usb_buffer_map_sg — create scatterlist DMA mapping(s) for an endpoint
int usb_buffer_map_sg ( | const struct usb_device * | dev, |
| int | is_in, | |
| struct scatterlist * | sg, | |
| int | nents); |
devdevice to which the scatterlist will be mapped
is_inmapping transfer direction
sgthe scatterlist to map
nentsthe number of entries in the scatterlist
Return value is either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist.
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.