From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>,
Christoph Hellwig <hch@lst.de>
Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
bart.vanassche@sandisk.com,
Alexander Duyck <alexander.h.duyck@intel.com>,
Krzysztof Kozlowski <k.kozlowski@samsung.com>,
Andrew Morton <akpm@linux-foundation.org>,
lkml <linux-kernel@vger.kernel.org>,
arcml <linux-snps-arc@lists.infradead.org>
Subject: Re: semantics of dma_map_single()
Date: Mon, 17 Jul 2017 09:46:30 -0700 [thread overview]
Message-ID: <1500309990.3244.2.camel@HansenPartnership.com> (raw)
In-Reply-To: <23203d16-da54-99c7-0eba-c082eba120d7@synopsys.com>
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
regions.
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
callers.
James
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prev parent 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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