The PCI Bus under Open Firmware

Practically everything on a PowerMac except the processor and the RAM is a device which is connected to the processor through the PCI bus, which maps the device's registers to memory through a somewhat convoluted process. At least from an Open Firmware point of view.

If you look at an OF device node for a PCI Bus, you will see that it has the #address-cells property set to 3, and the #size-cells property set to 2. The reg property of a node is a series of addr/size pairs, so a PCI child node's reg property will have entries which are 5 cells in size.

Note that this is not a main memory address -- it is an OF PCI bus address. To get an actual memory address, you have to take the bus specifier (the first cell of the PCI address), and find the matching entry in the assigned-addresses property (bus, device, function, and register fields must match). assigned-addresses has the same format as reg, except that the PCI address indicates that this is NOT a relocatable address, so the low cell (or two cells for a 64-bit Memory Space address) are an actual physical memory address.

Then you just have to map the physical address to get a virtual address that you can use to access it from the processor. I'm currently calling the map-in method of the root node ("/") of the OF device tree. However, PearPC won't open the root node, so presumably there are other ways to do this. It looks like the Linux kernel goes up through the device tree until it finds a pci bridge node (device-type = "pci"), and then looks up the address in the ranges property. But...that still leaves you with a physical address. Grr. How do you get from a physical to a virtual address?