struct_usb_request (9) - Linux Manuals

struct_usb_request: describes one i/o request


struct_usb_request - describes one i/o request


struct usb_request {
  void * buf;
  unsigned length;
  dma_addr_t dma;
  struct scatterlist * sg;
  unsigned num_sgs;
  unsigned num_mapped_sgs;
  unsigned stream_id:16;
  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;



Buffer used for data. Always provide this; some controllers only use PIO, or don't use DMA for some endpoints.


Length of that data


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.


a scatterlist for SG-capable controllers.


number of SG entries


number of SG entries mapped to DMA (internal)


The stream id, when USB3.0 bulk streams are being used


If true, hints that no completion irq is needed. Helpful sometimes with deep request queues that are handled directly by DMA controllers.


If true, when writing data, makes the last packet be "short" by adding a zero length packet as needed;


When reading data, makes short packets be treated as errors (queue stops advancing till cleanup).


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.


For use by the completion callback


For use by the gadget driver.


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.


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.


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.


this is analogous to 'struct urb' on the host side, except that it's thinner and promotes more pre-allocation.


David Brownell <dbrownell [at]>