fuel_gauge: add basic support for AXP2101 chip

The AXP2101 chip is a multi functional power chip offering a regulator,
charge controller and a fuel gauge (battery percentage and voltage).
Hereby, the fuel gauge provides much more reliable data compared to using
an ADC.

We implement minimal support for this chip (connected state, voltage and
gauge) and bind it to the fuel gauge subsystem.

Closes: #89158
Signed-off-by: Felix Moessbauer <felix.moessbauer@gmail.com>

drivers/fuel_gauge/CMakeLists.txt

@@ -6,6 +6,7 @@ add_subdirectory_ifdef(CONFIG_SBS_GAUGE_NEW_API		sbs_gauge)
 add_subdirectory_ifdef(CONFIG_FUEL_GAUGE_COMPOSITE	composite)
 add_subdirectory_ifdef(CONFIG_MAX17048		max17048)
 add_subdirectory_ifdef(CONFIG_BQ27Z746		bq27z746)
+add_subdirectory_ifdef(CONFIG_FUEL_GAUGE_AXP2101	axp2101)
 
 zephyr_library_sources_ifdef(CONFIG_USERSPACE fuel_gauge_syscall_handlers.c)
 

drivers/fuel_gauge/Kconfig

@@ -23,5 +23,6 @@ source "drivers/fuel_gauge/max17048/Kconfig"
 source "drivers/fuel_gauge/sbs_gauge/Kconfig"
 source "drivers/fuel_gauge/bq27z746/Kconfig"
 source "drivers/fuel_gauge/composite/Kconfig"
+source "drivers/fuel_gauge/axp2101/Kconfig"
 
 endif # FUEL_GAUGE

drivers/fuel_gauge/axp2101/CMakeLists.txt

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
+
+zephyr_library()
+zephyr_library_sources(fuel_gauge_axp2101.c)

drivers/fuel_gauge/axp2101/Kconfig

@@ -0,0 +1,14 @@
+# Zephyr axp2101 fuel-gauge device
+
+# Copyright (c) 2025 Felix Moessbauer
+# SPDX-License-Identifier: Apache-2.0
+
+config FUEL_GAUGE_AXP2101
+	bool "X-Powers AXP2101 fuel gauge"
+	default y
+	depends on DT_HAS_X_POWERS_AXP2101_FUEL_GAUGE_ENABLED
+	depends on DT_HAS_X_POWERS_AXP2101_ENABLED
+	select I2C
+	select MFD
+	help
+	  Enable driver for the x-powers axp2101 fuel gauge device.

drivers/fuel_gauge/axp2101/fuel_gauge_axp2101.c

@@ -0,0 +1,203 @@
+/*
+ * Copyright (c) 2025, Felix Moessbauer
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Note: The functions that query the raw values via i2c are named similar
+ * to the ones in the reference implementation in the XPowersLib. This also
+ * applies to the register names (defines).
+ *
+ * Datasheet:
+ * https://github.com/lewisxhe/XPowersLib/blob/a7d06b98c1136c8fee7854b1d29a9f012b2aba83/datasheet/AXP2101_Datasheet_V1.0_en.pdf
+ */
+
+#define DT_DRV_COMPAT x_powers_axp2101_fuel_gauge
+
+#include <zephyr/device.h>
+#include <zephyr/drivers/fuel_gauge.h>
+#include <zephyr/drivers/i2c.h>
+#include <zephyr/kernel.h>
+#include <zephyr/logging/log.h>
+
+LOG_MODULE_REGISTER(fuel_gauge_axp2101, CONFIG_FUEL_GAUGE_LOG_LEVEL);
+
+/* clang-format off */
+/* registers */
+#define AXP2101_STATUS1			0x00
+	#define BAT_PRESENT_MASK	BIT(3)
+#define AXP2101_CHARGE_GAUGE_WDT_CTRL	0x18
+	#define GAUGE_ENABLE_MASK	BIT(3)
+#define AXP2101_ADC_DATA_VBAT_H		0x34
+	#define GAUGE_VBAT_H_MASK	0x1F
+#define AXP2101_ADC_DATA_VBAT_L		0x35
+#define AXP2101_BAT_DET_CTRL		0x68
+	#define BAT_TYPE_DET_MASK	BIT(0)
+#define AXP2101_BAT_PERCENT_DATA	0xA4
+
+/* internal feature flags */
+#define GAUGE_FEATURE_BAT_DET		BIT(0)
+#define GAUGE_FEATURE_GAUGE		BIT(1)
+#define GAUGE_FEATURE_ALL		(GAUGE_FEATURE_BAT_DET | GAUGE_FEATURE_GAUGE)
+/* clang-format on */
+
+struct axp2101_config {
+	struct i2c_dt_spec i2c;
+};
+
+struct axp2101_data {
+	uint8_t features;
+};
+
+static int enable_fuel_gauge(const struct device *dev)
+{
+	const struct axp2101_config *cfg = dev->config;
+	struct axp2101_data *data = dev->data;
+	int ret = 0;
+
+	ret = i2c_reg_update_byte_dt(&cfg->i2c, AXP2101_CHARGE_GAUGE_WDT_CTRL, GAUGE_ENABLE_MASK,
+				     GAUGE_ENABLE_MASK);
+	if (ret < 0) {
+		data->features &= ~GAUGE_FEATURE_GAUGE;
+	}
+	return ret;
+}
+
+static int enable_batt_detection(const struct device *dev)
+{
+	const struct axp2101_config *cfg = dev->config;
+	struct axp2101_data *data = dev->data;
+	int ret = 0;
+
+	ret = i2c_reg_update_byte_dt(&cfg->i2c, AXP2101_BAT_DET_CTRL, BAT_TYPE_DET_MASK,
+				     BAT_TYPE_DET_MASK);
+	if (ret < 0) {
+		data->features &= ~GAUGE_FEATURE_BAT_DET;
+	}
+	return ret;
+}
+
+static int is_battery_connect(const struct device *dev, union fuel_gauge_prop_val *val)
+{
+	const struct axp2101_config *cfg = dev->config;
+	struct axp2101_data *data = dev->data;
+	uint8_t tmp;
+	int ret;
+
+	if ((data->features & GAUGE_FEATURE_BAT_DET) == 0) {
+		return -ENOTSUP;
+	}
+
+	ret = i2c_reg_read_byte_dt(&cfg->i2c, AXP2101_STATUS1, &tmp);
+	if (ret < 0) {
+		return ret;
+	}
+
+	val->present_state = tmp & BAT_PRESENT_MASK;
+	return 0;
+}
+
+static int get_battery_percent(const struct device *dev, union fuel_gauge_prop_val *val)
+{
+	const struct axp2101_config *cfg = dev->config;
+	struct axp2101_data *data = dev->data;
+	uint8_t tmp;
+	int ret;
+
+	if ((data->features & GAUGE_FEATURE_GAUGE) == 0) {
+		return -ENOTSUP;
+	}
+
+	ret = i2c_reg_read_byte_dt(&cfg->i2c, AXP2101_BAT_PERCENT_DATA, &tmp);
+	if (ret < 0) {
+		return ret;
+	}
+
+	val->relative_state_of_charge = tmp;
+	return 0;
+}
+
+static int get_bat_voltage(const struct device *dev, union fuel_gauge_prop_val *val)
+{
+	const struct axp2101_config *cfg = dev->config;
+	struct axp2101_data *data = dev->data;
+	uint8_t h5, l8;
+	int ret;
+
+	if ((data->features & GAUGE_FEATURE_GAUGE) == 0) {
+		return -ENOTSUP;
+	}
+
+	ret = i2c_reg_read_byte_dt(&cfg->i2c, AXP2101_ADC_DATA_VBAT_H, &h5);
+	if (ret < 0) {
+		return ret;
+	}
+
+	ret = i2c_reg_read_byte_dt(&cfg->i2c, AXP2101_ADC_DATA_VBAT_L, &l8);
+	if (ret < 0) {
+		return ret;
+	}
+
+	val->voltage = (((h5 & GAUGE_VBAT_H_MASK) << 8) | l8) * 1000;
+	return 0;
+}
+
+static int axp2101_get_prop(const struct device *dev, fuel_gauge_prop_t prop,
+			    union fuel_gauge_prop_val *val)
+{
+	switch (prop) {
+	case FUEL_GAUGE_PRESENT_STATE:
+	case FUEL_GAUGE_CONNECT_STATE:
+		return is_battery_connect(dev, val);
+	case FUEL_GAUGE_VOLTAGE:
+		return get_bat_voltage(dev, val);
+	case FUEL_GAUGE_ABSOLUTE_STATE_OF_CHARGE:
+	case FUEL_GAUGE_RELATIVE_STATE_OF_CHARGE:
+		return get_battery_percent(dev, val);
+	default:
+		return -ENOTSUP;
+	}
+}
+
+static int axp2101_init(const struct device *dev)
+{
+	struct axp2101_data *data = dev->data;
+	const struct axp2101_config *cfg;
+	int ret = 0;
+
+	cfg = dev->config;
+
+	if (!device_is_ready(cfg->i2c.bus)) {
+		LOG_ERR("Bus device is not ready");
+		return -ENODEV;
+	}
+
+	ret = enable_fuel_gauge(dev);
+	if (ret < 0) {
+		LOG_WRN("Failed to enable fuel gauge");
+		data->features &= ~GAUGE_FEATURE_GAUGE;
+	}
+
+	ret = enable_batt_detection(dev);
+	if (ret < 0) {
+		LOG_WRN("Failed to enable battery detection");
+		data->features &= ~GAUGE_FEATURE_BAT_DET;
+	}
+	return 0;
+}
+
+static DEVICE_API(fuel_gauge, axp2101_api) = {
+	.get_property = axp2101_get_prop,
+};
+
+#define AXP2101_INIT(inst)                                                                         \
+	static const struct axp2101_config axp2101_config_##inst = {                               \
+		.i2c = I2C_DT_SPEC_GET(DT_PARENT(DT_INST(inst, DT_DRV_COMPAT))),                   \
+	};                                                                                         \
+	static struct axp2101_data axp2101_data_##inst = {                                         \
+		.features = GAUGE_FEATURE_ALL,                                                     \
+	};                                                                                         \
+	DEVICE_DT_INST_DEFINE(inst, &axp2101_init, NULL, &axp2101_data_##inst,                     \
+			      &axp2101_config_##inst, POST_KERNEL,                                 \
+			      CONFIG_FUEL_GAUGE_INIT_PRIORITY, &axp2101_api);
+
+DT_INST_FOREACH_STATUS_OKAY(AXP2101_INIT)

dts/bindings/fuel-gauge/x-powers,axp2101-fuel-gauge.yaml

@@ -0,0 +1,9 @@
+# Copyright (c) 2025 Felix Moessbauer <felix.moessbauer@gmail.com>
+# SPDX-License-Identifier: Apache-2.0
+
+description: |
+  X-Powers AXP2101 Fuel Gauge
+
+compatible: "x-powers,axp2101-fuel-gauge"
+
+include: [fuel-gauge.yaml]