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

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Date:   Tue, 1 Aug 2023 11:54:43 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Limonciello, Mario" <mario.limonciello@....com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
        linux-acpi@...r.kernel.org, Iain Lane <iain@...ngesquash.org.uk>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v7 2/2] PCI: Don't put non-power manageable PCIe root
 ports into D3

On Sat, Jul 15, 2023 at 1:00 AM Limonciello, Mario
<mario.limonciello@....com> wrote:
>
>
> On 7/14/2023 2:17 PM, Rafael J. Wysocki wrote:
>
> Well, this looks like a spec interpretation difference.
>
> We thought that _SxD/_SxW would only be relevant for devices with ACPI
> PM support, but the firmware people seem to think that those objects
> are also relevant for PCI devices that don't have ACPI PM support
> (because those devices are still power-manageable via PMCSR).  If
> Windows agrees with that viewpoint, we'll need to adjust, but not
> through adding _SxW checks in random places.
>
> I think that depends upon how you want to handle the lack of _S0W.

If _S0W is not present, _S0D should return the deepest state that can be used.

If that is not present, it is a matter of OS policy.

> On these problematic devices there is no _S0W under the PCIe
> root port.  As I said; Windows puts them into D0 in this case though.

Do you know what the rationale for that is?  Maybe Windows takes the
lack of _S0W/_S0D as the indication that the device could not go into
low-power states in D0, but do you actually know that this is the
case?

Surely, for non-bridge devices the lack of _S0W/_S0D does not mean
that the device should not be programmed into low-power states via
PMCSR, but maybe Root Ports are an exception?

> So acpi_dev_power_state_for_wake should return ACPI_STATE_UNKNOWN.

And then who'll decide what to do with that return value?

> Can you suggest where you think adding a acpi_dev_power_state_for_wake() does make sense?
>
> Two areas that I think would work would be in: pci_pm_suspend_noirq() (to avoid calling pci_prepare_to_sleep)
>
> or
>
> directly in pci_prepare_to_sleep() to check that value in lieu of pci_target_state().

I'm not sure that this is a core problem TBH.  It looks like this is
an exception made specifically for ports, so this check seems to
belong to where ports are handled, so that would be
acpi_pci_bridge_d3() after all.

However, _S0D needs to be checked too when _S0W is not present.