There are some use cases where USB devices work very well with EHCI (and even UHCI+OHCI) but fail to work with XHCI. This is mostly due to XHCI controllers handling more of the USB protocol internally (with related non-fixable firmware bugs/shortcomings), while predecessors are more software-defined.
So in such use cases, one normally wants to route the USB traces of the physical USB connectors to the EHCI controller IP core, rather than the XHCI controller. For Intel XHCI there are some more or less standard ways to do this via PCI configuration space. One good explanation is found at https://www.systutorials.com/h…in-usb-2-0-mode-in-linux/
Now the question is: How to achieve this on an APU2? I have unloaded xhci_pci and now I only have the EHCI linux driver bound to the two EHCI controllers. However, the attached low/full/high speed devices do not show up on the EHCI controllers.
I couldn't find any register-level documentation of the AMD XHCI, but maybe somebody at PC Engines could check how to determine where the USB 2 DN/DP of the physical connectors should be routed internally (XHCI vs. EHCI) and share this information with the APU users.
Thanks in advance!