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From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Subject: Re: [Linaro-mm-sig] [PATCH 6/8] drivers: add Contiguous Memory Allocator
Date: Wed, 6 Jul 2011 16:59:45 +0200
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Cc: Nicolas Pitre <nicolas.pitre@linaro.org>,
        "Russell King - ARM Linux" <linux@arm.linux.org.uk>,
        "'Daniel Walker'" <dwalker@codeaurora.org>,
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        linaro-mm-sig@lists.linaro.org,
        "'Jesse Barker'" <jesse.barker@linaro.org>,
        "'Kyungmin Park'" <kyungmin.park@samsung.com>,
        "'Ankita Garg'" <ankita@in.ibm.com>,
        "'Andrew Morton'" <akpm@linux-foundation.org>, linux-mm@kvack.org,
        "'KAMEZAWA Hiroyuki'" <kamezawa.hiroyu@jp.fujitsu.com>,
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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