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

Bartosz Golaszewski
2016-09-26T09:54:35Z

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>
---
drivers/gpio/gpio-pca953x.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/drivers/gpio/gpio-pca953x.c b/drivers/gpio/gpio-pca953x.c
index 018f39c..f306909 100644
--- a/drivers/gpio/gpio-pca953x.c
+++ b/drivers/gpio/gpio-pca953x.c
@@ -786,6 +786,22 @@ static int pca953x_probe(struct i2c_client *client,
chip->chip_type = PCA_CHIP_TYPE(chip->driver_data);

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));

--
2.7.4