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

[Mika Westerberg]

On Fri, Jun 28, 2019 at 04:53:02PM +0200, Timur Kristóf wrote:
> 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.

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.