a question on coherent_dma_mask

[Chan Kim]

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
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