Configuring PCI-PCI Bridges - Assigning PCI Bus Numbers

   

  
Figure: Configuring a PCI System: Part 1

For PCI-PCI bridges to pass PCI I/O, PCI Memory or PCI Configuration address space reads and writes across them, they need to know the following:

Primary Bus Number

The bus number immediately upstream of the PCI-PCI Bridge,

Secondary Bus Number

The bus number immediately downstream of the PCI-PCI Bridge,

Subordinate Bus Number

The highest bus number of all of the busses that can be reached downstream of the bridge.

PCI I/O and PCI Memory Windows

The window base and size for PCI I/O address space and PCI Memory address space for all addresses downstream of the PCI-PCI Bridge.

The problem is that at the time when you wish to configure any given PCI-PCI bridge you do not know the subordinate bus number for that bridge. You do not know if there are further PCI-PCI bridges downstream and if you did, you do not know what numbers will be assigned to them. The answer is to use a depthwise recursive algorithm and scan each bus for any PCI-PCI bridges assigning them numbers as they are found. As each PCI-PCI bridge is found and its secondary bus numbered, assign it a temporary subordinate number of 0xFF and scan and assign numbers to all PCI-PCI bridges downstream of it. This all seems complicated but the worked example below makes this process clearer.

PCI-PCI Bridge Numbering: Step 1

Taking the topology in Figure , the first bridge the scan would find is . The PCI bus downstream of would be numbered as 1 and assigned a secondary bus number of 1 and a temporary subordinate bus number of 0xFF. This means that all Type 1 PCI Configuration addresses specifying a PCI bus number of 1 or higher would be passed across and onto PCI Bus 1. They would be translated into Type 0 Configuration cycles if tehy have a bus number of 1 but left untranslated for all other bus numbers. This is exactly what the Linux PCI initialisation code needs to do in order to go and scan PCI Bus 1.

  
Figure: Configuring a PCI System: Part 2

PCI-PCI Bridge Numbering: Step 2

Linux uses a depthwise algorithm and so the initialisation code goes on to scan PCI Bus 1. Here it finds PCI-PCI . There are no further PCI-PCI bridges beyond PCI-PCI , so it is assigned a subordinate bus number of 2 which matches the number assigned to its secondary interface. Figure  shows how the busses and PCI-PCI bridges are numbered at this point.

  
Figure: Configuring a PCI System: Part 3

PCI-PCI Bridge Numbering: Step 3

The PCI initialisation code returns to scanning PCI Bus 1 and finds another PCI-PCI bridge, . It is assigned 1 as its primary bus interface number, 3 as its secondary bus interface number and 0xFF as its subordinate bus number. Figure  on page  shows how the system is configured now. Type 1 PCI configuration cycles with a bus number of 1, 2 or 3 wil be correctly delivered to the appropriate PCI buses.

  
Figure: Configuring a PCI System: Part 4

PCI-PCI Bridge Numbering: Step 4

Linux starts scanning PCI Bus 3, downstream of PCI-PCI . PCI Bus 3 has another PCI-PCI bridge ( ) on it, it is assigned 3 as its primary bus number and 4 as its secondary bus number. It is the last bridge on this branch and so it is assigned a subordinate bus interface number of 4. The initialisation code returns to PCI-PCI and assigns it a subordinate bus number of 4. Finally, the PCI initialisation code can assign 4 as the subordinate bus number for PCI-PCI . Figure  on page  shows the final bus numbers.