linux-kernel - RE: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings

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Date:	Tue, 16 Aug 2011 12:17:30 +0200
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'Arnd Bergmann' <arnd@...db.de>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	'Michal Nazarewicz' <mina86@...a86.com>,
	'Kyungmin Park' <kyungmin.park@...sung.com>,
	'Russell King' <linux@....linux.org.uk>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>,
	'Ankita Garg' <ankita@...ibm.com>,
	'Daniel Walker' <dwalker@...eaurora.org>,
	'Mel Gorman' <mel@....ul.ie>,
	'Jesse Barker' <jesse.barker@...aro.org>,
	'Jonathan Corbet' <corbet@....net>,
	'Shariq Hasnain' <shariq.hasnain@...aro.org>,
	'Chunsang Jeong' <chunsang.jeong@...aro.org>
Subject: RE: [PATCH 7/9] ARM: DMA: steal memory for DMA coherent mappings

Hello,

On Friday, August 12, 2011 2:53 PM Arnd Bergmann wrote:

> On Friday 12 August 2011, Marek Szyprowski wrote:
> >
> > From: Russell King <rmk+kernel@....linux.org.uk>
> >
> > Steal memory from the kernel to provide coherent DMA memory to drivers.
> > This avoids the problem with multiple mappings with differing attributes
> > on later CPUs.
> >
> > Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
> > [m.szyprowski: rebased onto 3.1-rc1]
> > Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> 
> Hi Marek,
> 
> Is this the same patch that Russell had to revert because it didn't
> work on some of the older machines, in particular those using
> dmabounce?

Yes.
 
> I thought that our discussion ended with the plan to use this only
> for ARMv6+ (which has a problem with double mapping) but not on ARMv5
> and below (which don't have this problem but might need dmabounce).

Ok, my fault. I've forgot to mention that this patch was almost ready 
during Linaro meeting, but I didn't manage to post it that time. Of course 
it doesn't fulfill all the agreements from that discussion.

I was only unsure if we should care about the case where CMA is not enabled
for ARMv6+ or not. This patch was prepared in assumption that 
dma_alloc_coherent should work in both cases - with and without CMA.

Now I assume that for ARMv6+ the CMA should be enabled unconditionally.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center

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