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

[Timur Kristóf]

On Fri, 2019-07-05 at 09:36 -0400, Alex Deucher wrote:
> On Thu, Jul 4, 2019 at 6:55 AM Michel Dänzer <michel at daenzer.net>
> wrote:
> > On 2019-07-03 1:04 p.m., Timur Kristóf wrote:
> > > > > There may be other factors, yes. I can't offer a good
> > > > > explanation
> > > > > on
> > > > > what exactly is happening, but it's pretty clear that amdgpu
> > > > > can't
> > > > > take
> > > > > full advantage of the TB3 link, so it seemed like a good idea
> > > > > to
> > > > > start
> > > > > investigating this first.
> > > > 
> > > > Yeah, actually it would be consistent with ~16-32 KB
> > > > granularity
> > > > transfers based on your measurements above, which is plausible.
> > > > So
> > > > making sure that the driver doesn't artificially limit the PCIe
> > > > bandwidth might indeed help.
> > > 
> > > Can you point me to the place where amdgpu decides the PCIe link
> > > speed?
> > > I'd like to try to tweak it a little bit to see if that helps at
> > > all.
> > 
> > I'm not sure offhand, Alex or anyone?
> 
> amdgpu_device_get_pcie_info() in amdgpu_device.c.


Hi Alex,

I took a look at amdgpu_device_get_pcie_info() and found that it uses
pcie_bandwidth_available to determine the capabilities of the PCIe
port. However, pcie_bandwidth_available gives you only the current
bandwidth as set by the PCIe link status register, not the maximum
capability.

I think something along these lines would fix it:
https://pastebin.com/LscEMKMc

It seems to me that the PCIe capabilities are only used in a few places
in the code, so this patch fixes pp_dpm_pcie. However it doesn't affect
the actual performance.

What do you think?

Best regards,
Tim