linux-kernel - Re: [PATCH] pci: Don't call resume callback for nearly bound devices

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Date:   Tue, 9 Nov 2021 07:42:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Robert Święcki <robert@...ecki.net>,
        linux-i2c@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pci: Don't call resume callback for nearly bound devices

On Mon, Nov 08, 2021 at 08:56:19PM -0600, Bjorn Helgaas wrote:
> [+cc Greg: new device_is_bound() use]
> 
> On Mon, Nov 08, 2021 at 10:22:26PM +0100, Uwe Kleine-König wrote:
> > pci_pm_runtime_resume() exits early when the device to resume isn't
> > bound yet:
> > 
> > 	if (!to_pci_driver(dev->driver))
> > 		return 0;
> > 
> > This however isn't true when the device currently probes and
> > local_pci_probe() calls pm_runtime_get_sync() because then the driver
> > core already setup dev->driver. As a result the driver's resume callback
> > is called before the driver's probe function is called and so more often
> > than not required driver data isn't setup yet.
> > 
> > So replace the check for the device being unbound by a check that only
> > becomes true after .probe() succeeded.
> 
> I like the fact that this patch is short and simple.
> 
> But there are 30+ users of to_pci_driver().  This patch asserts that
> *one* of them, pci_pm_runtime_resume(), is special and needs to test
> device_is_bound() instead of using to_pci_driver().
> 
> It's special because the current PM implementation calls it via
> pm_runtime_get_sync() before the driver's .probe() method.  That
> connection is a little bit obscure and fragile.  What if the PM
> implementation changes?
> 
> Maybe we just need a comment there about why it looks different than
> the other PM interfaces?
> 
> I also notice that the only other uses of device_is_bound()
> outside the driver core are in iommu_group_store_type() and
> regulator_resolve_supply().  This patch seems like a reasonable use,
> but I always look twice when we do something unique.

I agree that this looks really odd.  No one should care outside of the
driver core to call device_is_bound(), as if a driver is being called,
implicitly you know that the device is bound to that driver.

Why does the PCI core care if a device is bound to a pci driver at this
point in time?

But, this does feel like an odd use of to_pci_driver() here, what needs
to be known here, if a pci driver is in control of a device here or not?

thanks,

greg k-h