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.
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
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
Operations on multitudes of 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.
All URBs of an anchor are unanchored en masse.
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.
Returns true if no URBs are associated with an anchor. Locking is the caller's responsibility.
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.