Re: Transparent bridges (a. k. a. HUB-to-PCI bridges)?

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