On Sun, Feb 11, 2018 at 04:05:15AM -0800, Matthew Wilcox wrote: > On Sun, Feb 11, 2018 at 03:28:08AM -0800, Matthew Wilcox wrote: > > Now, longer-term, perhaps we should do the following: > > > > #ifdef CONFIG_ZONE_DMA32 > > #define OPT_ZONE_DMA32 ZONE_DMA32 > > #elif defined(CONFIG_64BIT) > > #define OPT_ZONE_DMA OPT_ZONE_DMA > > #else > > #define OPT_ZONE_DMA32 ZONE_NORMAL > > #endif > > For consistent / coherent memory, we have an allocation function. > But we don't have an allocation function for streaming memory, which is > what these drivers want. They also flush the DMA memory and then access > the memory through a different virtual mapping, which I'm not sure is > going to work well on virtually-indexed caches like SPARC and PA-RISC > (maybe not MIPS either?)
Perhaps I (and a number of other people ...) have misunderstood the semantics of GFP_DMA32. Perhaps GFP_DMA32 is not "allocate memory below 4GB", perhaps it's "allocate memory which can be mapped below 4GB". Machines with an IOMMU can use ZONE_NORMAL. Machines with no IOMMU can choose to allocate memory with a physical address below 4GB. After all, it has 'DMA' right there in the name. If someone's relying on it to allocate physical memory below 4GB, they're arguably misusing it.
