USB4/TBT3 support

Hello,

Is anyone working on USB4 / Thunderbolt 3 (TBT3) support?

Scott Long did a lot of work on this a few years ago, but he had to move
on to other things, so he passed things on to Hans Petter Selasky.
Fortunately, Hans Petter dropped the code in a public repository.

https://github.com/hselasky/usb4
https://github.com/hselasky/usb4/commit/dd85c216a2a6bee5361c7166595ba6ca461578b5

Here is an overview of what Scott shared with me.

Mostly completed work:

- Debug/Trace framework
- NHI controller driver
- PCIe bridge driver
- WMI driver
- Integrated Connection Manager handshake and authentication handling
- Router and Config Space layer handling (in progress, almost complete)

Remaining work:

- tbtconfig (userland tool)
- man pages
- DMAR/IOMMU integration, PCIe tunnelling control
- Support for resetting and firmware flashing on the NHI via out-of-band control
- Host Connection Manager
- Cross-domain login
- ThunderboltIP

Here are the details that Scott shared.

     The driver originally targeted the Thunderbolt 3 controllers that
     were sold under the names “AlpineRidge" and “Icelake”, in the late
     2010's, before the USB standards group publicly released the USB4
     spec.  The driver set I wrote was complete enough to activate
     Thunderbolt3 peripherals that otherwise would be disabled by
     default when plugged in.  The driver also attempted to make it
     easier to identify things like PCIe tunnels in the topology, but
     that was mostly cosmetic.  Unfortunately, the AlpineRidge chips
     proved to be extremely hard to work with despite their wide
     availability, and I spent way too much time fighting them and not
     enough time developing more useful functionality.  The WMI driver
     was written to work around vexing problems with the Alpine Ridge
     controller that I never figured out.

     Much of the infrastructure from the TBT3 support extends to modern
     USB4 controllers, but there are still a lot of missing pieces.  The
     NHI driver doesn't know how to probe a USB4 controller yet, but
     that should be easy to fix.  Even more important, though, is that
     the code lacks a functional USB4 Connection Manager.  Most of the
     pieces required to traverse the topology, discover routers and
     adapters, read and write their properties, and build routes between
     endpoints exists now, but there's no state machine yet that
     integrates those pieces together into a real Connection Manager.
     Without that, no attached peripherals will actually run.  The TBT3
     controllers like AlpineRidge and IceLake have a connection manager
     in firmware, so that's why those controllers function even with
     minimal host OS support.  This isn't especially hard code to write,
     but it's missing nonetheless.

     Once the connection manager is written, it'll need to configure
     connections with the USB3, PCIe, and DisplayPort devices that
     operate over tunnels, and it'll need a cross domain handler for
     connecting to another host.  USB3 tunnel support might require
     significant changes in the USB3 stack in order to work with USB4.

     Additionally you might need to write a USB-PD driver.  Without it,
     negotiation on USB-C connectors for power delivery advertisements,
     cable orientation, alt mode configuration, and USB3 vs USB4 lane
     assignment might not work.  If those negotiations are not handled
     then nothing that you plug into the port will even be seen by the
     controller.

     I'd totally stay away from spending time on supporting Falcon
     Ridge, Alpine Ridge, and Titan Ridge controllers.  They're old,
     they're extremely difficult to work with, and they're not worth the
     headache.  In fact, just ignore all TBT3 controllers, and remove
     the Internal Connection Manager code.  The ICM module isn't code
     that I'm all that proud of anyways =-). Focus on writing an HCM,
     supporting PCIe and DP tunneling, and integrating IOMMU protections
     into both the NHI driver and the PCIe tunnel drivers.

     One thing that would be pretty awesome is ThunderboltIP support.
     It turns out that these controllers are really cheap 40Gbps
     devices, and have the potential to perform pretty well at line rate
     as a replacement for traditional 40Gb ethernet controllers, at
     least in a point-to-point configuration.

In short, a lot of work has been done, but a lot of work remains to be
done.  The USB4 spec is complicated, and requires intimate knowledge
of the USB-PD, USB-C, and USB3 specs.  There's also a spec for
writing a Host Connection Manager that you’ll need to get familiar with.

If anyone has already started or would like to continue Scott's work,
could you please let me know?

Joe