PCI-PCI Bridges: PCI Configuration Cycles and PCI Bus Numbering

   

  
Figure: Type 0 PCI Configuration Cycle

  
Figure: Type 1 PCI Configuration Cycle

So that the CPU's PCI initialization code can address devices that are not on the main PCI bus, there has to be a mechanism that allows bridges to decide whether or not to pass Configuration cycles from their primary interface to their secondary interface. A cycle is just an address as it appears on the PCI bus. The PCI specification defines two formats for the PCI Configuration addresses; Type 0 and Type 1; these are shown in Figure  and Figure  respectively. Type 0 PCI Configuration cycles do not contain a bus number and these are interpretted by all devices as being for PCI configuration addresses on this PCI bus. Bits 31:11 of the Type 0 configuraration cycles are treated as the device select field. One way to design a system is to have each bit select a different device. In this case bit 11 would select the PCI device in slot 0, bit 12 would select the PCI device in slot 1 and so on. Another way is to write the device's slot number directly into bits 31:11. Which mechanism is used in a system depends on the system's PCI memory controller.

Type 1 PCI Configuration cycles contain a PCI bus number and this type of configuration cycle are ignored by all PCI devices except the PCI-PCI bridges. All of the PCI-PCI Bridges seeing Type 1 configuration cycles may choose to pass them to the PCI buses downstream of themselves. Whether the PCI-PCI Bridge ignores the Type 1 configuration cycle or passes it onto the downstream PCI bus depends on how the PCI-PCI Bridge has been configured. Every PCI-PCI bridge has a primary bus interface number, and a secondary bus interface number. The primary bus interface being the one nearest the CPU and the secondary bus interface being the one furthest away. Each PCI-PCI Bridge also has a subordinate bus number and this is the maximum bus number of all the PCI busses that are bridged beyond the secondary bus interface. Or to put it another way, the subordinate bus number is the highest numbered PCI bus downstream of the PCI-PCI bridge. When the PCI-PCI bridge sees a Type 1 PCI configuration cycle it does one of the following things:

• Ignore it if the bus number specified is not in between the bridge's secondary bus number and subordinate bus number (inclusive),
• Convert it to a Type 0 configuration command if the bus number specified matches the secondary bus number of the bridge,
• Pass it onto the secondary bus interface unchanged if the bus number specified is greater than the secondary bus number and less than or equal to the subordinate bus number.

So, if we want to address Device 1 on bus 3 of the topology Figure  on page  we must generate a Type 1 Configuration command from the CPU. passes this unchanged onto Bus 1. ignores it but converts it into a Type 0 Configuration command and sends it out on Bus 3 where Device 1 responds to it.

It is up to each individual operating system to allocate bus numbers during PCI configuration but whatever the numbering scheme used the following statement must be true for all of the PCI-PCI bridges in the system:

``All PCI busses located behind a PCI-PCI bridge must reside between the seondary bus number and the subordinate bus number (inclusive).''  

If this rule is broken then the PCI-PCI Bridges will not pass and translate Type 1 PCI configuration cycles correctly and the system will fail to find and initialise the PCI devices in the system. To achieve this numbering scheme, Linux configures these special devices in a particular order. Section  on page  describes Linux's PCI bridge and bus numbering scheme in detail together with a worked example.