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Re: Memory allocation/sharing when DMA operations used..

From: Peter 'p2' De Schrijver
Subject: Re: Memory allocation/sharing when DMA operations used..
Date: Fri, 1 Apr 2005 12:00:33 +0200
User-agent: Mutt/1.5.6+20040907i

On Fri, Apr 01, 2005 at 11:22:21AM +0200, Bas Wijnen wrote:
> Hi,
> I'm not very familiar with DMA, but I remembered that only certain pages
> can be used for DMA.  Or was that only for ISA?  Anyway, if that is not
> the case, it becomes a lot simpler.

It's true for ISA but also for PCI in some 64bit systems. In general we
have the following magic barriers on IA32/AMD64 systems :

1MB  : 8bit ISA DMA 
16MB : 16bit ISA DMA
4GB  : Non-DAC capable PCI cards on 64bit systems lacking an IO-MMU.
       (DAC stands for Dual Address Cycle here. Ie. the ability to use a
       64bit address by doing two 32bit address cycles).

Additionally 8 and 16bit ISA DMA transfers can't cross a 64KB or a 128KB
boundary respectively. 


Peter (p2).

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