On Tue, Mar 15, 2011 at 10:45:50AM +0900, InKi Dae wrote: > 2011/3/14 Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>: > > On Mon, Mar 14, 2011 at 09:37:51PM +0900, KyongHo Cho wrote: > >> I have also noticed that dma_map_single/page/sg() can map physical > >> memory into an arbitrary device address region. > >> But it is not enough solution for various kinds of IOMMUs. > >> As Kukjin Kim addressed, we need to support larger page size than 4KB > >> because we can reduce TLB miss when we have larger page size. > >> > >> Our IOMMU(system mmu) supports all page size of ARM architecture > >> including 16MB, 1MB, 64KB and 4KB. > >> Since the largest size supported by buddy system of 32-bit architecture is 4MB, > >> our system support all page sizes except 16MB. > >> We proved that larger page size is helpful for DMA performance > >> significantly (more than 10%, approximately). > >> Big page size is not a problem for peripheral devices > >> because their address space is not suffer from external fragmentation. > > > > 1. dma_map_single() et.al. is used for mapping *system* *RAM* for devices > > using whatever is necessary. It must not be used for trying to setup > > arbitary other mappings. > > > > 2. It doesn't matter where the memory for dma_map_single() et.al. comes > > from provided the virtual address is a valid system RAM address or > > the struct page * is a valid struct page in the memory map (iow, you > > can't create this yourself.) > > You mean that we cannot have arbitrary virtual address mapping for > iommu based device? No. I mean exactly what I said - I'm talking about the DMA API in the above two points. The implication is that you can not create arbitary mappings of non-system RAM with the DMA API. > actually, we have memory mapping to arbitrary device virtual address > space, not kernel virtual address space. > > > > > 3. In the case of an IOMMU, the DMA API does not limit you to only using > > 4K pages to setup the IOMMU mappings. You can use whatever you like > > provided the hardware can cope with it. You can coalesce several > > existing entries together provided you track what you're doing and can > > undo what's been done when the mapping is no longer required. > > > > So really there's no reason not to use 64K, 1M and 16M IOMMU entries if > > that's the size of buffer which has been passed to the DMA API. > > > > _______________________________________________ > > linux-arm-kernel mailing list > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel > > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
