[openib-general] mthca and DDR not hidden

Tue Oct 5 08:53:01 PDT 2004

On Tue, Oct 05, 2004 at 05:32:26PM +0200, Michael S. Tsirkin wrote:
> > The argument about IOMMU resources seems to be an argument
> > in favor of letting the consumer handle allocation.
> 
> Are you saying consumer shall play with IOMMU?

Yes, if "play with" means something above the HCA driver deals with
DMA mappings.

> But then
> PCI addresses of memory will change each time consumer
> does this and you wont be able to use your default region?

Are regions defined by physical (DMA) or virtual addresses?

In any case, a "default region" implies a long term DMA mapping.
Documentation/DMA-mapping.txt is pretty clear that "streaming" DMA mappings
are only intended for transient use - ie map something, do the DMA,
then unmap. "Coherent" DMA mappings are intended to live longer
periods but only for control structures like descriptor rings
or other shared data.  Linux tries to be "stingy" with DMA mappings
because they are a scarce resource on some platforms.

hth,
grant