USB_BUFFER_MAP_SG(9)                      USB Core APIs                      USB_BUFFER_MAP_SG(9)



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

SYNOPSIS
       int usb_buffer_map_sg(const struct usb_device * dev, int is_in, struct scatterlist * sg,
                             int nents);

ARGUMENTS
       dev
           device to which the scatterlist will be mapped

       is_in
           mapping transfer direction

       sg
           the scatterlist to map

       nents
           the number of entries in the scatterlist

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

NOTE
       The caller is responsible for placing the resulting DMA addresses from the scatterlist
       into URB transfer buffer pointers, and for setting the URB_NO_TRANSFER_DMA_MAP transfer
       flag in each of those URBs.

       Top I/O rates come from queuing URBs, instead of waiting for each one to complete before
       starting the next I/O. This is particularly easy to do with scatterlists. Just allocate
       and submit one URB for each DMA mapping entry returned, stopping on the first error or
       when all succeed. Better yet, use the usb_sg_*() calls, which do that (and more) for you.

       This call would normally be used when translating scatterlist requests, rather than
       usb_buffer_map, since on some hardware (with IOMMUs) it may be able to coalesce mappings
       for improved I/O efficiency.

       Reverse the effect of this call with usb_buffer_unmap_sg.

COPYRIGHT
Kernel Hackers Manual 4.8.                 January 2017                      USB_BUFFER_MAP_SG(9)