Gigabyte AX370-Gaming 1.0, enabled "Above 4G Decode", RealTek onboard GigE NIC fails?

[VirtualLarry]

Has this happened to anyone else?

I had, in Device Manager, PCI-E Downstream Switch Port (Code 12, no resources available), yellow triangle. And my RealTek PCI-E GigE Network Controller wasn't showing up any more, and the "Ethernet" graph in Task Manager was missing.

I remembered that I had recently changed "Above 4G Decode" in the UEFI to Enabled.

I rebooted, changed it back, and the NIC was back.

Strange.

 

[RLGL]

Interrresting.

 

[VirtualLarry]

The thing is, the problem didn't immediately appear once I set the option. Only after some USB devices re-detected / enumerated or something, then I guess the PCI port resources didn't re-balance correctly. I'm not on the newest BIOS/UEFI, I've stuck to F21 for now, as I don't plan on running APUs on that board.

But it sure seems curious.

My understanding of what the "Above 4G Decoding" means, is that PCI resources (including PCI-E), originally were 32-bit address sizes, so they clustered the PCI hardware towards the top of the 4GB address range (much like hardware resources for the ISA bus, were sandwiched between 640KiB of base RAM, and the top of the 1MiB address range).

But when PCI-E and 64-bit address support came around, and more than 4GB of RAM, then they added the option to set those resources above the 4GiB mark, which I guess would mean after the entire block of system RAM, which is 16GiB in this system.

So, either the PCI-E RealTek LAN chip can't handle the address that it was assigned to, or otherwise, the PCI-E bridge (?) / Switch Port, couldn't find an address range for a resource assignment.

These are the really stupid and annoying things that keep Platform System Designers up at night, I'm sure.