Re: [PATCH v7 2/2] PCI: Don't put non-power manageable PCIe root ports into D3

[Mario Limonciello]

On 8/2/23 00:26, Mika Westerberg wrote:
> 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.


I scoured the web looking for acpidumps a bit an Apollo Lake system and 
came across this random bug report:

https://bugzilla.redhat.com/show_bug.cgi?id=1591307

"Intel(R) Celeron(R) CPU N3450 @ 1.10GHz (family: 0x6, model: 0x5c, 
stepping: 0x9)"

I looked at the acpidump, and I notice:

Low Power S0 Idle (V5) : 0

That means that Windows wouldn't actually be putting it into Modern 
Standby at suspend but would rather use S3.

Considering that result, could we perhaps adjust the check to:

if ((c->x86_vendor == X86_VENDOR_INTEL) && !(acpi_gbl_FADT.flags & 
ACPI_FADT_LOW_POWER_S0))

Or could we quirk the PCI root ports from Apollo Lake to opt into D3?