gpio: pca953x: add a comment explaining the need for a lockdep subclass

This is a follow-up to commit 559b46990e76 ("gpio: pca953x: fix an
incorrect lockdep warning"). The reason for calling
lockdep_set_subclass() in pca953x_probe() is not explained in
the code.

Add a comment describing the problem, partial solution and required
future extensions.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/gpio/gpio-pca953x.c

@@ -794,6 +794,22 @@ static int pca953x_probe(struct i2c_client *client,
 	}
 
 	mutex_init(&chip->i2c_lock);
+	/*
+	 * In case we have an i2c-mux controlled by a GPIO provided by an
+	 * expander using the same driver higher on the device tree, read the
+	 * i2c adapter nesting depth and use the retrieved value as lockdep
+	 * subclass for chip->i2c_lock.
+	 *
+	 * REVISIT: This solution is not complete. It protects us from lockdep
+	 * false positives when the expander controlling the i2c-mux is on
+	 * a different level on the device tree, but not when it's on the same
+	 * level on a different branch (in which case the subclass number
+	 * would be the same).
+	 *
+	 * TODO: Once a correct solution is developed, a similar fix should be
+	 * applied to all other i2c-controlled GPIO expanders (and potentially
+	 * regmap-i2c).
+	 */
 	lockdep_set_subclass(&chip->i2c_lock,
 			     i2c_adapter_depth(client->adapter));