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From: Marek Szyprowski <>
To: Robin Murphy <>, Christoph Hellwig <>
Cc: "Horia Geantă" <>,
	"Konrad Rzeszutek Wilk" <>,,,
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
Date: Thu, 23 May 2019 07:35:07 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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

  reply	other threads:[~2019-05-23  5:35 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-22  7:20 [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE Horia Geantă
2019-05-22 12:32 ` Christoph Hellwig
2019-05-22 12:50   ` Robin Murphy
2019-05-22 13:09     ` Christoph Hellwig
2019-05-22 13:25       ` Robin Murphy
2019-05-22 13:34         ` Christoph Hellwig
2019-05-22 13:55           ` Robin Murphy
2019-05-23  5:35             ` Marek Szyprowski [this message]
2019-05-23 16:25               ` Horia Geanta
2019-05-23 16:43               ` Christoph Hellwig
2019-05-23 17:53                 ` Horia Geanta
2019-05-23 18:05                 ` Robin Murphy

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