Hi Geert,

On Thu, 26 Feb 2026 10:32:30 +0100
Geert Uytterhoeven [off-list ref] wrote:

Hi Hugo,

On Wed, 25 Feb 2026 at 16:54, Hugo Villeneuve [off-list ref] wrote:

quoted

From: Hugo Villeneuve <hvilleneuve@dimonoff.com>

Add DT bindings for GPIO charlieplex keypad.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>

Thanks for your patch!

quoted

--- /dev/null
+++ b/Documentation/devicetree/bindings/input/gpio-charlieplex-keypad.yaml

@@ -0,0 +1,106 @@
+# 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 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.
+  This mechanism doesn't allow to detect simultaneous key presses.

Indeed, e.g. pressing S1 and S2 simultaneously will show a ghost
S5 keypress.

quoted

+
+  Wiring example for 3 lines keyboard with 6 switches and 3 diodes:
+
+  L0  --+---------------------+----------------------+
+        |                     |                      |
+  L1  -------+-----------+---------------------+     |
+        |    |           |    |                |     |
+  L2  -------------+----------------+-----+    |     |
+        |    |     |     |    |     |     |    |     |
+        |    |     |     |    |     |     |    |     |
+        |  S1 \  S2 \    |  S3 \  S4 \    |  S5 \  S6 \
+        |    |     |     |    |     |     |    |     |
+        |    +--+--+     |    +--+--+     |    +--+--+
+        |       |        |       |        |       |
+        |    D1 v        |    D2 v        |    D3 v
+        |       - (k)    |       - (k)    |       - (k)
+        |       |        |       |        |       |
+        +-------+        +-------+        +-------+

Don't you need pull-down resistors on L[0-2], and/or a way to specify
in DT to enable internal poll-down on GPIO controllers that support it?
Some controllers may support internal pull-up only, but I guess that
can be handled using GPIO_ACTIVE_LOW?

Yes, I did not put any resistors in the diagram in order to keep it
simple and clear.

You need pull-down resistors either on the board, or specified in the
DT like we do for our board (we use the PCAL6416 with pull-up/down
support):

  line-gpios = <&gpio20 0 (GPIO_ACTIVE_HIGH | GPIO_PULL_DOWN)
                 ...

I will add this to the example, and add a few lines explaining this in
the description.

quoted

+
+  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>
+
+    charlieplex-keypad {

"keyboard", as per Devicetree Specification Generic Names
Recommendation.

Ok.

Thank you,
Hugo.

quoted

+        compatible = "gpio-charlieplex-keypad";
+        debounce-delay-ms = <20>;
+        poll-interval = <5>;
+        settling-time-us = <2>;
+
+        line-gpios = <&gpio2 25 GPIO_ACTIVE_HIGH
+                      &gpio2 26 GPIO_ACTIVE_HIGH
+                      &gpio2 27 GPIO_ACTIVE_HIGH>;
+
+        /* 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 */
+        >;
+    };

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

-- 
Hugo Villeneuve