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