linux-kernel - Re: [PATCH v2 4/4] gpio: pca953x: fix an incorrect lockdep warning

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Date:   Wed, 21 Sep 2016 07:45:21 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Vignesh R <vigneshr@...com>, Yong Li <yong.b.li@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Peter Rosin <peda@...ntia.se>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [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@...libre.com>

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


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