echo xxx > /dev/ttySxxx to send command to tty. When I do this first time, I must set baut rate. After once, I needn't. So, where does the kernel keep the serial port config?
On Debian I found a copy of /etc/serial.conf in /var/lib/setserial. After using
# dpkg-reconfigure setserial
the /etc/serial.conf is gone, and appears in /var/lib/setserial/etc.serial.conf