Re: [PATCH V3 1/2] PCI/ASPM: Add host-bridge API to override default ASPM/CLKPM link state

[Bjorn Helgaas <helgaas@kernel.org>]

On Sun, Aug 31, 2025 at 06:28:53PM +0530, Manivannan Sadhasivam wrote:
> On Thu, Aug 28, 2025 at 03:43:45PM GMT, Bjorn Helgaas wrote:
> > On Mon, Aug 25, 2025 at 01:35:22PM -0700, David E. Box wrote:
> > > Synthetic PCIe hierarchies, such as those created by Intel VMD, are not
> > > enumerated by firmware and do not receive BIOS-provided ASPM or CLKPM
> > > defaults. Devices in such domains may therefore run without the intended
> > > power management.
> > > 
> > > Add a host-bridge mechanism that lets controller drivers supply their own
> > > defaults. A new aspm_default_link_state field in struct pci_host_bridge is
> > > set via pci_host_set_default_pcie_link_state(). During link initialization,
> > > if this field is non-zero, ASPM and CLKPM defaults come from it instead of
> > > BIOS.
> > > 
> > > This enables drivers like VMD to align link power management with platform
> > > expectations and avoids embedding controller-specific quirks in ASPM core
> > > logic.
> > 
> > I think this kind of sidesteps the real issue.  Drivers for host
> > controllers or PCI devices should tell us about *broken* things, but
> > not about things advertised by the hardware and available for use.
> > 
> > The only documented policy controls I'm aware of for ASPM are:
> > 
> >   - FADT "PCIe ASPM Controls" bit ("if set, OS must not enable ASPM
> >     control on this platform")
> > 
> >   - _OSC negotiation for control of the PCIe Capability (OS is only
> >     allowed to write PCI_EXP_LNKCTL if platform has granted control to
> >     the OS)
> > 
> > I think what we *should* be doing is enabling ASPM when it's
> > advertised, subject to those platform policy controls and user choices
> > like CONFIG_PCIEASPM_PERFORMANCE/POWERSAVE/etc and sysfs attributes.
> > 
> > So basically I think link->aspm_default should be PCIE_LINK_STATE_ALL
> > without drivers doing anything at all.  Maybe we have to carve out
> > exceptions, e.g., "VMD hierarchies are exempt from _OSC," or "devices
> > on x86 systems before 2026 can't enable more ASPM than BIOS did," or
> > whatever.  Is there any baby step we can make in that direction?
> 
> I'm not sure about the ACPI world, but for devicetree platforms,
> BIOS or the bootloader won't configure ASPM for the devices
> (mostly). So the baby step would be to set PCIE_LINK_STATE_ALL for
> all devicetree platforms :)

Yes.  How likely would this be to break something?

Before doing that, I think we need to add some logging, at least at
pci_dbg(), of what is already enabled and what we change, so we have
some kind of hint when things do break.