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

Robin Murphy
2022-03-25T19:43:03Z

On 2022-03-25 18:30, Linus Torvalds wrote:
> On Fri, Mar 25, 2022 at 3:25 AM Maxime Bizon <mbizon@freebox.fr> wrote:
>>
>> In the non-cache-coherent scenario, and assuming dma_map() did an
>> initial cache invalidation, you can write this:
>
> .. but the problem is that the dma mapping code is supposed to just
> work, and the driver isn't supposed to know or care whether dma is
> coherent or not, or using bounce buffers or not.
>
> And currently it doesn't work.
>
> Because what that ath9k driver does is "natural", but it's wrong for
> the bounce buffer case.
>
> And I think the problem is squarely on the dma-mapping side for two reasons:
>
> (a) this used to work, now it doesn't, and it's unclear how many
> other drivers are affected
>
> (b) the dma-mapping naming and calling conventions are horrible and
> actively misleading
>
> That (a) is a big deal. The reason the ath9k issue was found quickly
> is very likely *NOT* because ath9k is the only thing affected. No,
> it's because ath9k is relatively common.
>
> Just grep for dma_sync_single_for_device() and ask yourself: how many
> of those other drivers have you ever even HEARD of, much less be able
> to test?
>
> And that's just one "dma_sync" function. Admittedly it's likely one of
> the more common ones, but still..
>
> Now, (b) is why I think driver nufgt get this so wrong - or, in this
> case, possibly the dma-mapping code itself.
>
> The naming - and even the documentation(!!!) - implies that what ath9k
> does IS THE RIGHT THING TO DO.
>
> The documentation clearly states:
>
> "Before giving the memory to the device, dma_sync_single_for_device() needs
> to be called, and before reading memory written by the device,
> dma_sync_single_for_cpu(), just like for streaming DMA mappings that are
> reused"

Except that's documentation for the non-coherent allocation API, rather
than the streaming API in question here. I'll refrain from commenting on
having at least 3 DMA APIs, with the same set of sync functions serving
two of them, and just stand back a safe distance...

Anyway, the appropriate part of that document is probably:

"You must do this:

- Before reading values that have been written by DMA from the device
(use the DMA_FROM_DEVICE direction)"

I'm not saying it constitutes *good* documentation, but I would note how
it says "have been written", and not "are currently being written".
Similarly from the HOWTO:

"If you need to use the same streaming DMA region multiple times and
touch the data in between the DMA transfers, the buffer needs to be
synced properly..."

Note "between the DMA transfers", and not "during the DMA transfers".
The fundamental assumption of the streaming API is that only one thing
is ever accessing the mapping at any given time, which is what the whole
notion of ownership is about.

Thanks,
Robin.