Ugh. This website saved my ass trying to make HID devices work on top of the completely non-conforming Linaro DwUsbHostDxe driver (look at https://github.com/andreiw/RaspberryPiPkg/commits/master/Dri... for a laugh). I did end up having to hit the USB spec in the end, but it's a legit resource to get a birds-eye view of the whole USB1 and USB2 thing. It doesn't make USB simpler, but sure makes it more approachable.