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

David Laight
2022-03-28T08:15:41Z

From: Christoph Hellwig
> Sent: 28 March 2022 07:37
>
> 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.

I don't think it is really 'ownership' but more about who has
write access.
Only one side can have write access (to a cache line [1]) at any
one time.

Read access is different.
You need a 'synchronise' action to pick up newly written data.
This might be a data copy, cache flush or cache invalidate.
It only need affect the area that needs to be read - not
full buffer.
Partial cache flush/invalidate will almost certainly speed
up receipt of short network packets that are copied into a
new skb - leaving the old one mapped for another receive.

[1] The cache line size might be a property of the device
and dma subsystem, not just the cpu.
I have used hardware when the effective size was 1kB.

David

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