Where the driver for CDMA/GSM hardware? i.e. I have a phone in CDMA from HTC, who provides the CDMA hardware driver (radio-firmware ?),Google Android source tree or HTC?

If the driver is provided by Google Android source tree, then I can compile a custom version of Android for my phone, If from HTC, then I don't know how can I compile an Android and drive the HTC CDMA hardware.
The radio is closed source and provided by the manufacturer.