about consistent_init function

Hello, In http://lxr.free-electrons.com/source/arch/arm/mm/dma-mapping.c?v=3.3#L162, in line 208, Could anybody kindly explain why it increases the variable 'base' by PMD_SIZE? Is the consistent memory chunck initialized by PMD_SIZE? It seems to me the consistent_init allocates only the skeleton of the page table (with final pte entries empty) for the consistent memory region. Then when is the actual pte entries filled? In ARM how does this work? I'm trying to allocate memory for dma (uncached region) for our sparc processor using dma_alloc_writecombine which is from arm. Is this a correct approach? (dma_alloc_coherent seems to be for smaller size), I need almost 2MB buffer. Any help or suggestion will be appreciated. Thanks, Chan