Transparent bridges (a. k. a. HUB-to-PCI bridges)?
Chuck Swiger
cswiger at mac.com
Wed Nov 24 06:28:58 PST 2004
M. Warner Losh wrote:
> In message: <41A416E7.4030107 at mac.com>
> Chuck Swiger <cswiger at mac.com> writes:
[ ... ]
> : A host-PCI bridge is typically part of the "southbridge" chip of modern
> : motherboards; on Intel motherboards this is also called the ICH chip, such as
> : the 82801AA/BA/CA/etc. VIA Southbridges include the VT8233/8235/8237/etc.
> :
> : A PCI-PCI bridge is commonly found on multifunction PCI cards, an example
> : would be the DEC 21151 chip found on various four-port NICs.
>
> Newer laptops (and other machines) typically have a PCI PCI bridge
> that all the builtin hardware lives behind. Many, but not all, of
> these bridges are transparent pci pci bridges, maning they act much
> like a subtractively decoded bridge.
You are absolutely right; the impression I got was that laptops like to have
PCI-PCI bridges in order to make it easier to route interrupts for devices on
a docking station or the like. If they don't use such a PCI-PCI bridge chip,
then the laptop's BIOS needs to set up a $PIR table which routes interrupts
properly for _all_ of the possible docking station configurations and devices
to which the laptop might be attached to.
Making things work right with a known configuration seems to be hard enough
for some vendors, so it's not surprising that pre-planning for possible future
configurations is difficult to do without using a PCI-PCI bridge to aggregate
the devices lurking behind it.
--
-Chuck
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