linux-kernel - Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is get_user_pages() enough to prevent pages from being swapped out ?")

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Date:	Fri, 7 Aug 2009 08:15:01 -0500
From:	Robin Holt <holt@....com>
To:	Laurent Desnogues <laurent.desnogues@...il.com>
Cc:	Jamie Lokier <jamie@...reable.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	David Xiao <dxiao@...adcom.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Robin Holt <holt@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	v4l2_linux <linux-media@...r.kernel.org>,
	"linux-arm-kernel@...ts.arm.linux.org.uk" 
	<linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Is
	get_user_pages() enough to prevent pages from being swapped out ?")

On Fri, Aug 07, 2009 at 02:07:43PM +0200, Laurent Desnogues wrote:
> On Fri, Aug 7, 2009 at 11:54 AM, Jamie Lokier<jamie@...reable.org> wrote:
> >
> > 1. Does the architecture not prevent speculative instruction
> > prefetches from crossing a page boundary?  It would be handy under the
> > circumstances.
> 
> There's no such restriction in ARMv7 architecture.

Doesn't it prevent them for uncached areas?  I _THOUGHT_ there was an
alloc_consistent (or something like that) call on ARM which gave you
an uncached mapping where you could do DMA.  I also thought there was
a dma_* set of functions which remapped as uncached before DMA begins
and remapped as normal after DMA has been completed.

Sorry for the fuzzy recollection.  I am dredging from 2.6.21 timeframe.

Robin
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