RFC bus_dma_tag_parent

[Matthew Dillon]

: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.
:
:Joerg

    Yes, that makes sense.  I think the best approach to the allocation issue
    is to solve the real problem, and that is to allow a dma tag to be 
    flagged as being just a topological specification, and panicing if
    someone actually tries to initiate DMA using the topological
    specification.  I would rather not delay the allocation of the 
    scatter/gather array to hack around the lack of knowledge.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>