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

[Timur Kristóf]

> > Sure, though in this case 3 of those downstream ports are not
> > exposed
> > by the hardware, so it's a bit surprising to see them there.
> 
> They lead to other peripherals on the TBT host router such as the TBT
> controller and xHCI. Also there are two downstream ports for
> extension
> from which you eGPU is using one.

If you look at the device tree from my first email, you can see that
both the GPU and the XHCI uses the same port: 04:04.0 - in fact I can
even remove the other 3 ports from the system without any consequences.

> > Like I said the device really is limited to 2.5 GT/s even though it
> > should be able to do 8 GT/s.
> 
> There is Thunderbolt link between the host router (your host system)
> and
> the eGPU box. That link is not limited to 2.5 GT/s so even if the
> slot
> claims it is PCI gen1 the actual bandwidth can be much higher because
> of
> the virtual link.

Not sure I understand correctly, are you saying that TB3 can do 40
Gbit/sec even though the kernel thinks it can only do 8 Gbit / sec?

I haven't found a good way to measure the maximum PCIe throughput
between the CPU and GPU, but I did take a look at AMD's sysfs interface
at /sys/class/drm/card1/device/pcie_bw which while running the
bottlenecked game. The highest throughput I saw there was only 2.43
Gbit /sec.

One more thought. I've also looked at
/sys/class/drm/card1/device/pp_dpm_pcie - which tells me that amdgpu
thinks it is running on a 2.5GT/s x8 link (as opposed to the expected 8
GT/s x4). Can this be a problem?

> 
> > > > 3. Is it possible to manually set them to 8 GT/s?
> > > 
> > > No idea.
> > > 
> > > Are you actually seeing some performance issue because of this or
> > > are
> > > you just curious?
> > 
> > Yes, I see a noticable performance hit: some games have very low
> > frame
> > rate while neither the CPU nor the GPU are fully utilized.
> 
> Is that problem in Linux only or do you see the same issue in Windows
> as
> well?


I admit I don't have Windows on this computer now and it has been some
time since I last tried it, but when I did, I didn't see this problem.

Best regards,
Tim