UART_MCR_TCRTLR
serial8250_out_MCR(up, mcr | UART_MCR_TCRTLR);
up->mcr = serial_in(up, UART_MCR) & ~UART_MCR_TCRTLR;
serial_out(up, UART_MCR, up->mcr | UART_MCR_TCRTLR);
serial_out(up, UART_MCR, up->mcr | UART_MCR_TCRTLR);