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Documentation / usb / anchors.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

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