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

In message: <41A49A7B.6020009_at_mac.com>
            Chuck Swiger <cswiger_at_mac.com> writes:
: 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.

Having a pci-pci bridge is no guarantee that the interrupts will be
routed correctly.  Many laptops route them directly, even in the face
of a pci-pci bridge.  The $PIR will often lists devices behind the
bridge.

Warner