"The USB subsystem is based on message passing transactions. The messages are called URBs, which stands for USB request block. URBs are sent by calling the usb_submit_urb method, struct urb *urb, int mem_flags). This is an asynchronous call, and it returns immediately. The URB is put in a queue, and later, it reaches a completion handler. The completion handler is a member of the URB structure called complete, a pointer to a function in the URB struct. In the completion handler, you can check urb -> status to see if any errors have been detected..."
