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

[Timur Kristóf]

On Friday, 5 July 2019, Marek Olšák wrote:
> On Fri, Jul 5, 2019 at 5:27 AM Timur Kristóf <timur.kristof at gmail.com>
> wrote:
> 
> > 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?).
> >
> 
> Based on the SDMA results, you have 2.4 GB/s. For 128KB, it's 2.2 GB/s for
> GTT->VRAM copies.
> 
> Marek

That's interesting, AFAIU that would be 17.6 Gbit/sec. But how can that be so much faster than the 5 Gbit/sec result from amdgpu.benchmark?