And I’m back.
Now, if we go into the Asus BIOS, under “Advanced Settings”, there’s an “AMD PBS” entry. And in there, you’ll find a single PCIe redriver setting that is set by default to “auto”.
It’s quite misleading. You’d think this means that the BIOS is somewhat capable of adapting redriver settings to the hardware you plug in. I have seen no evidence of that, and furthermore I cannot conceive of how the BIOS would even be able to do that. The PCIe negotiation protocols are internal to the PCIe subsystem hardware, as far as I know it’s not something you can look into from software running in the CPU cores.
Let’s switch to “manual”. As my interest is in slot 7, let’s look at what we can configure now :
Bingo ! We have access to the FIR filter coefficients, the amplifier gain and the output voltage amplitude settings. We can even disable the redrivers, and we have separate settings for the TX and RX paths.
As a test, I booted my machine with those fixed settings (and GPU set to PCIe 3.0 and 4.0). It behaved exactly the same : rock stable at 3.0, very crappy at 4.0, which to me is confirmation that indeed there is no magic happening when you use the default “auto” setting. In fact, if you look at the settings in manual mode you’ll find that they are “middle of the road”, as I suspected earlier :
And if you’re wondering, those settings aren’t coming from nowhere, these are the redrivers’ presets. Here’s an excerpt from the datasheet :
What can we conclude ? That Asus’ “auto” settings for the redrivers should actually be called “default” settings. They just couldn’t make any assumption as to what we’d put in those PCIe slots, so they went for something beige. Although the user manual does make it a point to tell you how many GPU you could use and which slots to put them in, so they could have made some assumptions there.
At least, they did write a BIOS that gives us full control, so we may have some room to tailor our PCIe lanes’ performance to what we’re trying to accomplish.
Fair warning, though : it’s not going to be easy nor trivial. This isn’t just a matter of cranking everything up to eleven. And to be honest and dispel any illusion I might have given, I’m not a PCIe expert, I’m just a PCIe tourist. I know what I’m doing, but I don’t know if what I want to do is doable. FFS the PCIe base spec alone is 1300 pages ! 
For now I’m gonna eat dinner and then start studying the redrivers in depth. And maybe give a chance to any PCIe experts around here to come upon this thread and save us all from my madness 