Login via serial console in linux system

When a user want to log in linux system via physical serial port (eg ttyS0), a login program (agetty or mingetty) should have already been waiting for user input (username and password) on that port.
To make the login program ready on serial port, append the following line in /etc/inittab:
1:23:respawn:/sbin/mingetty ttyS0

The next step is to add parameter console=ttyS0 at kernel boot command in GRUB, the kernel will expose interaction on that serial port so that user can login.