linux-kernel - Re: [PATCH] docs: gpio: explain GPIOD_OUT

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Date:   Wed, 21 Jul 2021 15:21:57 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Hannu Hartikainen <hannu@...k.in>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        linux-doc <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] docs: gpio: explain GPIOD_OUT_* values and toggling
 active low

On Thu, Jul 8, 2021 at 5:21 PM Hannu Hartikainen <hannu@...k.in> wrote:
>
> 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@...k.in>
> ---
>  Documentation/driver-api/gpio/consumer.rst | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/driver-api/gpio/consumer.rst b/Documentation/driver-api/gpio/consumer.rst
> index 3366a991b4aa..47869ca8ccf0 100644
> --- a/Documentation/driver-api/gpio/consumer.rst
> +++ b/Documentation/driver-api/gpio/consumer.rst
> @@ -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.
> --
> 2.32.0
>

Makes sense, applied!

Bartosz