This change adds support for specifying device tree buttons emitting
abs/rel events.

ABS events were previously supported, but only via platform data, so add
the missing device tree property to allow axis values to be emitted with
abs/rel events.

Also emit 0 on button releases for REL and ABS keys. This is a must-have
for supporting digital joysticks, and aligns the driver whith
gpio-keys-polled.

Finally, report min/max values for abs axes to the input framework.

Signed-off-by: Hans Holmberg <redacted>
---

RFC because it alters the behaviour of abs-buttons making them
un-sticky, reporting 0 on button releases - just like gpio-keys-polled.

FWIW, i've looked through the in-tree board files and not found a
single occurence of abs gpio-buttons.

Changes since v1:
* Corrected cc list - now the mailing lists are looped in.

 .../devicetree/bindings/input/gpio-keys.txt        | 50 +++++++++++++++++++++-
 drivers/input/keyboard/gpio_keys.c                 | 29 +++++++++++--
 2 files changed, 75 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/input/gpio-keys.txt b/Documentation/devicetree/bindings/input/gpio-keys.txt
index a949404..1b53eed 100644
--- a/Documentation/devicetree/bindings/input/gpio-keys.txt
+++ b/Documentation/devicetree/bindings/input/gpio-keys.txt

@@ -22,6 +22,13 @@ both at the same time. Specifying both properties is allowed.
 Optional subnode-properties:
 	- linux,input-type: Specify event type this button/key generates.
 	  If not specified defaults to <1> == EV_KEY.
+	- linux,input-value: If linux,input-type is EV_ABS or EV_REL then this
+	  value is sent for events this button generates when pressed.
+	  EV_ABS/EV_REL axis will generate an event with a value of 0 when
+	  all buttons with linux,input-type == type and linux,code == axis
+	  are released. This value is interpreted as a signed 32 bit value,
+	  e.g. to make a button generate a value of -1 use:
+	  linux,input-value = <0xffffffff>; /* -1 */
 	- debounce-interval: Debouncing interval time in milliseconds.
 	  If not specified defaults to 5.
 	- wakeup-source: Boolean, button can wake-up the system.

@@ -47,4 +54,45 @@ Example nodes:
 				linux,code = <108>;
 				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;
 			};
-			...
+	...
+
+	gpio-joystick: {