Re: [PATCH v2 1/7] thunderbolt: Hold a router reference for each path hop

[Mika Westerberg]

Hi,

On Mon, Aug 24, 2026 at 01:06:12PM +0200, Sven Peter wrote:
> So unfortunately the entire USB hardware is best described as horribly
> broken :(

:-(

> For USB2 already we only ever receive a single hotplug interrupt in dwc3. If
> the first device is unplugged and another one plugged in nothing happens.
> The only way around that is to tear down dwc3/xhci and the PHY completely,
> assert all external reset lines and then bring them up again.
> Upstream drivers/usb/dwc3/dwc3-apple.c contains more details in the very
> first comment at the top.
> 
> With USB4 this gets a bit worse: There's a block called "ACIO" (Apple
> Converged I/O) which can only be brought up correctly after the PHY has been
> switched to Thunderbolt/USB4 mode. This block has a co-processor which then
> exposes the host router, NHI, IOMMU, etc. to our address space.
> We then have to write cable information into a vendor-specific capabilities
> register and only then do we get the hotplug event and the link comes up.
> This register appears to be effectively write-once after each boot of the
> co-processor. If I try to write 0 that still works but trying to write the
> value for the next connection then crashes the co-processor with a very
> helpful message along the lines of "assert 4357 violated".
> So there's no way around first tearing everything down, then shutting down
> the co-processor and the entire ACIO block and then finally bringing it all
> up again after the next cable is connected. I've tried to find ways around
> this but didn't succeed without documentation and could only reproduce what
> XNU does.

Thanks for sharing the interesting details! I totally understand the pain
of reverse engineering things. The initial USB4/TB driver was also result
of a huge reverse engineering effort by Andreas Noever, but it was and
still is functional, and we still support Apple TB1/2 hardware (although
not too much testing is done these days, I fire up my Cactus Ridge based
Mac maybe once a year).

But okay it does not sound like the PC ACPI hotplug "solution" we had at
least. It's just a much more complex subsystem with many interactions that
need to be done in certain order which is not documented anywhere ;-)