Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu | Date | Tue, 7 Jul 2009 16:06:48 +0200 |
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On Tuesday 07 July 2009, Ming Lei wrote: > > ARM has two (normal, and dma bounce), and in the long run we need to do > > OK, Can we use dma-mapping-common.h on ARM?
It should work in principle. It may be a good idea to also move to the generic swiotlb instead of the traditional dma bounce at the same time.
Note that dma-mapping-common.h is only needed if you want to support two or more different DMA implementations in a single kernel, which I'm not sure is needed for ARM.
> > cache handling on unmap as well as map due to CPU speculative fetches. > > IMHO, it seems we can fix this problem now. > > For DMA_TO_DEVICE transfer, clean cache in dma map, but does nothing in > dma unmap; > > For DMA_FROM_DEVICE, we may do nothing in dma map, but invaliate cache > in dma unmap.
A number of other architectures do this already. You also need to have dma_sync_*_for_cpu and dma_sync_*_for_device, where the *_for_device operation needs to do the same flushing as dma_map_* and *_for_cpu does the same as dma_unmap_*.
Note that actually you need to do writeback+invalidate in DMA_TO_DEVICE and at least an invalidate in DMA_FROM_DEVICE during dma_map_*. For the unmap, I don't think you ever need to invalidate the cache. If you invalidate only at unmap time for DMA_FROM_DEVICE, a dirty cache line might be accidentally flushed to the buffer after the device has written to it.
Arnd <><
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