[coreboot] Help with devicetree.cb

Hello!
I'm new to coreboot, and I'm trying to port it to the Dell Latitude E6400. It is fairly similar to the Thinkpad T400, so I thought this would be a relatively easy first project. I currently have all code copied from the T400. Right now I'm working on the devicetree.cb, but I can't seem to figure out exactly what everything means and what to set it to. Some things, like device, chip, and subsystemid are fairly obvious, but other things, such as register and ioapic_irq are less so. 
I understand what register does, I just don't know where to get the values I should set them to, such as for gfx.did and pirqa_routing.
I have no idea where to find what values go into ioapic_irq
Can someone point me in the right direction?