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

[Rafael J. Wysocki]

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.