The USB composite framework

As I've mentioned, Mass Storage Function is merely a conversion of the File-Storage Gadget so most of the code in MSF is copied with modifications from FSG. As such, your understanding is correct, by MSF I mean all the functions in f_mass_storage.c as well as storage_common.c (the latter is code that is identical (or nearly identical) in both MSF and FSG).

The prefix of the function names may be confusing. At one point I even wanted to use fsg_ for FSG stuff, msf_ for MSF stuff and stor_ for commons but Alan convinced my that it's too much hustle and modification not worth the effort. Hence, the fsg_ prefix stayed.

As far as including “.c” files... to be honest, I'm not entirely convinced why this is done this way but that's the way it is done in other composite gadget drivers as well. The comment you may find in those is:

> Kbuild is not very cooperative with respect to linking separately compiled library objects into one module. So for now we won't use separate compilation ... ensuring init/exit sections work to shrink the runtime footprint, and giving us at least some parts of what a "gcc --combine ... part1.c part2.c part3.c ... " build would.

Going further in your comment, the situation with composite is just like iabervon described. It makes uninteresting things trivial. I would also add that it makes the more interesting part a bit harder.

This is because composite function has to worry about registering interface and string IDs and never knows in how many configurations it will be present or ever how many configurations gadget has.

One of the most complicated thing about converting File-Storage Gadget to Mass Storage Function was to allow for it to be added to several USB configurations. To achieve that I have created the fsg_common structure with reference counting and some such.

As of next articles I can only thank you for interest and get back to work. ;) Nonetheless, the my next article probably won't cover most of your questions but should be valuable addition to this one.

I may only quickly point out that you deal with hardware limitations by obeying them. :P If your UDC has limited number of endpoints you won't be able to stash many composite functions in a single configuration in your gadget. Mass storage is rather simple since in bulk-only mode (the one implemented by MSF) it uses only one IN and one OUT endpoint but generally functions may need more endpoints.