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Date:   Wed, 22 May 2019 15:34:00 +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: <20190522133400.GA27229@lst.de>
References: <20190522072018.10660-1-horia.geanta@nxp.com> <20190522123243.GA26390@lst.de> <6cbe5470-16a6-17e9-337d-6ba18b16b6e8@arm.com> <20190522130921.GA26874@lst.de> <fdfd7318-7999-1fe6-01b6-ae1fb7ba8c30@arm.com>
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On Wed, May 22, 2019 at 02:25:38PM +0100, Robin Murphy wrote:
> Sure, but that should be irrelevant since the effective problem here is in 
> the sync_*_for_cpu direction, and it's the unmap which nobbles the buffer. 
> If the driver does this:
>
> 	dma_map_single(whole buffer);
> 	<device writes to part of buffer>
> 	dma_unmap_single(whole buffer);
> 	<contents of rest of buffer now undefined>
>
> then it could instead do this and be happy:
>
> 	dma_map_single(whole buffer, SKIP_CPU_SYNC);
> 	<device writes to part of buffer>
> 	dma_sync_single_for_cpu(updated part of buffer);
> 	dma_unmap_single(whole buffer, SKIP_CPU_SYNC);
> 	<contents of rest of buffer still valid>

Assuming the driver knows how much was actually DMAed this would
solve the issue.  Horia, does this work for you?