linux-kernel - Re: Define CONFIG_BOUNCE to avoid useless inclusion of bounce buffer logic.

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Message-ID: <18005.2746.374990.29287@cargo.ozlabs.ibm.com>
Date:	Thu, 24 May 2007 13:47:06 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Russell King <rmk+lkml@....linux.org.uk>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: Define CONFIG_BOUNCE to avoid useless inclusion of bounce buffer
 logic.

Christoph Lameter writes:

> > The DMA zone is for memory allocations _for_ _DMA_.  If all your memory
> > is DMA-able then it belongs in the DMA zone.
> 
> Nope. The DMA zone is for crappy DMA devices that can only use a portion 
> of memory.

That is (presumably) true today, but is in fact a redefinition of what
ZONE_DMA historically was for.

Also there is the problem that some drivers use ZONE_DMA allocations
because their device can only generate addresses below some limit, but
on a platform with an IOMMU there is in fact no restriction on what
memory the device can access.

Paul.
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