Device-tree support for ARM

Hi all,

The following series of patches is an early prototype to add device tree
support to the ARM architecture.

If you're not familiar with device trees: a device tree is a data
structure describing the hardware that the kernel is booted on. On boot,
the bootloader can pass a 'device tree blob' to the kernel; the kernel
then parses the blob and extracts data used to initialise the machine.
Think ATAGS with a free-form structure and arbitrary property names and
values.

For cases where we don't want to rely on the bootloader's device tree
blob[1] (maybe we have an updated one with fixes), we can supply our own
device tree, attached to the kernel and passed through a boot wrapper.

Device trees are currently used in the boot process for the powerpc,
sparc and microblaze architectures.

This work is aimed at reducing the effort required to port to a new
board. Some hardware changes to may not require a new kernel at all;
these may be described by a corresponding change to the device
tree.

We'd also hope to see an overall decrease in code required as drivers
can be made generic enough to support a set of compatible devices, with
the differences being represented in the device tree.

At this stage, I've just added a mechanism to probe a platform
(Versatile PB) using a device tree; this is just a proof of concept for
machine probe, the interesting stuff happens when we can add DT support
to drivers that the DT-enabled platforms use. Also todo is the boot
wrapper to enable us to embed an updated DT in a bootable image.

These patches are based on Grant Likely's test-devicetree repo[2], which
is an effort to merge the common device tree code between the powerpc,
sparc and microblaze architectures. I have a git tree[3] of these
patches up too, based on his tree.

The approach I've chosen for a boot interface is to add an ATAG that
points to a device tree blob. If this ATAG is found, we use the
device-tree based discovery. There are a couple of other options, but I
think this is the least-intrusive method of passing the DT to the
kernel, and will continue to work with non-DT bootloaders.

If you'd like to try the code out, I have a page[4] up that has
details of the kernel, a link to DT-enabled qemu sources and a sample
device tree for the versatile machine.

This is only an initial prototype, but I'd appreciate any comments,
feedback or review.

Cheers,


Jeremy

1: http://lkml.org/lkml/2009/5/27/446
2: http://git.secretlab.ca/?p=linux-2.6.git;a=shortlog;h=ref...
3: http://kernel.ubuntu.com/git?p=jk/dt/linux-2.6.git;a=summary
4: https://wiki.ubuntu.com/KernelTeam/ARMDeviceTrees

---
Jeremy Kerr (11):
      arm: change command_line to cmd_line, and export it
      arm: use generic infrastructure for early params
      arm: export arm_add_memory
      arm-dt: Add ATAG_DEVTREE tag
      arm-dt: Add CONFIG_ARM_DEVTREE
      arm-dt: postpone machine detection until setup_arch with CONFIG_ARM_DEVTREE
      arm-dt: parse devtree pointer on boot
      arm-dt: parse memory info from DT
      arm-dt: probe for device-tree enabled platforms
      arm-dt: parse initrd from device tree
      arm/versatile: probe via device tree