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    SubjectRe: [patch] gpiolib: fix oops in gpio_get_value_cansleep()
    Date
    On Thursday 16 October 2008, Anton Vorontsov wrote:
    > > -     return chip->get(chip, gpio - chip->base);
    > > +     return chip->get ? chip->get(chip, gpio - chip->base) : 0;
    >
    > Why don't we check the .set in the gpio_set_value? Because
    > we must always call gpio_direction_output()?

    Yes. The output driver needs to be explicitly enabled,
    to avoid misbehaving electic circuitry.


    > It is not exactly the
    > same we work with the input direction.. is this documented anywhere?

    In Documentation/gpio.txt since the very first versions.

    See the section on "Spinlock-Safe GPIO access", where
    special cases for reading the value of output GPIOs are
    desribed. Other aspects are mentioned in other spots.

    For example, open drain signals -- as used with I2C and
    other protocols -- only drive the "low" signal level,
    and if code wants a "high" level it's got to verify that
    nobody else is driving that shared line to low. By
    reading it back, even though it's configured as output.

    - Dave


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