What is anchor?
===============

A USB driver needs to support some callbacks requiring
a driver to cease all IO to an interface. To do so, a
driver has to keep track of the URBs it has submitted
to know they've all completed or to call usb_kill_urb
for them. The anchor is a data structure takes care of
keeping track of URBs and provides methods to deal with
multiple URBs.

Allocation and Initialisation
=============================

There's no API to allocate an anchor. It is simply declared
as struct usb_anchor. init_usb_anchor() must be called to
initialise the data structure.

Deallocation
============

Once it has no more URBs associated with it, the anchor can be
freed with normal memory management operations.

Association and disassociation of URBs with anchors
===================================================

An association of URBs to an anchor is made by an explicit
call to usb_anchor_urb(). The association is maintained until
an URB is finished by (successfull) completion. Thus disassociation
is automatic. A function is provided to forcibly finish (kill)
all URBs associated with an anchor.
Furthermore, disassociation can be made with usb_unanchor_urb()

Operations on multitudes of URBs
================================

usb_kill_anchored_urbs()
------------------------

This function kills all URBs associated with an anchor. The URBs
are called in the reverse temporal order they were submitted.
This way no data can be reordered.

usb_wait_anchor_empty_timeout()
-------------------------------

This function waits for all URBs associated with an anchor to finish
or a timeout, whichever comes first. Its return value will tell you
whether the timeout was reached.