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

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Message-ID: <ed26de5e-aee4-4e19-095c-cc551012d475@arm.com>
Date:   Wed, 22 May 2019 14:55:50 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 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.