usb_ep_queue — queues (submits) an I/O request to an endpoint.
int usb_ep_queue ( | struct usb_ep * | ep, |
struct usb_request * | req, | |
gfp_t | gfp_flags) ; |
ep
the endpoint associated with the request
req
the request being submitted
gfp_flags
GFP_* flags to use in case the lower level driver couldn't pre-allocate all necessary memory with the request.
This tells the device controller to perform the specified request through
that endpoint (reading or writing a buffer). When the request completes,
including being canceled by usb_ep_dequeue
, the request's completion
routine is called to return the request to the driver. Any endpoint
(except control endpoints like ep0) may have more than one transfer
request queued; they complete in FIFO order. Once a gadget driver
submits a request, that request may not be examined or modified until it
is given back to that driver through the completion callback.
Each request is turned into one or more packets. The controller driver never merges adjacent requests into the same packet. OUT transfers will sometimes use data that's already buffered in the hardware. Drivers can rely on the fact that the first byte of the request's buffer always corresponds to the first byte of some USB packet, for both IN and OUT transfers.
Bulk endpoints can queue any amount of data; the transfer is packetized automatically. The last packet will be short if the request doesn't fill it out completely. Zero length packets (ZLPs) should be avoided in portable protocols since not all usb hardware can successfully handle zero length packets. (ZLPs may be explicitly written, and may be implicitly written if the request 'zero' flag is set.) Bulk endpoints may also be used for interrupt transfers; but the reverse is not true, and some endpoints won't support every interrupt transfer. (Such as 768 byte packets.)
Interrupt-only endpoints are less functional than bulk endpoints, for example by not supporting queueing or not handling buffers that are larger than the endpoint's maxpacket size. They may also treat data toggle differently.
Control endpoints ... after getting a setup
callback, the driver queues
one response (even if it would be zero length). That enables the
status ack, after transfering data as specified in the response. Setup
functions may return negative error codes to generate protocol stalls.
(Note that some USB device controllers disallow protocol stall responses
in some cases.) When control responses are deferred (the response is
written after the setup callback returns), then usb_ep_set_halt
may be
used on ep0 to trigger protocol stalls.
For periodic endpoints, like interrupt or isochronous ones, the usb host arranges to poll once per interval, and the gadget driver usually will have queued some data to transfer at that time.
Returns zero, or a negative error code. Endpoints that are not enabled report errors; errors will also be reported when the usb peripheral is disconnected.