Re: [PATCH v3] PCI: Add D3 support for PCI bridges in DT based platforms

Manivannan Sadhasivam
2024-03-05T16:25:48Z

On Thu, Feb 22, 2024 at 10:40:52AM +0100, Lukas Wunner wrote:
> On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> > 1) D3hot doesn't work per spec. This sounds like a hardware
> > defect in the device that should be a quirk based on
> > Vendor/Device ID, not something in DT. I don't actually know if
> > this is common, although there are several existing quirks that
> > mention issues with D3.
>
> My recollection is that putting Root Ports into D3hot on older x86
> systems would raise MCEs, which is why pci_bridge_d3_possible() only
> allows D3hot in cases which are known to work (e.g. Thunderbolt
> controllers, machines with a recent BIOS). It was a conservative
> policy chosen to avoid regressions.
>

So pci_bridge_d3_possible() is only checking for D3hot capability? If so, I'd
rename it to pci_bridge_d3hot_possible() and also 'bridge_d3' to 'bridge_d3hot'
to make it explicit.

Since the default value of 'd3cold_allowed' is true, I believe the code expects
all devices to support D0 and D3cold. Please correct me if I'm wrong.

- Mani

> I don't know if similar issues exist on non-ACPI systems. If they
> don't exist, platform_pci_bridge_d3() could just return true for
> all devicetree-based systems. Might be worth testing if any systems
> can be found which exhibit issues with such a policy. That would
> obviate the need to specify "supports-d3" in the devicetree.
> Quite the opposite, ports which are known not to work could be
> blacklisted. Of course if it turns out that's the majority then
> whitelisting via "supports-d3" is a better option.
>
>
> > 2) The platform doesn't support putting the bridge in D3cold and
> > back to D0. I don't understand this either because I assumed DT
> > would describe *hardware*, and "supports-d3" might imply the
> > presence of hardware power control, but doesn't tell us how to
> > operate it, and it must be up to a native driver to know how to
> > do it.
>
> I think we're putting devices into D3hot first before cutting power
> (i.e. putting them into D3cold), so knowing that D3hot is safe is
> basically a prerequisite for D3cold.
>
> Thanks,
>
> Lukas

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