you've tried that already. When the situation is not one of limited PCIe lanes,  PCIe slots not being x16 electrically, or a UEFI setting, the cause and fixes tend to be mysterious, which is why this can be a difficult issue to deal with.