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[124.171.87.207]) by smtp.gmail.com with ESMTPSA id o187sm24624950pfb.12.2020.04.19.07.46.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 19 Apr 2020 07:46:09 -0700 (PDT) Subject: Re: [PATCH 3/4] dma-mapping: add a dma_ops_bypass flag to struct device To: Joerg Roedel , Christoph Hellwig Cc: iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org, Lu Baolu , Greg Kroah-Hartman , Robin Murphy , linux-kernel@vger.kernel.org References: <20200414122506.438134-1-hch@lst.de> <20200414122506.438134-4-hch@lst.de> <20200418124205.GD6113@8bytes.org> <20200419080058.GB12222@lst.de> <20200419122502.GI21900@8bytes.org> From: Alexey Kardashevskiy Autocrypt: addr=aik@ozlabs.ru; keydata= mQINBE+rT0sBEADFEI2UtPRsLLvnRf+tI9nA8T91+jDK3NLkqV+2DKHkTGPP5qzDZpRSH6mD EePO1JqpVuIow/wGud9xaPA5uvuVgRS1q7RU8otD+7VLDFzPRiRE4Jfr2CW89Ox6BF+q5ZPV /pS4v4G9eOrw1v09lEKHB9WtiBVhhxKK1LnUjPEH3ifkOkgW7jFfoYgTdtB3XaXVgYnNPDFo 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ZhSFtHopZnBSCUSNDbB+QGy4B/E++Bb02IBTGl/JxmOwG+kZUnymsPvTtnNIeTLHxN/H/ae0 c7E5M+/NpslPCmYnDjs5qg0/3ihh6XuOGggZQOqrYPC3PnsNs3NxirwOkVPQgO6mXxpuifvJ DG9EMkK8IBXnLulqVk54kf7fE0jT/d8RTtJIA92GzsgdK2rpT1MBKKVffjRFGwN7nQVOzi4T XrB5p+6ML7Bd84xOEGsj/vdaXmz1esuH7BOZAGEZfLRCHJ0GVCSssg== Message-ID: <9a2fbf06-49aa-e6df-1ff0-cf94753180bf@ozlabs.ru> Date: Mon, 20 Apr 2020 00:46:04 +1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <20200419122502.GI21900@8bytes.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19/04/2020 22:25, Joerg Roedel wrote: > On Sun, Apr 19, 2020 at 10:00:58AM +0200, Christoph Hellwig wrote: >> The difference is that NULL ops mean imply the direct mapping is always >> used, dma_ops_bypass means a direct mapping is used if no bounce buffering >> using swiotlb is needed, which should also answer your first question. >> The idea is to consolidate code in the core to use an opportunistic >> direct mapping instead of the dynamic iommu mapping. I though the cover >> letter and commit log explained this well enough, but maybe I need to >> do a better job. > > Ah right, now I see it, when dma_ops_bypass is set it will only use > direct mapping when the available memory fits into the device's > dma_masks, and calls into dma_ops otherwise. > > I wonder how that will interact with an IOMMU driver, which has to make > sure that the direct mapping is accessible for the device at all. It > can either put the device into a passthrough domain for direct mapping > or into a re-mapped domain, but then all DMA-API calls need to use dma-ops. > When the dma_mask covers available memory but coherent_mask doesn't, > the streaming calls will use dma-direct and alloc_coherent() calls into > dma-ops. There is no way for the IOMMU driver to ensure both works. > > So what are the conditions under which an IOMMU driver would set > dma_ops_bypass to 1 and get a different result as to when setting > dev->dma_ops to NULL? One example is powerpc64/pseries (arch/powerpc/kernel/dma-iommu.c) where dma_iommu_ops::dma_iommu_dma_supported() (i.e. need ops) decides whether to set dma_ops_bypass to 1. It tries creating a DMA window with 1:1 mapping to fit maximum possible RAM address, if that works, then ops is not needed. -- Alexey