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

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

On Fri, 9 Feb 2024 10:35:21 -0600
Bjorn Helgaas <helgaas@kernel.org> wrote:

> On Tue, Jan 23, 2024 at 11:55:31AM -0700, Alex Williamson 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.  
> 
> This apparently fixes a problem: the bugzilla says something about
> disks attached to Thunderbolt/USB4 docks not working, but I doubt it's
> actually specific to Thunderbolt/USB4 or to disks.

Right, AIUI it's simply a PCIe hierarchy where a bridge was previously
scanned in response to a PME and no longer is because of the invalid
requirement added in d3fcd7360338 that the runtime power management
status of the device is active.

> The bugzilla also indicates that d3fcd7360338 was a regression.
> d3fcd7360338 appeared in v6.6, so this fix is likely a candidate for
> the current release (v6.8).

Agreed.

> I'd like to mention both the user-visible problem being fixed and 
> the fact that it fixes a regression here in the commit log so we can
> make the case for putting this in v6.8.

Ok, I've not experienced the regression myself, but I can add a
paragraph describing my understanding of the bugzilla.  I'd probably
just say:

	This resolves a regression reported in the bugzilla below where
	a Thunderbolt/USB4 hierarchy fails to scan for an attached NVMe
	endpoint downstream of a bridge in a D3hot power state.

If you'd like a respin including that or if you have further
phrasing/info suggestions, please let me know.  Thanks,

Alex

> > 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)
> > +					continue;
> > +
> > +				if (bridge->current_state != PCI_D0)
> > +					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);
> > -- 
> > 2.43.0
> >   
>