gpio: Document GPIO hogging mechanism

Add GPIO hogging documentation to gpio.txt

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/gpio/gpio.txt b/Documentation/devicetree/bindings/gpio/gpio.txt
index f7a158d85862..5788d5cf1252 100644
--- a/Documentation/devicetree/bindings/gpio/gpio.txt
+++ b/Documentation/devicetree/bindings/gpio/gpio.txt
@@ -116,6 +116,29 @@ Every GPIO controller node must contain both an empty "gpio-controller"
 property, and a #gpio-cells integer property, which indicates the number of
 cells in a gpio-specifier.
 
+The GPIO chip may contain GPIO hog definitions. GPIO hogging is a mechanism
+providing automatic GPIO request and configuration as part of the
+gpio-controller's driver probe function.
+
+Each GPIO hog definition is represented as a child node of the GPIO controller.
+Required properties:
+- gpio-hog:   A property specifying that this child node represent a GPIO hog.
+- gpios:      Store the GPIO information (id, flags, ...). Shall contain the
+              number of cells specified in its parent node (GPIO controller
+              node).
+Only one of the following properties scanned in the order shown below.
+This means that when multiple properties are present they will be searched
+in the order presented below and the first match is taken as the intended
+configuration.
+- input:      A property specifying to set the GPIO direction as input.
+- output-low  A property specifying to set the GPIO direction as output with
+              the value low.
+- output-high A property specifying to set the GPIO direction as output with
+              the value high.
+
+Optional properties:
+- line-name:  The GPIO label name. If not present the node name is used.
+
 Example of two SOC GPIO banks defined as gpio-controller nodes:
 
         qe_pio_a: gpio-controller@1400 {
@@ -123,6 +146,13 @@ Example of two SOC GPIO banks defined as gpio-controller nodes:
                 reg = <0x1400 0x18>;
                 gpio-controller;
                 #gpio-cells = <2>;
+
+                line_b {
+                        gpio-hog;
+                        gpios = <6 0>;
+                        output-low;
+                        line-name = "foo-bar-gpio";
+                };
         };
 
         qe_pio_e: gpio-controller@1460 {