Another person asking that PCIe 5.0 @ PCIe 4.0 x8 question
Posted by munkimatt@reddit | buildapc | View on Reddit | 16 comments
Hi all
First of all, I apologise for asking a question that I know has been asked countless times, but I'm several hours into reading threads on it and I'm absolutely none the wiser.
I've recently picked up a 5080 and popped it into my machine. GPU-Z is reporting the bus interface as PCIe x 16 5.0 @ PCIe 4.0 x8 even under load. I (vaguely) understand the concept of shared lanes, so I've tried stripping out various innards of the PC, such as a seperate 2.5gb ethernet card and the second M.2, but that doesn't seem to make a difference.
Specs as follows;
5800X3D
X570 Crosshair VIII Hero
2 x 4.0 M.2 drives
2 x SATA drives
The GPU is in the top PCIe slot. The M.2s populate both slots, and when I removed one I took the bottom M.2 out because the top one is the boot drive.
I've also tried a 9070XT card in the same slot (after DDUing and everything), GPU-Z is reporting the same.
I've looked in the BIOS and the PCIe birfucation mode is set to Auto, which the toolbox says should come back at x16.
Just hopeing someone can point me in the right direction...and preferably in ELI5 terms because I'm apparently struggling with this!
Narrow-Prompt-4626@reddit
I don't see any mention of a configuration in your manual that would set pcie lanes in the SATA buses to be shared unless you have multiple pcie devices. Is your BIOS up to date? Can you explicitly set the bifurcation mode to x16? Your gpu is in the top-most slot right, not the middle one?
Page 24 of the pdf or 1-8 of the physical
munkimatt@reddit (OP)
Yes, I shouldn't have mentioned that the BIOS is up to date.
I can't see any options to specifically set it to x16. The only two options under the setting are Auto or RAID. Unless there's another setting that I've not found!
Narrow-Prompt-4626@reddit
Something just clicked for me. You are almost certainly setting the bifurcation mode for lanes going to your m2 drives, not your display driver. The raid option does not make sense here. It should give choices for x16, x8/x8, or x4/x4/x4/x4
munkimatt@reddit (OP)
That's...a very good point.
I reset the bios back the defaults last night, which made no difference. Let me check again tonight for anything that suggests it's more for the GPU.
Narrow-Prompt-4626@reddit
Prayge
munkimatt@reddit (OP)
No luck. I'm like 99.999% sure it is the GPU slots. The Raid mode seems to be used if you use a Hyper M.2 x16 card in one of them. Photo from BiOS
Narrow-Prompt-4626@reddit
I'm with you. It has to be poor contact
munkimatt@reddit (OP)
Yeah, I think so. I've tried a different CPU and GPU in there with no change, so I'm blaming the mobo. Contacts look fine and I've blown it with compressed air with no change, so I guess there's something properly broken there.
Realistically, how much performances am I losing here?
Narrow-Prompt-4626@reddit
Can you double check the slot? It looks like that's the only other limiting factor. There should be two below the slot you have it in since there are three
munkimatt@reddit (OP)
Yep, 100% certainly in the top slot.
Narrow-Prompt-4626@reddit
The only thing left I can think of is check for debris in the slot. Bad contact on specific contact surfaces can cause this to happen
reckless150681@reddit
Did you try explicitly disabling bifurcation?
munkimatt@reddit (OP)
I've got two options in the BIOS, either Auto or Raid mode. Couldn't see a disabled option!
Both PCIe 1 and 2 have the same options, but 1 says Auto sets it to x16 and 2 says Auto sets it at x8.
ViruliferousBadger@reddit
1 should always be 16x unless you put anything in slot 2, then both are 8x.
This is a x570/CPU limitation, IIRC.
So make sure you don't have anything in slot 2, no extra SATA cards, etc.
munkimatt@reddit (OP)
Definitely nothing in any of the other slots, none of the full sized ones of the small one.
Dorfdad@reddit
I’ve had this exact same issue on different motherboards. I had to open chat gpt and type in my exact motherboard make and model and asked it to help me configure bios so my pci ports allow both my 5090 and my nvme 5.0 both worked at full speeds. Some motherboard will allow just 2 paths to full pci speeds and some seem to limit or share based on the board price etc.
You can use crystal disk and mygpu apps to check if your getting full speeds on those ports!
Took me like 3-4 bios settings to tweak it properly but I finally got it all working at full speeds! Hope you get it working and ask chat gpt it really is good as a manual replacement!!