Date:	Fri, 11 Sep 2015 00:08:02 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Thierry Reding <thierry.reding@...il.com>,
	Alan Stern <stern@...land.harvard.edu>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Thursday, September 10, 2015 12:19:03 PM Thierry Reding wrote:
> From: Thierry Reding <treding@...dia.com>
> 
> Deferred probe can lead to strange situations where a device that is a
> dependency for others will be moved to the end of the dpm_list. At the
> same time the dependers may not be moved because at the time they will
> be probed the dependee may already have been successfully reprobed and
> they will not have to defer the probe themselves.

So there's a bug in the implementation of deferred probing IMO.

> One example where this happens is the Jetson TK1 board (Tegra124). The
> gpio-keys driver exposes the power key of the board as an input device
> that can also be used as a wakeup source. Commit 17cdddf0fb68 ("ARM:
> tegra: Add gpio-ranges property") results in the gpio-tegra driver
> deferring probe because one of its dependencies, the pinctrl-tegra
> driver, has not successfully completed probing. Currently the deferred
> probe code will move the corresponding gpio-tegra device to the end of
> the dpm_list, but by the time the gpio-keys device, depending on the
> gpio-tegra device, is probed, gpio-tegra has already been reprobed, so
> the gpio-keys device is not moved to the end of dpm_list itself. As a
> result, the suspend ordering becomes pinctrl-tegra -> gpio-keys ->
> gpio-tegra. That's problematic because the gpio-keys driver requests
> the power key to be a wakeup source. However, the programming of the
> wakeup interrupt registers happens in the gpio-tegra driver's suspend
> callback, which is now called before that of the gpio-keys driver. The
> result is that the wrong values are programmed and leaves the system
> unable to be resumed using the power key.
> 
> To fix this situation, always move devices to the end of the dpm_list
> before probing them. Technically this should only be done for devices
> that have been successfully probed, but that won't work for recursive
> probing of devices (think an I2C master that instantiates children in
> its ->probe()). Effectively the dpm_list will end up ordered the same
> way that devices were probed, hence taking care of dependencies.
> 
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
> Note that this commit is kind of the PM equivalent of 52cdbdd49853
> ("driver core: correct device's shutdown order) and that we have two
> lists that are essentially the same (dpm_list and devices_kset). I'm
> wondering if it would be worth looking into getting rid of one of
> them? I don't see any reason why the ordering for shutdown and
> suspend/resume should be different, and having a single list would
> help keep this in sync.

We move away things from dpm_list during suspend and add them back to it
during resume to handle the situations in which some devices go away or
appear during suspend/resume.  That makes this idea potentially problematic.

> 
>  drivers/base/dd.c | 33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index be0eb4639128..56291b11049b 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -88,16 +88,6 @@ static void deferred_probe_work_func(struct work_struct *work)
>  		 */
>  		mutex_unlock(&deferred_probe_mutex);
>  
> -		/*
> -		 * Force the device to the end of the dpm_list since
> -		 * the PM code assumes that the order we add things to
> -		 * the list is a good order for suspend but deferred
> -		 * probe makes that very unsafe.
> -		 */
> -		device_pm_lock();
> -		device_pm_move_last(dev);
> -		device_pm_unlock();
> -
>  		dev_dbg(dev, "Retrying from deferred list\n");
>  		bus_probe_device(dev);
>  
> @@ -312,6 +302,29 @@ static int really_probe(struct device *dev, struct device_driver *drv)
>  	 */
>  	devices_kset_move_last(dev);
>  
> +	/*
> +	 * Force the device to the end of the dpm_list since the PM code
> +	 * assumes that the order we add things to the list is a good order
> +	 * for suspend but deferred probe makes that very unsafe.
> +	 *
> +	 * Deferred probe can also cause situations in which a device that is
> +	 * a dependency for others gets moved further down the dpm_list as a
> +	 * result of probe deferral. In that case the dependee will end up
> +	 * getting suspended before any of its dependers.
> +	 *
> +	 * To ensure proper ordering of suspend/resume, move every device that
> +	 * is being probed to the end of the dpm_list. Note that technically
> +	 * only successfully probed devices need to be moved, but that breaks
> +	 * for recursively added devices because they would end up in the list
> +	 * in reverse of the desired order, so we simply do it unconditionally
> +	 * for all devices before they are being probed. In the worst case the
> +	 * list will be reordered a couple more times than necessary, which
> +	 * should be an insignificant amount of work.
> +	 */
> +	device_pm_lock();
> +	device_pm_move_last(dev);
> +	device_pm_unlock();

So I don't agree with doing that for every driver being probed against the
same device.  That's just wasteful IMO.

> +
>  	if (dev->bus->probe) {
>  		ret = dev->bus->probe(dev);
>  		if (ret)
> 

Alan, what do you think about this?

Thanks,
Rafael

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