Hi Mario,
On Tue, Aug 01, 2023 at 10:17:11PM -0500, Mario Limonciello wrote:
> > Consequently, platform_pci_bridge_d3() will return false and the only
> > thing that may allow the port to go into D0 is the dmi_get_bios_year()
> > check at the end of pci_bridge_d3_possible().
> >
> > However, that was added, because there are Intel platforms on which
> > Root Ports need to be programmed into D3hot on suspend (which allows
> > the whole platform to reduce power significantly) and there are no
> > ACPI device power management objects associated with them (Mika should
> > know the gory details related to this). It looks like under Windows
> > the additional power reduction would not be possible on those systems,
> > but that would be a problem, wouldn't it?
> >
>
> I've been thinking on this today, and I at least have a hypothesis about
> this behavior. Perhaps Windows is actually utilizing enabled PEP
> constraints to enforce what state device should be put into over Modern
> Standby cycles in the absence of ACPI objects.
>
> In the case of one of my problematic system the PEP constraints for the root
> port are:
>
> Package (0x04)
> {
> 0x00,
> "\\_SB.PCI0.GP17",
> 0x00,
> 0x00
> },
>
> That first 0x00 means the constraint isn't actually enabled for the root
> port.
>
> Mika,
>
> Could you get an acpidump from one of these problematic Intel systems so we
> can check the PEP constraints to see if this theory works? Or maybe you have
> some other ideas why this is different?
The patch adding this was merged in 2016 and unfortunately I don't have
any of the ACPI dumps from them available anymore (and do not recall the
details either). I think these were Apollo Lake-P based systems with the
initial runtime D3cold and S0ix support at the time.
