Input: charlieplex_keypad - add GPIO charlieplex keypad

Add support for GPIO-based charlieplex keypad, allowing to control
N^2-N keys using N GPIO lines.

Reuse matrix keypad keymap to simplify, even if there is no concept
of rows and columns in this type of keyboard.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://patch.msgid.link/20260312180304.3865850-5-hugo@hugovil.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

MAINTAINERS

@@ -5962,6 +5962,13 @@ S:	Maintained
 F:	Documentation/hwmon/powerz.rst
 F:	drivers/hwmon/powerz.c
 
+CHARLIEPLEX KEYPAD DRIVER
+M:	Hugo Villeneuve <hvilleneuve@dimonoff.com>
+S:	Supported
+W:	http://www.mosaic-industries.com/embedded-systems/microcontroller-projects/electronic-circuits/matrix-keypad-scan-decode
+F:	Documentation/devicetree/bindings/input/gpio-charlieplex-keypad.yaml
+F:	drivers/input/keyboard/charlieplex_keypad.c
+
 CHECKPATCH
 M:	Andy Whitcroft <apw@canonical.com>
 M:	Joe Perches <joe@perches.com>

drivers/input/keyboard/Kconfig

@@ -289,6 +289,20 @@ config KEYBOARD_MATRIX
 	  To compile this driver as a module, choose M here: the
 	  module will be called matrix_keypad.
 
+config KEYBOARD_CHARLIEPLEX
+	tristate "GPIO driven charlieplex keypad support"
+	depends on GPIOLIB || COMPILE_TEST
+	select INPUT_MATRIXKMAP
+	help
+	  Enable support for GPIO driven charlieplex keypad. A charlieplex
+	  keypad allows to use fewer GPIO lines to interface to key switches.
+	  For example, an N lines charlieplex keypad can be used to interface
+	  to N^2-N different key switches. However, this type of keypad
+	  cannot detect more than one key press at a time.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called charlieplex_keypad.
+
 config KEYBOARD_HIL_OLD
 	tristate "HP HIL keyboard support (simple driver)"
 	depends on GSC || HP300

drivers/input/keyboard/Makefile

@@ -15,6 +15,7 @@ obj-$(CONFIG_KEYBOARD_ATARI)		+= atakbd.o
 obj-$(CONFIG_KEYBOARD_ATKBD)		+= atkbd.o
 obj-$(CONFIG_KEYBOARD_BCM)		+= bcm-keypad.o
 obj-$(CONFIG_KEYBOARD_CAP11XX)		+= cap11xx.o
+obj-$(CONFIG_KEYBOARD_CHARLIEPLEX)	+= charlieplex_keypad.o
 obj-$(CONFIG_KEYBOARD_CLPS711X)		+= clps711x-keypad.o
 obj-$(CONFIG_KEYBOARD_CROS_EC)		+= cros_ec_keyb.o
 obj-$(CONFIG_KEYBOARD_CYPRESS_SF)	+= cypress-sf.o

drivers/input/keyboard/charlieplex_keypad.c

@@ -0,0 +1,232 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * GPIO driven charlieplex keypad driver
+ *
+ * Copyright (c) 2026 Hugo Villeneuve <hvilleneuve@dimonoff.com>
+ *
+ * Based on matrix_keyboard.c
+ */
+
+#include <linux/bitops.h>
+#include <linux/delay.h>
+#include <linux/dev_printk.h>
+#include <linux/device/devres.h>
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/input.h>
+#include <linux/input/matrix_keypad.h>
+#include <linux/math.h>
+#include <linux/module.h>
+#include <linux/mod_devicetable.h>
+#include <linux/platform_device.h>
+#include <linux/property.h>
+#include <linux/string_helpers.h>
+#include <linux/types.h>
+
+struct charlieplex_keypad {
+	struct input_dev *input_dev;
+	struct gpio_descs *line_gpios;
+	unsigned int nlines;
+	unsigned int settling_time_us;
+	unsigned int debounce_threshold;
+	unsigned int debounce_count;
+	int debounce_code;
+	int current_code;
+};
+
+static void charlieplex_keypad_report_key(struct input_dev *input)
+{
+	struct charlieplex_keypad *keypad = input_get_drvdata(input);
+	const unsigned short *keycodes = input->keycode;
+
+	if (keypad->current_code > 0) {
+		input_event(input, EV_MSC, MSC_SCAN, keypad->current_code);
+		input_report_key(input, keycodes[keypad->current_code], 0);
+		input_sync(input);
+	}
+
+	if (keypad->debounce_code) {
+		input_event(input, EV_MSC, MSC_SCAN, keypad->debounce_code);
+		input_report_key(input, keycodes[keypad->debounce_code], 1);
+		input_sync(input);
+	}
+
+	keypad->current_code = keypad->debounce_code;
+}
+
+static void charlieplex_keypad_check_switch_change(struct input_dev *input,
+						   unsigned int code)
+{
+	struct charlieplex_keypad *keypad = input_get_drvdata(input);
+
+	if (code != keypad->debounce_code) {
+		keypad->debounce_count = 0;
+		keypad->debounce_code = code;
+	}
+
+	if (keypad->debounce_code != keypad->current_code) {
+		if (keypad->debounce_count++ >= keypad->debounce_threshold)
+			charlieplex_keypad_report_key(input);
+	}
+}
+
+static int charlieplex_keypad_scan_line(struct charlieplex_keypad *keypad,
+					unsigned int oline)
+{
+	struct gpio_descs *line_gpios = keypad->line_gpios;
+	DECLARE_BITMAP(values, MATRIX_MAX_ROWS);
+	int err;
+
+	/* Activate only one line as output at a time. */
+	gpiod_direction_output(line_gpios->desc[oline], 1);
+
+	if (keypad->settling_time_us)
+		fsleep(keypad->settling_time_us);
+
+	/* Read input on all other lines. */
+	err = gpiod_get_array_value_cansleep(line_gpios->ndescs, line_gpios->desc,
+					     line_gpios->info, values);
+
+	gpiod_direction_input(line_gpios->desc[oline]);
+
+	if (err)
+		return err;
+
+	for (unsigned int iline = 0; iline < keypad->nlines; iline++) {
+		if (iline == oline)
+			continue; /* Do not read active output line. */
+
+		/* Check if GPIO is asserted. */
+		if (test_bit(iline, values))
+			return MATRIX_SCAN_CODE(oline, iline,
+						get_count_order(keypad->nlines));
+	}
+
+	return 0;
+}
+
+static void charlieplex_keypad_poll(struct input_dev *input)
+{
+	struct charlieplex_keypad *keypad = input_get_drvdata(input);
+	int code = 0;
+
+	for (unsigned int oline = 0; oline < keypad->nlines; oline++) {
+		code = charlieplex_keypad_scan_line(keypad, oline);
+		if (code != 0)
+			break;
+	}
+
+	if (code >= 0)
+		charlieplex_keypad_check_switch_change(input, code);
+}
+
+static int charlieplex_keypad_init_gpio(struct platform_device *pdev,
+					struct charlieplex_keypad *keypad)
+{
+	char **pin_names;
+	char label[32];
+
+	snprintf(label, sizeof(label), "%s-pin", pdev->name);
+
+	keypad->line_gpios = devm_gpiod_get_array(&pdev->dev, "line", GPIOD_IN);
+	if (IS_ERR(keypad->line_gpios))
+		return PTR_ERR(keypad->line_gpios);
+
+	keypad->nlines = keypad->line_gpios->ndescs;
+
+	if (keypad->nlines > MATRIX_MAX_ROWS)
+		return -EINVAL;
+
+	pin_names = devm_kasprintf_strarray(&pdev->dev, label, keypad->nlines);
+	if (IS_ERR(pin_names))
+		return PTR_ERR(pin_names);
+
+	for (unsigned int i = 0; i < keypad->line_gpios->ndescs; i++)
+		gpiod_set_consumer_name(keypad->line_gpios->desc[i], pin_names[i]);
+
+	return 0;
+}
+
+static int charlieplex_keypad_probe(struct platform_device *pdev)
+{
+	struct charlieplex_keypad *keypad;
+	struct input_dev *input_dev;
+	unsigned int debounce_interval_ms = 5;
+	unsigned int poll_interval_ms;
+	int err;
+
+	keypad = devm_kzalloc(&pdev->dev, sizeof(*keypad), GFP_KERNEL);
+	if (!keypad)
+		return -ENOMEM;
+
+	input_dev = devm_input_allocate_device(&pdev->dev);
+	if (!input_dev)
+		return -ENOMEM;
+
+	keypad->input_dev = input_dev;
+
+	err = device_property_read_u32(&pdev->dev, "poll-interval", &poll_interval_ms);
+	if (err)
+		return dev_err_probe(&pdev->dev, err,
+				     "failed to parse 'poll-interval' property\n");
+
+	if (poll_interval_ms == 0)
+		return dev_err_probe(&pdev->dev, -EINVAL, "invalid 'poll-interval' value\n");
+
+	device_property_read_u32(&pdev->dev, "debounce-delay-ms", &debounce_interval_ms);
+	device_property_read_u32(&pdev->dev, "settling-time-us", &keypad->settling_time_us);
+
+	keypad->current_code = -1;
+	keypad->debounce_code = -1;
+	keypad->debounce_threshold = DIV_ROUND_UP(debounce_interval_ms, poll_interval_ms);
+
+	err = charlieplex_keypad_init_gpio(pdev, keypad);
+	if (err)
+		return err;
+
+	input_dev->name = pdev->name;
+	input_dev->id.bustype = BUS_HOST;
+
+	err = matrix_keypad_build_keymap(NULL, NULL, keypad->nlines,
+					 keypad->nlines, NULL, input_dev);
+	if (err)
+		return dev_err_probe(&pdev->dev, err, "failed to build keymap\n");
+
+	if (device_property_read_bool(&pdev->dev, "autorepeat"))
+		__set_bit(EV_REP, input_dev->evbit);
+
+	input_set_capability(input_dev, EV_MSC, MSC_SCAN);
+
+	err = input_setup_polling(input_dev, charlieplex_keypad_poll);
+	if (err)
+		return dev_err_probe(&pdev->dev, err, "unable to set up polling\n");
+
+	input_set_poll_interval(input_dev, poll_interval_ms);
+
+	input_set_drvdata(input_dev, keypad);
+
+	err = input_register_device(keypad->input_dev);
+	if (err)
+		return err;
+
+	return 0;
+}
+
+static const struct of_device_id charlieplex_keypad_dt_match[] = {
+	{ .compatible = "gpio-charlieplex-keypad" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, charlieplex_keypad_dt_match);
+
+static struct platform_driver charlieplex_keypad_driver = {
+	.probe		= charlieplex_keypad_probe,
+	.driver		= {
+		.name	= "charlieplex-keypad",
+		.of_match_table = charlieplex_keypad_dt_match,
+	},
+};
+module_platform_driver(charlieplex_keypad_driver);
+
+MODULE_AUTHOR("Hugo Villeneuve <hvilleneuve@dimonoff.com>");
+MODULE_DESCRIPTION("GPIO driven charlieplex keypad driver");
+MODULE_LICENSE("GPL");