_DSD Device Properties Related to GPIO
--------------------------------------

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
by _CRS.  Previously, we were only able to use an integer index to find
the corresponding GPIO, which is pretty error prone (it depends on
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:

  // Bluetooth device with reset and shutdown GPIOs
  Device (BTH)
  {
      Name (_HID, ...)

      Name (_CRS, ResourceTemplate ()
      {
          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
                  "\\_SB.GPO0", 0, ResourceConsumer) {27, 31}
      })

      Name (_DSD, Package ()
      {
          ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
          Package ()
	  {
              Package () {"reset-gpio", Package() {^BTH, 1, 1, 0 }},
              Package () {"shutdown-gpio", Package() {^BTH, 0, 0, 0 }},
          }
      })
  }

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,
        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.

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
it to 1 marks the GPIO as active low.

In our Bluetooth example the "reset-gpio" refers to the second GpioIo()
resource, second pin in that resource with the GPIO number of 31.

ACPI GPIO Mappings Provided by Drivers
--------------------------------------

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
them.

In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV,
available to the driver can be used to identify the device and that is supposed
to be sufficient to determine the meaning and purpose of all of the GPIO lines
listed by the GpioIo()/GpioInt() resources returned by _CRS.  In other words,
the driver is supposed to know what to use the GpioIo()/GpioInt() resources for
once it has identified the device.  Having done that, it can simply assign names
to the GPIO lines it is going to use and provide the GPIO subsystem with a
mapping between those names and the ACPI GPIO resources corresponding to them.

To do that, the driver needs to define a mapping table as a NULL-terminated
array of struct acpi_gpio_mapping objects that each contain a name, a pointer
to an array of line data (struct acpi_gpio_params) objects and the size of that
array.  Each struct acpi_gpio_params object consists of three fields,
crs_entry_index, line_index, active_low, representing the index of the target
GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target
line in that resource starting from zero, and the active-low flag for that line,
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:

static const struct acpi_gpio_params reset_gpio = { 1, 1, false };
static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };

static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = {
  { "reset-gpio", &reset_gpio, 1 },
  { "shutdown-gpio", &shutdown_gpio, 1 },
  { },
};

Next, the mapping table needs to be passed as the second argument to
acpi_dev_add_driver_gpios() that will register it with the ACPI device object
pointed to by its first argument.  That should be done in the driver's .probe()
routine.  On removal, the driver should unregister its GPIO mapping table by
calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
table was previously registered.