Messages in this thread | | | From | David Brownell <> | Subject | Re: [patch] gpiolib: fix oops in gpio_get_value_cansleep() | Date | Thu, 16 Oct 2008 10:06:21 -0700 |
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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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