usb_ep_queue(9) [centos man page]
USB_EP_QUEUE(9) Kernel Mode Gadget API USB_EP_QUEUE(9) NAME
usb_ep_queue - queues (submits) an I/O request to an endpoint. SYNOPSIS
int usb_ep_queue(struct usb_ep * ep, struct usb_request * req, gfp_t gfp_flags); ARGUMENTS
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. DESCRIPTION
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 transferring 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. Depending on the controller, it may not be possible to trigger a status-stage protocol stall when the data stage is over, that is, from within the response's completion routine. 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. AUTHOR
David Brownell <dbrownell@users.sourceforge.net> Author. COPYRIGHT
Kernel Hackers Manual 3.10 June 2014 USB_EP_QUEUE(9)
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STRUCT
USB_REQUEST(9) Kernel Mode Gadget API STRUCT USB_REQUEST(9) NAME
struct_usb_request - describes one i/o request SYNOPSIS
struct usb_request { void * buf; unsigned length; dma_addr_t dma; unsigned no_interrupt:1; unsigned zero:1; unsigned short_not_ok:1; void (* complete) (struct usb_ep *ep,struct usb_request *req); void * context; struct list_head list; int status; unsigned actual; }; MEMBERS
buf Buffer used for data. Always provide this; some controllers only use PIO, or don't use DMA for some endpoints. length Length of that data dma DMA address corresponding to 'buf'. If you don't set this field, and the usb controller needs one, it is responsible for mapping and unmapping the buffer. no_interrupt If true, hints that no completion irq is needed. Helpful sometimes with deep request queues that are handled directly by DMA controllers. zero If true, when writing data, makes the last packet be "short" by adding a zero length packet as needed; short_not_ok When reading data, makes short packets be treated as errors (queue stops advancing till cleanup). complete Function called when request completes, so this request and its buffer may be re-used. The function will always be called with interrupts disabled, and it must not sleep. Reads terminate with a short packet, or when the buffer fills, whichever comes first. When writes terminate, some data bytes will usually still be in flight (often in a hardware fifo). Errors (for reads or writes) stop the queue from advancing until the completion function returns, so that any transfers invalidated by the error may first be dequeued. context For use by the completion callback list For use by the gadget driver. status Reports completion code, zero or a negative errno. Normally, faults block the transfer queue from advancing until the completion callback returns. Code "-ESHUTDOWN" indicates completion caused by device disconnect, or when the driver disabled the endpoint. actual Reports bytes transferred to/from the buffer. For reads (OUT transfers) this may be less than the requested length. If the short_not_ok flag is set, short reads are treated as errors even when status otherwise indicates successful completion. Note that for writes (IN transfers) some data bytes may still reside in a device-side FIFO when the request is reported as complete. DESCRIPTION
These are allocated/freed through the endpoint they're used with. The hardware's driver can add extra per-request data to the memory it returns, which often avoids separate memory allocations (potential failures), later when the request is queued. Request flags affect request handling, such as whether a zero length packet is written (the "zero" flag), whether a short read should be treated as an error (blocking request queue advance, the "short_not_ok" flag), or hinting that an interrupt is not required (the "no_interrupt" flag, for use with deep request queues). Bulk endpoints can use any size buffers, and can also be used for interrupt transfers. interrupt-only endpoints can be much less functional. NOTE
this is analagous to 'struct urb' on the host side, except that it's thinner and promotes more pre-allocation. AUTHOR
David Brownell <dbrownell@users.sourceforge.net> Author. COPYRIGHT
Kernel Hackers Manual 2.6. July 2010 STRUCT USB_REQUEST(9)