USB_SG_WAIT(9) USB Core APIs USB_SG_WAIT(9)NAMEusb_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.
COPYRIGHTKernel Hackers Manual 2.6. November 2013 USB_SG_WAIT(9)
