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

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Message-ID: <CAJZ5v0juq2AsqxBAGN4oOsQp7-73+TP67Ppjz2DWq-pVEAxTrA@mail.gmail.com>
Date:   Thu, 17 Aug 2023 21:31:54 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Mario Limonciello <mario.limonciello@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-acpi@...r.kernel.org,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Iain Lane <iain@...ngesquash.org.uk>,
        Shyam-sundar S-k <Shyam-sundar.S-k@....com>
Subject: Re: [PATCH v12 9/9] ACPI: x86: s2idle: Enforce LPS0 constraints for
 PCI devices

On Thu, Aug 17, 2023 at 9:25 PM Bjorn Helgaas <helgaas@...nel.org> 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.
>
> > +         (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.

That's correct, it is not expected to have 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.

It seems that we are entering bikeshedding territory here ...