--- /dev/null
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * The driver for Measurement Specialties HTU31 Temperature and Humidity sensor.
+ *
+ * Copyright (C) 2025
+ * Author: Andrei Lalaev <andrey.lalaev@gmail.com>
+ */
+
+#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 <andrey.lalaev@gmail.com>");
+MODULE_DESCRIPTION("HTU31 Temperature and Humidity sensor driver");
+MODULE_LICENSE("GPL");
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