[DRAFT][opentitanlib] Introduce abstraction over the USB backend

[pamaury]

Currently there is a UsbBackend struct which wraps around rusb and provides some utility functions. In the future, we may want some transports to implement USB in a different way (e.g. QEMU implements USB over a virtual channel). To do, the first step is to create an abstraction over the existing rusb implementation.

This PR split UsbBackend into two: a context trait and a device trait. The rusb implementation becomes just a particular implementation of those traits.

[cfrantz]

Overall, this looks like nice straightforward wrapper around the functionality of a client usb library.

My only concern is that the wrapper exposes the underlying rusb::ConfigurationDescriptor, which itself exposes the other rusb descriptor types (Interface, Endpoint, etc). These rusb descriptor types appear to be structs that hold a pointer to some C-struct from libusb.

If you write a fake usb shim to speak directly to qemu, what will you do to provide access to the various descriptors?

[pamaury]

Overall, this looks like nice straightforward wrapper around the functionality of a client usb library.

My only concern is that the wrapper exposes the underlying rusb::ConfigurationDescriptor, which itself exposes the other rusb descriptor types (Interface, Endpoint, etc). These rusb descriptor types appear to be structs that hold a pointer to some C-struct from libusb.

If you write a fake usb shim to speak directly to qemu, what will you do to provide access to the various descriptors?

You are correct, I mistakenly assumed that rusb provided a generic implementation of tge descriptors but this is indeed tied to libusb which is not good. This means that we need to provide a way to parse the descriptors which is... not ideal.

Out of curiosity, I looked at nusb and they do provide a generic interface but maybe it's too much to import a dependency just for that? Or we can reimplement just the bare minimum of descriptor parsing ourselves.

[jwnrt]

Out of curiosity, I looked at nusb and they do provide a generic interface but maybe it's too much to import a dependency just for that? Or we can reimplement just the bare minimum of descriptor parsing ourselves.

Aside: we were considering nusb for master for its async API and to help with concerns about LGPL dependencies (libusb).