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       usb_sg_wait - synchronously execute scatter/gather request


       void usb_sg_wait  (struct usb_sg_request * io);


       io     request  block  handle,  as  initialized  with usb_sg_init. some
              fields become accessible when this call returns.


       !in_interrupt ()


       This function blocks until the specified I/O  operation  completes.  It
       leverages the grouping of the related I/O requests to get good transfer
       rates, by queueing the requests. At higher  speeds,  such  queuing  can
       significantly improve USB throughput.

       There  are  three  kinds  of completion for this function. (1) success,
       where io->status is zero. The number of  io->bytes  transferred  is  as
       requested.  (2)  error, where io->status is a negative errno value. The
       number of io->bytes transferred before the error is usually  less  than
       requested,  and  can be nonzero. (3) cancellation, a type of error with
       status -ECONNRESET that is initiated by usb_sg_cancel.

       When this function returns, all memory allocated through usb_sg_init or
       this  call  will have been freed. The request block parameter may still
       be passed to usb_sg_cancel, or it  may  be  freed.  It  could  also  be
       reinitialized and then reused.


       Bulk  transfers  are  valid  for full or high speed endpoints. The best
       full speed data rate is 19 packets of 64 bytes each per frame, or  1216
       bytes  per  millisecond. The best high speed data rate is 13 packets of
       512 bytes each per microframe, or 52 KBytes per millisecond.

       The reason to use interrupt  transfers  through  this  API  would  most
       likely  be  to  reserve high speed bandwidth, where up to 24 KBytes per
       millisecond could be transferred. That capability is  less  useful  for
       low  or  full speed interrupt endpoints, which allow at most one packet
       per millisecond, of at most 8 or 64 bytes (respectively).