This driver is named "option"

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.