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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>
84 lines
2.7 KiB
ReStructuredText
84 lines
2.7 KiB
ReStructuredText
USB Anchors
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~~~~~~~~~~~
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What is anchor?
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===============
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A USB driver needs to support some callbacks requiring
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a driver to cease all IO to an interface. To do so, a
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driver has to keep track of the URBs it has submitted
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to know they've all completed or to call usb_kill_urb
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for them. The anchor is a data structure takes care of
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keeping track of URBs and provides methods to deal with
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multiple URBs.
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Allocation and Initialisation
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=============================
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There's no API to allocate an anchor. It is simply declared
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as struct usb_anchor. :c:func:`init_usb_anchor` must be called to
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initialise the data structure.
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Deallocation
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============
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Once it has no more URBs associated with it, the anchor can be
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freed with normal memory management operations.
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Association and disassociation of URBs with anchors
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===================================================
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An association of URBs to an anchor is made by an explicit
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call to :c:func:`usb_anchor_urb`. The association is maintained until
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an URB is finished by (successful) completion. Thus disassociation
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is automatic. A function is provided to forcibly finish (kill)
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all URBs associated with an anchor.
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Furthermore, disassociation can be made with :c:func:`usb_unanchor_urb`
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Operations on multitudes of URBs
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================================
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:c:func:`usb_kill_anchored_urbs`
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--------------------------------
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This function kills all URBs associated with an anchor. The URBs
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are called in the reverse temporal order they were submitted.
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This way no data can be reordered.
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:c:func:`usb_unlink_anchored_urbs`
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----------------------------------
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This function unlinks all URBs associated with an anchor. The URBs
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are processed in the reverse temporal order they were submitted.
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This is similar to :c:func:`usb_kill_anchored_urbs`, but it will not sleep.
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Therefore no guarantee is made that the URBs have been unlinked when
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the call returns. They may be unlinked later but will be unlinked in
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finite time.
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:c:func:`usb_scuttle_anchored_urbs`
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-----------------------------------
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All URBs of an anchor are unanchored en masse.
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:c:func:`usb_wait_anchor_empty_timeout`
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---------------------------------------
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This function waits for all URBs associated with an anchor to finish
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or a timeout, whichever comes first. Its return value will tell you
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whether the timeout was reached.
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:c:func:`usb_anchor_empty`
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--------------------------
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Returns true if no URBs are associated with an anchor. Locking
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is the caller's responsibility.
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:c:func:`usb_get_from_anchor`
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-----------------------------
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Returns the oldest anchored URB of an anchor. The URB is unanchored
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and returned with a reference. As you may mix URBs to several
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destinations in one anchor you have no guarantee the chronologically
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first submitted URB is returned.
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