linux/Documentation/usb/anchors.txt
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   1What is anchor?
   2===============
   3
   4A USB driver needs to support some callbacks requiring
   5a driver to cease all IO to an interface. To do so, a
   6driver has to keep track of the URBs it has submitted
   7to know they've all completed or to call usb_kill_urb
   8for them. The anchor is a data structure takes care of
   9keeping track of URBs and provides methods to deal with
  10multiple URBs.
  11
  12Allocation and Initialisation
  13=============================
  14
  15There's no API to allocate an anchor. It is simply declared
  16as struct usb_anchor. init_usb_anchor() must be called to
  17initialise the data structure.
  18
  19Deallocation
  20============
  21
  22Once it has no more URBs associated with it, the anchor can be
  23freed with normal memory management operations.
  24
  25Association and disassociation of URBs with anchors
  26===================================================
  27
  28An association of URBs to an anchor is made by an explicit
  29call to usb_anchor_urb(). The association is maintained until
  30an URB is finished by (successful) completion. Thus disassociation
  31is automatic. A function is provided to forcibly finish (kill)
  32all URBs associated with an anchor.
  33Furthermore, disassociation can be made with usb_unanchor_urb()
  34
  35Operations on multitudes of URBs
  36================================
  37
  38usb_kill_anchored_urbs()
  39------------------------
  40
  41This function kills all URBs associated with an anchor. The URBs
  42are called in the reverse temporal order they were submitted.
  43This way no data can be reordered.
  44
  45usb_unlink_anchored_urbs()
  46--------------------------
  47
  48This function unlinks all URBs associated with an anchor. The URBs
  49are processed in the reverse temporal order they were submitted.
  50This is similar to usb_kill_anchored_urbs(), but it will not sleep.
  51Therefore no guarantee is made that the URBs have been unlinked when
  52the call returns. They may be unlinked later but will be unlinked in
  53finite time.
  54
  55usb_scuttle_anchored_urbs()
  56---------------------------
  57
  58All URBs of an anchor are unanchored en masse.
  59
  60usb_wait_anchor_empty_timeout()
  61-------------------------------
  62
  63This function waits for all URBs associated with an anchor to finish
  64or a timeout, whichever comes first. Its return value will tell you
  65whether the timeout was reached.
  66
  67usb_anchor_empty()
  68------------------
  69
  70Returns true if no URBs are associated with an anchor. Locking
  71is the caller's responsibility.
  72
  73usb_get_from_anchor()
  74---------------------
  75
  76Returns the oldest anchored URB of an anchor. The URB is unanchored
  77and returned with a reference. As you may mix URBs to several
  78destinations in one anchor you have no guarantee the chronologically
  79first submitted URB is returned.
  80
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