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

[Sven Peter <sven@kernel.org>]

On 8/24/26 13:37, Mika Westerberg wrote:
> 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).

Hah, nice! I wasn't aware the initial version of this driver was built 
for Apple hardware!
Let's see if we can the Apple Silicon changes into a shape where they 
still work 10+ years from now as well :-)


Sven