option.c
This driver exists because the "normal" serial driver doesn't work too well with GSM modems. Issues: - data loss -- one single Receive URB is not nearly enough - nonstandard flow (Option devices) control - controlling the baud rate doesn't make sense
This driver is named "option" because the most common device it's used for is a PC-Card (with an internal OHCI-USB interface, behind which the GSM interface sits), made by Option Inc.
usb-serial.c
the normal usb convert serial driver.