On Thu, Jan 12, 2017 at 12:56:07PM +0100, Arnd Bergmann wrote: > That is an interesting question: We actually have the > "DMA_ATTR_NO_KERNEL_MAPPING" for this case, and ARM implements > it in the coherent interface, so that might be a good fit. Yes. my WIP HMB patch uses DMA_ATTR_NO_KERNEL_MAPPING, although I'm workin on x86 at the moment where it's a no-op. > Implementing it in the streaming API makes no sense since we > already have a kernel mapping here, but using a normal allocation > (possibly with DMA_ATTR_NON_CONSISTENT or DMA_ATTR_SKIP_CPU_SYNC, > need to check) might help on other architectures that have > limited amounts of coherent memory and no CMA. Though about that - but in the end DMA_ATTR_NO_KERNEL_MAPPING implies those, so instead of using lots of flags in driver I'd rather fix up more dma_ops implementations to take advantage of DMA_ATTR_NO_KERNEL_MAPPING.
