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Date: Mon, 6 May 2024 18:19:06 +0200
From: Christoph Hellwig <hch@lst.de>
To: "T.J. Mercier" <tjmercier@google.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>, isaacmanjarres@google.com,
	Catalin Marinas <catalin.marinas@arm.com>, iommu@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] dma-direct: Set SG_DMA_SWIOTLB flag for dma-direct
Message-ID: <20240506161906.GA17237@lst.de>
References: <20240503183713.1557480-1-tjmercier@google.com> <20240506052955.GA4923@lst.de> <CABdmKX1XNTtoPTvfsJRobim8pHdDjPsKx=qVovVZDh5GEbKCfQ@mail.gmail.com> <20240506160244.GA16248@lst.de> <CABdmKX1n98+bw+1kewz=wdqq2Nbpaxao_Lx-Gq8oKGNUEP4ytQ@mail.gmail.com>
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On Mon, May 06, 2024 at 09:10:40AM -0700, T.J. Mercier wrote:
> 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.

You should not check, you simply must handle it by doing the proper
DMA API based ownership management.