usb/anchors.txt: convert to ReST and add to driver-api book

This document describe some USB core functions. Add it to the
driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/usb/anchors.rst

@@ -0,0 +1,83 @@
+USB Anchors
+~~~~~~~~~~~
+
+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. :c:func:`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 :c:func:`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 :c:func:`usb_unanchor_urb`
+
+Operations on multitudes of URBs
+================================
+
+:c:func:`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.
+
+:c:func:`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 :c:func:`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.
+
+:c:func:`usb_scuttle_anchored_urbs`
+-----------------------------------
+
+All URBs of an anchor are unanchored en masse.
+
+:c:func:`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.
+
+:c:func:`usb_anchor_empty`
+--------------------------
+
+Returns true if no URBs are associated with an anchor. Locking
+is the caller's responsibility.
+
+:c:func:`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.

Documentation/driver-api/usb/index.rst

@@ -6,6 +6,7 @@ Linux USB API
 
    usb
    gadget
+   anchors
    writing_usb_driver
    writing_musb_glue_layer