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

[Timur Kristóf]

On Fri, 2019-06-28 at 17:14 +0300, Mika Westerberg wrote:
> 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).

That's unfortunate, I would have expected there to be some sort of PCIe
speed test utility.

Now that I gave it a try, I can measure ~20 Gbit/sec when I run Gnome
Wayland on this system (which forces the eGPU to send the framebuffer
back and forth all the time - for two 4K monitors). But it still
doesn't give me 40 Gbit/sec.

> 
> > 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?

I had a brief discussion with Marek about this earlier, and he said
that this has to do with latency too, not just bandwidth, but he didn't
explain any further.