Provided by: linux-doc-2.6.15_2.6.15-23.39_all bug


       struct usb_request - describes one i/o request


       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;


       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.

              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;

              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. 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

              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.


       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


       David Brownell <>.