Hi Alan, Mike, Thanks for your help! 2015-12-31 19:25 GMT+09:00 One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>: >> >> In a system like Fig.2, is the memory non-consistent? > > dma_alloc_coherent will always provide you with coherent memory. On a > machine with good cache interfaces it will provide you with normal > memory. On some systems it may be memory from a special window, in other > cases it will fall back to providing uncached memory for this. > > If the platform genuinely cannot support this (even by marking those areas > uncacheable) then it will fail the allocation. > > What it does mean is that you need to use non-coherent mappings when > accessing a lot of data. On hardware without proper cache coherency it > may be quite expensive to access coherent memory. Now, it is clearer to me. The following is what I understood. (Please point out if I am wrong.) I think, roughly, there are two ways for handling DMA: (At first, I was so confused that I was thinking about [1] and [2] mixed.) [1] DMA-coherent buffers Allocate buffers with dma_alloc_coherent() and just have access to the buffers without cache synchronization. There is no need to call dma_sync_single_for_*(). [2] Streaming DMA Allocate buffers with kmalloc() or friends, and then map them for DMA with dma_map_single(). The buffers are cached, so they are non-consitent unless there exists hardware assist such as Cache Coherency Interconnect. The drivers must invoke cache operations by calling dma_sync_single_for_*(). Is there any guideline about which way should be used in drivers? I think, if the buffer size is small, [1] is more efficient because it need not invoke cache operations. If the buffer is large, [2] seems better because the cost of uncached memory access gets more expensive than that of cache operations. (If devices are connected to the memory controller via Cache Coherency Interconnect, [1] always works very well. But drivers should be written in a portable way, so such a hardware implementation should not be expected.) I am not sure about the border line between [1] and [2], though... BTW, I am studying the DMA APIs in order to write a new MMC host driver for my ARM SoC. I grepped under drivers/mmc/host, and I found many drivers call dma_alloc_coherent(), but there are also some drivers that use dma_map_single(). -- Best Regards Masahiro Yamada -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
