a question on coherent_dma_mask

Dear folks, I have a basic question, in __dma_alloc_buffer function (arch/arm/mm/dma-mapping.c, v3.3) u64 mask = get_coherent_dma_mask(dev); if (!mask) return NULL; if (mask < 0xffffffffULL) gfp |= GFP_DMA; page = alloc_pages(gfp, order); if (!page) return NULL; ... what does the second if statement mean? (I'm accessing this function to get memory for DMA. then should I have coherent_dma_mask < 0xffffffff ? if so, what does it mean? ) Thanks, Chan