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Re: Introduce a want_64bit_pci support [was: PAE support]

On Tue, Feb 23, 2010 at 06:51:56PM -0600, David Young wrote:
> On Wed, Feb 24, 2010 at 01:19:41AM +0100, Joerg Sonnenberger wrote:
> > Hi,
> > based on the PAE discussion there is the question of what to do for
> > drives like twa(4). E.g. drivers that can use a more compact 32bit
> > layout or support the full 64bit PCI address space.
> > 
> > Would it be reasonable to add a new MI variable
> >     bool pci_use_64bit_addressing;
> > that gets set by MD to true if drivers should use 64bit support?
> > For x86, it would be enabled if physical memory above 4GB is found.
> > Other architectures might have similiar rules or have a IOMMU that makes
> > it irrelevant. The other reason for avoiding 64bit is the increased
> > bus traffic after all.
> This seems like a property that the MD mainbus_attach() routine should
> set on the pcibus_attach_args before it attaches the host bridge(s).  It
> may suffice simply to initialize pba_dmat64 to non-NULL.

...and the child checks for pa_dmat64 != NULL?


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