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

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

On Tue, 23 Jan 2024 20:59:50 +0100
"Rafael J. Wysocki" <rafael@kernel.org> wrote:

> On Tue, Jan 23, 2024 at 8:51 PM Alex Williamson
> <alex.williamson@redhat.com> wrote:
> >
> > 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,  
> 
> Well, CONFIG_PCIE_PME depends on CONFIG_PM, so I'm not sure how
> dev->pme_poll can be set without it.

I only see that drivers/pci/pci.c:pci_pm_init() sets pme_poll true and
I'm not spotting a dependency on either PCIE_PME or PM to get there.  I
see a few places where pme.c, governed by PCIE_PME, can set pme_poll
false though.

It's also not clear to me that we should skip scanning a device if
pm_runtime_get_if_active() returns -EINVAL for the bridge due to
power.disable_depth.  If runtime PM isn't enabled on the bridge,
shouldn't we be able to test current_state and assume it won't change?
Thanks,

Alex