platform/chrome: cros_ec: Use per-device lockdep key

Lockdep reports a bogus possible deadlock on MT8192 Chromebooks due to
the following lock sequences:

1. lock(i2c_register_adapter) [1]; lock(&ec_dev->lock)
2. lock(&ec_dev->lock); lock(prepare_lock);

The actual dependency chains are much longer. The shortened version
looks somewhat like:

1. cros-ec-rpmsg on mtk-scp
ec_dev->lock -> prepare_lock
2. In rt5682_i2c_probe() on native I2C bus:
prepare_lock -> regmap->lock -> (possibly) i2c_adapter->bus_lock
3. In rt5682_i2c_probe() on native I2C bus:
regmap->lock -> i2c_adapter->bus_lock
4. In sbs_probe() on i2c-cros-ec-tunnel I2C bus attached on cros-ec:
i2c_adapter->bus_lock -> ec_dev->lock

While lockdep is correct that the shared lockdep classes have a circular
dependency, it is bogus because

a) 2+3 happen on a native I2C bus
b) 4 happens on the actual EC on ChromeOS devices
c) 1 happens on the SCP coprocessor on MediaTek Chromebooks that just
happens to expose a cros-ec interface, but does not have an
i2c-cros-ec-tunnel I2C bus

In short, the "dependencies" are actually on different devices.

Setup a per-device lockdep key for cros_ec devices so lockdep can tell
the two instances apart. This helps with getting rid of the bogus
lockdep warning. For ChromeOS devices that only have one cros-ec
instance this doesn't change anything.

Also add a missing mutex_destroy, just to make the teardown complete.

[1] This is likely the per I2C bus lock with shared lockdep class

Signed-off-by: Chen-Yu Tsai <[e-mail]>
Signed-off-by: Tzung-Bi Shih <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/platform/chrome/cros_ec.c

···
 	if (!ec_dev->dout)
 		return -ENOMEM;
 
+	lockdep_register_key(&ec_dev->lockdep_key);
 	mutex_init(&ec_dev->lock);
+	lockdep_set_class(&ec_dev->lock, &ec_dev->lockdep_key);
 
 	err = cros_ec_query_all(ec_dev);
 	if (err) {
 		dev_err(dev, "Cannot identify the EC: error %d\n", err);
-		return err;
+		goto destroy_mutex;
 	}
 
 	if (ec_dev->irq > 0) {
···
 		if (err) {
 			dev_err(dev, "Failed to request IRQ %d: %d\n",
 				ec_dev->irq, err);
-			return err;
+			goto destroy_mutex;
 		}
 	}
 
···
 	if (IS_ERR(ec_dev->ec)) {
 		dev_err(ec_dev->dev,
 			"Failed to create CrOS EC platform device\n");
-		return PTR_ERR(ec_dev->ec);
+		err = PTR_ERR(ec_dev->ec);
+		goto destroy_mutex;
 	}
 
 	if (ec_dev->max_passthru) {
···
 exit:
 	platform_device_unregister(ec_dev->ec);
 	platform_device_unregister(ec_dev->pd);
+destroy_mutex:
+	mutex_destroy(&ec_dev->lock);
+	lockdep_unregister_key(&ec_dev->lockdep_key);
 	return err;
 }
 EXPORT_SYMBOL(cros_ec_register);
···
 	if (ec_dev->pd)
 		platform_device_unregister(ec_dev->pd);
 	platform_device_unregister(ec_dev->ec);
+	mutex_destroy(&ec_dev->lock);
+	lockdep_unregister_key(&ec_dev->lockdep_key);
 }
 EXPORT_SYMBOL(cros_ec_unregister);
 

include/linux/platform_data/cros_ec_proto.h

···
 #define __LINUX_CROS_EC_PROTO_H
 
 #include <linux/device.h>
+#include <linux/lockdep_types.h>
 #include <linux/mutex.h>
 #include <linux/notifier.h>
 
···
  *            command. The caller should check msg.result for the EC's result
  *            code.
  * @pkt_xfer: Send packet to EC and get response.
+ * @lockdep_key: Lockdep class for each instance. Unused if CONFIG_LOCKDEP is
+ *		 not enabled.
  * @lock: One transaction at a time.
  * @mkbp_event_supported: 0 if MKBP not supported. Otherwise its value is
  *                        the maximum supported version of the MKBP host event
···
 			struct cros_ec_command *msg);
 	int (*pkt_xfer)(struct cros_ec_device *ec,
 			struct cros_ec_command *msg);
+	struct lock_class_key lockdep_key;
 	struct mutex lock;
 	u8 mkbp_event_supported;
 	bool host_sleep_v1;