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

[R. Koot]

Peter 'p2' De Schrijver wrote:
> 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 

Note that 8bit ISA card (which are almost as (un)common as the 16bit 
variants) can still access the entier 16MB memory when plugged into a
16bit bus (which every system capable of running the HURD has (or had)).

> 16MB : 16bit ISA DMA

I know of at least one southbridge (Intel SIO, predecessor of the Intel 
PIIX) which allows for 29- or 32-bit addressing for ISA cards.

> 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. 
>
> Cheers,
>
> Peter (p2).
>
>
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