From mboxrd@z Thu Jan  1 00:00:00 1970
From: m.szyprowski@samsung.com (Marek Szyprowski)
Date: Thu, 28 Apr 2011 12:32:32 +0200
Subject: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1
In-Reply-To: <20110428093741.GV17290@n2100.arm.linux.org.uk>
References: <201104212129.17013.arnd@arndb.de>
	<20110427073514.GH17290@n2100.arm.linux.org.uk>
	<1303940271.2513.187.camel@pasglop>
	<20110428093741.GV17290@n2100.arm.linux.org.uk>
Message-ID: <003e01cc058f$94beb490$be3c1db0$%szyprowski@samsung.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Hello,

On Thursday, April 28, 2011 11:38 AM Russell King - ARM Linux wrote:

> > > > 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
> > > >    that this is needed, and it currently is not implemented, with
> > > >    an outdated comment explaining why it used to not be possible
> > > >    to do it.
> > >
> > > dma_alloc_noncoherent is an entirely pointless API afaics.
> >
> > I was about to ask what the point is ... (what is the expected
> > semantic ? Memory that is reachable but not necessarily cache
> > coherent ?)
> 
> As far as I can see, dma_alloc_noncoherent() should just be a wrapper
> around the normal page allocation function.  I don't see it ever needing
> to do anything special - and the advantage of just being the normal
> page allocation function is that its properties are well known and
> architecture independent.

If there is IOMMU chip that supports pages larger than 4KiB then
dma_alloc_noncoherent() might try to allocate such larger pages what will
result in faster access to the buffer (lower iommu tlb miss ratio).
For large buffers even 64KiB 'pages' gives a significant performance
improvement.

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