Documentation: ACPI: move gpio-properties.txt to firmware-guide/acpi and convert to reST

.. SPDX-License-Identifier: GPL-2.0

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_DSD Device Properties Related to GPIO

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======================================

With the release of ACPI 5.1, the _DSD configuration object finally

allows names to be given to GPIOs (and other things as well) returned

the _CRS output ordering, for example).

With _DSD we can now query GPIOs using a name instead of an integer

index, like the ASL example below shows:

index, like the ASL example below shows::

  // Bluetooth device with reset and shutdown GPIOs

  Device (BTH)

      })

  }

The format of the supported GPIO property is:

The format of the supported GPIO property is::

  Package () { "name", Package () { ref, index, pin, active_low }}

  ref - The device that has _CRS containing GpioIo()/GpioInt() resources,

ref

  The device that has _CRS containing GpioIo()/GpioInt() resources,

  typically this is the device itself (BTH in our case).

  index - Index of the GpioIo()/GpioInt() resource in _CRS starting from zero.

  pin - Pin in the GpioIo()/GpioInt() resource. Typically this is zero.

  active_low - If 1 the GPIO is marked as active_low.

index

  Index of the GpioIo()/GpioInt() resource in _CRS starting from zero.

pin

  Pin in the GpioIo()/GpioInt() resource. Typically this is zero.

active_low

  If 1 the GPIO is marked as active_low.

Since ACPI GpioIo() resource does not have a field saying whether it is

active low or high, the "active_low" argument can be used here.  Setting

cases like with SPI host controllers where some chip selects may be

implemented as GPIOs and some as native signals. For example a SPI host

controller can have chip selects 0 and 2 implemented as GPIOs and 1 as

native:

native::

  Package () {

      "cs-gpios",

  }

Other supported properties

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==========================

Following Device Tree compatible device properties are also supported by

_DSD device properties for GPIO controllers:

- input

- line-name

Example:

Example::

  Name (_DSD, Package () {

      // _DSD Hierarchical Properties Extension UUID

- gpio-line-names

Example:

Example::

  Package () {

      "gpio-line-names",

about these properties.

ACPI GPIO Mappings Provided by Drivers

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There are systems in which the ACPI tables do not contain _DSD but provide _CRS

with GpioIo()/GpioInt() resources and device drivers still need to work with

respectively, in analogy with the _DSD GPIO property format specified above.

For the example Bluetooth device discussed previously the data structures in

question would look like this:

question would look like this::

  static const struct acpi_gpio_params reset_gpio = { 1, 1, false };

  static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };

table was previously registered.

Using the _CRS fallback

-----------------------

=======================

If a device does not have _DSD or the driver does not create ACPI GPIO

mapping, the Linux GPIO framework refuses to return any GPIOs. This is

because the driver does not know what it actually gets. For example if we

have a device like below:

have a device like below::

  Device (BTH)

  {

      })

  }

The driver might expect to get the right GPIO when it does:

The driver might expect to get the right GPIO when it does::

  desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);

objects, as listed in the above chapter, of the device in question.

Getting GPIO descriptor

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=======================

There are two main approaches to get GPIO resource from ACPI::

There are two main approaches to get GPIO resource from ACPI:

  desc = gpiod_get(dev, connection_id, flags);

  desc = gpiod_get_index(dev, connection_id, index, flags);

We may consider two different cases here, i.e. when connection ID is

provided and otherwise.

Case 1:

Case 1::

  desc = gpiod_get(dev, "non-null-connection-id", flags);

  desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);

Case 2:

Case 2::

  desc = gpiod_get(dev, NULL, flags);

  desc = gpiod_get_index(dev, NULL, index, flags);

   enumeration

   osi

   DSD-properties-rules

   gpio-properties

L:	linux-gpio@vger.kernel.org

L:	linux-acpi@vger.kernel.org

S:	Maintained

F:	Documentation/acpi/gpio-properties.txt

F:	Documentation/firmware-guide/acpi/gpio-properties.rst

F:	drivers/gpio/gpiolib-acpi.c

GPIO IR Transmitter