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docs: gpio: explain GPIOD_OUT_* values and toggling active low

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I was confused about the gpiod_flags values and thought that
GPIOD_OUT_LOW and GPIOD_OUT_HIGH set the line to be active low / active
high. This is not true, but I got the misconception because the flags
GPIOD_OUT_*_OPEN_DRAIN do change line configuration and there's a
subchapter about *active low* and *open drain* semantics.

Add an explicit mention that the initial value is a logical value (and
not the line configuration or physical line level). Also add a mention
of the function gpiod_toggle_active_low which was previously missing
from this document.

Signed-off-by: Hannu Hartikainen <hannu@hrtk.in>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
This commit is contained in:
Hannu Hartikainen 2021-07-08 18:20:54 +03:00 committed by Bartosz Golaszewski
parent 17ce60b2e4
commit f3f1017a98
1 changed files with 9 additions and 1 deletions
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10
Documentation/driver-api/gpio/consumer.rst
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@ -72,6 +72,10 @@ for the GPIO. Values can be:
* GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line
to be electrically used with open drain.
Note that the initial value is *logical* and the physical line level depends on
whether the line is configured active high or active low (see
:ref:`active_low_semantics`).
The two last flags are used for use cases where open drain is mandatory, such
as I2C: if the line is not already configured as open drain in the mappings
(see board.txt), then open drain will be enforced anyway and a warning will be
@ -252,6 +256,8 @@ that can't be accessed from hardIRQ handlers, these calls act the same as the
spinlock-safe calls.
.. _active_low_semantics:
The active low and open drain semantics
---------------------------------------
As a consumer should not have to care about the physical line level, all of the
@ -309,9 +315,11 @@ work on the raw line value::
void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
The active low state of a GPIO can also be queried using the following call::
The active low state of a GPIO can also be queried and toggled using the
following calls::
int gpiod_is_active_low(const struct gpio_desc *desc)
void gpiod_toggle_active_low(struct gpio_desc *desc)
Note that these functions should only be used with great moderation; a driver
should not have to care about the physical line level or open drain semantics.