RFC bus_dma_tag_parent

Joerg Sonnenberger joerg at britannica.bec.de
Fri Nov 12 10:44:34 PST 2004

On Fri, Nov 12, 2004 at 09:44:39AM -0800, Chuck Tuffli wrote:
>     parent_dmat = bus_dma_tag_parent(dev);

parent_dmat = bus_dma_tag_parent(dev, lowaddr, highaddr, maxsegments);

>     bus_dma_tag_create(parent_dmat, ..., &mybuffer_dmat);
> What kind of situations would specifying the max number of segments be
> useful?

This is useful if the bus DMA engine supports scatter/gather itself.
This can be done IIRC on the Alpha, maybe on other architectures too.
I don't mind if we have a UNKNOWN value, e.g. 0. This is not a problem
here, but for many drivers we know this in advantage and it might be
used to allocate the segment maps in the parent tag.


> -- 
> Chuck Tuffli
> Agilent Technologies

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