Per Matias Edlund: I recently used the tcflush function and found that it failed with ENOTTY. The reason is that the TCFLSH case in the function uart_ioctl in the file drivers/serial/serial.c does not update the ret value." On success, the TCFLSH IOCTL logic needs to explicitly set the return value (ret) to zero.

drivers/serial/serial.c

@@ -1339,6 +1339,7 @@ static int uart_ioctl(FAR struct file *filep, int cmd, unsigned long arg)
                 }
 
               leave_critical_section(flags);
+              ret = 0;
             }
             break;