RFC bus_dma_tag_parent

Chuck Tuffli chuck_tuffli at agilent.com
Fri Nov 12 09:46:08 PST 2004

On Fri, Nov 12, 2004 at 10:54:25AM +0100, Joerg Sonnenberger wrote:
> On Wed, Nov 10, 2004 at 02:20:08PM -0800, Chuck Tuffli wrote:
> >   bus_dma_tag_parent(dev)
> > 
> >   Returns the DMA tag of the bus dev connects to. Drivers can use the
> >   DMA tag for the parent parameter in subsequent calls to
> >   bus_dma_tag_create(9). All devices on a bus share this tag as their
> >   parent and should not destroy it.
> > 
> >   Returns NULL if tag does not exist or a bus_dma_tag_t on success.
> I still think having an option to specify the number of maximum number of
> segments would be useful. This is not a problem here, because e.g. for
> PCI it can be ignored or used as INVARIANT.
> I also think we want to have the address constraints in the parent tag.
> Joerg

The intent is that drivers use this tag for parent instead of creating
their own parent tag by doing something like

    bus_dma_tag_create(NULL, ..., &parent_dmat);
    bus_dma_tag_create(parent_dmat, ..., &mybuffer_dmat);

This should allow the bus driver to specify the address constraints as
well as the number of segments and allow device drivers to inherit
sane parameters for their calls to bus_dma_tag_create(). The current
sequence of events looks something like

    bus_dma_tag_create(NULL, ..., &parent_dmat);
    bus_dma_tag_create(parent_dmat, ..., &mybuffer_dmat);

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

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