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From: Horia Geanta <horia.geanta@nxp.com>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	dl-linux-imx <linux-imx@nxp.com>
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
Date: Thu, 23 May 2019 16:25:54 +0000	[thread overview]
Message-ID: <AM0PR0402MB3476949ECC57B188E83B820B98010@AM0PR0402MB3476.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <0c79721a-11cb-c945-5626-3d43cc299fe6@samsung.com>

On 5/23/2019 8:35 AM, Marek Szyprowski wrote:
> 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?
In my particular case, input is provided as a scatterlist, out of which first N
bytes are problematic (not written to by device and corrupted when swiotlb
bouncing is needed), while remaining bytes (Total - N) are updated by the device.

>>
>> 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?
> 
Or, put more bluntly, driver must not rely on previous data in the area mapped
DMA_FROM_DEVICE. This limitation stems from the buffer bouncing mechanism of the
swiotlb DMA API backend, which other backends might not suffer from (e.g. IOMMU).

Btw, the device I am working on (caam crypto engine) is deployed in several SoCs
configured differently - with or without an IOMMU (and coherent or non-coherent
etc.). IOW it's a "power user" of the DMA API and I appreciate all the help in
solving / clarifying this kind of implicit assumptions.

Thanks,
Horia

  reply	other threads:[~2019-05-23 16:26 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-22  7:20 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
2019-05-23 16:25               ` Horia Geanta [this message]
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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