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Documentation: gpio: consumer: describe active low property 

I've been searching for any documentation of 'the active-low property of a GPIO'
already mentioned in this documenation. But couldn't find any. Add it.

Sigend-off-by: Dirk Behme <dirk.behme@gmail.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
[Spelling, grammar fixes]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
This commit is contained in:
Dirk Behme 2015-07-18 08:02:07 +02:00 committed by Linus Walleij
parent 929550b9f9
commit ac49fbd1f9
1 changed files with 33 additions and 0 deletions
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33
Documentation/gpio/consumer.txt
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@ -237,6 +237,39 @@ Note that these functions should only be used with great moderation ; a driver
should not have to care about the physical line level.
The active-low property
-----------------------
As a driver should not have to care about the physical line level, all of the
gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
the *logical* value. With this they take the active-low property into account.
This means that they check whether the GPIO is configured to be active-low,
and if so, they manipulate the passed value before the physical line level is
driven.
With this, all the gpiod_set_(array)_value_xxx() functions interpret the
parameter "value" as "active" ("1") or "inactive" ("0"). The physical line
level will be driven accordingly.
As an example, if the active-low property for a dedicated GPIO is set, and the
gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level
will be driven low.
To summarize:
Function (example) active-low proporty physical line
gpiod_set_raw_value(desc, 0); don't care low
gpiod_set_raw_value(desc, 1); don't care high
gpiod_set_value(desc, 0); default (active-high) low
gpiod_set_value(desc, 1); default (active-high) high
gpiod_set_value(desc, 0); active-low high
gpiod_set_value(desc, 1); active-low low
Please note again that the set_raw/get_raw functions should be avoided as much
as possible, especially by drivers which should not care about the actual
physical line level and worry about the logical value instead.
Set multiple GPIO outputs with a single function call
-----------------------------------------------------
The following functions set the output values of an array of GPIOs: