MAX_DMA32_PFN
#define MAX_DMA32_PFN (1UL << (32 - PAGE_SHIFT))
#define MAX_DMA32_PFN (1UL << (32 - PAGE_SHIFT))
#define MAX_DMA32_PFN (((1ULL << 32) - (1ULL << 28)) >> PAGE_SHIFT)
#define MAX_DMA32_PFN (1UL << (32 - PAGE_SHIFT))