dma_alloc_coherent
static void *dma_alloc_coherent(struct device *, size_t, dma_addr_t *, int);
xmm->cp = dma_alloc_coherent(xmm->dev, sizeof(struct control_page), &xmm->cp_phys, GFP_KERNEL);
ring->tds = dma_alloc_coherent(xmm->dev, sizeof(struct td_ring_entry)*depth, &ring->tds_phys, GFP_KERNEL);
ring->pages[i] = dma_alloc_coherent(xmm->dev, ring->page_size, &ring->pages_phys[i], GFP_KERNEL);