Greg KH pointed out on 18 Nov 2004 in the Linux Kernel mailing list a GPL Violation by Silicon Laboratories. See also his blog entry

In talking with the company, they insist that they will not release the source code to this module, and they claim that they are not infringing on any rights by not doing so. I claim that this is not true, as to write a usb to serial driver for Linux you have to use the drivers/usb/serial/usb-serial.h header file which is specifically licensed under the GPL v2. This file contains inline functions and structures that all usb-serial drivers need to use in order to work properly.

In short, there's no way you can write a Linux usb-serial driver, that uses the usbserial interface, without it being a derived work of other, GPL only code.

So, they are in violation, so what. Well, I can't do much about this (due to my employer's rules about suing companies). But I can do my best to spread the word that the CP2102 device is not supported on Linux, and should be avoided at all costs by anyone considering such a device in a future design. I encourage everyone else to help spread this information too.

Word spread. I had to talk about that as well. I don't like blatant license violations.