Re: [PATCH v12 9/9] ACPI: x86: s2idle: Enforce LPS0 constraints for PCI devices

[Limonciello, Mario]

On 8/17/2023 2:37 PM, Rafael J. Wysocki wrote:
> On Thu, Aug 17, 2023 at 9:30 PM Limonciello, Mario
> <mario.limonciello@xxxxxxx> wrote:
> > On 8/17/2023 2:25 PM, Bjorn Helgaas wrote:
> > > On Wed, Aug 16, 2023 at 03:41:43PM -0500, Mario Limonciello wrote:
> > > > Since commit 9d26d3a8f1b0 ("PCI: Put PCIe ports into D3 during suspend")
> > > > PCIe ports from modern machines (>=2015) are allowed to be put into D3 by
> > > > storing a value to the `bridge_d3` variable in the `struct pci_dev`
> > > > structure.
> > > > ...
> > >
> > > > +static void lpi_check_pci_dev(struct lpi_constraints *entry, struct pci_dev *pdev)
> > > > +{
> > > > +    pci_power_t target = entry->enabled ? entry->min_dstate : PCI_D0;
> > > > +
> > > > +    if (pdev->current_state == target)
> > > > +            return;
> > > > +
> > > > +    /* constraint of ACPI D3hot means PCI D3hot _or_ D3cold */
> > > > +    if (target == ACPI_STATE_D3_HOT &&
> > >
> > > ACPI_STATE_D3_HOT is not a valid pci_power_t value.
> >
> > Based on this, kernel robot sparse complaints and your comments on v11's
> > last patch I am going to split off to another function that returns the
> > pci_power_t state based upon the situation and better comment the reason
> > for the D0 when not enabled.
> >
> > > > +        (pdev->current_state == PCI_D3hot ||
> > > > +         pdev->current_state == PCI_D3cold))
> > > > +            return;
> > > > +
> > > > +    if (pm_debug_messages_on)
> > > > +            acpi_handle_info(entry->handle,
> > > > +                             "LPI: PCI device in %s, not in %s\n",
> > > > +                             acpi_power_state_string(pdev->current_state),
> > > > +                             acpi_power_state_string(target));
> > > > +
> > > > +    /* don't try with things that PCI core hasn't touched */
> > > > +    if (pdev->current_state == PCI_UNKNOWN) {
> > > > +            entry->handle = NULL;
> > > > +            return;
> > > > +    }
> > > > +
> > > > +    pci_set_power_state(pdev, target);
> > >
> > > It doesn't seem logical for a "check_constraints()" function that
> > > takes no parameters and returns nothing to actively set the PCI power
> > > state.
> > >
> > > lpi_check_constraints() returns nothing, and from the fact that it was
> > > previously only called when "pm_debug_messages_on", I infer that it
> > > should have no side effects.
> > >
> > > IMHO "lpi_check_constraints" is not a great name because "check"
> > > doesn't suggest anything specific about what it does.
> > > "dump_constraints()" -- fine.  "log_unmet_constraints()" -- fine
> > > (seems like the original intention of 726fb6b4f2a8 ("ACPI / PM: Check
> > > low power idle constraints for debug only"), which added it.
> >
> > Great feedback, thanks.  I'm thinking to instead change it to:
> >
> > lpi_enforce_constraints()
>
> Don't even try to go this way, please.
>
> Originally, the LPI constraints are there to indicate to Windows
> whether or not it should attempt to enter Connected/Modern Standby.
>
> Because Linux doesn't do Modern Standby, it doesn't use the LPI
> constraints the way Windows does and it really shouldn't do that.
>
> I think that the exercise here is to use the information from the
> constraints list as an indication whether or not a given PCI Root Port
> is supposed to be put into D3hot/cold on suspend-to-idle and this has
> nothing to do with enforcement.

What do you think about me making the changes to pci_prepare_to_sleep()?

Something like this:

@@ -2733,11 +2742,17 @@ int pci_prepare_to_sleep(struct pci_dev *dev)
 {
        bool wakeup = device_may_wakeup(&dev->dev);
        pci_power_t target_state = pci_target_state(dev, wakeup);
+       pci_power_t constraint;
        int error;

        if (target_state == PCI_POWER_ERROR)
                return -EIO;

+       /* if platform indicates device constraint for suspend, use it */
+       constraint = platform_check_constraint(dev, target_state);
+       if (constraint != PCI_POWER_ERROR)
+               target_state = constraint;
+
        pci_enable_wake(dev, target_state, wakeup);

        error = pci_set_power_state(dev, target_state);