Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link state

["David E. Box" <david.e.box@linux.intel.com>]

On Tue, 2023-12-12 at 15:27 -0600, Bjorn Helgaas wrote:
> On Tue, Dec 12, 2023 at 11:48:27AM +0800, Kai-Heng Feng wrote:
> > On Fri, Dec 8, 2023 at 4:47 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > ...
> 
> > > I hope we can obsolete this whole idea someday.  Using pci_walk_bus()
> > > in qcom and vmd to enable ASPM is an ugly hack to work around this
> > > weird idea that "the OS isn't allowed to enable more ASPM states than
> > > the BIOS did because the BIOS might have left ASPM disabled because it
> > > knows about hardware issues."  More history at
> > > https://lore.kernel.org/linux-pci/20230615070421.1704133-1-kai.heng.feng@canonical.com/T/#u
> > > 
> > > I think we need to get to a point where Linux enables all supported
> > > ASPM features by default.  If we really think x86 BIOS assumes an
> > > implicit contract that the OS will never enable ASPM more
> > > aggressively, we might need some kind of arch quirk for that.
> > 
> > The reality is that PC ODM toggles ASPM to workaround hardware
> > defects, assuming that OS will honor what's set by the BIOS.
> > If ASPM gets enabled for all devices, many devices will break.
> 
> That's why I mentioned some kind of arch quirk.  Maybe we're forced to
> do that for x86, for instance.  But even that is a stop-gap.
> 
> The idea that the BIOS ASPM config is some kind of handoff protocol is
> really unsupportable.

To be clear, you are not talking about a situation where ACPI_FADT_NO_ASPM or
_OSC PCIe disallow OS ASPM control, right? Everyone agrees that this should be
honored? The question is what to do by default when the OS is not restricted by
these mechanisms?

Reading the mentioned thread, I too think that using the BIOS config as the
default would be the safest option, but only to avoid breaking systems, not
because of an implied contract between the BIOS and OS. However, enabling all
capable ASPM features is the ideal option. If the OS isn't limited by
ACPI_FADT_NO_ASPM or _OSC PCIe, then ASPM enabling is fully under its control.
If this doesn't work for some devices then they are broken and need a quirk.

David