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

6       usb_buffer_map_sg - create scatterlist DMA mapping(s) for an endpoint
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SYNOPSIS

9       int usb_buffer_map_sg(const struct usb_device * dev, int is_in,
10                             struct scatterlist * sg, int nents);
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ARGUMENTS

13       dev
14           device to which the scatterlist will be mapped
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16       is_in
17           mapping transfer direction
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19       sg
20           the scatterlist to map
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22       nents
23           the number of entries in the scatterlist
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RETURN

26       Either < 0 (indicating no buffers could be mapped), or the number of
27       DMA mapping array entries in the scatterlist.
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NOTE

30       The caller is responsible for placing the resulting DMA addresses from
31       the scatterlist into URB transfer buffer pointers, and for setting the
32       URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.
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34       Top I/O rates come from queuing URBs, instead of waiting for each one
35       to complete before starting the next I/O. This is particularly easy to
36       do with scatterlists. Just allocate and submit one URB for each DMA
37       mapping entry returned, stopping on the first error or when all
38       succeed. Better yet, use the usb_sg_*() calls, which do that (and more)
39       for you.
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41       This call would normally be used when translating scatterlist requests,
42       rather than usb_buffer_map, since on some hardware (with IOMMUs) it may
43       be able to coalesce mappings for improved I/O efficiency.
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45       Reverse the effect of this call with usb_buffer_unmap_sg.
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48Kernel Hackers Manual 3.10         June 2019              USB_BUFFER_MAP_SG(9)
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