I guess I'm not entirely clear on what I would need to implement to use ALSA's PCM interface with an arbitrary transport, given a defined memory area and buffer

I guess I'm not entirely clear on what I would need to implement to use ALSA's PCM interface with an arbitrary transport, given a defined memory area and buffer size. I appear to be fuzzy on the topic of what parts I need to write and what is already provided, generally. The class-compliant USB interfaces do indeed generally "just work" (with the configuration issues I mentioned on my particular system, but I'm not clear on where this occurs, "channel mapping" or somesuch calling my 8 outputs a singular surround interface etc...)