[deliverable/linux.git] / Documentation / usb / anchors.txt

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 (successfull) 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_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.
Commit	Line	Data
e6a79f1f ON	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 (successfull) 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_wait_anchor_empty_timeout()
	46	-------------------------------
	47
	48	This function waits for all URBs associated with an anchor to finish
	49	or a timeout, whichever comes first. Its return value will tell you
	50	whether the timeout was reached.