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Date:   Tue, 15 Jan 2019 16:20:29 +0100
From:   "hch@lst.de" <hch@lst.de>
To:     christian.koenig@amd.com
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Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator
Message-ID: <20190115152029.GB2325@lst.de>
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On Tue, Jan 15, 2019 at 03:24:55PM +0100, Christian K?nig wrote:
> Yeah, indeed. Bounce buffers are an absolute no-go for GPUs.
>
> If the DMA API finds that a piece of memory is not directly accessible by 
> the GPU we need to return an error and not try to use bounce buffers behind 
> the surface.
>
> That is something which always annoyed me with the DMA API, which is 
> otherwise rather cleanly defined.

That is exactly what I want to fix with my series to make
DMA_ATTR_NON_CONSISTENT more useful and always available:

https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031985.html

With that you allocate the memory using dma_alloc_attrs with
DMA_ATTR_NON_CONSISTENT, and use dma_sync_single_* to transfer
ownership to the cpu and back to the device, with a gurantee that
there won't be any bouncing.  So far the interest by the parties that
requested the feature has been rather lacklustre, though.