1USB_SG_WAIT(9)                   USB Core APIs                  USB_SG_WAIT(9)
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NAME

6       usb_sg_wait - synchronously execute scatter/gather request
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SYNOPSIS

9       void usb_sg_wait(struct usb_sg_request * io);
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ARGUMENTS

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

17       !in_interrupt ()
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DESCRIPTION

20       This function blocks until the specified I/O operation completes. It
21       leverages the grouping of the related I/O requests to get good transfer
22       rates, by queueing the requests. At higher speeds, such queuing can
23       significantly improve USB throughput.
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25       There are three kinds of completion for this function.
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27       (1) success, where io->status is zero. The number of io->bytes
28       transferred is as requested. (2) error, where io->status is a negative
29       errno value. The number of io->bytes transferred before the error is
30       usually less than requested, and can be nonzero. (3) cancellation, a
31       type of error with status -ECONNRESET that is initiated by
32       usb_sg_cancel.
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34       When this function returns, all memory allocated through usb_sg_init or
35       this call will have been freed. The request block parameter may still
36       be passed to usb_sg_cancel, or it may be freed. It could also be
37       reinitialized and then reused.
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DATA TRANSFER RATES

40       Bulk transfers are valid for full or high speed endpoints. The best
41       full speed data rate is 19 packets of 64 bytes each per frame, or 1216
42       bytes per millisecond. The best high speed data rate is 13 packets of
43       512 bytes each per microframe, or 52 KBytes per millisecond.
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45       The reason to use interrupt transfers through this API would most
46       likely be to reserve high speed bandwidth, where up to 24 KBytes per
47       millisecond could be transferred. That capability is less useful for
48       low or full speed interrupt endpoints, which allow at most one packet
49       per millisecond, of at most 8 or 64 bytes (respectively).
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51       It is not necessary to call this function to reserve bandwidth for
52       devices under an xHCI host controller, as the bandwidth is reserved
53       when the configuration or interface alt setting is selected.
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56Kernel Hackers Manual 3.10         June 2019                    USB_SG_WAIT(9)
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