hwmon: (sht3x) read out sensor serial number

The temperature/humidity sensors of the STS3x/SHT3x family are
calibrated and factory-programmed with a unique serial number.
For some sensors, this serial number can be used to obtain a calibration
certificate via an API provided by the manufacturer (Sensirion).
Expose the serial number via debugfs.

Tested with: 2x STS31, 1x STS32, 1x SHT31

Signed-off-by: Stefan Gloor <code@stefan-gloor.ch>
Link: https://lore.kernel.org/r/20240131111512.25321-2-code@stefan-gloor.ch
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/sht3x.rst

@@ -65,6 +65,10 @@ When the temperature and humidity readings move back between the hysteresis
 values, the alert bit is set to 0 and the alert pin on the sensor is set to
 low.
 
+The serial number exposed to debugfs allows for unique identification of the
+sensors. For sts32, sts33 and sht33, the manufacturer provides calibration
+certificates through an API.
+
 sysfs-Interface
 ---------------
 
@@ -99,3 +103,10 @@ repeatability:      write or read repeatability, higher repeatability means
                         - 1: medium repeatability
                         - 2: high repeatability
 =================== ============================================================
+
+debugfs-Interface
+-----------------
+
+=================== ============================================================
+serial_number:      unique serial number of the sensor in decimal
+=================== ============================================================

drivers/hwmon/sht3x.c

@@ -10,6 +10,7 @@
 
 #include <asm/page.h>
 #include <linux/crc8.h>
+#include <linux/debugfs.h>
 #include <linux/delay.h>
 #include <linux/err.h>
 #include <linux/hwmon.h>
@@ -41,6 +42,9 @@ static const unsigned char sht3x_cmd_heater_off[]              = { 0x30, 0x66 };
 /* other commands */
 static const unsigned char sht3x_cmd_read_status_reg[]         = { 0xf3, 0x2d };
 static const unsigned char sht3x_cmd_clear_status_reg[]        = { 0x30, 0x41 };
+static const unsigned char sht3x_cmd_read_serial_number[]      = { 0x37, 0x80 };
+
+static struct dentry *debugfs;
 
 /* delays for single-shot mode i2c commands, both in us */
 #define SHT3X_SINGLE_WAIT_TIME_HPM  15000
@@ -163,12 +167,14 @@ struct sht3x_data {
 	enum sht3x_chips chip_id;
 	struct mutex i2c_lock; /* lock for sending i2c commands */
 	struct mutex data_lock; /* lock for updating driver data */
+	struct dentry *sensor_dir;
 
 	u8 mode;
 	const unsigned char *command;
 	u32 wait_time;			/* in us*/
 	unsigned long last_update;	/* last update in periodic mode*/
 	enum sht3x_repeatability repeatability;
+	u32 serial_number;
 
 	/*
 	 * cached values for temperature and humidity and limits
@@ -831,6 +837,40 @@ static int sht3x_write(struct device *dev, enum hwmon_sensor_types type,
 	}
 }
 
+static void sht3x_debugfs_init(struct sht3x_data *data)
+{
+	char name[32];
+
+	snprintf(name, sizeof(name), "i2c%u-%02x",
+		 data->client->adapter->nr, data->client->addr);
+	data->sensor_dir = debugfs_create_dir(name, debugfs);
+	debugfs_create_u32("serial_number", 0444,
+			   data->sensor_dir, &data->serial_number);
+}
+
+static void sht3x_debugfs_remove(void *sensor_dir)
+{
+	debugfs_remove_recursive(sensor_dir);
+}
+
+static int sht3x_serial_number_read(struct sht3x_data *data)
+{
+	int ret;
+	char buffer[SHT3X_RESPONSE_LENGTH];
+	struct i2c_client *client = data->client;
+
+	ret = sht3x_read_from_command(client, data,
+				      sht3x_cmd_read_serial_number,
+				      buffer,
+				      SHT3X_RESPONSE_LENGTH, 0);
+	if (ret)
+		return ret;
+
+	data->serial_number = (buffer[0] << 24) | (buffer[1] << 16) |
+			      (buffer[3] << 8) | buffer[4];
+	return ret;
+}
+
 static const struct hwmon_ops sht3x_ops = {
 	.is_visible = sht3x_is_visible,
 	.read = sht3x_read,
@@ -899,6 +939,18 @@ static int sht3x_probe(struct i2c_client *client)
 	if (ret)
 		return ret;
 
+	ret = sht3x_serial_number_read(data);
+	if (ret) {
+		dev_dbg(dev, "unable to read serial number\n");
+	} else {
+		sht3x_debugfs_init(data);
+		ret = devm_add_action_or_reset(dev,
+					       sht3x_debugfs_remove,
+					       data->sensor_dir);
+		if (ret)
+			return ret;
+	}
+
 	hwmon_dev = devm_hwmon_device_register_with_info(dev,
 							 client->name,
 							 data,
@@ -917,7 +969,19 @@ static struct i2c_driver sht3x_i2c_driver = {
 	.id_table    = sht3x_ids,
 };
 
-module_i2c_driver(sht3x_i2c_driver);
+static int __init sht3x_init(void)
+{
+	debugfs = debugfs_create_dir("sht3x", NULL);
+	return i2c_add_driver(&sht3x_i2c_driver);
+}
+module_init(sht3x_init);
+
+static void __exit sht3x_cleanup(void)
+{
+	debugfs_remove_recursive(debugfs);
+	i2c_del_driver(&sht3x_i2c_driver);
+}
+module_exit(sht3x_cleanup);
 
 MODULE_AUTHOR("David Frey <david.frey@sensirion.com>");
 MODULE_AUTHOR("Pascal Sachs <pascal.sachs@sensirion.com>");