[PATCH] driver core: Ensure proper suspend/resume ordering

[Thierry Reding]

From: Thierry Reding <treding@xxxxxxxxxx>

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.

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@xxxxxxxxxx>
---
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.

 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();
+
 	if (dev->bus->probe) {
 		ret = dev->bus->probe(dev);
 		if (ret)
-- 
2.5.0

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