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

[Lukas Wunner <lukas@wunner.de>]

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?

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.

Thanks,

Lukas