Re: [PATCH v3 3/4] dt-bindings: input: add GPIO charlieplex keypad

[Hugo Villeneuve <hugo@hugovil.com>]

Hi Geert,

On Thu, 26 Feb 2026 10:32:30 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> Hi Hugo,
> 
> On Wed, 25 Feb 2026 at 16:54, Hugo Villeneuve <hugo@hugovil.com> wrote:
> > 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!
> 
> > --- /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.
> 
> > +
> > +  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, using something like this:

  line-gpios = <&gpio2 25 (GPIO_ACTIVE_LOW | GPIO_PULL_UP)

should work I think, although with my hardware unfortunately I cannot
test it.

Hugo.


> 
> > +
> > +  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.
> 
> > +        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