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

[Timur Kristóf]

> > 
> > 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.
> 
> How do you measure that? Is there a DP stream also? As I said the
> bandwidth is dynamically shared between the consumers so you probably
> do
> not get the full bandwidth for PCIe only because it needs to reserve
> something for possible DP streams and so on.

I'm measuring it using AMD's pcie_bw sysfs interface which shows how
many packets were sent and received by the GPU, and the max packet
size. So it's not an exact measurement but a good estimate.

AFAIK there is no DP stream. Only the eGPU is connected to the TB3 port
and nothing else. The graphics card inside the TB3 enclosure does have
a DP connector which is in use, but I assume that's not what you mean.

It also doesn't seem to make a difference whether or not anything is
plugged into the USB ports provided by the eGPU. (Some online posts
suggest that not using those ports would allow higher throughput to the
eGPU, but I don't see that it would make any difference here.)