On Mon, Jul 03, 2017 at 11:27:32AM +0200, Marek Szyprowski wrote: > The main question here if we want to merge incomplete solution or not. As > for now, there is no support in ARM/ARM64 for NON_CONSISTENT attribute. > Also none of the v4l2 drivers use it. Sadly support for NON_CONSISTENT > attribute is not fully implemented nor even defined in mainline. > DMA_ATTR_NON_CONSISTENT is the way to get the dma_alloc_noncoherent semantics through the dma_alloc_attr API, and as such I think it is pretty well defined, although the documentation in Documentation/DMA-attributes.txt is really bad and we need to improve it, by merging it with the dma_alloc_noncoherent description in Documentation/DMA-API.txt. My series to remove dma_alloc_noncoherent updates the latter to mention DMA_ATTR_NON_CONSISTENT, but we should probably merge Documentation/DMA-API.txt, Documentation/DMA-attributes.txt and Documentation/DMA-API-HOWTO.txt into a single coherent document. > I know that it works fine for some vendor kernel trees, but supporting it in > mainline was a bit controversial. There is no proper way to sync cache for > such > buffers. Calling dma_sync_sg worked so far, but it has to be first agreed as > a proper DMA API. As documented in Documentation/DMA-API.txt the proper way to sync noncoherent/nonconsistent regions is to call dma_cache_sync. It seems like it generally is the same as dma_sync_range/sg so if we could eventually merge these APIs that should reduce the confusion further.
