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From: James Bottomley <>
To: Vineet Gupta <>,
	Christoph Hellwig <>
Cc: "" <>,
	"" <>,,
	Alexander Duyck <>,
	Krzysztof Kozlowski <>,
	Andrew Morton <>,
	lkml <>,
	arcml <>
Subject: Re: semantics of dma_map_single()
Date: Mon, 17 Jul 2017 09:46:30 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, 2017-07-17 at 09:06 -0700, Vineet Gupta wrote:
> Hi Christoph,
> On 07/16/2017 11:42 PM, Christoph Hellwig wrote:
> > 
> > I would expect that it would support any contiguous range in
> > the kernel mapping (e.g. no vmalloc and friends).A A But it's not
> > documented anywhere, and if no in kernel users makes use of that
> > fact at the moment it might be better to document a page size
> > limitation and add asserts to enforce it.
> My first thought was indeed to add a BUG_ON for @size > PAGE_SIZE
> (also accounting for offset etc), but I have a feeling this will
> cause too many breakages. So perhaps it would be better to add the
> fact to Documentation that it can handle any physically contiguous
> range.

Actually, that's not historically right. A dma_map_single() was
originally designed to be called on any region that was kmalloc'd
meaning it was capable of mapping physically contiguous > PAGE_SIZE

For years (decades?) we've been eliminating the specialised
dma_map_single() calls in favour of dma_map_sg, so it's possible there
may not be any large region consumers anymore, so it *may* be safe to
enforce a PAGE_SIZE limit, but not without auditing the remaining


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      reply	other threads:[~2017-07-17 16:46 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-16  1:24 semantics of dma_map_single() Vineet Gupta
2017-07-17  6:42 ` Christoph Hellwig
2017-07-17 16:06   ` Vineet Gupta
2017-07-17 16:46     ` James Bottomley [this message]

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