Re: [PATCH] PCI: Fix active state requirement in PME polling

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Jan 23, 2024 at 7:56 PM Alex Williamson
<alex.williamson@redhat.com> wrote:
>
> The commit noted in fixes added a bogus requirement that runtime PM
> managed devices need to be in the RPM_ACTIVE state for PME polling.
> In fact, only devices in low power states should be polled.
>
> However there's still a requirement that the device config space must
> be accessible, which has implications for both the current state of
> the polled device and the parent bridge, when present.  It's not
> sufficient to assume the bridge remains in D0 and cases have been
> observed where the bridge passes the D0 test, but the PM state
> indicates RPM_SUSPENDING and config space of the polled device becomes
> inaccessible during pci_pme_wakeup().
>
> Therefore, since the bridge is already effectively required to be in
> the RPM_ACTIVE state, formalize this in the code and elevate the PM
> usage count to maintain the state while polling the subordinate
> device.
>
> Cc: Lukas Wunner <lukas@wunner.de>
> Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
> Cc: Rafael J. Wysocki <rafael@kernel.org>
> Fixes: d3fcd7360338 ("PCI: Fix runtime PM race with PME polling")
> Reported-by: Sanath S <sanath.s@amd.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218360
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  drivers/pci/pci.c | 37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index bdbf8a94b4d0..764d7c977ef4 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -2433,29 +2433,36 @@ static void pci_pme_list_scan(struct work_struct *work)
>                 if (pdev->pme_poll) {
>                         struct pci_dev *bridge = pdev->bus->self;
>                         struct device *dev = &pdev->dev;
> -                       int pm_status;
> +                       struct device *bdev = bridge ? &bridge->dev : NULL;
> +                       int bref = 0;
>
>                         /*
> -                        * If bridge is in low power state, the
> -                        * configuration space of subordinate devices
> -                        * may be not accessible
> +                        * If we have a bridge, it should be in an active/D0
> +                        * state or the configuration space of subordinate
> +                        * devices may not be accessible or stable over the
> +                        * course of the call.
>                          */
> -                       if (bridge && bridge->current_state != PCI_D0)
> -                               continue;
> +                       if (bdev) {
> +                               bref = pm_runtime_get_if_active(bdev, true);
> +                               if (!bref)

I would check bref <= 0 here.

> +                                       continue;
> +
> +                               if (bridge->current_state != PCI_D0)

Isn't the power state guaranteed to be PCI_D0 at this point?  If it
isn't, then why?

> +                                       goto put_bridge;
> +                       }
>
>                         /*
> -                        * If the device is in a low power state it
> -                        * should not be polled either.
> +                        * The device itself should be suspended but config
> +                        * space must be accessible, therefore it cannot be in
> +                        * D3cold.
>                          */
> -                       pm_status = pm_runtime_get_if_active(dev, true);
> -                       if (!pm_status)
> -                               continue;
> -
> -                       if (pdev->current_state != PCI_D3cold)
> +                       if (pm_runtime_suspended(dev) &&
> +                           pdev->current_state != PCI_D3cold)
>                                 pci_pme_wakeup(pdev, NULL);
>
> -                       if (pm_status > 0)
> -                               pm_runtime_put(dev);
> +put_bridge:
> +                       if (bref > 0)
> +                               pm_runtime_put(bdev);
>                 } else {
>                         list_del(&pme_dev->list);
>                         kfree(pme_dev);
> --