USB_SG_WAIT(9)							   USB Core APIs						    USB_SG_WAIT(9)

NAME

       usb_sg_wait - synchronously execute scatter/gather request

SYNOPSIS

       void usb_sg_wait(struct usb_sg_request * io);

ARGUMENTS

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

CONTEXT

       !in_interrupt ()

DESCRIPTION

       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.

DATA TRANSFER RATES

       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).

       It is not necessary to call this function to reserve bandwidth for devices under an xHCI host controller, as the bandwidth is reserved when
       the configuration or interface alt setting is selected.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014							    USB_SG_WAIT(9)