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

[Timur Kristóf]

Hi Mika,

Thanks for your quick reply.

> > 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.
> 
> Because it is standard PCIe switch with one upstream port and n
> downstream ports.

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.

Why I asked about it is because I have a suspicion that maybe the
bandwidth is allocated equally between the 4 downstream ports, even
though only one of them is used.

> 
> > 2. Why are some of the bridge devices only capable of 2.5 GT/s
> > according to lspci?
> 
> You need to talk to lspci maintainer.

Sorry if the question was unclear.
It's not only lspci, the kernel also prints a warning about it.

Like I said the device really is limited to 2.5 GT/s even though it
should be able to do 8 GT/s.

> 
> > 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.

(Side note: mesa 19.1 has a radeonsi patch that reduces the bandwidth
use, which does help. However it doesn't solve the underlying problem
of the slow TB3 interface.)

Best regards,
Tim