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From: Robin Murphy <>
To: Christoph Hellwig <>
Cc: "Horia Geantă" <>,
	"Konrad Rzeszutek Wilk" <>,
	"Marek Szyprowski" <>,,,
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
Date: Wed, 22 May 2019 14:25:38 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 2019-05-22 2:09 pm, Christoph Hellwig wrote:
> On Wed, May 22, 2019 at 01:50:47PM +0100, Robin Murphy wrote:
>> Would that work out any different from the existing DMA_ATTR_SKIP_CPU_SYNC?
>> If drivers are prepared to handle this issue from their end, they can
>> already do so for single mappings by using that attr along with explicit
>> partial syncs via dma_sync_single(). For page/sg mappings we'd still have
>> the problem of identifying what part of "partial" actually matters, and
>> probably having to add some additional new sync operations to cope.
> Except that the same optimization we are tripping over here is also
> present in dma_sync_* - dma_sync_*_to_device with DMA_FROM_DEVICE is a
> no-op in swiotlb.

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>


  reply	other threads:[~2019-05-22 13:25 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 [this message]
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
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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