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.\"     Title: usb_sg_wait
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: USB Core APIs
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "USB_SG_WAIT" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "USB Core APIs"
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.SH "NAME"
usb_sg_wait \- synchronously execute scatter/gather request
.SH "SYNOPSIS"
.HP \w'void\ usb_sg_wait('u
.BI "void usb_sg_wait(struct\ usb_sg_request\ *\ " "io" ");"
.SH "ARGUMENTS"
.PP
\fIio\fR
.RS 4
request block handle, as initialized with
\fBusb_sg_init\fR\&. some fields become accessible when this call returns\&.
.RE
.SH "CONTEXT"
.PP
!in_interrupt ()
.SH "DESCRIPTION"
.PP
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\&.
.PP
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
\fBusb_sg_cancel\fR\&.
.PP
When this function returns, all memory allocated through
\fBusb_sg_init\fR
or this call will have been freed\&. The request block parameter may still be passed to
\fBusb_sg_cancel\fR, or it may be freed\&. It could also be reinitialized and then reused\&.
.PP
Data Transfer Rates:
.PP
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\&.
.PP
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)\&.
.PP
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\&.
.SH "COPYRIGHT"
.br