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SubjectRe: [PATCH 1/4] dma-mapping: Add bounced DMA ops
On Mon, Jul 13, 2020 at 12:55:43PM +0100, Robin Murphy wrote:
> On 2020-07-13 10:12, Claire Chang wrote:
>> The bounced DMA ops provide an implementation of DMA ops that bounce
>> streaming DMA in and out of a specially allocated region. Only the
>> operations relevant to streaming DMA are supported.
>
> I think there are too many implicit assumptions here - apparently that
> coherent allocations will always be intercepted by
> dma_*_from_dev_coherent(), and that calling into dma-direct won't actually
> bounce things a second time beyond where you thought they were going,
> manage coherency for a different address, and make it all go subtly wrong.
> Consider "swiotlb=force", for instance...
>
> Again, plumbing this straight into dma-direct so that SWIOTLB can simply
> target a different buffer and always bounce regardless of masks would seem
> a far better option.

I haven't really had time to read through the details, but I agree that
any bouncing scheme should reuse the swiotlb code and not invent a
parallel infrastructure.

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