[EMAIL PROTECTED] wrote:
Hummm, let me try again, I may be tripping up on my ignorance of the kernel. According to Linux Device Drivers 3, the DMA_TO/FROM_DEVICE macros impact when the dma_map/unmap_single functions copy data and do cache flushes to ensure cache coherency. DMA_TO_DEVICE tells dma_map_single() to ensure that all data is copied out to memory and cache is flushed before doing the transfer and DMA_FROM_DEVICE tell dma_unmap_single() to ensure all data is in main memory after the transfer.
Right.
(My understanding is that this is really only important on archs that use bounce buffers which I'm not).
It's also important on architectures where DMA is non-coherent (mpc83xx DMA is coherent).
So, no as far as "the device" (and I do mean the DMA engine) is concerned, the "mapping" of the src and dest pointers make zero difference, the hardware doesn't care since it's just acting on raw addresses. However, it does (or at least I think it does) matter to the kernel and making sure that the cache doesn't get screwed up.
Right.
But it sounds like you're saying those macros are unimportant.
No, they're very important on certain hardware.
Can dma_async_memcpy_buf_to_buf() _safely_ be used to DMA either direction (CPU <=> peripheral) _without_ cache coherency problems?
Any given memcpy operation involves *both* directions. Data is copied out of host memory into the DMA engine's internal buffer (this is the src mapping), and then it its copied back into host memory at a different address (this is the dest mapping).
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