On Thu, Jul 13, 2017 at 01:13:28PM +0200, Marek Szyprowski wrote: > Hi Christoph, > > On 2017-07-05 19:33, Christoph Hellwig wrote: > >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. > > Right. I started conversion of dma_alloc_noncoherent to NON_CONSISTENT > DMA attribute, but later I got stuck at the details of cache > synchronization. > > >>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. > > Original dma_alloc_noncoherent utilized dma_cache_sync() function, which had > some flaws, which prevented me to continue that task and introduce it to ARM > architecture. The dma_alloc_noncoherent() and dma_cache_sync() API lacks > buffer ownership and imprecisely defines how and when the caches has to be > synchronized. dma_cache_sync() also lacks DMA address argument, what also > complicates potential lightweight implementation. My conclusion of the dma_alloc_noncoherent() and dma_cache_sync() API when it was introduced is that it's basically a completely broken interface, and I've never seen any point to it. Maybe some of that is because it's badly documented - which in turn makes it badly designed (because there's no specification detailing what it's supposed to be doing.) I'd like to see that thing die... -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.
