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

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Date:	Thu, 24 May 2007 10:57:19 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Christoph Lameter <clameter@....com>,
	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.

> 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.

Bzzzzzt - have to call your bluff on that one

The IOMMU mapping window itself may be out of range of some devices. This
is a big problem on AMD64 where the GART window is above the 2GB boundary
- you can do 64->32 nicely but 32->31/30/28/24, all of which turn up on
PC hardware, are no solved by the IOMMU only by GFP_DMA

Alan
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