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-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.
-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 (successful) 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
-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.
-This function unlinks all URBs associated with an anchor. The URBs
-are processed in the reverse temporal order they were submitted.
-This is similar to usb_kill_anchored_urbs(), but it will not sleep.
-Therefore no guarantee is made that the URBs have been unlinked when
-the call returns. They may be unlinked later but will be unlinked in
-finite time.
-All URBs of an anchor are unanchored en masse.
-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.
-Returns true if no URBs are associated with an anchor. Locking
-is the caller's responsibility.
-Returns the oldest anchored URB of an anchor. The URB is unanchored
-and returned with a reference. As you may mix URBs to several
-destinations in one anchor you have no guarantee the chronologically
-first submitted URB is returned.