Linking multiple Git repositories for version tracking

Hi,

I'm developing firmware that is composed of multiple projects. Each section of the firmware has it's own git repository (each section correlates to a physical processor). But the firmware as a whole is getting to the point where I have to remember which version of Firmware A is compatible with Firmware B. If I add a feature to B that requires some tweaks in A, I need to know that both A v3.04 and B v2.7 need to be used together.

I'm starting to move into alpha with this code, so I really need to have a system that keeps track of compatible firmware versions. The best I can come up with is a plain text file (or Excel spreadsheet) that lists the overall firmware version, and the Git hash for each individual project. That way, if I want to load up a particular firmware version, I can checkout each hash for that version. Seems foolproof, but not terribly easy, and somewhat annoying.

I know submodules might be used, but it's not super obvious how to make their paradigm work nicely here. (You check out a version you want, and then list all the submodule git hashes for that version? What happens if one hash needs updating? Do you branch it?) It seems more complicated than I'd like.

But this can't be a problem that only I have. What have you done to keep track of software bundle versions?

Thanks! James