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Date:   Wed, 7 Jun 2023 09:49:39 +0200
From:   Lukas Wunner <lukas@wunner.de>
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-pm@vger.kernel.org, rafael@kernel.org,
        Bjorn Helgaas <bhelgaas@google.com>,
        Johannes Berg <johannes.berg@intel.com>
Subject: Re: [PATCH v2] PCI/PM: enable runtime PM later during device scanning
Message-ID: <20230607074939.GA17700@wunner.de>
References: <20230605121621.4259f1be6cd2.Idbaa55b93f780838af44ebccb84c36f60716df04@changeid>
 <20230605203519.bc4232207449.Idbaa55b93f780838af44ebccb84c36f60716df04@changeid>
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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