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

[Timur Kristóf]

On Mon, 2019-07-01 at 16:54 +0200, Michel Dänzer wrote:
> On 2019-06-28 2:21 p.m., Timur Kristóf wrote:
> > I haven't found a good way to measure the maximum PCIe throughput
> > between the CPU and GPU,
> 
> amdgpu.benchmark=3
> 
> on the kernel command line will measure throughput for various
> transfer
> sizes during driver initialization.

Thanks, I will definitely try that.
Is this the only way to do this, or is there a way to benchmark it
after it already booted?

> > 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.
> 
> PCIe bandwidth generally isn't a bottleneck for games, since they
> don't
> constantly transfer large data volumes across PCIe, but store them in
> the GPU's local VRAM, which is connected at much higher bandwidth.

There are reasons why I think the problem is the bandwidth:
1. The same issues don't happen when the GPU is not used with a TB3
enclosure.
2. In case of radeonsi, the problem was mitigated once Marek's SDMA
patch was merged, which hugely reduces the PCIe bandwidth use.
3. In less optimized cases (for example D9VK), the problem is still
very noticable.

Best regards,
Tim