8250_port.c
	uses port->mutex to mutually exclude ->fcr changes



serial core:
       mutually excluded operations guaranteed by port->mutex
       		->config_port()
		->set_termios()

		uart_configure_port()
		uart_resume_port()
		uart_suspend_port()
		uart_open() => uart_startup() => uart_port_startup
		uart_hangup() => uart_shutdown()  ** too much coverage **
		uart_close() => uart_shutdown()  ** see uart_close.analysis **
		uart_break_ctl()
		uart_tiocmset()
		uart_tiocmget()
		uart_add_one_port() ** most of this including tty_port_register_device_attr **

		/* ioctls */
		uart_get_info()		// TIOCGSERIAL
		uart_set_info()		// TIOCSSERIAL
		uart_do_autoconfig()	// TIOCSERCONFIG
		uart_get_lsr_info()	// TIOCSERGETLSR
		uart_get_rs485_config	// TIOCGRS485
		uart_set_rs485_config	// TIOCSRS485
		// any serial driver ioctl


		uart_change_pm()

		link/unlink uart_state <=> uart_port
		access to uart_port->flags


		tty_port_tty_set() for uart == incidental


TODO

TIOCMIWAIT handling is messy; simplify