Re: [PATCH] PCI/ASPM: Enable L0s/L1 for removable devices when BIOS didn't configure ASPM

[Mario Limonciello]

On 5/5/26 16:42, Bjorn Helgaas wrote:
> [+cc Mika]
>
> On Tue, May 05, 2026 at 11:08:14AM -0500, Mario Limonciello wrote:
> > On 5/5/26 11:05, Bjorn Helgaas wrote:
> > > On Mon, May 04, 2026 at 05:52:46PM -0500, Mario Limonciello wrote:
> > > > When comparing lspci output between Windows and Linux for hotplugged
> > > > Thunderbolt 5 eGPU devices, Windows enables ASPM L1 but Linux doesn't:
> > > >
> > > >     Windows: LnkCtl: ASPM L1 Enabled
> > > >     Linux:   LnkCtl: ASPM Disabled
> > > >
> > > > This difference in ASPM configuration can cause behavioral differences
> > > > between the two operating systems for the same hardware.
> > >
> > > A tangent, not a comment on the patch itself, but what sort of
> > > behavioral differences are these?  If ASPM is working correctly, the
> > > only differences *should* be in power consumption and performance.
> >
> > This originally stemmed from a significant performance difference that was
> > observed between Windows and Linux with eGPUs.  The link in the patch points
> > at that bug if you want to look more closely at it.
>
> Hmm.  The bug (https://bugzilla.kernel.org/show_bug.cgi?id=221319)
> reports "instant reboot", which is definitely a behavioral difference.
> But AFAICS this patch would just fix something noticed along the way
> but not the reboot itself.
>
> To avoid confusion, I would use "performance difference" or "power
> difference" when describing this patch.

There is a lot of traffic in that bug and similar eGPU bugs; but some 
people have narrowed down that using NVIDIA's GSP "causes the instant 
reboot" but the performance difference is tangential to the reboot (or 
maybe it's part of the cause - I don't actually know).

The reboots /seem/ to be caused by sync floods which I originally 
hypothesized to be caused by Linux using AER and Windows not using it 
(potentially leading to a flood of errors in Linux), but turning off AER 
from kernel command line didn't change that.

> > I was hopeful that aligning ASPM would align the behavior, but alas this
> > didn't.
> >
> > It was still a difference that I figured we should discuss whether it should
> > be changed to be consistent.
>
> Definitely.  I hope we can at least enable L1.1.  L1.2 is a whole
> 'nother issue.

Yup.