serial.c - is the device driver for bare UARTS (8250-16550) others are present for various cards like stallion, digi, specialix et.al. but you probably can't get the tech doc for those. This is the interface with the hardware.
n_tty.c is the code for the line discipline which does the processing of the input/output stream as well as some control function. This is the interface between the "user" and the driver.
tty_io.c and tty_ioctl.c provide some common support functions
tty.h, termio[s].h, termbits.h, ioctl.h, serial.h contain the structure definitions and defines.