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>
---
 Documentation/gpio/consumer.txt | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

On Sun, Jul 5, 2015 at 1:19 AM, Dirk Behme <dirk.behme@gmail.com> wrote:
> 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>
> ---
>  Documentation/gpio/consumer.txt | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/Documentation/gpio/consumer.txt b/Documentation/gpio/consumer.txt
> index 75542b9..f1a6e20 100644
> --- a/Documentation/gpio/consumer.txt
> +++ b/Documentation/gpio/consumer.txt
> @@ -236,6 +236,32 @@ The active-low state of a GPIO can also be queried using the following call:
>  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_xxx_value_xxx() functions do take the active-low property into account.
> +This does mean 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.

This would be "all of the gpiod_set_value_xxx() functions, since
gpiod_set_raw_value_xxx() ignores the active-low setting.

I think it would also be worth to explain 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.

> +
> +With this, all the gpiod_set_xxx_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 poperty for a dedicated GPIO is set, and the
> +gpiod_set_xxx_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);              n/a              low
> +gpiod_set_value(desc, 1);              n/a              high

What is n/a here? Shouldn't it be "active high (default)"?

Once clarified,

Acked-by: Alexandre Courbot <acourbot@nvidia.com>
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