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

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Message-ID: <Pine.LNX.4.64.0705231603470.23000@schroedinger.engr.sgi.com>
Date:	Wed, 23 May 2007 16:07:12 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Russell King <rmk+lkml@....linux.org.uk>
cc:	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.

On Wed, 23 May 2007, Russell King wrote:

> > GFP_DMA allocations are an exception and that exception can be removed 
> > from the core VM by not defining ZONE_DMA. You cannot switch off the 
> > NORMAL zone.
> 
> I'd like to be able to switch off the normal and highmem zones and leave
> just the DMA zone behind.  The normal and highmem zones are just a waste
> of space on ARM.

If you do not have exceptional memory requirements then you do not need a 
DMA zone. The core code allows you to switch off ZONE_DMA but not 
ZONE_NORMAL. For a description of the roles of the zones see 
include/linux/mmzone.h

> > Nope. The DMA zone is for crappy DMA devices that can only use a portion 
> > of memory.
> 
> I'm sorry, we're going to have to agree to disagree then.
> 
> As for your assertion that it's "crappy DMA devices" there are modern
> PCI based platforms being produced by Marvell (which were designed by
> Intel) which can only DMA from 64MB of memory, inspite of the PCI
> device having full PCI busmastering capabilities.

Ok sorry for the word choice.

> At the end of the day, it is _far_ simpler from an architectural point
> of view for memory to live in the DMA zone and disable the normal and
> highmem zones than it is to selectively populate the normal zone
> depending on whether device X is configured, and then also have to hack
> around various drivers which decide they want to use __GFP_DMA because
> of some antequated x86ism which doesn't apply on non-x86.

If you switch off CONFIG_ZONE_DMA then __GFP_DMA becomes a no op. So no 
problem. Many of us want to rid the kernel of __GFP_DMA. Please join the 
club and nuke the useless ZONE_DMA on your platforms.


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