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

[Bjorn Helgaas <helgaas@kernel.org>]

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.

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).

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.

> 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
>