If you plan to write your own serial port driver, you will need to actually write the driver for your special needs. If you just need to use the port, you can stay in the user space. Of course, you can write it as driver, too. Your program will look differently as module an as user-space program.
To access serial port you can use a tool like minicom. The port is just a file, so you can read/write to it. Serial ports are /dev/ttyS0, /dev/ttyS1 and so on.