hwmon: (ad7314) Validate leading zero bits and return error

Leading zero bits are sent on the bus before the temperature value is
transmitted. If any of these bits are high, the connection might be
unstable or there could be no AD7314 / ADT730x (or compatible) at all.
Return -EIO in that case.

Signed-off-by: Erik Schumacher <erik.schumacher@iris-sensing.com>
Fixes: 4f3a659 ("hwmon: AD7314 driver (ported from IIO)")
Link: https://lore.kernel.org/r/24a50c2981a318580aca8f50d23be7987b69ea00.camel@iris-sensing.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: iris-ersc

drivers/hwmon/ad7314.c

@@ -22,11 +22,13 @@
  */
 #define AD7314_TEMP_MASK		0x7FE0
 #define AD7314_TEMP_SHIFT		5
+#define AD7314_LEADING_ZEROS_MASK	BIT(15)
 
 /*
  * ADT7301 and ADT7302 temperature masks
  */
 #define ADT7301_TEMP_MASK		0x3FFF
+#define ADT7301_LEADING_ZEROS_MASK	(BIT(15) | BIT(14))
 
 enum ad7314_variant {
 	adt7301,
@@ -65,12 +67,20 @@ static ssize_t ad7314_temperature_show(struct device *dev,
 		return ret;
 	switch (spi_get_device_id(chip->spi_dev)->driver_data) {
 	case ad7314:
+		if (ret & AD7314_LEADING_ZEROS_MASK) {
+			/* Invalid read-out, leading zero part is missing */
+			return -EIO;
+		}
 		data = (ret & AD7314_TEMP_MASK) >> AD7314_TEMP_SHIFT;
 		data = sign_extend32(data, 9);
 
 		return sprintf(buf, "%d\n", 250 * data);
 	case adt7301:
 	case adt7302:
+		if (ret & ADT7301_LEADING_ZEROS_MASK) {
+			/* Invalid read-out, leading zero part is missing */
+			return -EIO;
+		}
 		/*
 		 * Documented as a 13 bit twos complement register
 		 * with a sign bit - which is a 14 bit 2's complement