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SubjectRe: [PATCH v2 4/4] gpio: pca953x: fix an incorrect lockdep warning
On Fri, Sep 16, 2016 at 06:02:45PM +0200, Bartosz Golaszewski wrote:
> If an I2C GPIO multiplexer is driven by a GPIO provided by an expander
> when there's a second expander using the same device driver on one of
> the I2C bus segments, lockdep prints a deadlock warning when trying to
> set the direction or the value of the GPIOs provided by the second
> expander.
>
> The below diagram presents the setup:
>
> - - - - -
> ------- --------- Bus segment 1 | |
> | | | |--------------- Devices
> | | SCL/SDA | | | |
> | Linux |-----------| I2C MUX | - - - - -
> | | | | | Bus segment 2
> | | | | |-------------------
> ------- | --------- |
> | | - - - - -
> ------------ | MUX GPIO | |
> | | | Devices
> | GPIO | | | |
> | Expander 1 |---- - - - - -
> | | |
> ------------ | SCL/SDA
> |
> ------------
> | |
> | GPIO |
> | Expander 2 |
> | |
> ------------
>
> The reason for lockdep warning is that we take the chip->i2c_lock in
> pca953x_gpio_set_value() or pca953x_gpio_direction_output() and then
> come right back to pca953x_gpio_set_value() when the GPIO mux kicks
> in. The locks actually protect different expanders, but for lockdep
> both are of the same class, so it says:
>
> Possible unsafe locking scenario:
>
> CPU0
> ----
> lock(&chip->i2c_lock);
> lock(&chip->i2c_lock);
>
> *** DEADLOCK ***
>
> May be due to missing lock nesting notation
>
> In order to get rid of the warning, retrieve the adapter nesting depth
> and use it as lockdep subclass for chip->i2c_lock.
>
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Linus, we'd like that in 4.9. Can I get your ack for the gpio part?

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