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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 23 Jan 2024 20:40:32 +0100
"Rafael J. Wysocki" <rafael@kernel.org> wrote:

> 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?

Both of these seem necessary to support !CONFIG_PM, where bref would be
-EINVAL and provides no indication of the current_state.  Is that
incorrect?  Thanks,

Alex


> > +                                       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);
> > --  
>