add gpio-key overlay (raspberrypi#2329)

An overlay that allows a Linux key to be bound to a GPIO.

arch/arm/boot/dts/overlays/Makefile

@@ -31,6 +31,7 @@ dtbo-$(CONFIG_ARCH_BCM2835) += \
 	googlevoicehat-soundcard.dtbo \
 	gpio-ir.dtbo \
 	gpio-ir-tx.dtbo \
+	gpio-key.dtbo \
 	gpio-poweroff.dtbo \
 	gpio-shutdown.dtbo \
 	hifiberry-amp.dtbo \

arch/arm/boot/dts/overlays/README

@@ -515,6 +515,27 @@ Params: gpio_pin                Output GPIO (default 18)
                                 Default is "0" (active-high).
 
 
+Name:   gpio-key
+Info:   This is a generic overlay for activating GPIO keypresses using
+        the gpio-keys library and this dtoverlay. Multiple keys can be
+        set up using multiple calls to the overlay for configuring
+        additional buttons or joysticks. You can see available keycodes
+        at https://github.com/torvalds/linux/blob/v4.12/include/uapi/
+        linux/input-event-codes.h#L64
+Load:   dtoverlay=gpio-key,<param>=<val>
+Params: gpio                    GPIO pin to trigger on (default 3)
+        active_low              When this is 1 (active low), a falling
+                                edge generates a key down event and a
+                                rising edge generates a key up event.
+                                When this is 0 (active high), this is
+                                reversed. The default is 1 (active low)
+        gpio_pull               Desired pull-up/down state (off, down, up)
+                                Default is "up". Note that the default pin
+                                (GPIO3) has an external pullup
+        label                   Set a label for the key
+        keycode                 Set the key code for the button
+
+
 Name:   gpio-poweroff
 Info:   Drives a GPIO high or low on poweroff (including halt). Enabling this
         overlay will prevent the ability to boot by driving GPIO3 low.

arch/arm/boot/dts/overlays/gpio-key-overlay.dts

@@ -0,0 +1,48 @@
+// Definitions for gpio-key module
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2708";
+
+	fragment@0 {
+		// Configure the gpio pin controller
+		target = <&gpio>;
+		__overlay__ {
+			pin_state: button_pins@0 {
+				brcm,pins = <3>; // gpio number
+				brcm,function = <0>; // 0 = input, 1 = output
+				brcm,pull = <2>; // 0 = none, 1 = pull down, 2 = pull up
+			};
+		};
+	};
+	fragment@1 {
+		target-path = "/";
+		__overlay__ {
+			button: button@0 {
+				compatible = "gpio-keys";
+				pinctrl-names = "default";
+				pinctrl-0 = <&pin_state>;
+				status = "okay";
+
+				key: key {
+					linux,code = <116>;
+					gpios = <&gpio 3 1>;
+					label = "KEY_POWER";
+				};
+			};
+		};
+	};
+
+	__overrides__ {
+		gpio =       <&key>,"gpios:4",
+		             <&button>,"reg:0",
+		             <&pin_state>,"brcm,pins:0",
+		             <&pin_state>,"reg:0";
+		label =      <&key>,"label";
+		keycode =    <&key>,"linux,code:0";
+		gpio_pull =  <&pin_state>,"brcm,pull:0";
+		active_low = <&key>,"gpios:8";
+	};
+
+};