Re: [PATCH v2] gpio: pca953x: fix a lockdep warning

[Bartosz Golaszewski]

2016-08-29 10:33 GMT+02:00 Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>:
> 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 lockdep
> doesn't see this and says:
>
>   Possible unsafe locking scenario:
>
>         CPU0
>         ----
>    lock(&chip->i2c_lock);
>    lock(&chip->i2c_lock);
>
>   *** DEADLOCK ***
>
>   May be due to missing lock nesting notation
>
> To shut lockdep up, use mutex_lock_nested() and use the GPIO base
> number as the subclass argument (it has the same type).
>
> NOTE: this only fixes a specific issue we're experiencing with our
> setup. The problem would probably occur as well with other I2C
> expanders under similar circumstances. A proper fix would probably be
> to implement an I2C-GPIO expander framework that would unduplicate
> common code for all drivers.
>
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@xxxxxxxxxxxx>
> ---
> v1 -> v2:
> - tweaked the commit message
> - expanded the comment
>
>  drivers/gpio/gpio-pca953x.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpio/gpio-pca953x.c b/drivers/gpio/gpio-pca953x.c
> index 02f2a56..3387bdd 100644
> --- a/drivers/gpio/gpio-pca953x.c
> +++ b/drivers/gpio/gpio-pca953x.c
> @@ -329,7 +329,15 @@ static void pca953x_gpio_set_value(struct gpio_chip *gc, unsigned off, int val)
>         u8 reg_val;
>         int ret, offset = 0;
>
> -       mutex_lock(&chip->i2c_lock);
> +       /*
> +        * We're using mutex_lock_nested() here to avoid a lockdep warning
> +        * when there are two pca953x expanders, of which one is used to
> +        * control an i2c gpio mux.
> +        *
> +        * We're using the GPIO base number to distinguish the lock
> +        * subclasses.
> +        */
> +       mutex_lock_nested(&chip->i2c_lock, chip->gpio_start);
>         if (val)
>                 reg_val = chip->reg_output[off / BANK_SZ]
>                         | (1u << (off % BANK_SZ));
> --
> 2.7.4
>

I didn't notice it before, but this patch triggers a different lockdep
warning due to exceeding the max allowed subclass number. I'm working
on converting the driver to regmap, which should fix the issue.

Best regards,
Bartosz Golaszewski