Messages in this thread | | | Date | Sat, 12 Mar 2005 12:34:02 +1100 | From | Paul Mackerras <> | Subject | Re: AGP bogosities |
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Bjorn Helgaas writes:
> I still don't quite understand this. If the host bridge is not a > PCI device, what PCI device contains the AGP capability that controls > the host bridge? I assume you're saying that you are required to
The AGP spec shows an example northbridge implementation that has the AGP interface circuitry as a PCI-to-PCI bridge. You don't have to do it that way, but in any case you need some sort of PCI device to represent the target (host) end of the AGP link.
> have TWO PCI devices that have the AGP capability, one for the AGP > device and one for the bridge.
Yes, exactly.
> HP boxes certainly don't have that (zx6000 sample below). I guess > it wouldn't be the first time we deviated from a spec ;-) > > Can you point me to the relevant section?
In the AGP 2.0 spec, the clearest statement I can find is in section 6.1.5, on page 248, where it says "Configuration registers are used by the operating system to initialize A.G.P. features. These features must be supported by both A.G.P. master and target devices in the following registers.", followed by a description of various PCI config space registers. In the context, "configuration registers" means registers in the config space of a PCI device. The AGP 3.0 spec is a delta from the AGP 2.0 spec and doesn't revoke that requirement anywhere that I could see.
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