Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

[T.J. Mercier]

On Mon, May 6, 2024 at 9:02 AM Christoph Hellwig <hch@xxxxxx> wrote:
>
> On Mon, May 06, 2024 at 09:00:59AM -0700, T.J. Mercier wrote:
> > Oh, that's disappointing. I'm looking for a way to quickly check if
> > any addresses point at a SWIOTLB buffer without doing a potentially
> > expensive call to iommu_iova_to_phys. Since it's meant to be dma-iommu
> > only I guess I could use sg_dma_is_swiotlb if iommu_get_domain_for_dev
> > returns a domain, and is_swiotlb_buffer otherwise for dma-direct, but
> > it'd be nice to have just one way to check which it looked like the
> > SG_DMA_SWIOTLB flag could be used for.
>
> This sounds like you're trying to do that from a consumer of the
> DMA API, which is simply wrong.  What is the actual problem you are
> trying to solve?

I want to reject mapping a dma_buf for a device if any of the pages
that back the buffer require SWIOTLB. AFAICT there's no way to know
whether SWIOTLB is used until after calling dma_map_sg, so afterwards
I'm trying to check.