linux-kernel - Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

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Date:   Fri, 16 Jul 2021 07:24:03 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Sven Peter <sven@...npeter.dev>, Will Deacon <will@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Arnd Bergmann <arnd@...nel.org>, devicetree@...r.kernel.org,
        Hector Martin <marcan@...can.st>, linux-kernel@...r.kernel.org,
        Marc Zyngier <maz@...nel.org>,
        Mohamed Mediouni <mohamed.mediouni@...amail.com>,
        Stan Skowronek <stan@...ellium.com>,
        linux-arm-kernel@...ts.infradead.org,
        Mark Kettenis <mark.kettenis@...all.nl>,
        iommu@...ts.linux-foundation.org, Alexander Graf <graf@...zon.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Herring <robh+dt@...nel.org>, r.czerwinski@...gutronix.de,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

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.