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

[Timur Kristóf]

Hi guys,

I use an AMD RX 570 in a Thunderbolt 3 external GPU box.
dmesg gives me the following message:
pci 0000:3a:00.0: 8.000 Gb/s available PCIe bandwidth, limited by 2.5 GT/s x4 link at 0000:04:04.0 (capable of 31.504 Gb/s with 8 GT/s x4 link)

Here is a tree view of the devices as well as the output of lspci -vvv:
https://pastebin.com/CSsS2akZ

The critical path of the device tree looks like this:

00:1c.4 Intel Corporation Sunrise Point-LP PCI Express Root Port #5 (rev f1)
03:00.0 Intel Corporation JHL6540 Thunderbolt 3 Bridge (C step) [Alpine Ridge 4C 2016] (rev 02)
04:04.0 Intel Corporation JHL6540 Thunderbolt 3 Bridge (C step) [Alpine Ridge 4C 2016] (rev 02)
3a:00.0 Intel Corporation DSL6540 Thunderbolt 3 Bridge [Alpine Ridge 4C 2015]
3b:01.0 Intel Corporation DSL6540 Thunderbolt 3 Bridge [Alpine Ridge 4C 2015]
3c:00.0 Advanced Micro Devices, Inc. [AMD/ATI] Ellesmere [Radeon RX 470/480/570/570X/580/580X/590] (rev ef)

Here is the weird part:

Accoding to lspci, all of these devices report in their LnkCap that
they support 8 GT/s, except the 04:04.0 and 3a:00.0 which say they only
support 2.5 GT/s. Contradictory to lspci, sysfs on the other hand says
that both of them are capable of 8 GT/s as well:
"/sys/bus/pci/devices/0000:04:04.0/max_link_speed" and
"/sys/bus/pci/devices/0000:3a:00.0/max_link_speed" are 8 GT/s.
It seems that there is a discrepancy between what lspci thinks and what
the devices are actually capable of.

Questions:

1. Why are there four bridge devices? 04:00.0, 04:01.0 and 04:02.0 look
superfluous to me and nothing is connected to them. It actually gives
me the feeling that the TB3 driver creates 4 devices with 2.5 GT/s
each, instead of one device that can do the full 8 GT/s.

2. Why are some of the bridge devices only capable of 2.5 GT/s
according to lspci?

3. Is it possible to manually set them to 8 GT/s?

Thanks in advance for your answers!

Best regards,
Tim