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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.
- 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 (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
- ================================
- 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_unlink_anchored_urbs()
- --------------------------
- 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.
- usb_scuttle_anchored_urbs()
- ---------------------------
- All URBs of an anchor are unanchored en masse.
- 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.
- usb_anchor_empty()
- ------------------
- Returns true if no URBs are associated with an anchor. Locking
- is the caller's responsibility.
- usb_get_from_anchor()
- ---------------------
- 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.
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