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 3.10					     June 2014						      USB_BUFFER_MAP_SG(9)