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

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Message-ID: <CABdmKX1n98+bw+1kewz=wdqq2Nbpaxao_Lx-Gq8oKGNUEP4ytQ@mail.gmail.com>
Date: Mon, 6 May 2024 09:10:40 -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 Mon, May 6, 2024 at 9:02 AM Christoph Hellwig <hch@....de> 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.