Re: [RFC] PCI/AER: Block runtime suspend when handling errors

Rafael J. Wysocki
2024-02-01T14:10:47Z

On Thu, Feb 1, 2024 at 10:36 AM Stanislaw Gruszka
<stanislaw.gruszka@linux.intel.com> wrote:
>
> On Mon, Jan 29, 2024 at 06:14:36PM -0600, Bjorn Helgaas wrote:
> > On Tue, Jan 23, 2024 at 10:00:16AM +0100, Stanislaw Gruszka wrote:
> > > PM runtime can be done simultaneously with AER error handling.
> > > Avoid that by using pm_runtime_get_sync() just after pci_dev_get()
> > > and pm_runtime_put() just before pci_dev_put() in AER recovery
> > > procedures.
> >
> > I guess there must be a general rule here, like "PCI core must use
> > pm_runtime_get_sync() whenever touching the device asynchronously,
> > i.e., when it's doing something unrelated to a call from the driver"?
>
> Clear rules would be nice, that's for sure.
>
> > Probably would apply to all subsystem cores, not just PCI.
>
> If they have something similar like AER.
>
> > > I'm not sure about DPC case since I do not see get/put there. It
> > > just call pci_do_recovery() from threaded irq dcd_handler().
> > > I think pm_runtime* should be added to this handler as well.
> >
> > s/dcd_handler/dpc_handler/
> >
> > I'm guessing the "threaded" part really doesn't matter; just the fact
> > that this is in response to an interrupt, not something directly
> > called by a driver?
>
> Yes. I added "threaded" just to emphasis that it's safe to add sleeping
> functions there. In context of possible solution, not related to
> the problem itself.
>
> > > pm_runtime_get_sync() will increase dev->power.usage_count counter to
> > > prevent any rpm actives. When there is suspending pending, it will wait
> > > for it and do the rpm resume. Not sure if that problem, on my testing
> > > I did not encounter issues with that.
> >
> > Sorry, I didn't catch your meaning here.
> I tired to write two things:
>
> First, pm_runtime_get_sync() after exit prevents possibility of
> runtime suspend, so we are sure device will not be powered off
>
> Second, if during pm_runtime_get_sync(), there is pending attempt
> to suspend device, it will be synchronized and device will
> be resumed.

Not exactly. pm_runtime_get_sync() will resume the device
synchronously no matter what.

> This can be problematic as device is in error state.

If this is a real possibility (I mean, device in a low-power state and
in an error state at the same time), it would be better to call
__pm_runtime_disable(dev, false) instead of pm_runtime_get_sync(), as
that would prevent runtime PM from changing the device state.

> But at least from software perspective we should end in device
> being in active state and then we can perform reset of it.

I'm not sure about this. It may be better to power-cycle the device
in D3hot instead of attempting to put it into D0 beforehand.