linux-kernel - Re: [PATCH 6/8] drivers: add Contiguous Memory Allocator

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Date:	Wed, 6 Jul 2011 18:02:29 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	'Daniel Walker' <dwalker@...eaurora.org>,
	'Jonathan Corbet' <corbet@....net>,
	'Mel Gorman' <mel@....ul.ie>,
	'Chunsang Jeong' <chunsang.jeong@...aro.org>,
	'Jesse Barker' <jesse.barker@...aro.org>,
	'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	'Michal Nazarewicz' <mina86@...a86.com>,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	'Kyungmin Park' <kyungmin.park@...sung.com>,
	'Ankita Garg' <ankita@...ibm.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-media@...r.kernel.org
Subject: Re: [PATCH 6/8] drivers: add Contiguous Memory Allocator

On Wed, Jul 06, 2011 at 11:05:00AM -0500, Christoph Lameter wrote:
> On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:
> 
> > > > they typically don't fall into the highmem zone.  As the dmabounce
> > > > code allocates from the DMA coherent allocator to provide it with
> > > > guaranteed DMA-able memory, that would be rather inconvenient.
> > >
> > > True. The dmabounce code would consequently have to allocate
> > > the memory through an internal function that avoids the
> > > contiguous allocation area and goes straight to ZONE_DMA memory
> > > as it does today.
> >
> > CMA's whole purpose for existing is to provide _dma-able_ contiguous
> > memory for things like cameras and such like found on crippled non-
> > scatter-gather hardware.  If that memory is not DMA-able what's the
> > point?
> 
> ZONE_DMA is a zone for memory of legacy (crippled) devices that cannot DMA
> into all of memory (and so is ZONE_DMA32). Memory from ZONE_NORMAL can be
> used for DMA as well and a fully capable device would be expected to
> handle any memory in the system for DMA transfers.
> 
> "guaranteed" dmaable memory? DMA abilities are device specific. Well maybe
> you can call ZONE_DMA memory to be guaranteed if you guarantee that any
> device must at mininum be able to perform DMA into ZONE_DMA memory. But
> there may not be much of that memory around so you would want to limit
> the use of that scarce resource.

Precisely, which is what ZONE_DMA is all about.  I *have* been a Linux
kernel hacker for the last 18 years and do know these things, especially
as ARM has had various issues with DMA memory limitations over those
years - and have successfully had platforms working reliably given that
and ZONE_DMA.
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