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

[Timur Kristóf]

> > > > 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?
> 
> We limit the speed of the link the the driver to the max speed of any
> upstream links.  So if there are any links upstream limited to 2.5
> GT/s, it doesn't make sense to clock the local link up to faster
> speeds.
> 
> Alex

Hi Alex,

I have two concerns about it:

1. Why does amdgpu think that the link has 8 lanes, when it only has 4?

2. As far as I understood what Mika said, there isn't really a 2.5 GT/s
limitation there, since the virtual link should be running at 40 Gb/s
regardless of the reported speed of that device. Would it be possible
to run the AMD GPU at 8 GT/s in this case?

Best regards,
Tim