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

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Date:   Tue, 9 Nov 2021 18:18:18 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Robert Święcki <robert@...ecki.net>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] pci: Don't call resume callback for nearly bound devices

On Tue, Nov 9, 2021 at 7:59 AM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
>
> 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()?

Yes, in principle it might be replaced with pm_runtime_get_noresume().

In theory, that may be problematic if a device is put into a low-power
state on remove and then the driver is bound again to it.

> 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.

If it is called after ->remove() and before clearing the device's
driver pointer, then yes.

If this is turned into pm_runtime_put_noidle(), all should work.

> > 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.