question about interrupt firing in controller

Dear all guys and Greg:
I have a question about when will the the hub_irq be called?
 
In hub_configure, we register the hub->urb as urb->complete function below:
	usb_fill_int_urb(hub->urb, hdev, pipe, *hub->buffer, maxp, hub_irq,
		hub, endpoint->bInterval);
and in hub_activate(hub), we submit it.
	status = usb_submit_urb(hub->urb, GFP_NOIO);

But the hub_irq is only called after usb_hcd_irq, the irq handler we register for the ehci.
It seems we submit the hub->urb to some place and it is been called after interrupt triggered. But where does it hide? 
Appreciate your help,
Cckuo

Ps:Below is the message I get from log and is what my surmise comes from.

calling hub_activate
hcd_submit_urb called   //we submit the hub->urb as mention above
xxxxx     beginning hub_events     //the hub thread is working on
hub 1-0:1.0: state 7 ports 6 chg 0000 evt 0000
xxxxx     beginning each port 
calling hub_port_status
ehci_hcd 0000:00:03.3: GetStatus port 1 status 001000 POWER sig=se0
usb_hcd_giveback_urb called
xxxxx   end of each port
xxxxx     beginning each port 
..........................................................
...................................................  //some device plug in
usb_hcd_irq called
ehci_irq called
ehci_hcd 0000:00:03.3: irq status 0004 PCD  //detect change port state interrupt
usb_hcd_giveback_urb called  //it seems the hub->urb that we submited before finished right now
calling hub_irq       //calling hub_irq
xxxx   calling hub_event
hcd_submit_urb called
xxxxx     beginning hub_events
hub 1-0:1.0: state 7 ports 6 chg 0000 evt 0002
xxxxx     beginning each port 
calling hub_port_status
calling usb_start_wait_urb
hcd_submit_urb called
rh_call_control called
ehci_hcd 0000:00:03.3: GetStatus port 1 status 001803 POWER sig=j CSC CONNECT
usb_hcd_giveback_urb called

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