On Fri, Mar 25, 2022 at 11:46:09AM -0700, Linus Torvalds wrote: > I think my list of three different sync cases (not just two! It's not > just about whether to sync for the CPU or the device, it's also about > what direction the data itself is taking) is correct. > > But maybe I'm wrong. At the high level you are correct. It is all about which direction the data is taking. That is the direction argument that all the map/unmap/sync call take. The sync calls then just toggle the ownership. You seem to hate that ownership concept, but I don't see how things could work without that ownership concept as we're going to be in trouble without having that. And yes, a peek operation could work in some cases, but it would have to be at the cache line granularity. arch/arc/mm/dma.c has a really good comment how these transfers relate to actual cache operations btw> * * | map == for_device | unmap == for_cpu * |---------------------------------------------------------------- * TO_DEV | writeback writeback | none none * FROM_DEV | invalidate invalidate | invalidate* invalidate* * BIDIR | writeback+inv writeback+inv | invalidate invalidate * * [*] needed for CPU speculative prefetches
