On Tue, Jan 29, 2013 at 10:47:09AM -0700, Bjorn Helgaas wrote:

As I understand it, the DT is a description of the hardware, so in
that sense, the DT can't set aside physical address space.  It can
describe what the hardware does with the address space, and I assume
that's what you mean.  Maybe the hardware isn't configurable, e.g., it
is hard-wired to route certain address ranges to PCIe?

The DT is largely a description of the hardware, but when it comes to
addresses, particularly HW programmable addresess, there is an general
expectation that the driver/bootloader will program HW address
decoders to either match the addresses given in the DT, or to new
values guided by the DT addresses.

In a real sense that means the DT also describes the physical address
map the kernel should use.

In the PCI-E case the DT PCI-E HW description includes physical
address ranges to use for the MMIO/IO/PREFETCH PCI-E interface windows
and the driver is expected to program the internal HW address decoders
based on those address ranges.

The catch is that the hardware decoders are on a link-by-link basis,
not on a root-complex basis, so the programming can only take place
once the Linux kernel has done PCI resource assignment.

So when I say set aside, I mean for instance, the PCI-E entry in DT
has 128M of physical address space marked for PCI MMIO use. The kernel
does PCI resource allocation and the HW decoders in each link will be
set to claim some portion of the 128M - based on the MMIO windows
programmed on the PCI-PCI root port bridges. The reamining part of the
128M is dead address space, not claimed by any hardware block at all.

Jason