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Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
To:     Christoph Hellwig <hch@lst.de>
Cc:     =?UTF-8?Q?Horia_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
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>
 <20190522133400.GA27229@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <ed26de5e-aee4-4e19-095c-cc551012d475@arm.com>
Date:   Wed, 22 May 2019 14:55:50 +0100
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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 :)

Robin.