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

[Bjorn Helgaas <helgaas@kernel.org>]

[+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.

> 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.