i2c: Fall back to emulated SMBus if the operation isn't supported natively

Adapter drivers might support only a subset of the SMBus operations
natively. Those drivers currently have to manually emulate unsupported
operations using I2C.

Make the i2c_smbus_xfer() function fall back to
i2c_smbus_xfer_emulated() when the adapter's .smbus_xfer() operation
returns -EOPNOTSUPP, like it already does when the .smbus_xfer()
operation isn't available at all.

[JD: Minor optimization.]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>

drivers/i2c/i2c-core.c

@@ -2140,11 +2140,17 @@ s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr, unsigned short flags,
 				break;
 		}
 		i2c_unlock_adapter(adapter);
-	} else
-		res = i2c_smbus_xfer_emulated(adapter, addr, flags, read_write,
-					      command, protocol, data);
 
-	return res;
+		if (res != -EOPNOTSUPP || !adapter->algo->master_xfer)
+			return res;
+		/*
+		 * Fall back to i2c_smbus_xfer_emulated if the adapter doesn't
+		 * implement native support for the SMBus operation.
+		 */
+	}
+
+	return i2c_smbus_xfer_emulated(adapter, addr, flags, read_write,
+				       command, protocol, data);
 }
 EXPORT_SYMBOL(i2c_smbus_xfer);