void * usb_buffer_alloc (struct usb_device * dev, size_t size, unsigned mem_flags, dma_addr_t * dma);
Return value is either null (indicating no buffer could be allocated), or the cpu-space pointer to a buffer that may be used to perform DMA to the specified device. Such cpu-space buffers are returned along with the DMA address (through the pointer provided).
These buffers are used with URB_NO_xxx_DMA_MAP set in urb->transfer_flags to avoid behaviors like using ``DMA bounce buffers'', or tying down I/O mapping hardware for long idle periods. The implementation varies between platforms, depending on details of how DMA will work to this device. Using these buffers also helps prevent cacheline sharing problems on architectures where CPU caches are not DMA-coherent.
When the buffer is no longer used, free it with usb_buffer_free.