hwmon: HIH-6130: Support I2C bus drivers without I2C_FUNC_SMBUS_QUICK

Some I2C bus drivers do not allow zero-length data transfers which are
required to start a measurement with the HIH6130/1 sensor. Nevertheless,
we can overcome this limitation by writing a zero dummy byte. This byte
is ignored by the sensor and was verified to be working with the OMAP
I2C bus driver in a BeagleBone board.
Signed-off-by: NJosé Miguel Gonçalves <jose.goncalves@inov.pt>
[Guenter Roeck: Simplified complexity of write_length initialization]
Cc: stable@vger.kernel.org # v3.10+
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

drivers/hwmon/hih6130.c

@@ -43,6 +43,7 @@
  * @last_update: time of last update (jiffies)
  * @temperature: cached temperature measurement value
  * @humidity: cached humidity measurement value
+ * @write_length: length for I2C measurement request
  */
 struct hih6130 {
 	struct device *hwmon_dev;
@@ -51,6 +52,7 @@ struct hih6130 {
 	unsigned long last_update;
 	int temperature;
 	int humidity;
+	size_t write_length;
 };
 
 /**
@@ -121,8 +123,15 @@ static int hih6130_update_measurements(struct i2c_client *client)
 	 */
 	if (time_after(jiffies, hih6130->last_update + HZ) || !hih6130->valid) {
 
-		/* write to slave address, no data, to request a measurement */
-		ret = i2c_master_send(client, tmp, 0);
+		/*
+		 * Write to slave address to request a measurement.
+		 * According with the datasheet it should be with no data, but
+		 * for systems with I2C bus drivers that do not allow zero
+		 * length packets we write one dummy byte to allow sensor
+		 * measurements on them.
+		 */
+		tmp[0] = 0;
+		ret = i2c_master_send(client, tmp, hih6130->write_length);
 		if (ret < 0)
 			goto out;
 
@@ -252,6 +261,9 @@ static int hih6130_probe(struct i2c_client *client,
 		goto fail_remove_sysfs;
 	}
 
+	if (!i2c_check_functionality(client->adapter, I2C_FUNC_SMBUS_QUICK))
+		hih6130->write_length = 1;
+
 	return 0;
 
 fail_remove_sysfs: