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

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Message-Id: <201107061659.45253.arnd@arndb.de>
Date:	Wed, 6 Jul 2011 16:59:45 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	"'Daniel Walker'" <dwalker@...eaurora.org>,
	"'Jonathan Corbet'" <corbet@....net>,
	"'Mel Gorman'" <mel@....ul.ie>,
	"'Chunsang Jeong'" <chunsang.jeong@...aro.org>,
	linux-kernel@...r.kernel.org,
	"'Michal Nazarewicz'" <mina86@...a86.com>,
	linaro-mm-sig@...ts.linaro.org,
	"'Jesse Barker'" <jesse.barker@...aro.org>,
	"'Kyungmin Park'" <kyungmin.park@...sung.com>,
	"'Ankita Garg'" <ankita@...ibm.com>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	"'KAMEZAWA Hiroyuki'" <kamezawa.hiroyu@...fujitsu.com>,
	linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [PATCH 6/8] drivers: add Contiguous Memory Allocator

On Wednesday 06 July 2011, Nicolas Pitre wrote:
> On Wed, 6 Jul 2011, Russell King - ARM Linux wrote:
> 
> > Another issue is that when a platform has restricted DMA regions,
> > 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.
> 
> Do we encounter this in practice i.e. do those platforms requiring large 
> contiguous allocations motivating this work have such DMA restrictions?

You can probably find one or two of those, but we don't have to optimize
for that case. I would at least expect the maximum size of the allocation
to be smaller than the DMA limit for these, and consequently mandate that
they define a sufficiently large CONSISTENT_DMA_SIZE for the crazy devices,
or possibly add a hack to unmap some low memory and call
dma_declare_coherent_memory() for the device.

	Arnd
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