IPP-USB Gadget Support

[michaelrsweet]

Develop a Linux USB gadget and corresponding support code in PAPPL to allow communication with the default printer in PAPPL using the IPP-USB protocol. References:

• Kernel Gadget API
• IPP-USB specification

There are two parts to this:

• [x] A kernel driver, either an updated version of the USB printer gadget or a new g_ippusb gadget. The gadget needs to expose at least three pairs of endpoints to support printing and scanning simultaneously. Descriptor limits may be an issue.
• [x] Support code in PAPPL to basically map the endpoints to local sockets, either localhost or (better) a domain socket listener.

This code should coexist with the USB printer gadget - the client device can only select the legacy printer class or the new IPP-USB class (not both), but the PAPPL code needs to properly handle that selection.

[michaelrsweet]

OK, so I've done some prototyping using the FunctionFS gadget driver which allows me to export N instances of IPP-USB descriptors and endpoints (tested with N=3 which is the typical configuration for printers), and then no need to do a separate kernel driver!

Documentation for FunctionFS sucks. But it looks like it works well.

[michaelrsweet]

Additional issues found during testing:

• [x] Wi-Fi test harness in testpappl
• [x] When connected over localhost, only advertise http/ipp URLs
• [x] Should use domain socket when available (but need to store the domain socket path in the server object)

[michaelrsweet]

This work is now done.