Question about usb3 host code

Hello linux-usb community,

  I am new to linux-usb code and I hope that my question is not silly.
I am interested in real-time usb.

  I know that on USB3, the gadget can initiate transactions
and I am searching for the function that is called in the usb3 host
when an USB3 gadget wants to start a transaction.
For example for raspberry pi 4 or other host.
Is it on the xhci host code or on the USB3 controller code (for example dwc3) ?

  Do you have any hints?
I appreciate any help.

Thank you,
Laurentiu-Cristian