Re: [patch] gpiolib: fix oops in gpio_get_value_cansleep()

From: David Brownell
Date: Thu Oct 16 2008 - 13:10:07 EST

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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