Source for the ASP firmware is at https://github.com/amd/AMD-ASPFW.

It has a number of gaps, but it is mostly there. It doesn't build, it doesn't have source for some of the service calls iirc (SVC_.*), and the AGESA source isn't open (though a replacement is in progress, openSIL).

This is the source of only a single application (out of 30 or 40?). :)