RFC bus_dma_tag_parent

Matthew Dillon dillon at apollo.backplane.com
Fri Nov 12 09:56:00 PST 2004

:while the proposal would look like
:    parent_dmat = bus_dma_tag_parent(dev);
:    bus_dma_tag_create(parent_dmat, ..., &mybuffer_dmat);
:What kind of situations would specifying the max number of segments be
:Chuck Tuffli
:Agilent Technologies

    For PCI the PCI BUS itself doesn't control DMA, so the pci bus would
    specify '0' there (we'd make '0' officially mean 'unlimited or
    unspecified').  Other busses would depend, but for the most part the
    only thing that truely matters for their tags are the address 
    limitations of the bus.

    The device driver would also specify limitations based on knowledge of
    the device chip.  e.g. if a PCI chipset's dma chaining structure only
    has room for e.g. 32 bit addresses, then the device driver would also
    specify a 32 bit limitation for its tag.

					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>

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