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Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
To:     Christoph Hellwig <hch@lst.de>,
        =?UTF-8?Q?Horia_Geant=c4=83?= <horia.geanta@nxp.com>
Cc:     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
References: <20190522072018.10660-1-horia.geanta@nxp.com>
 <20190522123243.GA26390@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <6cbe5470-16a6-17e9-337d-6ba18b16b6e8@arm.com>
Date:   Wed, 22 May 2019 13:50:47 +0100
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On 22/05/2019 13:32, Christoph Hellwig wrote:
> I'm a little worried about this.  While it looks functionally correct
> we have surived without it, and doing another copy for every swiotlb
> dma mapping from the device looks extremely painful for the typical use
> cases where we expect the device to transfer the whole mapping.
> 
> I'd be tempted to instead properl document the current behavior and
> introduce a new DMA_ATTR_PARTIAL flag to allow for partial mappings.

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.

Robin.