[PATCH v6 2/3] counter: add GPIO-based counter driver

[Wadim Mueller <wafgo01@gmail.com>]

Add a platform driver that turns plain GPIOs into a counter device.
Edge interrupts on signal-a, signal-b (and optional index) are
decoded in software. Quadrature steps are validated with a 2-bit
Gray-code parity check; direction comes from the same identity.

The driver exposes two Counts on the same wires:

  "Count 1" - Synapses on A, B and optional Index 1. Supports
              QUADRATURE_X1_{A,B} / X2_{A,B} / X4, PULSE_DIRECTION,
              INCREASE and DECREASE.
  "Count 2" - Synapse on B and optional Index 2. Supports INCREASE
              and DECREASE only.

Per-Count state (value, ceiling, preset, enable, function, direction)
lives in struct gpio_counter_count_priv so new per-Count features
(e.g. floor) can be added without touching parallel arrays.

Sleepable GPIO providers (I2C/SPI expanders) are rejected at probe
since the ISRs run in hardirq context.

Signed-off-by: Wadim Mueller <wafgo01@gmail.com>
---
 drivers/counter/Kconfig        |   17 +
 drivers/counter/Makefile       |    1 +
 drivers/counter/gpio-counter.c | 1015 ++++++++++++++++++++++++++++++++
 3 files changed, 1033 insertions(+)
 create mode 100644 drivers/counter/gpio-counter.c

diff --git a/drivers/counter/Kconfig b/drivers/counter/Kconfig
index d30d22dfe..c20044032 100644
--- a/drivers/counter/Kconfig
+++ b/drivers/counter/Kconfig
@@ -68,6 +68,23 @@ config INTEL_QEP
 	  To compile this driver as a module, choose M here: the module
 	  will be called intel-qep.
 
+config GPIO_COUNTER
+	tristate "GPIO-based counter driver"
+	depends on GPIOLIB
+	help
+	  Select this option to enable the GPIO-based counter driver.  It
+	  reads A/B and an optional index signal via edge-triggered GPIO
+	  interrupts and decodes them according to the selected mode:
+	  Quadrature X1/X2/X4 (rotary or linear encoders), pulse-direction,
+	  and pure increase / decrease pulse counters.
+
+	  This is useful on SoCs that lack a dedicated hardware quadrature
+	  decoder or pulse counter, or where the signals are wired to
+	  generic GPIO pins rather than to a dedicated peripheral.
+
+	  To compile this driver as a module, choose M here: the
+	  module will be called gpio-counter.
+
 config INTERRUPT_CNT
 	tristate "Interrupt counter driver"
 	depends on GPIOLIB
diff --git a/drivers/counter/Makefile b/drivers/counter/Makefile
index fa3c1d08f..3959d69fb 100644
--- a/drivers/counter/Makefile
+++ b/drivers/counter/Makefile
@@ -14,6 +14,7 @@ obj-$(CONFIG_STM32_TIMER_CNT)	+= stm32-timer-cnt.o
 obj-$(CONFIG_STM32_LPTIMER_CNT)	+= stm32-lptimer-cnt.o
 obj-$(CONFIG_TI_EQEP)		+= ti-eqep.o
 obj-$(CONFIG_FTM_QUADDEC)	+= ftm-quaddec.o
+obj-$(CONFIG_GPIO_COUNTER)		+= gpio-counter.o
 obj-$(CONFIG_MICROCHIP_TCB_CAPTURE)	+= microchip-tcb-capture.o
 obj-$(CONFIG_INTEL_QEP)		+= intel-qep.o
 obj-$(CONFIG_TI_ECAP_CAPTURE)	+= ti-ecap-capture.o
diff --git a/drivers/counter/gpio-counter.c b/drivers/counter/gpio-counter.c
new file mode 100644
index 000000000..8d9fa0b61
--- /dev/null
+++ b/drivers/counter/gpio-counter.c
@@ -0,0 +1,1015 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * GPIO-based Counter Driver
+ *
+ * Software A/B (+ optional Z) edge decoding exposed as two Counts:
+ *   Count 1: A + B (+ Index 1). Quadrature, pulse-direction, inc/dec.
+ *   Count 2: B (+ Index 2). Inc / dec.
+ *
+ * Copyright (C) 2026 CMBlu Energy AG
+ * Author: Wadim Mueller <wafgo01@gmail.com>
+ */
+
+#include <linux/cleanup.h>
+#include <linux/counter.h>
+#include <linux/gpio/consumer.h>
+#include <linux/interrupt.h>
+#include <linux/irq.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+#include <linux/spinlock.h>
+#include <linux/types.h>
+
+enum gpio_counter_signal_id {
+	GPIO_COUNTER_SIGNAL_A = 0,
+	GPIO_COUNTER_SIGNAL_B,
+	GPIO_COUNTER_SIGNAL_INDEX1,
+	GPIO_COUNTER_SIGNAL_INDEX2,
+	GPIO_COUNTER_NUM_SIGNALS_MAX,
+};
+
+enum gpio_counter_count_id {
+	GPIO_COUNTER_COUNT_1 = 0,
+	GPIO_COUNTER_COUNT_2,
+	GPIO_COUNTER_NUM_COUNTS,
+};
+
+#define GPIO_COUNTER_COUNT1_NUM_SYNAPSES	3	/* A, B, Index 1 */
+#define GPIO_COUNTER_COUNT2_NUM_SYNAPSES	2	/* B, Index 2 */
+#define GPIO_COUNTER_MAX_INDEX			2
+
+/* per-count state */
+struct gpio_counter_count_priv {
+	u64				value;
+	u64				ceiling;
+	u64				preset;
+	bool				preset_enabled;
+	bool				enabled;
+	enum counter_function		function;
+	enum counter_count_direction	direction;
+};
+
+struct gpio_counter_priv {
+	struct gpio_desc *gpio_a;
+	struct gpio_desc *gpio_b;
+	struct gpio_desc *gpio_idx[GPIO_COUNTER_MAX_INDEX];
+
+	int irq_a;
+	int irq_b;
+	int irq_idx[GPIO_COUNTER_MAX_INDEX];
+
+	int n_idx;
+
+	/* protects count state, prev_a, prev_b */
+	spinlock_t lock;
+
+	/* serialises enable_write() callers and b_irq_users */
+	struct mutex enable_lock;
+
+	/* irq_b is shared by Count 1 and Count 2 (under enable_lock) */
+	int b_irq_users;
+
+	int prev_a;
+	int prev_b;
+
+	struct gpio_counter_count_priv count_priv[GPIO_COUNTER_NUM_COUNTS];
+
+	struct counter_signal  signals[GPIO_COUNTER_NUM_SIGNALS_MAX];
+	struct counter_synapse synapses_count1[GPIO_COUNTER_COUNT1_NUM_SYNAPSES];
+	struct counter_synapse synapses_count2[GPIO_COUNTER_COUNT2_NUM_SYNAPSES];
+	struct counter_count   counts[GPIO_COUNTER_NUM_COUNTS];
+};
+
+/* Gray-code: valid step <=> exactly one of A/B toggled. */
+#define GPIO_COUNTER_STATE_CHANGED(pa, pb, ca, cb)	\
+	((pa) ^ (pb) ^ (ca) ^ (cb))
+#define GPIO_COUNTER_GET_DIRECTION(pb, ca)			\
+	(((pb) ^ (ca)) ? COUNTER_COUNT_DIRECTION_FORWARD :	\
+			 COUNTER_COUNT_DIRECTION_BACKWARD)
+
+/* priv->lock held. */
+static void gpio_counter_value_step(struct gpio_counter_count_priv *cp)
+{
+	if (cp->direction == COUNTER_COUNT_DIRECTION_FORWARD) {
+		if (cp->value < cp->ceiling)
+			cp->value++;
+	} else {
+		if (cp->value > 0)
+			cp->value--;
+	}
+}
+
+/* priv->lock held. */
+static void gpio_counter_quadrature_x1_step(struct gpio_counter_count_priv *cp,
+					    int level)
+{
+	if (level && cp->direction == COUNTER_COUNT_DIRECTION_FORWARD)
+		gpio_counter_value_step(cp);
+	else if (!level && cp->direction == COUNTER_COUNT_DIRECTION_BACKWARD)
+		gpio_counter_value_step(cp);
+}
+
+static irqreturn_t gpio_counter_a_isr(int irq, void *dev_id)
+{
+	struct counter_device *counter = dev_id;
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp =
+		&priv->count_priv[GPIO_COUNTER_COUNT_1];
+	unsigned long flags;
+	bool enabled;
+	int a, b;
+
+	/* !! drops negative gpiod_get_value() errors. */
+	a = !!gpiod_get_value(priv->gpio_a);
+	b = !!gpiod_get_value(priv->gpio_b);
+
+	spin_lock_irqsave(&priv->lock, flags);
+
+	enabled = cp->enabled;
+	if (!enabled)
+		goto out;
+
+	switch (cp->function) {
+	case COUNTER_FUNCTION_QUADRATURE_X4:
+	case COUNTER_FUNCTION_QUADRATURE_X2_A:
+	case COUNTER_FUNCTION_QUADRATURE_X1_A:
+		if (!GPIO_COUNTER_STATE_CHANGED(priv->prev_a, priv->prev_b,
+						a, b))
+			break;
+		cp->direction = GPIO_COUNTER_GET_DIRECTION(priv->prev_b, a);
+		if (cp->function == COUNTER_FUNCTION_QUADRATURE_X1_A)
+			gpio_counter_quadrature_x1_step(cp, a);
+		else
+			gpio_counter_value_step(cp);
+		break;
+
+	case COUNTER_FUNCTION_PULSE_DIRECTION:
+		/* direction set in the B-ISR */
+		if (a)
+			gpio_counter_value_step(cp);
+		break;
+
+	case COUNTER_FUNCTION_INCREASE:
+	case COUNTER_FUNCTION_DECREASE:
+		gpio_counter_value_step(cp);
+		break;
+
+	default:
+		break;
+	}
+
+out:
+	priv->prev_a = a;
+	priv->prev_b = b;
+
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	if (enabled)
+		counter_push_event(counter, COUNTER_EVENT_CHANGE_OF_STATE,
+				   GPIO_COUNTER_COUNT_1);
+
+	return IRQ_HANDLED;
+}
+
+static irqreturn_t gpio_counter_b_isr(int irq, void *dev_id)
+{
+	struct counter_device *counter = dev_id;
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *c1 =
+		&priv->count_priv[GPIO_COUNTER_COUNT_1];
+	struct gpio_counter_count_priv *c2 =
+		&priv->count_priv[GPIO_COUNTER_COUNT_2];
+	bool c1_enabled, c2_enabled;
+	unsigned long flags;
+	int a, b;
+
+	a = !!gpiod_get_value(priv->gpio_a);
+	b = !!gpiod_get_value(priv->gpio_b);
+
+	spin_lock_irqsave(&priv->lock, flags);
+
+	c1_enabled = c1->enabled;
+	c2_enabled = c2->enabled;
+
+	/* Count 1: A+B decoding on this B edge */
+	if (c1_enabled) {
+		switch (c1->function) {
+		case COUNTER_FUNCTION_QUADRATURE_X4:
+		case COUNTER_FUNCTION_QUADRATURE_X2_B:
+		case COUNTER_FUNCTION_QUADRATURE_X1_B:
+			if (!GPIO_COUNTER_STATE_CHANGED(priv->prev_a,
+							priv->prev_b, a, b))
+				break;
+			c1->direction =
+				GPIO_COUNTER_GET_DIRECTION(priv->prev_b, a);
+			if (c1->function == COUNTER_FUNCTION_QUADRATURE_X1_B)
+				gpio_counter_quadrature_x1_step(c1, b);
+			else
+				gpio_counter_value_step(c1);
+			break;
+
+		case COUNTER_FUNCTION_PULSE_DIRECTION:
+			/* B is the direction line */
+			c1->direction = b ? COUNTER_COUNT_DIRECTION_BACKWARD
+					  : COUNTER_COUNT_DIRECTION_FORWARD;
+			break;
+
+		default:
+			break;
+		}
+	}
+
+	/* Count 2: B-only */
+	if (c2_enabled)
+		gpio_counter_value_step(c2);
+
+	priv->prev_a = a;
+	priv->prev_b = b;
+
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	if (c1_enabled)
+		counter_push_event(counter, COUNTER_EVENT_CHANGE_OF_STATE,
+				   GPIO_COUNTER_COUNT_1);
+	if (c2_enabled)
+		counter_push_event(counter, COUNTER_EVENT_CHANGE_OF_STATE,
+				   GPIO_COUNTER_COUNT_2);
+
+	return IRQ_HANDLED;
+}
+
+static void gpio_counter_index_pulse(struct counter_device *counter,
+				     enum gpio_counter_count_id count_id)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count_id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	if (cp->preset_enabled) {
+		cp->value = cp->preset;
+		if (cp->value > cp->ceiling)
+			cp->value = cp->ceiling;
+	}
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	counter_push_event(counter, COUNTER_EVENT_INDEX, count_id);
+}
+
+static irqreturn_t gpio_counter_index1_isr(int irq, void *dev_id)
+{
+	gpio_counter_index_pulse(dev_id, GPIO_COUNTER_COUNT_1);
+	return IRQ_HANDLED;
+}
+
+static irqreturn_t gpio_counter_index2_isr(int irq, void *dev_id)
+{
+	gpio_counter_index_pulse(dev_id, GPIO_COUNTER_COUNT_2);
+	return IRQ_HANDLED;
+}
+
+static int gpio_counter_count_read(struct counter_device *counter,
+				   struct counter_count *count, u64 *val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*val = cp->value;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_count_write(struct counter_device *counter,
+				    struct counter_count *count, u64 val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+
+	if (val > cp->ceiling) {
+		spin_unlock_irqrestore(&priv->lock, flags);
+		return -EINVAL;
+	}
+	cp->value = val;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static const enum counter_function gpio_counter_count1_functions[] = {
+	COUNTER_FUNCTION_INCREASE,
+	COUNTER_FUNCTION_DECREASE,
+	COUNTER_FUNCTION_PULSE_DIRECTION,
+	COUNTER_FUNCTION_QUADRATURE_X1_A,
+	COUNTER_FUNCTION_QUADRATURE_X1_B,
+	COUNTER_FUNCTION_QUADRATURE_X2_A,
+	COUNTER_FUNCTION_QUADRATURE_X2_B,
+	COUNTER_FUNCTION_QUADRATURE_X4,
+};
+
+static const enum counter_function gpio_counter_count2_functions[] = {
+	COUNTER_FUNCTION_INCREASE,
+	COUNTER_FUNCTION_DECREASE,
+};
+
+static int gpio_counter_function_read(struct counter_device *counter,
+				      struct counter_count *count,
+				      enum counter_function *function)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*function = cp->function;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_function_write(struct counter_device *counter,
+				       struct counter_count *count,
+				       enum counter_function function)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	cp->function = function;
+
+	switch (function) {
+	case COUNTER_FUNCTION_INCREASE:
+		cp->direction = COUNTER_COUNT_DIRECTION_FORWARD;
+		break;
+	case COUNTER_FUNCTION_DECREASE:
+		cp->direction = COUNTER_COUNT_DIRECTION_BACKWARD;
+		break;
+	case COUNTER_FUNCTION_PULSE_DIRECTION:
+		cp->direction = !!gpiod_get_value(priv->gpio_b)
+					? COUNTER_COUNT_DIRECTION_BACKWARD
+					: COUNTER_COUNT_DIRECTION_FORWARD;
+		break;
+	default:
+		/* quadrature: direction is set in the ISR */
+		break;
+	}
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_action_read(struct counter_device *counter,
+				    struct counter_count *count,
+				    struct counter_synapse *synapse,
+				    enum counter_synapse_action *action)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	enum gpio_counter_signal_id signal_id = synapse->signal->id;
+	enum counter_function function;
+	enum counter_count_direction direction;
+	unsigned long flags;
+
+	if (signal_id == GPIO_COUNTER_SIGNAL_INDEX1 ||
+	    signal_id == GPIO_COUNTER_SIGNAL_INDEX2) {
+		*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
+		return 0;
+	}
+
+	spin_lock_irqsave(&priv->lock, flags);
+	function = cp->function;
+	direction = cp->direction;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	*action = COUNTER_SYNAPSE_ACTION_NONE;
+
+	switch (function) {
+	case COUNTER_FUNCTION_QUADRATURE_X4:
+		*action = COUNTER_SYNAPSE_ACTION_BOTH_EDGES;
+		return 0;
+
+	case COUNTER_FUNCTION_QUADRATURE_X2_A:
+		if (signal_id == GPIO_COUNTER_SIGNAL_A)
+			*action = COUNTER_SYNAPSE_ACTION_BOTH_EDGES;
+		return 0;
+
+	case COUNTER_FUNCTION_QUADRATURE_X2_B:
+		if (signal_id == GPIO_COUNTER_SIGNAL_B)
+			*action = COUNTER_SYNAPSE_ACTION_BOTH_EDGES;
+		return 0;
+
+	case COUNTER_FUNCTION_QUADRATURE_X1_A:
+		if (signal_id == GPIO_COUNTER_SIGNAL_A) {
+			if (direction == COUNTER_COUNT_DIRECTION_FORWARD)
+				*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
+			else
+				*action = COUNTER_SYNAPSE_ACTION_FALLING_EDGE;
+		}
+		return 0;
+
+	case COUNTER_FUNCTION_QUADRATURE_X1_B:
+		if (signal_id == GPIO_COUNTER_SIGNAL_B) {
+			if (direction == COUNTER_COUNT_DIRECTION_FORWARD)
+				*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
+			else
+				*action = COUNTER_SYNAPSE_ACTION_FALLING_EDGE;
+		}
+		return 0;
+
+	case COUNTER_FUNCTION_PULSE_DIRECTION:
+		if (signal_id == GPIO_COUNTER_SIGNAL_A)
+			*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
+		return 0;
+
+	case COUNTER_FUNCTION_INCREASE:
+	case COUNTER_FUNCTION_DECREASE:
+		if ((count->id == GPIO_COUNTER_COUNT_1 &&
+		     signal_id == GPIO_COUNTER_SIGNAL_A) ||
+		    (count->id == GPIO_COUNTER_COUNT_2 &&
+		     signal_id == GPIO_COUNTER_SIGNAL_B))
+			*action = COUNTER_SYNAPSE_ACTION_BOTH_EDGES;
+		return 0;
+
+	default:
+		return -EINVAL;
+	}
+}
+
+static int gpio_counter_signal_read(struct counter_device *counter,
+				    struct counter_signal *signal,
+				    enum counter_signal_level *level)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_desc *gpio;
+	int ret;
+
+	switch (signal->id) {
+	case GPIO_COUNTER_SIGNAL_A:
+		gpio = priv->gpio_a;
+		break;
+	case GPIO_COUNTER_SIGNAL_B:
+		gpio = priv->gpio_b;
+		break;
+	case GPIO_COUNTER_SIGNAL_INDEX1:
+		gpio = priv->gpio_idx[0];
+		break;
+	case GPIO_COUNTER_SIGNAL_INDEX2:
+		gpio = priv->gpio_idx[1];
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	ret = gpiod_get_value(gpio);
+	if (ret < 0)
+		return ret;
+
+	*level = ret ? COUNTER_SIGNAL_LEVEL_HIGH : COUNTER_SIGNAL_LEVEL_LOW;
+	return 0;
+}
+
+static int gpio_counter_watch_validate(struct counter_device *counter,
+				       const struct counter_watch *watch)
+{
+	if (watch->channel >= GPIO_COUNTER_NUM_COUNTS)
+		return -EINVAL;
+
+	switch (watch->event) {
+	case COUNTER_EVENT_CHANGE_OF_STATE:
+	case COUNTER_EVENT_INDEX:
+		return 0;
+	default:
+		return -EINVAL;
+	}
+}
+
+static const struct counter_ops gpio_counter_ops = {
+	.count_read	= gpio_counter_count_read,
+	.count_write	= gpio_counter_count_write,
+	.function_read	= gpio_counter_function_read,
+	.function_write	= gpio_counter_function_write,
+	.action_read	= gpio_counter_action_read,
+	.signal_read	= gpio_counter_signal_read,
+	.watch_validate	= gpio_counter_watch_validate,
+};
+
+static int gpio_counter_ceiling_read(struct counter_device *counter,
+				     struct counter_count *count, u64 *val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*val = cp->ceiling;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_ceiling_write(struct counter_device *counter,
+				      struct counter_count *count, u64 val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	cp->ceiling = val;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_enable_read(struct counter_device *counter,
+				    struct counter_count *count, u8 *enable)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*enable = cp->enabled;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+/* enable_lock held */
+static void gpio_counter_enable_b(struct gpio_counter_priv *priv)
+{
+	if (priv->b_irq_users++ == 0)
+		enable_irq(priv->irq_b);
+}
+
+/* enable_lock held */
+static void gpio_counter_disable_b(struct gpio_counter_priv *priv)
+{
+	if (--priv->b_irq_users == 0)
+		disable_irq(priv->irq_b);
+}
+
+static void gpio_counter_enable_irqs(struct gpio_counter_priv *priv,
+				     enum gpio_counter_count_id count_id)
+{
+	switch (count_id) {
+	case GPIO_COUNTER_COUNT_1:
+		enable_irq(priv->irq_a);
+		gpio_counter_enable_b(priv);
+		if (priv->n_idx >= 1)
+			enable_irq(priv->irq_idx[0]);
+		break;
+	case GPIO_COUNTER_COUNT_2:
+		gpio_counter_enable_b(priv);
+		if (priv->n_idx >= 2)
+			enable_irq(priv->irq_idx[1]);
+		break;
+	default:
+		break;
+	}
+}
+
+static void gpio_counter_disable_irqs(struct gpio_counter_priv *priv,
+				      enum gpio_counter_count_id count_id)
+{
+	switch (count_id) {
+	case GPIO_COUNTER_COUNT_1:
+		if (priv->n_idx >= 1)
+			disable_irq(priv->irq_idx[0]);
+		gpio_counter_disable_b(priv);
+		disable_irq(priv->irq_a);
+		break;
+	case GPIO_COUNTER_COUNT_2:
+		if (priv->n_idx >= 2)
+			disable_irq(priv->irq_idx[1]);
+		gpio_counter_disable_b(priv);
+		break;
+	default:
+		break;
+	}
+}
+
+static int gpio_counter_enable_write(struct counter_device *counter,
+				     struct counter_count *count, u8 enable)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+	bool current_state;
+
+	guard(mutex)(&priv->enable_lock);
+
+	spin_lock_irqsave(&priv->lock, flags);
+	current_state = cp->enabled;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	if (current_state == enable)
+		return 0;
+
+	/* disable_irq() may sleep, so it can not run under the spinlock. */
+	if (enable) {
+		spin_lock_irqsave(&priv->lock, flags);
+		priv->prev_a = !!gpiod_get_value(priv->gpio_a);
+		priv->prev_b = !!gpiod_get_value(priv->gpio_b);
+		cp->enabled = true;
+		spin_unlock_irqrestore(&priv->lock, flags);
+		gpio_counter_enable_irqs(priv, count->id);
+	} else {
+		spin_lock_irqsave(&priv->lock, flags);
+		cp->enabled = false;
+		spin_unlock_irqrestore(&priv->lock, flags);
+		gpio_counter_disable_irqs(priv, count->id);
+	}
+
+	return 0;
+}
+
+static int gpio_counter_direction_read(struct counter_device *counter,
+				       struct counter_count *count,
+				       u32 *direction)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*direction = cp->direction;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_preset_read(struct counter_device *counter,
+				    struct counter_count *count, u64 *val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*val = cp->preset;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_preset_write(struct counter_device *counter,
+				     struct counter_count *count, u64 val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+	int ret = 0;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	if (val > cp->ceiling) {
+		ret = -EINVAL;
+		goto out;
+	}
+	cp->preset = val;
+out:
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return ret;
+}
+
+static int gpio_counter_preset_enable_read(struct counter_device *counter,
+					   struct counter_count *count,
+					   u8 *val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	*val = cp->preset_enabled;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static int gpio_counter_preset_enable_write(struct counter_device *counter,
+					    struct counter_count *count,
+					    u8 val)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	struct gpio_counter_count_priv *cp = &priv->count_priv[count->id];
+	unsigned long flags;
+
+	spin_lock_irqsave(&priv->lock, flags);
+	cp->preset_enabled = val;
+	spin_unlock_irqrestore(&priv->lock, flags);
+
+	return 0;
+}
+
+static struct counter_comp gpio_counter_count_ext[] = {
+	COUNTER_COMP_CEILING(gpio_counter_ceiling_read,
+			     gpio_counter_ceiling_write),
+	COUNTER_COMP_ENABLE(gpio_counter_enable_read,
+			    gpio_counter_enable_write),
+	COUNTER_COMP_DIRECTION(gpio_counter_direction_read),
+	COUNTER_COMP_PRESET(gpio_counter_preset_read,
+			    gpio_counter_preset_write),
+	COUNTER_COMP_PRESET_ENABLE(gpio_counter_preset_enable_read,
+				   gpio_counter_preset_enable_write),
+};
+
+static const enum counter_synapse_action gpio_counter_synapse_actions[] = {
+	COUNTER_SYNAPSE_ACTION_NONE,
+	COUNTER_SYNAPSE_ACTION_RISING_EDGE,
+	COUNTER_SYNAPSE_ACTION_FALLING_EDGE,
+	COUNTER_SYNAPSE_ACTION_BOTH_EDGES,
+};
+
+static const enum counter_synapse_action gpio_counter_index_synapse_actions[] = {
+	COUNTER_SYNAPSE_ACTION_RISING_EDGE,
+};
+
+static int gpio_counter_get_index_gpios(struct device *dev,
+					struct gpio_counter_priv *priv)
+{
+	struct gpio_descs *descs;
+	int i;
+
+	descs = devm_gpiod_get_array_optional(dev, "index", GPIOD_IN);
+	if (IS_ERR(descs))
+		return dev_err_probe(dev, PTR_ERR(descs),
+				     "failed to get index GPIOs\n");
+	if (!descs) {
+		priv->n_idx = 0;
+		return 0;
+	}
+
+	if (descs->ndescs < 1 || descs->ndescs > GPIO_COUNTER_MAX_INDEX)
+		return dev_err_probe(dev, -EINVAL,
+				     "index-gpios: expected 1..%d entries, got %u\n",
+				     GPIO_COUNTER_MAX_INDEX, descs->ndescs);
+
+	for (i = 0; i < descs->ndescs; i++)
+		priv->gpio_idx[i] = descs->desc[i];
+	priv->n_idx = descs->ndescs;
+
+	return 0;
+}
+
+static int gpio_counter_setup_signals(struct gpio_counter_priv *priv)
+{
+	priv->signals[GPIO_COUNTER_SIGNAL_A].id   = GPIO_COUNTER_SIGNAL_A;
+	priv->signals[GPIO_COUNTER_SIGNAL_A].name = "Signal A";
+	priv->signals[GPIO_COUNTER_SIGNAL_B].id   = GPIO_COUNTER_SIGNAL_B;
+	priv->signals[GPIO_COUNTER_SIGNAL_B].name = "Signal B";
+
+	if (priv->n_idx >= 1) {
+		priv->signals[GPIO_COUNTER_SIGNAL_INDEX1].id =
+			GPIO_COUNTER_SIGNAL_INDEX1;
+		priv->signals[GPIO_COUNTER_SIGNAL_INDEX1].name = "Index 1";
+	}
+	if (priv->n_idx >= 2) {
+		priv->signals[GPIO_COUNTER_SIGNAL_INDEX2].id =
+			GPIO_COUNTER_SIGNAL_INDEX2;
+		priv->signals[GPIO_COUNTER_SIGNAL_INDEX2].name = "Index 2";
+	}
+
+	return 2 + priv->n_idx;
+}
+
+static int gpio_counter_setup_synapses(struct gpio_counter_priv *priv)
+{
+	int n_c1 = 2, n_c2 = 1;
+
+	priv->synapses_count1[0].actions_list = gpio_counter_synapse_actions;
+	priv->synapses_count1[0].num_actions =
+		ARRAY_SIZE(gpio_counter_synapse_actions);
+	priv->synapses_count1[0].signal = &priv->signals[GPIO_COUNTER_SIGNAL_A];
+
+	priv->synapses_count1[1].actions_list = gpio_counter_synapse_actions;
+	priv->synapses_count1[1].num_actions =
+		ARRAY_SIZE(gpio_counter_synapse_actions);
+	priv->synapses_count1[1].signal = &priv->signals[GPIO_COUNTER_SIGNAL_B];
+
+	if (priv->n_idx >= 1) {
+		priv->synapses_count1[2].actions_list =
+			gpio_counter_index_synapse_actions;
+		priv->synapses_count1[2].num_actions =
+			ARRAY_SIZE(gpio_counter_index_synapse_actions);
+		priv->synapses_count1[2].signal =
+			&priv->signals[GPIO_COUNTER_SIGNAL_INDEX1];
+		n_c1 = 3;
+	}
+
+	priv->synapses_count2[0].actions_list = gpio_counter_synapse_actions;
+	priv->synapses_count2[0].num_actions =
+		ARRAY_SIZE(gpio_counter_synapse_actions);
+	priv->synapses_count2[0].signal = &priv->signals[GPIO_COUNTER_SIGNAL_B];
+
+	if (priv->n_idx >= 2) {
+		priv->synapses_count2[1].actions_list =
+			gpio_counter_index_synapse_actions;
+		priv->synapses_count2[1].num_actions =
+			ARRAY_SIZE(gpio_counter_index_synapse_actions);
+		priv->synapses_count2[1].signal =
+			&priv->signals[GPIO_COUNTER_SIGNAL_INDEX2];
+		n_c2 = 2;
+	}
+
+	priv->counts[GPIO_COUNTER_COUNT_1].id = GPIO_COUNTER_COUNT_1;
+	priv->counts[GPIO_COUNTER_COUNT_1].name = "Count 1";
+	priv->counts[GPIO_COUNTER_COUNT_1].functions_list =
+		gpio_counter_count1_functions;
+	priv->counts[GPIO_COUNTER_COUNT_1].num_functions =
+		ARRAY_SIZE(gpio_counter_count1_functions);
+	priv->counts[GPIO_COUNTER_COUNT_1].synapses = priv->synapses_count1;
+	priv->counts[GPIO_COUNTER_COUNT_1].num_synapses = n_c1;
+	priv->counts[GPIO_COUNTER_COUNT_1].ext = gpio_counter_count_ext;
+	priv->counts[GPIO_COUNTER_COUNT_1].num_ext =
+		ARRAY_SIZE(gpio_counter_count_ext);
+
+	priv->counts[GPIO_COUNTER_COUNT_2].id = GPIO_COUNTER_COUNT_2;
+	priv->counts[GPIO_COUNTER_COUNT_2].name = "Count 2";
+	priv->counts[GPIO_COUNTER_COUNT_2].functions_list =
+		gpio_counter_count2_functions;
+	priv->counts[GPIO_COUNTER_COUNT_2].num_functions =
+		ARRAY_SIZE(gpio_counter_count2_functions);
+	priv->counts[GPIO_COUNTER_COUNT_2].synapses = priv->synapses_count2;
+	priv->counts[GPIO_COUNTER_COUNT_2].num_synapses = n_c2;
+	priv->counts[GPIO_COUNTER_COUNT_2].ext = gpio_counter_count_ext;
+	priv->counts[GPIO_COUNTER_COUNT_2].num_ext =
+		ARRAY_SIZE(gpio_counter_count_ext);
+
+	return 0;
+}
+
+static int gpio_counter_request_irqs(struct device *dev,
+				     struct counter_device *counter)
+{
+	struct gpio_counter_priv *priv = counter_priv(counter);
+	irq_handler_t idx_handlers[GPIO_COUNTER_MAX_INDEX] = {
+		gpio_counter_index1_isr,
+		gpio_counter_index2_isr,
+	};
+	const char *idx_names[GPIO_COUNTER_MAX_INDEX] = {
+		"gpio-counter-index1",
+		"gpio-counter-index2",
+	};
+	int ret, i;
+
+	ret = devm_request_irq(dev, priv->irq_a, gpio_counter_a_isr,
+			       IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING |
+				       IRQF_NO_AUTOEN,
+			       "gpio-counter-a", counter);
+	if (ret)
+		return dev_err_probe(dev, ret,
+				     "failed to request IRQ for signal-a\n");
+
+	ret = devm_request_irq(dev, priv->irq_b, gpio_counter_b_isr,
+			       IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING |
+				       IRQF_NO_AUTOEN,
+			       "gpio-counter-b", counter);
+	if (ret)
+		return dev_err_probe(dev, ret,
+				     "failed to request IRQ for signal-b\n");
+
+	for (i = 0; i < priv->n_idx; i++) {
+		ret = devm_request_irq(dev, priv->irq_idx[i], idx_handlers[i],
+				       IRQF_TRIGGER_RISING | IRQF_NO_AUTOEN,
+				       idx_names[i], counter);
+		if (ret)
+			return dev_err_probe(dev, ret,
+					     "failed to request IRQ for index %d\n",
+					     i);
+	}
+
+	return 0;
+}
+
+static int gpio_counter_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct counter_device *counter;
+	struct gpio_counter_priv *priv;
+	int num_signals;
+	int i, ret;
+
+	counter = devm_counter_alloc(dev, sizeof(*priv));
+	if (!counter)
+		return -ENOMEM;
+
+	priv = counter_priv(counter);
+	spin_lock_init(&priv->lock);
+	ret = devm_mutex_init(dev, &priv->enable_lock);
+	if (ret)
+		return ret;
+
+	priv->gpio_a = devm_gpiod_get(dev, "signal-a", GPIOD_IN);
+	if (IS_ERR(priv->gpio_a))
+		return dev_err_probe(dev, PTR_ERR(priv->gpio_a),
+				     "failed to get signal-a GPIO\n");
+
+	priv->gpio_b = devm_gpiod_get(dev, "signal-b", GPIOD_IN);
+	if (IS_ERR(priv->gpio_b))
+		return dev_err_probe(dev, PTR_ERR(priv->gpio_b),
+				     "failed to get signal-b GPIO\n");
+
+	ret = gpio_counter_get_index_gpios(dev, priv);
+	if (ret)
+		return ret;
+
+	if (gpiod_cansleep(priv->gpio_a) || gpiod_cansleep(priv->gpio_b))
+		return dev_err_probe(dev, -EINVAL,
+				     "signal GPIO may sleep, not usable in hardirq\n");
+	for (i = 0; i < priv->n_idx; i++) {
+		if (gpiod_cansleep(priv->gpio_idx[i]))
+			return dev_err_probe(dev, -EINVAL,
+					     "index GPIO %d may sleep, not usable in hardirq\n",
+					     i);
+	}
+
+	priv->irq_a = gpiod_to_irq(priv->gpio_a);
+	if (priv->irq_a < 0)
+		return dev_err_probe(dev, priv->irq_a,
+				     "failed to get IRQ for signal-a\n");
+
+	priv->irq_b = gpiod_to_irq(priv->gpio_b);
+	if (priv->irq_b < 0)
+		return dev_err_probe(dev, priv->irq_b,
+				     "failed to get IRQ for signal-b\n");
+
+	for (i = 0; i < priv->n_idx; i++) {
+		priv->irq_idx[i] = gpiod_to_irq(priv->gpio_idx[i]);
+		if (priv->irq_idx[i] < 0)
+			return dev_err_probe(dev, priv->irq_idx[i],
+					     "failed to get IRQ for index %d\n",
+					     i);
+	}
+
+	priv->prev_a = !!gpiod_get_value(priv->gpio_a);
+	priv->prev_b = !!gpiod_get_value(priv->gpio_b);
+
+	priv->count_priv[GPIO_COUNTER_COUNT_1].function =
+		COUNTER_FUNCTION_QUADRATURE_X4;
+	priv->count_priv[GPIO_COUNTER_COUNT_1].direction =
+		COUNTER_COUNT_DIRECTION_FORWARD;
+	priv->count_priv[GPIO_COUNTER_COUNT_1].ceiling = U64_MAX;
+
+	priv->count_priv[GPIO_COUNTER_COUNT_2].function =
+		COUNTER_FUNCTION_INCREASE;
+	priv->count_priv[GPIO_COUNTER_COUNT_2].direction =
+		COUNTER_COUNT_DIRECTION_FORWARD;
+	priv->count_priv[GPIO_COUNTER_COUNT_2].ceiling = U64_MAX;
+
+	num_signals = gpio_counter_setup_signals(priv);
+	gpio_counter_setup_synapses(priv);
+
+	counter->name		= dev_name(dev);
+	counter->parent		= dev;
+	counter->ops		= &gpio_counter_ops;
+	counter->signals	= priv->signals;
+	counter->num_signals	= num_signals;
+	counter->counts		= priv->counts;
+	counter->num_counts	= ARRAY_SIZE(priv->counts);
+
+	ret = gpio_counter_request_irqs(dev, counter);
+	if (ret)
+		return ret;
+
+	ret = devm_counter_add(dev, counter);
+	if (ret < 0)
+		return dev_err_probe(dev, ret, "failed to add counter\n");
+
+	dev_info(dev,
+		 "GPIO counter registered (signals: A, B%s%s)\n",
+		 priv->n_idx >= 1 ? ", Index 1" : "",
+		 priv->n_idx >= 2 ? ", Index 2" : "");
+
+	return 0;
+}
+
+static const struct of_device_id gpio_counter_of_match[] = {
+	{ .compatible = "gpio-counter" },
+	{}
+};
+MODULE_DEVICE_TABLE(of, gpio_counter_of_match);
+
+static struct platform_driver gpio_counter_driver = {
+	.probe = gpio_counter_probe,
+	.driver = {
+		.name = "gpio-counter",
+		.of_match_table = gpio_counter_of_match,
+	},
+};
+module_platform_driver(gpio_counter_driver);
+
+MODULE_ALIAS("platform:gpio-counter");
+MODULE_AUTHOR("Wadim Mueller <wafgo01@gmail.com>");
+MODULE_DESCRIPTION("GPIO-based counter driver");
+MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS("COUNTER");
-- 
2.52.0