| Message ID | 1472459600-10815-1-git-send-email-bgolaszewski@baylibre.com |
|---|---|
| State | New |
| Headers | show |
2016-08-29 10:33 GMT+02:00 Bartosz Golaszewski <bgolaszewski@baylibre.com>: > 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@baylibre.com> > --- > 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
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));
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@baylibre.com> --- v1 -> v2: - tweaked the commit message - expanded the comment drivers/gpio/gpio-pca953x.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) -- 2.7.4