linux-kernel - Re: [PATCHv8 00/12] Contiguous Memory Allocator

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Message-ID: <20101223121917.GG3636@n2100.arm.linux.org.uk>
Date:	Thu, 23 Dec 2010 12:19:18 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	'Kyungmin Park' <kmpark@...radead.org>,
	'Michal Nazarewicz' <m.nazarewicz@...sung.com>,
	linux-arm-kernel@...ts.infradead.org,
	'Daniel Walker' <dwalker@...eaurora.org>,
	'Johan MOSSBERG' <johan.xx.mossberg@...ricsson.com>,
	'Mel Gorman' <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	'Michal Nazarewicz' <mina86@...a86.com>, linux-mm@...ck.org,
	'Ankita Garg' <ankita@...ibm.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	linux-media@...r.kernel.org,
	'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCHv8 00/12] Contiguous Memory Allocator

On Thu, Dec 23, 2010 at 11:58:08AM +0100, Marek Szyprowski wrote:
> Actually this contiguous memory allocator is a better replacement for
> alloc_pages() which is used by dma_alloc_coherent(). It is a generic
> framework that is not tied only to ARM architecture.

... which is open to abuse.  What I'm trying to find out is - if it
can't be used for DMA, what is it to be used for?

Or are we inventing an everything-but-ARM framework?

> > In other words, do we _actually_ have a use for this which doesn't
> > involve doing something like allocating 32MB of memory from it,
> > remapping it so that it's DMA coherent, and then performing DMA
> > on the resulting buffer?
> 
> This is an arm specific problem, also related to dma_alloc_coherent()
> allocator. To be 100% conformant with ARM specification we would
> probably need to unmap all pages used by the dma_coherent allocator
> from the LOW MEM area. This is doable, but completely not related
> to the CMA and this patch series.

You've already been told why we can't unmap pages from the kernel
direct mapping.

Okay, so I'm just going to assume that CMA has _no_ _business_ being
used on ARM, and is not something that should interest anyone in the
ARM community.
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