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Date:   Fri, 16 Jul 2021 07:24:03 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Sven Peter <sven@svenpeter.dev>, Will Deacon <will@kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Arnd Bergmann <arnd@kernel.org>, devicetree@vger.kernel.org,
        Hector Martin <marcan@marcan.st>, linux-kernel@vger.kernel.org,
        Marc Zyngier <maz@kernel.org>,
        Mohamed Mediouni <mohamed.mediouni@caramail.com>,
        Stan Skowronek <stan@corellium.com>,
        linux-arm-kernel@lists.infradead.org,
        Mark Kettenis <mark.kettenis@xs4all.nl>,
        iommu@lists.linux-foundation.org, Alexander Graf <graf@amazon.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
        Rob Herring <robh+dt@kernel.org>, r.czerwinski@pengutronix.de,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org
Subject: Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver
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References: <20210627143405.77298-1-sven@svenpeter.dev>
 <7261df01-34a9-4e53-37cd-ae1aa15b1fb4@arm.com>
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On Wed, Jul 14, 2021 at 07:19:50PM +0100, Robin Murphy wrote:
> Even at the DMA API level you could hide *some* of it (at the cost of
> effectively only having 1/4 of the usable address space), but there are
> still cases like where v4l2 has a hard requirement that a page-aligned
> scatterlist can be mapped into a contiguous region of DMA addresses.

Where does v4l2 make that broken assumption?  Plenty of dma mapping
implementations including dma-direct do not support that.

Drivers need to call dma_get_merge_boundary() to check for that kind of
behavior.