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Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
To:     Robin Murphy <robin.murphy@arm.com>, Christoph Hellwig <hch@lst.de>
Cc:     =?UTF-8?Q?Horia_Geant=c4=83?= <horia.geanta@nxp.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-imx@nxp.com
From:   Marek Szyprowski <m.szyprowski@samsung.com>
Message-ID: <0c79721a-11cb-c945-5626-3d43cc299fe6@samsung.com>
Date:   Thu, 23 May 2019 07:35:07 +0200
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Hi Robin,

On 2019-05-22 15:55, Robin Murphy wrote:
> On 22/05/2019 14:34, Christoph Hellwig wrote:
>> 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?
>
> Ohhh, and now I've just twigged what you were suggesting - your 
> DMA_ATTR_PARTIAL flag would mean "treat this as a read-modify-write of 
> the buffer because we *don't* know exactly which parts the device may 
> write to". So indeed if we did go down that route we wouldn't need any 
> of the sync stuff I was worrying about (but I might suggest naming it 
> DMA_ATTR_UPDATE instead). Apologies for being slow :)

Don't we have DMA_BIDIRECTIONAL for such case? Maybe we should update 
documentation a bit to point that DMA_FROM_DEVICE expects the whole 
buffer to be filled by the device?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland