hwmon: (pmbus) Add comments explaining internal driver API return values

Return values for functions reading/writing manufacturer specific registers are
poorly explained. Add comments to improve documentation.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>

drivers/hwmon/pmbus/pmbus.h

@@ -134,8 +134,16 @@
  * Semantics:
  * Virtual registers are all word size.
  * READ registers are read-only; writes are either ignored or return an error.
- * RESET registers are read/write. Reading returns zero (used for detection),
- * writing any value causes the associated history to be reset.
+ * RESET registers are read/write. Reading reset registers returns zero
+ * (used for detection), writing any value causes the associated history to be
+ * reset.
+ * Virtual registers have to be handled in device specific driver code. Chip
+ * driver code returns non-negative register values if a virtual register is
+ * supported, or a negative error code if not. The chip driver may return
+ * -ENODATA or any other error code in this case, though an error code other
+ * than -ENODATA is handled more efficiently and thus preferred. Either case,
+ * the calling PMBus core code will abort if the chip driver returns an error
+ * code when reading or writing virtual registers.
  */
 #define PMBUS_VIRT_BASE			0x100
 #define PMBUS_VIRT_READ_TEMP_MIN	(PMBUS_VIRT_BASE + 0)
@@ -320,6 +328,12 @@ struct pmbus_driver_info {
 	 * The following functions map manufacturing specific register values
 	 * to PMBus standard register values. Specify only if mapping is
 	 * necessary.
+	 * Functions return the register value (read) or zero (write) if
+	 * successful. A return value of -ENODATA indicates that there is no
+	 * manufacturer specific register, but that a standard PMBus register
+	 * may exist. Any other negative return value indicates that the
+	 * register does not exist, and that no attempt should be made to read
+	 * the standard register.
 	 */
 	int (*read_byte_data)(struct i2c_client *client, int page, int reg);
 	int (*read_word_data)(struct i2c_client *client, int page, int reg);