Re: [PATCH 0/8] Optional ZONE_DMA V1

[Christoph Lameter <christoph@engr.sgi.com>]

On Tue, 12 Sep 2006, Martin Bligh wrote:

> > This is wrong. All memory should be in ZONE_NORMAL since we have no DMA
> > restrictions on Altix.
> 
> PPC64 works the same way, I believe. All memory is DMA'able, therefore
> it all fits in ZONE_DMA.

ZONE_DMA is for broken/limited DMA controllers not for DMA controllers 
that can reach all of memory.
 
> The real problem is that there's no consistent definition of what the
> zones actually mean.

ZONE_DMA 	Special memory area for DMA controllers that can only
			do dma to a restricted memory area.

ZONE_DMA32	Second special memory area for DMA controllers that
		can only do dma to a restricted memory area that
		is different from ZONE_DMA

ZONE_NORMAL	Regular memory

ZONE_HIGHEM	Memory requires being mapped into kernel address space.


> 1. Is it DMA'able (this is stupid, as it doesn't say 'for what device'

That is *not* what ZONE_DMA means. We have always supported DMA to 
regular  memory.

> What is really needed is to pass a physical address limit from the
> caller, together with a flag that says whether the memory needs to be
> mapped into the permanent kernel address space or not. The allocator
> then finds the set of zones that will fulfill this criteria.
> But I suspect this level of change will cause too many people to squeak
> loudly.

Actually we could do this with the proposed change of passing an 
allocation_control struct instead of gfpflags to the allocator functions. 
See the discussion on linux-mm.

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