I noticed that the SPI API provides both spi_xchange() and spi_dma_xchange().  Ideally I would like to read/write the 
buffers directly from application memory. While I don't have DMA enabled, could  spi_dma_xchange() be used in my 
hardware interface to prevent the buffer copy from occurring?  If so will the dma_xfer function have access to this 
physical memory or am I limited to the ISR?

Thanks,
Devan