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 <20211109181224.GA1162053@bhelgaas>
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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Tue, 9 Nov 2021 19:52:08 +0100
Message-ID: <CAJZ5v0izp7462vpmJ+=AarPPLSBLmFea-GvZErobX5Q4Du-oeA@mail.gmail.com>
Subject: Re: [PATCH] pci: Don't call resume callback for nearly bound devices
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        =?UTF-8?B?Um9iZXJ0IMWad2nEmWNraQ==?= <robert@swiecki.net>,
        linux-i2c <linux-i2c@vger.kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Linux PCI <linux-pci@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Tue, Nov 9, 2021 at 7:12 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Tue, Nov 09, 2021 at 06:18:18PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Nov 9, 2021 at 7:59 AM Uwe Kleine-König
> > <u.kleine-koenig@pengutronix.de> wrote:
> > > 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.
>
> Yes, that is the case:
>
>   pci_device_remove
>     if (drv->remove) {
>       pm_runtime_get_sync
>       drv->remove()                # <-- driver ->remove() method
>       pm_runtime_put_noidle
>     }
>     ...
>     pm_runtime_put_sync            # <-- after ->remove()
>
> So pm_runtime_put_sync() is called after drv->remove(), and it may
> call drv->pm->runtime_idle().  I think the driver may not expect this.
>
> > If this is turned into pm_runtime_put_noidle(), all should work.
>
> pci_device_remove() already calls pm_runtime_put_noidle() immediately
> after calling the driver ->remove() method.
>
> Are you saying we should do this, which means pci_device_remove()
> would call pm_runtime_put_noidle() twice?

Well, they are both needed to keep the PM-runtime reference counting in balance.

This still has an issue, though, because user space would be able to
trigger a runtime suspend via sysfs after we've dropped the last
reference to the device in pci_device_remove().

So instead, we can drop the pm_runtime_get_sync() and
pm_runtime_put_sync() from local_pci_probe() and pci_device_remove(),
respectively, and add pm_runtine_get_noresume() to pci_pm_init(),
which will prevent PM-runtime from touching the device until it has a
driver that supports PM-runtime.

We'll lose the theoretical ability to put unbound devices into D3 this
way, but we learned some time ago that this isn't safe in all cases
anyway.

> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index 1d98c974381c..79c1a920fdc8 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -318,7 +318,7 @@ static long local_pci_probe(void *_ddi)
>          * count, in its probe routine and pm_runtime_get_noresume() in
>          * its remove routine.
>          */
> -       pm_runtime_get_sync(dev);
> +       pm_runtime_get_noresume(dev);
>         rc = pci_drv->probe(pci_dev, ddi->id);
>         if (!rc)
>                 return rc;
> @@ -465,7 +465,7 @@ static void pci_device_remove(struct device *dev)
>         pci_iov_remove(pci_dev);
>
>         /* Undo the runtime PM settings in local_pci_probe() */
> -       pm_runtime_put_sync(dev);
> +       pm_runtime_put_noidle(dev);
>
>         /*
>          * If the device is still on, set the power state as "unknown",