Re: [PATCH v2] PCI/PM: enable runtime PM later during device scanning

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2023-06-07 at 09:49 +0200, Lukas Wunner wrote:
> On Mon, Jun 05, 2023 at 08:35:45PM +0200, Johannes Berg wrote:
> > @@ -3139,6 +3139,7 @@ void pci_pm_init(struct pci_dev *dev)
> >  	u16 pmc;
> >  
> >  	pm_runtime_forbid(&dev->dev);
> > +	pm_runtime_get_noresume(&dev->dev);
> >  	pm_runtime_set_active(&dev->dev);
> >  	pm_runtime_enable(&dev->dev);
> >  	device_enable_async_suspend(&dev->dev);
> > @@ -335,9 +336,12 @@ void pci_bus_add_device(struct pci_dev *dev)
> >  	int retval;
> >  
> >  	/*
> > -	 * Can not put in pci_device_add yet because resources
> > -	 * are not assigned yet for some devices.
> > +	 * Allow runtime PM only here, since otherwise we may
> > +	 * try to suspend a device that isn't fully configured
> > +	 * yet, which causes problems.
> >  	 */
> > +	pm_runtime_put_noidle(&dev->dev);
> > +
> >  	pcibios_bus_add_device(dev);
> >  	pci_fixup_device(pci_fixup_final, dev);
> >  	pci_create_sysfs_dev_files(dev);
> 
> There seem to be many different callers that end up in pci_pm_init()
> and pci_bus_add_device().
> 
> Is it guaranteed that the two functions are always called in order?
> Do callers exist which only invoke the former but not the latter or
> vice-versa?  Can it happen that a caller of the former errors out,
> so the latter is never called, leading to a runtime PM ref imbalance?

I did ask myself that too, and honestly, I'm not entirely sure - need
somebody more familiar to really understand that, I think.

Most places elsewhere _do_ call both, and it feels like you have to call
both if you want to do something with the device.

However there are a few places that seem to call the first part and then
remove the device again immediately after. That also seems harmless
though.

> It would be easier to ascertain correctness if you could find a
> function at a higher level which (indirectly) calls both pci_pm_init()
> and pci_bus_add_device() so that you can acquire and release the
> runtimw PM ref in that single function.
> 

Unfortunately, there isn't such a place, since the scanning is done by
various bus walks.

johannes