dt-bindings: input: add GPIO charlieplex keypad

Add DT bindings for GPIO charlieplex keypad.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://patch.msgid.link/20260312180304.3865850-4-hugo@hugovil.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/gpio-charlieplex-keypad.yaml

@@ -0,0 +1,108 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+
+$id: http://devicetree.org/schemas/input/gpio-charlieplex-keypad.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GPIO charlieplex keypad
+
+maintainers:
+  - Hugo Villeneuve <hvilleneuve@dimonoff.com>
+
+description: |
+  The charlieplex keypad supports N^2)-N different key combinations (where N is
+  the number of I/O lines). Key presses and releases are detected by configuring
+  only one line as output at a time, and reading other line states. This process
+  is repeated for each line. Diodes are required to ensure current flows in only
+  one direction between any pair of pins, as well as pull-up or pull-down
+  resistors on all I/O lines.
+  This mechanism doesn't allow to detect simultaneous key presses.
+
+  Wiring example for 3 lines keyboard with 6 switches and 3 diodes (pull-up/down
+  resistors not shown but needed on L0, L1 and L2):
+
+  L0  --+---------------------+----------------------+
+        |                     |                      |
+  L1  -------+-----------+---------------------+     |
+        |    |           |    |                |     |
+  L2  -------------+----------------+-----+    |     |
+        |    |     |     |    |     |     |    |     |
+        |    |     |     |    |     |     |    |     |
+        |  S1 \  S2 \    |  S3 \  S4 \    |  S5 \  S6 \
+        |    |     |     |    |     |     |    |     |
+        |    +--+--+     |    +--+--+     |    +--+--+
+        |       |        |       |        |       |
+        |    D1 v        |    D2 v        |    D3 v
+        |       - (k)    |       - (k)    |       - (k)
+        |       |        |       |        |       |
+        +-------+        +-------+        +-------+
+
+  L: GPIO line
+  S: switch
+  D: diode (k indicates cathode)
+
+allOf:
+  - $ref: input.yaml#
+  - $ref: /schemas/input/matrix-keymap.yaml#
+
+properties:
+  compatible:
+    const: gpio-charlieplex-keypad
+
+  autorepeat: true
+
+  debounce-delay-ms:
+    default: 5
+
+  line-gpios:
+    description:
+      List of GPIOs used as lines. The gpio specifier for this property
+      depends on the gpio controller to which these lines are connected.
+
+  linux,keymap: true
+
+  poll-interval: true
+
+  settling-time-us: true
+
+  wakeup-source: true
+
+required:
+  - compatible
+  - line-gpios
+  - linux,keymap
+  - poll-interval
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+    #include <dt-bindings/input/input.h>
+
+    keyboard {
+        compatible = "gpio-charlieplex-keypad";
+        debounce-delay-ms = <20>;
+        poll-interval = <5>;
+        settling-time-us = <2>;
+
+        line-gpios = <&gpio2 25 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)
+                      &gpio2 26 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)
+                      &gpio2 27 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)>;
+
+        /* MATRIX_KEY(output, input, key-code) */
+        linux,keymap = <
+            /*
+             * According to wiring diagram above, if L1 is configured as
+             * output and HIGH, and we detect a HIGH level on input L0,
+             * then it means S1 is pressed: MATRIX_KEY(L1, L0, KEY...)
+             */
+            MATRIX_KEY(1, 0, KEY_F1) /* S1 */
+            MATRIX_KEY(2, 0, KEY_F2) /* S2 */
+            MATRIX_KEY(0, 1, KEY_F3) /* S3 */
+            MATRIX_KEY(2, 1, KEY_F4) /* S4 */
+            MATRIX_KEY(1, 2, KEY_F5) /* S5 */
+            MATRIX_KEY(0, 2, KEY_F6) /* S6 */
+        >;
+    };