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

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This is a follow-up to commit 559b46990e ("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>
This commit is contained in:
Bartosz Golaszewski 2016-09-26 11:54:15 +02:00 committed by Wolfram Sang
parent de34f4da7f
commit 74f47f07e5
1 changed files with 16 additions and 0 deletions
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16
drivers/gpio/gpio-pca953x.c
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@ -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));