linux-kernel - Re: [PATCH] dma-direct: Set SG_DMA

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Message-ID: <CABdmKX1XNTtoPTvfsJRobim8pHdDjPsKx=qVovVZDh5GEbKCfQ@mail.gmail.com>
Date: Mon, 6 May 2024 09:00:59 -0700
From: "T.J. Mercier" <tjmercier@...gle.com>
To: Christoph Hellwig <hch@....de>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>, Robin Murphy <robin.murphy@....com>, 
	isaacmanjarres@...gle.com, Catalin Marinas <catalin.marinas@....com>, iommu@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct

On Sun, May 5, 2024 at 10:29 PM Christoph Hellwig <hch@....de> wrote:
>
> On Fri, May 03, 2024 at 06:37:12PM +0000, T.J. Mercier wrote:
> > As of commit 861370f49ce4 ("iommu/dma: force bouncing if the size is
> > not cacheline-aligned") sg_dma_mark_swiotlb is called when
> > dma_map_sgtable takes the IOMMU path and uses SWIOTLB for some portion
> > of a scatterlist. It is never set for the direct path, so drivers
> > cannot always rely on sg_dma_is_swiotlb to return correctly after
> > calling dma_map_sgtable. Fix this by calling sg_dma_mark_swiotlb in the
> > direct path like it is in the IOMMU path.
>
> I don't think this is the right thing to do.  Despite it's name
> sg_dma_mark_swiotlb really is dma-iommu specific, and doesn't make sense
> in context of dma-direct.  If anything we need to find a better name
> for the flag.
>

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.