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SubjectRe: [PATCH 1/4] dma-mapping: Add bounced DMA ops
On Tue, Jul 14, 2020 at 7:01 PM Christoph Hellwig <hch@lst.de> wrote:
>
> 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.
Thanks for the feedback. I'll try to reuse SWIOTLB.

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