[PATCH] soundwire: bus_type: Avoid lockdep assert in sdw_drv_probe()

[Richard Fitzgerald]

Don't hold sdw_dev_lock while calling the peripheral driver
probe() and remove() callbacks.

Holding sdw_dev_lock around the probe() and remove() calls
causes a theoretical mutex inversion which lockdep will
assert on. The peripheral driver probe will probably register
a soundcard, which will take ALSA and ASoC locks. During
normal operation a runtime resume suspend can be triggered
while these locks are held and will then take sdw_dev_lock.

It's not necessary to hold sdw_dev_lock when calling the
probe() and remove(), it is only used to prevent the bus core
calling the driver callbacks if there isn't a driver or the
driver is removing.

If sdw_dev_lock is held while setting and clearing the
'probed' flag this is sufficient to guarantee the safety of
callback functions.

The potential race of a bus event happening while probe() is
executing is the same as the existing race of the bus event
handler taking the mutex first and processing the event
before probe() can run. In both cases the event has already
happened before the driver is probed and ready to accept
callbacks.

Signed-off-by: Richard Fitzgerald <rf at opensource.cirrus.com>
---
 drivers/soundwire/bus_type.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/soundwire/bus_type.c b/drivers/soundwire/bus_type.c
index 04b3529f8929..963498db0fd2 100644
--- a/drivers/soundwire/bus_type.c
+++ b/drivers/soundwire/bus_type.c
@@ -105,20 +105,19 @@ static int sdw_drv_probe(struct device *dev)
 	if (ret)
 		return ret;
 
-	mutex_lock(&slave->sdw_dev_lock);
-
 	ret = drv->probe(slave, id);
 	if (ret) {
 		name = drv->name;
 		if (!name)
 			name = drv->driver.name;
-		mutex_unlock(&slave->sdw_dev_lock);
 
 		dev_err(dev, "Probe of %s failed: %d\n", name, ret);
 		dev_pm_domain_detach(dev, false);
 		return ret;
 	}
 
+	mutex_lock(&slave->sdw_dev_lock);
+
 	/* device is probed so let's read the properties now */
 	if (drv->ops && drv->ops->read_prop)
 		drv->ops->read_prop(slave);
@@ -167,14 +166,12 @@ static int sdw_drv_remove(struct device *dev)
 	int ret = 0;
 
 	mutex_lock(&slave->sdw_dev_lock);
-
 	slave->probed = false;
+	mutex_unlock(&slave->sdw_dev_lock);
 
 	if (drv->remove)
 		ret = drv->remove(slave);
 
-	mutex_unlock(&slave->sdw_dev_lock);
-
 	dev_pm_domain_detach(dev, false);
 
 	return ret;
-- 
2.30.2