Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

[Christoph Hellwig]

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