1What is anchor? 2=============== 3 4A USB driver needs to support some callbacks requiring 5a driver to cease all IO to an interface. To do so, a 6driver has to keep track of the URBs it has submitted 7to know they've all completed or to call usb_kill_urb 8for them. The anchor is a data structure takes care of 9keeping track of URBs and provides methods to deal with 10multiple URBs. 11 12Allocation and Initialisation 13============================= 14 15There's no API to allocate an anchor. It is simply declared 16as struct usb_anchor. init_usb_anchor() must be called to 17initialise the data structure. 18 19Deallocation 20============ 21 22Once it has no more URBs associated with it, the anchor can be 23freed with normal memory management operations. 24 25Association and disassociation of URBs with anchors 26=================================================== 27 28An association of URBs to an anchor is made by an explicit 29call to usb_anchor_urb(). The association is maintained until 30an URB is finished by (successful) completion. Thus disassociation 31is automatic. A function is provided to forcibly finish (kill) 32all URBs associated with an anchor. 33Furthermore, disassociation can be made with usb_unanchor_urb() 34 35Operations on multitudes of URBs 36================================ 37 38usb_kill_anchored_urbs() 39------------------------ 40 41This function kills all URBs associated with an anchor. The URBs 42are called in the reverse temporal order they were submitted. 43This way no data can be reordered. 44 45usb_unlink_anchored_urbs() 46-------------------------- 47 48This function unlinks all URBs associated with an anchor. The URBs 49are processed in the reverse temporal order they were submitted. 50This is similar to usb_kill_anchored_urbs(), but it will not sleep. 51Therefore no guarantee is made that the URBs have been unlinked when 52the call returns. They may be unlinked later but will be unlinked in 53finite time. 54 55usb_scuttle_anchored_urbs() 56--------------------------- 57 58All URBs of an anchor are unanchored en masse. 59 60usb_wait_anchor_empty_timeout() 61------------------------------- 62 63This function waits for all URBs associated with an anchor to finish 64or a timeout, whichever comes first. Its return value will tell you 65whether the timeout was reached. 66 67usb_anchor_empty() 68------------------ 69 70Returns true if no URBs are associated with an anchor. Locking 71is the caller's responsibility. 72 73usb_get_from_anchor() 74--------------------- 75 76Returns the oldest anchored URB of an anchor. The URB is unanchored 77and returned with a reference. As you may mix URBs to several 78destinations in one anchor you have no guarantee the chronologically 79first submitted URB is returned. 80