linux-kernel - Re: [PATCH] swiotlb: sync buffer when mapping FROM

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Message-ID: <20190522133400.GA27229@lst.de>
Date:   Wed, 22 May 2019 15:34:00 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Horia Geantă <horia.geanta@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-imx@....com
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE

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?