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

Christoph Hellwig
2022-03-28T06:37:37Z

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