RFC: Improving alternate modes for captive cable compatibility.

[hardkrash]

The design currently has a single routing of USB 2.0 pairs. This can cause some adapters with alternate modes to fail to operate properly. Captive plug alternate modes can remap the alternate USB pair B6+B7 for its own purposes. reference Figure E2 in section e.2.1 of the document USB Type-C Spec R2.0 - August 2019.pdf

I propose mapping the USB 2.0 lines as 2x pairs for each A6+A7 and B7+B7.

The con of this is that a stub could be created with a device receptacle that has shorted the A6 to B6 and A7 to B7. This could be solved with resistor stuffing to allow the current configuration, and a pass through for those uses that require passthrough.

Future FW could alleviate this with a software controlled USB 2.0 muxing solution.

I'm unsure if the stub with the passive approach would cause much of an issue... I have successfully done stub monitoring with short 200mm cables to a Total Phase 480mbps beagle. The core difference here is that the stub would be floating with

[gregdavill]

This seems like a good idea.

[whitequark]

Excellent catch @hardkrash! I agree this is a good idea.

[dnschneid]

I've found that stubs of an inch or two can break high-speed USB. Muxing would be great, though.

[whitequark]

I think we can do muxing, though I think we should at least extract the firmware and make it easy to build as a part of this repo before committing to that. (I believe right now it can only be built out of a ChromeOS tree, which isn't something I would like to offer for general use.)