Interesting PCIe x16 Issue

[trentzb]

Has anyone run into PCIe x16 link issues when using it as x1?

I have several of these ASUS M4A79XTD boards and just begun populating them with more than 2 ATI cards each and have run into an interesting issue.

I can successfully run 3 5770's in these boards, 1 in each of the actual x16 slots (onboard) and 1 via an x1 slot/extender (offboard). No problems here.

When I lift the same 5770s offboard and connect 2 of them to the same x16 slots via x1 extenders I lose the second x16 slot entirely. The first x16 slot closest to CPU functions just fine using an x1 extender however the second slot is dead. Nothing in lspci for that slot/card. As expected, dropping 1 5770 back into the second x16 slot and connecting another 5770 via an x1 extender to the first x16 slot works just fine.

I noted in the specs on these Asus boards that 2 GPUs in the x16 slots drop them both to x8 but I doubt that is related unless it is forced x8 link. I have tried several (4) different x1 extenders, all exhibit the same behavior.

I have some x16 extenders on the way which I suspect will resolve the issue but I am curious if anyone else has run into anything similar. Am I missing something obvious?

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[shivansps]

Not every motherboard supports operating VGAs at x1 speeds, its a bios issue, it may support it on all or some slots, or it may not support it at all.

I have seem this issue on a Asus M3N78-PRO, no VGA supported on any of those x1 slots. You may try with other bios versions to check.

[[Tycho]]

I have some x16 extenders on the way which I suspect will resolve the issue but I am curious if anyone else has run into anything similar. Am I missing something obvious?

May it be caused by card detection issue mentioned in mrb's blog ?
He had to short out two contacts in x1 extender.

[shivansps]

I have some x16 extenders on the way which I suspect will resolve the issue but I am curious if anyone else has run into anything similar. Am I missing something obvious?

May it be caused by card detection issue mentioned in mrb's blog ?
He had to short out two contacts in x1 extender.

thats it... i forgot his name... card detection, thats the problem with some mbs trying to runs vga cards at x1 in all or some slots.

[trentzb]

I don't have any problem with x1 slots. I can run 57xx/58xx in the 2 x1 slots just fine. I can also run 57xx/58xx in both x16 slots when on the board. I can't run anything in x16 slot 2 when using x1 extenders but x16 slot 1 works fine with x1 extenders.

Were you referring to running an x16 VGA in x1 slot or x16 slot via extender?

I don't have the presence detect issue mrb noted on his page.

[shivansps]

Asus may never have expected that someone will want to run vgas at x1 in x16 slots...
It should work, but it may not. Or it may work is some slots, and no in another, or not at all in some mbs.

In your case, a VGA card doest work on the 2nd pci-e x16 slot at x1, and it does on every other slot, its normal, is a bios bug and no OEM will give you support to fix that.

I had mbs that not even support running VGAs at x1 on the main slot. The PCI-E specification doesnt enforce full retrocompatibility all the way down to x1 to operate a VGA device, so it may not work, its OEM desicion to incluide it on the bios, or not.

High end mbs generally does support it.

OEMs generally use this to control how many vgas cards can be placed on a MB, lately i havent see this problem with any medium-high end mb, but now with PCI-E 3.0 close by, i should be start worrying about, because pci-e x1 3.0 is enoght to run crossfire for gaming, and OEMs may not like that users try to do that.

[marcus_of_augustus]

Maybe the linux device
/dev/vga_arbiter

is causing issues?