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 4.8.                 January 2017                            USB_SG_WAIT(9)