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

[Timur Kristóf]

On Wed, 2019-07-03 at 14:44 -0400, Marek Olšák wrote:
> You can run:
> AMD_DEBUG=testdmaperf glxgears
> 
> It tests transfer sizes of up to 128 MB, and it tests ~60 slightly
> different methods of transfering data.
> 
> Marek


Thanks Marek, I didn't know about that option.
Tried it, here is the output: https://pastebin.com/raw/9SAAbbAA

I'm not quite sure how to interpret the numbers, they are inconsistent
with the results from both pcie_bw and amdgpu.benchmark, for example
GTT->VRAM at a 128 KB is around 1400 MB/s (I assume that is megabytes /
sec, right?).

It is also weird that unlike amdgpu.benchmark, the larger than 128 KB
transfers didn't actually get slightly slower.

Michel, can you make sense of this?

Best regards,
Tim