Why is Thunderbolt 3 limited to 2.5 GT/s on Linux?

[Mika Westerberg]

On Fri, Jun 28, 2019 at 03:33:56PM +0200, Timur Kristóf wrote:
> I have two more questions:
> 
> 1. What is the best way to test that the virtual link is indeed capable
> of 40 Gbit / sec? So far I've been unable to figure out how to measure
> its maximum throughput.

I don't think there is any good way to test it but the Thunderbolt gen 3
link is pretty much always 40 Gb/s (20 Gb/s x 2) from which the
bandwidth is shared dynamically between different tunnels (virtual links).

> 2. Why is it that the game can only utilize as much as 2.5 Gbit / sec
> when it gets bottlenecked? The same problem is not present on a desktop
> computer with a "normal" PCIe port.

This is outside of my knowledge, sorry. How that game even knows it can
"utilize" only 2.5 Gbit/s. Does it go over the output of "lspci" as well? :-)

The PCIe links itself should to get you the 8 GT/s x 4 and I'm quite
sure the underlying TBT link works fine as well so my guess is that the
issue lies somewhere else but where, I have no idea.

Maybe the problem is in the game itself?