usb_buffer_map — create DMA mapping(s) for an urb
struct urb * usb_buffer_map (struct urb * urb);
Return value is either null (indicating no buffer could be mapped), or the parameter. URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP are added to urb->transfer_flags if the operation succeeds. If the device is connected to this system through a non-DMA controller, this operation always succeeds.
This call would normally be used for an urb which is reused, perhaps
as the target of a large periodic transfer, with usb_buffer_dmasync
calls to synchronize memory and dma state.
Reverse the effect of this call with usb_buffer_unmap.