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Subject: Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver
To:     Christoph Hellwig <hch@infradead.org>
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
References: <20210627143405.77298-1-sven@svenpeter.dev>
 <7261df01-34a9-4e53-37cd-ae1aa15b1fb4@arm.com>
 <YPEmA0rdXHiGtHSx@infradead.org>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <0d20bd6b-d0a1-019c-6398-b12f83f4fdf7@arm.com>
Date:   Fri, 16 Jul 2021 16:32:50 +0100
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On 2021-07-16 07:24, Christoph Hellwig wrote:
> 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.

See vb2_dc_get_contiguous_size() and its callers. I still remember 
spending an entire work day on writing one email at the culmination of 
this discussion:

https://lore.kernel.org/linux-iommu/56409B6D.5090903@arm.com/

809eac54cdd6 was framed as an efficiency improvement because it 
technically was one (and something I had wanted to implement anyway), 
but it was also very much to save myself from any further email debates 
or customer calls about "regressing" code ported from 32-bit platforms...

Robin.