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Date:   Wed, 22 May 2019 15:09:21 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Horia =?utf-8?Q?Geant=C4=83?= <horia.geanta@nxp.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-imx@nxp.com
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
Message-ID: <20190522130921.GA26874@lst.de>
References: <20190522072018.10660-1-horia.geanta@nxp.com> <20190522123243.GA26390@lst.de> <6cbe5470-16a6-17e9-337d-6ba18b16b6e8@arm.com>
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On Wed, May 22, 2019 at 01:50:47PM +0100, Robin Murphy wrote:
> Would that work out any different from the existing DMA_ATTR_SKIP_CPU_SYNC? 
>
> If drivers are prepared to handle this issue from their end, they can 
> already do so for single mappings by using that attr along with explicit 
> partial syncs via dma_sync_single(). For page/sg mappings we'd still have 
> the problem of identifying what part of "partial" actually matters, and 
> probably having to add some additional new sync operations to cope.

Except that the same optimization we are tripping over here is also
present in dma_sync_* - dma_sync_*_to_device with DMA_FROM_DEVICE is a
no-op in swiotlb.