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

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Date:   Tue, 9 Nov 2021 07:59:27 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] pci: Don't call resume callback for nearly bound devices

Hello,

On Mon, Nov 08, 2021 at 08:56:19PM -0600, Bjorn Helgaas wrote:
> [+cc Greg: new device_is_bound() use]

ack, that's what I would have suggested now, too.

> 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().

Maybe for the other locations using device_is_bound(&pdev->dev) instead
of to_pci_driver(pdev) != NULL would be nice, too?

I have another doubt: device_is_bound() should (according to its
kernel-doc) be called with the device lock held. For the call stack that
is (maybe) fixed here, the lock is held (by __device_attach). We
probably should check if the lock is also held for the other calls of
pci_pm_runtime_resume().

Hmm, the device lock is a mutex, the pm functions might be called in
atomic context, right?

> 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 a saver bet would be to not use pm_runtime_get_sync() in
local_pci_probe()?

I wonder if the same problem exists on remove, i.e. pci_device_remove()
calls pm_runtime_put_sync() after the driver's .remove() callback was
called.

> Maybe we just need a comment there about why it looks different than
> the other PM interfaces?

A comment is a good idea for sure.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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