linux-kernel - Re: [PATCH v4 9/9] x86: convert dma_map_ops to support mapping a __pfn

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Date:	Tue, 9 Jun 2015 08:58:54 +0200
From:	Christoph Hellwig <hch@....de>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, boaz@...xistor.com,
	david@...morbit.com, linux-arch@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, arnd@...db.de,
	ross.zwisler@...ux.intel.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	benh@...nel.crashing.org, linux-nvdimm@...ts.01.org,
	heiko.carstens@...ibm.com, hch@....de, tj@...nel.org,
	paulus@...ba.org, hpa@...or.com, schwidefsky@...ibm.com,
	willy@...ux.intel.com, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, mingo@...nel.org
Subject: Re: [PATCH v4 9/9] x86: convert dma_map_ops to support mapping a
	__pfn_t.

On Fri, Jun 05, 2015 at 05:19:55PM -0400, Dan Williams wrote:
> As long as a dma_map_sg() implementation avoids sg_page() conversions it
> can support scatterlists that carry "page-less" __pfn_t entries.
> However, a couple implementations require that __pfn_t_has_page() is
> always true. The Xen swiotlb implementation's entanglements with ARM and
> the Calgary MMUs requirement to have a pre-existing virtual mapping make
> them unable to support this conversion (i.e. these now have 'depends on
> !HAVE_DMA_PFN').

That's why we really need a whole kernel conversion and not just a piecemail
one.  Given how trivial this patch is that doesn't look like a too big
task ayway.
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