[Bug 218525] New: Thunderbolt eGPU bad performance

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218525

            Bug ID: 218525
           Summary: Thunderbolt eGPU bad performance
           Product: Drivers
           Version: 2.5
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: USB
          Assignee: drivers_usb@kernel-bugs.kernel.org
          Reporter: gipawu@gmail.com
        Regression: No

I opened this bug report because I think there is a performance problem in the
Thunderbolt stack.

I have an eGPU R43SG-TB3 adapter, which can be connected either via Thunderbolt
3/4 port, or directly via PCIe M.2 x4 link.

Using the eGPU connected via Thunderbolt, game performance is often absymal,
especially with games that use DXVK.

The problem is described at length here:
https://gitlab.freedesktop.org/drm/amd/-/issues/2885

but in summary, the main symptoms that indicate the presence of a performance
problem with Thunderbolt eGPU are:

- Low and bumpy fps, GPU clock speeds and GPU usage.
- Low or no difference when downscaling the resolution or reducing graphic
settings.

Using the direct M.2 connection instead, performance is as good as expected,
almost on the level of Windows.

In Windows there is a performance gap of 5-10% between Thunderbolt and M.2
connection, but in Linux the gap is much wider, sometimes more than 50%. Very
few games seem to be unaffected by the problem (e.g. Shadows of Tomb Raider and
Doom Eternal).

I tested with an Nvidia RTX 2060 Super GPU, paired with an Intel Nuc 13 Pro
(i5-1340p and Thunderbolt 4) and with a Dell XPS 9570 (i7-8750H and Thunderbolt
3), both running Fedora 39 and kernel 6.7.5. I tried kernel 6.8.0 rc5 too, but
with no differences.

Since the problem emerges with both Nvidia and AMD GPUs (as evident from the
linked report) and that the problem is not reproducible by connecting the GPU
directly via M.2 connection, bypassing the Thunderbolt port, I suspect there is
a performance issue in the Thunderbolt stack, perhaps in proper bandwidth
allocation.

I hope that information given here, and in the link given, may help to resolve
the problem.

I am of course available to provide additional information, logs, and test any
patches if needed. Thank you.

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