linux-kernel - Re: [Linaro-mm-sig] [PATCH 08/10] mm: cma: Contiguous Memory Allocator added

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Message-ID: <20110614170158.GU2419@fooishbar.org>
Date:	Tue, 14 Jun 2011 18:01:58 +0100
From:	Daniel Stone <daniels@...labora.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Michal Nazarewicz <mina86@...a86.com>,
	'Ankita Garg' <ankita@...ibm.com>,
	'Daniel Walker' <dwalker@...eaurora.org>,
	'Jesse Barker' <jesse.barker@...aro.org>,
	'Mel Gorman' <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
	'Kyungmin Park' <kyungmin.park@...sung.com>,
	'KAMEZAWA Hiroyuki' <kamezawa.hiroyu@...fujitsu.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [PATCH 08/10] mm: cma: Contiguous Memory
 Allocator added

Hi,

On Tue, Jun 14, 2011 at 06:03:00PM +0200, Arnd Bergmann wrote:
> On Tuesday 14 June 2011, Michal Nazarewicz wrote:
> > On Tue, 14 Jun 2011 15:49:29 +0200, Arnd Bergmann <arnd@...db.de> wrote:
> > > Please explain the exact requirements that lead you to defining multiple
> > > contexts.
> > 
> > Some devices may have access only to some banks of memory.  Some devices
> > may use different banks of memory for different purposes.
> 
> For all I know, that is something that is only true for a few very special
> Samsung devices, and is completely unrelated of the need for contiguous
> allocations, so this approach becomes pointless as soon as the next
> generation of that chip grows an IOMMU, where we don't handle the special
> bank attributes. Also, the way I understood the situation for the Samsung
> SoC during the Budapest discussion, it's only a performance hack, not a
> functional requirement, unless you count '1080p playback' as a functional
> requirement.

Hm, I think that was something similar but not quite the same: talking
about having allocations split to lie between two banks of RAM to
maximise the read/write speed for performance reasons.  That's something
that can be handled in the allocator, rather than an API constraint, as
this is.

Not that I know of any hardware which is limited as such, but eh.

Cheers,
Daniel
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