hwmon: add driver for HTU31

Add base support for HTU31 temperature and humidity sensor.

Besides temperature and humidity values, the driver also exports a 24-bit
heater control to sysfs and serial number to debugfs.

Signed-off-by: Andrei Lalaev <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[groeck: Fixed continuation line alignment]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/hwmon/htu31.rst

···
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver HTU31
+====================
+
+Supported chips:
+
+  * Measurement Specialties HTU31
+
+    Prefix: 'htu31'
+
+    Addresses scanned: -
+
+    Datasheet: Publicly available from https://www.te.com/en/product-CAT-HSC0007.html
+
+Author:
+
+  - Andrei Lalaev <[e-mail]>
+
+Description
+-----------
+
+HTU31 is a humidity and temperature sensor.
+
+Supported temperature range is from -40 to 125 degrees Celsius.
+
+Communication with the device is performed via I2C protocol. Sensor's default address
+is 0x40.
+
+sysfs-Interface
+---------------
+
+=================== =================
+temp1_input:        temperature input
+humidity1_input:    humidity input
+heater_enable:      heater control
+=================== =================

Documentation/hwmon/index.rst

···
    hih6130
    hp-wmi-sensors
    hs3001
+   htu31
    ibmaem
    ibm-cffps
    ibmpowernv

MAINTAINERS

···
 F:	Documentation/devicetree/bindings/iio/humidity/st,hts221.yaml
 F:	drivers/iio/humidity/hts221*
 
+HTU31 Hardware Temperature and Humidity Sensor
+M:	Andrei Lalaev <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+F:	drivers/hwmon/htu31.c
+
 HUAWEI ETHERNET DRIVER
 M:	Cai Huoqing <[e-mail]>
 L:	[e-mail]

drivers/hwmon/Kconfig

···
 	  This driver can also be built as a module. If so, the module
 	  will be called hs3001.
 
+config SENSORS_HTU31
+	tristate "Measurement Specialties HTU31 humidity and temperature sensor"
+	depends on I2C
+	select CRC8
+	help
+	  If you say yes here you get support for the HTU31 humidity
+	  and temperature sensors.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called htu31.
+
 config SENSORS_IBMAEM
 	tristate "IBM Active Energy Manager temperature/power sensors and control"
 	select IPMI_SI

drivers/hwmon/Makefile

···
 obj-$(CONFIG_SENSORS_GXP_FAN_CTRL) += gxp-fan-ctrl.o
 obj-$(CONFIG_SENSORS_HIH6130)	+= hih6130.o
 obj-$(CONFIG_SENSORS_HS3001)	+= hs3001.o
+obj-$(CONFIG_SENSORS_HTU31)	+= htu31.o
 obj-$(CONFIG_SENSORS_ULTRA45)	+= ultra45_env.o
 obj-$(CONFIG_SENSORS_I5500)	+= i5500_temp.o
 obj-$(CONFIG_SENSORS_I5K_AMB)	+= i5k_amb.o

drivers/hwmon/htu31.c

···
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The driver for Measurement Specialties HTU31 Temperature and Humidity sensor.
+ *
+ * Copyright (C) 2025
+ * Author: Andrei Lalaev <[e-mail]>
+ */
+
+#include <linux/array_size.h>
+#include <linux/cleanup.h>
+#include <linux/crc8.h>
+#include <linux/debugfs.h>
+#include <linux/delay.h>
+#include <linux/hwmon.h>
+#include <linux/hwmon-sysfs.h>
+#include <linux/i2c.h>
+#include <linux/init.h>
+#include <linux/module.h>
+
+#define HTU31_READ_TEMP_HUM_CMD	0x00
+#define HTU31_READ_SERIAL_CMD		0x0a
+#define HTU31_CONVERSION_CMD		0x5e
+#define HTU31_HEATER_OFF_CMD		0x02
+#define HTU31_HEATER_ON_CMD		0x04
+
+#define HTU31_TEMP_HUM_LEN		6
+
+/* Conversion time for the highest resolution */
+#define HTU31_HUMIDITY_CONV_TIME	10000 /* us */
+#define HTU31_TEMPERATURE_CONV_TIME	15000 /* us */
+
+#define HTU31_SERIAL_NUMBER_LEN	3
+#define HTU31_SERIAL_NUMBER_CRC_LEN	1
+#define HTU31_SERIAL_NUMBER_CRC_OFFSET	3
+
+#define HTU31_CRC8_INIT_VAL		0
+#define HTU31_CRC8_POLYNOMIAL		0x31
+DECLARE_CRC8_TABLE(htu31_crc8_table);
+
+/**
+ * struct htu31_data - all the data required to operate a HTU31 chip
+ * @client: the i2c client associated with the HTU31
+ * @lock: a mutex to prevent parallel access to the data
+ * @wait_time: the time needed by sensor to convert values
+ * @temperature: the latest temperature value in millidegrees
+ * @humidity: the latest relative humidity value in millipercent
+ * @serial_number: the serial number of the sensor
+ * @heater_enable: the internal state of the heater
+ */
+struct htu31_data {
+	struct i2c_client *client;
+	struct mutex lock; /* Used to protect against parallel data updates */
+	long wait_time;
+	long temperature;
+	long humidity;
+	u8 serial_number[HTU31_SERIAL_NUMBER_LEN];
+	bool heater_enable;
+};
+
+static long htu31_temp_to_millicelsius(u16 val)
+{
+	return -40000 + DIV_ROUND_CLOSEST_ULL(165000ULL * val, 65535);
+}
+
+static long htu31_relative_humidity(u16 val)
+{
+	return DIV_ROUND_CLOSEST_ULL(100000ULL * val, 65535);
+}
+
+static int htu31_data_fetch_command(struct htu31_data *data)
+{
+	struct i2c_client *client = data->client;
+	u8 conversion_on = HTU31_CONVERSION_CMD;
+	u8 read_data_cmd = HTU31_READ_TEMP_HUM_CMD;
+	u8 t_h_buf[HTU31_TEMP_HUM_LEN] = {};
+	struct i2c_msg msgs[] = {
+		{
+			.addr = client->addr,
+			.flags = 0,
+			.len = 1,
+			.buf = &read_data_cmd,
+		},
+		{
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = sizeof(t_h_buf),
+			.buf = t_h_buf,
+		},
+	};
+	int ret;
+	u8 crc;
+
+	guard(mutex)(&data->lock);
+
+	ret = i2c_master_send(client, &conversion_on, 1);
+	if (ret != 1) {
+		ret = ret < 0 ? ret : -EIO;
+		dev_err(&client->dev,
+			"Conversion command is failed. Error code: %d\n", ret);
+		return ret;
+	}
+
+	fsleep(data->wait_time);
+
+	ret = i2c_transfer(client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (ret != ARRAY_SIZE(msgs)) {
+		ret = ret < 0 ? ret : -EIO;
+		dev_err(&client->dev,
+			"T&H command is failed. Error code: %d\n", ret);
+		return ret;
+	}
+
+	crc = crc8(htu31_crc8_table, &t_h_buf[0], 2, HTU31_CRC8_INIT_VAL);
+	if (crc != t_h_buf[2]) {
+		dev_err(&client->dev, "Temperature CRC mismatch\n");
+		return -EIO;
+	}
+
+	crc = crc8(htu31_crc8_table, &t_h_buf[3], 2, HTU31_CRC8_INIT_VAL);
+	if (crc != t_h_buf[5]) {
+		dev_err(&client->dev, "Humidity CRC mismatch\n");
+		return -EIO;
+	}
+
+	data->temperature = htu31_temp_to_millicelsius(be16_to_cpup((__be16 *)&t_h_buf[0]));
+	data->humidity = htu31_relative_humidity(be16_to_cpup((__be16 *)&t_h_buf[3]));
+
+	return 0;
+}
+
+static umode_t htu31_is_visible(const void *data, enum hwmon_sensor_types type,
+				u32 attr, int channel)
+{
+	switch (type) {
+	case hwmon_temp:
+	case hwmon_humidity:
+		return 0444;
+	default:
+		return 0;
+	}
+}
+
+static int htu31_read(struct device *dev, enum hwmon_sensor_types type,
+		      u32 attr, int channel, long *val)
+{
+	struct htu31_data *data = dev_get_drvdata(dev);
+	int ret;
+
+	ret = htu31_data_fetch_command(data);
+	if (ret < 0)
+		return ret;
+
+	switch (type) {
+	case hwmon_temp:
+		if (attr != hwmon_temp_input)
+			return -EINVAL;
+
+		*val = data->temperature;
+		break;
+	case hwmon_humidity:
+		if (attr != hwmon_humidity_input)
+			return -EINVAL;
+
+		*val = data->humidity;
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	return 0;
+}
+
+static int htu31_read_serial_number(struct htu31_data *data)
+{
+	struct i2c_client *client = data->client;
+	u8 read_sn_cmd = HTU31_READ_SERIAL_CMD;
+	u8 sn_buf[HTU31_SERIAL_NUMBER_LEN + HTU31_SERIAL_NUMBER_CRC_LEN];
+	struct i2c_msg msgs[] = {
+		{
+			.addr = client->addr,
+			.flags = 0,
+			.len = 1,
+			.buf = &read_sn_cmd,
+		},
+		{
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = sizeof(sn_buf),
+			.buf = sn_buf,
+		},
+	};
+	int ret;
+	u8 crc;
+
+	ret = i2c_transfer(client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (ret < 0)
+		return ret;
+
+	crc = crc8(htu31_crc8_table, sn_buf, HTU31_SERIAL_NUMBER_LEN, HTU31_CRC8_INIT_VAL);
+	if (crc != sn_buf[HTU31_SERIAL_NUMBER_CRC_OFFSET]) {
+		dev_err(&client->dev, "Serial number CRC mismatch\n");
+		return -EIO;
+	}
+
+	memcpy(data->serial_number, sn_buf, HTU31_SERIAL_NUMBER_LEN);
+
+	return 0;
+}
+
+static ssize_t heater_enable_show(struct device *dev,
+				  struct device_attribute *attr,
+				  char *buf)
+{
+	struct htu31_data *data = dev_get_drvdata(dev);
+
+	return sysfs_emit(buf, "%d\n", data->heater_enable);
+}
+
+static ssize_t heater_enable_store(struct device *dev,
+				   struct device_attribute *attr,
+				   const char *buf,
+				   size_t count)
+{
+	struct htu31_data *data = dev_get_drvdata(dev);
+	u8 heater_cmd;
+	bool status;
+	int ret;
+
+	ret = kstrtobool(buf, &status);
+	if (ret)
+		return ret;
+
+	heater_cmd = status ? HTU31_HEATER_ON_CMD : HTU31_HEATER_OFF_CMD;
+
+	guard(mutex)(&data->lock);
+
+	ret = i2c_master_send(data->client, &heater_cmd, 1);
+	if (ret < 0)
+		return ret;
+
+	data->heater_enable = status;
+
+	return count;
+}
+
+static DEVICE_ATTR_RW(heater_enable);
+
+static int serial_number_show(struct seq_file *seq_file,
+			      void *unused)
+{
+	struct htu31_data *data = seq_file->private;
+
+	seq_printf(seq_file, "%X%X%X\n", data->serial_number[0],
+		   data->serial_number[1], data->serial_number[2]);
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(serial_number);
+
+static struct attribute *htu31_attrs[] = {
+	&dev_attr_heater_enable.attr,
+	NULL
+};
+
+ATTRIBUTE_GROUPS(htu31);
+
+static const struct hwmon_channel_info * const htu31_info[] = {
+	HWMON_CHANNEL_INFO(temp, HWMON_T_INPUT),
+	HWMON_CHANNEL_INFO(humidity, HWMON_H_INPUT),
+	NULL
+};
+
+static const struct hwmon_ops htu31_hwmon_ops = {
+	.is_visible = htu31_is_visible,
+	.read = htu31_read,
+};
+
+static const struct hwmon_chip_info htu31_chip_info = {
+	.info = htu31_info,
+	.ops = &htu31_hwmon_ops,
+};
+
+static int htu31_probe(struct i2c_client *client)
+{
+	struct device *dev = &client->dev;
+	struct device *hwmon_dev;
+	struct htu31_data *data;
+	int ret;
+
+	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->client = client;
+	data->wait_time = HTU31_TEMPERATURE_CONV_TIME + HTU31_HUMIDITY_CONV_TIME;
+
+	ret = devm_mutex_init(dev, &data->lock);
+	if (ret)
+		return ret;
+
+	crc8_populate_msb(htu31_crc8_table, HTU31_CRC8_POLYNOMIAL);
+
+	ret = htu31_read_serial_number(data);
+	if (ret) {
+		dev_err(dev, "Failed to read serial number\n");
+		return ret;
+	}
+
+	debugfs_create_file("serial_number",
+			    0444,
+			    client->debugfs,
+			    data,
+			    &serial_number_fops);
+
+	hwmon_dev = devm_hwmon_device_register_with_info(dev,
+							 client->name,
+							 data,
+							 &htu31_chip_info,
+							 htu31_groups);
+
+	return PTR_ERR_OR_ZERO(hwmon_dev);
+}
+
+static const struct i2c_device_id htu31_id[] = {
+	{ "htu31" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, htu31_id);
+
+#if IS_ENABLED(CONFIG_OF)
+static const struct of_device_id htu31_of_match[] = {
+	{ .compatible = "meas,htu31" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, htu31_of_match);
+#endif
+
+static struct i2c_driver htu31_driver = {
+	.driver = {
+		.name = "htu31",
+		.of_match_table = of_match_ptr(htu31_of_match),
+	},
+	.probe = htu31_probe,
+	.id_table = htu31_id,
+};
+module_i2c_driver(htu31_driver);
+
+MODULE_AUTHOR("Andrei Lalaev <[e-mail]>");
+MODULE_DESCRIPTION("HTU31 Temperature and Humidity sensor driver");
+MODULE_LICENSE("GPL");