ACPI / GPIO: Document ACPI GPIO mappings API

Document the previously introduced method that can be used by device
drivers to provide the GPIO subsystem with mappings between GPIO names
(connection IDs) and GpioIo()/GpioInt() resources in _CRS.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/Documentation/acpi/gpio-properties.txt b/Documentation/acpi/gpio-properties.txt
index 3e45b8b7e4f25..ae36fcf86dc72 100644
--- a/Documentation/acpi/gpio-properties.txt
+++ b/Documentation/acpi/gpio-properties.txt
@@ -50,3 +50,47 @@ 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.