hwmon: (sht21) Add Electronic Identification Code retrieval

Expose the per-chip unique identifier so it can be used to identify the
sensor producing the measurements.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/sht21

@@ -35,6 +35,7 @@ sysfs-Interface
 
 temp1_input - temperature input
 humidity1_input - humidity input
+eic - Electronic Identification Code
 
 Notes
 -----
@@ -45,5 +46,5 @@ humidity and 66 ms for temperature. To keep self heating below 0.1 degree
 Celsius, the device should not be active for more than 10% of the time,
 e.g. maximum two measurements per second at the given resolution.
 
-Different resolutions, the on-chip heater, using the CRC checksum and reading
-the serial number are not supported yet.
+Different resolutions, the on-chip heater, and using the CRC checksum
+are not supported yet.

drivers/hwmon/sht21.c

@@ -34,23 +34,29 @@
 /* I2C command bytes */
 #define SHT21_TRIG_T_MEASUREMENT_HM  0xe3
 #define SHT21_TRIG_RH_MEASUREMENT_HM 0xe5
+#define SHT21_READ_SNB_CMD1 0xFA
+#define SHT21_READ_SNB_CMD2 0x0F
+#define SHT21_READ_SNAC_CMD1 0xFC
+#define SHT21_READ_SNAC_CMD2 0xC9
 
 /**
  * struct sht21 - SHT21 device specific data
  * @hwmon_dev: device registered with hwmon
  * @lock: mutex to protect measurement values
- * @valid: only 0 before first measurement is taken
  * @last_update: time of last update (jiffies)
  * @temperature: cached temperature measurement value
  * @humidity: cached humidity measurement value
+ * @valid: only 0 before first measurement is taken
+ * @eic: cached electronic identification code text
  */
 struct sht21 {
 	struct i2c_client *client;
 	struct mutex lock;
-	char valid;
 	unsigned long last_update;
 	int temperature;
 	int humidity;
+	char valid;
+	char eic[18];
 };
 
 /**
@@ -165,15 +171,97 @@ static ssize_t sht21_show_humidity(struct device *dev,
 	return sprintf(buf, "%d\n", sht21->humidity);
 }
 
+static ssize_t eic_read(struct sht21 *sht21)
+{
+	struct i2c_client *client = sht21->client;
+	u8 tx[2];
+	u8 rx[8];
+	u8 eic[8];
+	struct i2c_msg msgs[2] = {
+		{
+			.addr = client->addr,
+			.flags = 0,
+			.len = 2,
+			.buf = tx,
+		},
+		{
+			.addr = client->addr,
+			.flags = I2C_M_RD,
+			.len = 8,
+			.buf = rx,
+		},
+	};
+	int ret;
+
+	tx[0] = SHT21_READ_SNB_CMD1;
+	tx[1] = SHT21_READ_SNB_CMD2;
+	ret = i2c_transfer(client->adapter, msgs, 2);
+	if (ret < 0)
+		goto out;
+	eic[2] = rx[0];
+	eic[3] = rx[2];
+	eic[4] = rx[4];
+	eic[5] = rx[6];
+
+	tx[0] = SHT21_READ_SNAC_CMD1;
+	tx[1] = SHT21_READ_SNAC_CMD2;
+	msgs[1].len = 6;
+	ret = i2c_transfer(client->adapter, msgs, 2);
+	if (ret < 0)
+		goto out;
+	eic[0] = rx[3];
+	eic[1] = rx[4];
+	eic[6] = rx[0];
+	eic[7] = rx[1];
+
+	ret = snprintf(sht21->eic, sizeof(sht21->eic),
+		       "%02x%02x%02x%02x%02x%02x%02x%02x\n",
+		       eic[0], eic[1], eic[2], eic[3],
+		       eic[4], eic[5], eic[6], eic[7]);
+out:
+	if (ret < 0)
+		sht21->eic[0] = 0;
+
+	return ret;
+}
+
+/**
+ * eic_show() - show Electronic Identification Code in sysfs
+ * @dev: device
+ * @attr: device attribute
+ * @buf: sysfs buffer (PAGE_SIZE) where EIC is written
+ *
+ * Will be called on read access to eic sysfs attribute.
+ * Returns number of bytes written into buffer, negative errno on error.
+ */
+static ssize_t eic_show(struct device *dev,
+	struct device_attribute *attr,
+	char *buf)
+{
+	struct sht21 *sht21 = dev_get_drvdata(dev);
+	int ret;
+
+	ret = sizeof(sht21->eic) - 1;
+	mutex_lock(&sht21->lock);
+	if (!sht21->eic[0])
+		ret = eic_read(sht21);
+	if (ret > 0)
+		memcpy(buf, sht21->eic, ret);
+	mutex_unlock(&sht21->lock);
+	return ret;
+}
+
 /* sysfs attributes */
 static SENSOR_DEVICE_ATTR(temp1_input, S_IRUGO, sht21_show_temperature,
 	NULL, 0);
 static SENSOR_DEVICE_ATTR(humidity1_input, S_IRUGO, sht21_show_humidity,
 	NULL, 0);
+static DEVICE_ATTR_RO(eic);
 
 static struct attribute *sht21_attrs[] = {
 	&sensor_dev_attr_temp1_input.dev_attr.attr,
 	&sensor_dev_attr_humidity1_input.dev_attr.attr,
+	&dev_attr_eic.attr,
 	NULL
 };