Re: RFC bus_dma_tag_parent

[Joerg Sonnenberger <joerg@xxxxxxxxxxxxxxxxx>]

On Fri, Nov 12, 2004 at 11:04:27AM -0800, Matthew Dillon wrote:
> :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.

Even better, we just have to fail any attempt to load or allocate DMA
mappings, perhaps with a KASSERT for the debugging case :)

Joerg

> 					-Matt
> 					Matthew Dillon 
> 					<dillon@xxxxxxxxxxxxx>