linux-kernel - Re: GFP_DMA32 and PAE x86 machines

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Message-Id: <200705310024.25937.ak@suse.de>
Date:	Thu, 31 May 2007 00:24:25 +0200
From:	Andi Kleen <ak@...e.de>
To:	"Dave Airlie" <airlied@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"Linus Torvalds" <torvalds@...l.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: GFP_DMA32 and PAE x86 machines

On Thursday 31 May 2007 00:07:45 Dave Airlie wrote:
> Depending on split all lowmem is below 1GB which isn't exactly
> optimal, I'd llike all 4GB for DMA.

Well it would be for a quite specialized limited use case: 
- Memory the kernel doesn't need to map (after all kmap is evil) 
- You need more than 500MB or so.
- 32bit kernel and user cannot run 64bit kernel
- Machine has >3GB of RAM 

Is there clear evidence that is a common issue? If it is just a "would
be nice to have in theory" the cost of doing a GFP_DMA32 for i386 would be
probably not worth doing it. If it's a common issue it might be 
considered, although the "here's a nickle. buy yourself a 64bit CPU"
strategy would also sound attractive. 

Please give a very very strong rationale why you want it. Did you actually
run into such a situation yourself yet?

-Andi
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