dt: Linux DT usage model documentation

Linux and the Device Tree

-------------------------

The Linux usage model for device tree data

Author: Grant Likely <grant.likely@secretlab.ca>

This article describes how Linux uses the device tree.  An overview of

the device tree data format can be found on the device tree usage page

at devicetree.org[1].

[1] http://devicetree.org/Device_Tree_Usage

The "Open Firmware Device Tree", or simply Device Tree (DT), is a data

structure and language for describing hardware.  More specifically, it

is a description of hardware that is readable by an operating system

so that the operating system doesn't need to hard code details of the

machine.

Structurally, the DT is a tree, or acyclic graph with named nodes, and

nodes may have an arbitrary number of named properties encapsulating

arbitrary data.  A mechanism also exists to create arbitrary

links from one node to another outside of the natural tree structure.

Conceptually, a common set of usage conventions, called 'bindings',

is defined for how data should appear in the tree to describe typical

hardware characteristics including data busses, interrupt lines, GPIO

connections, and peripheral devices.

As much as possible, hardware is described using existing bindings to

maximize use of existing support code, but since property and node

names are simply text strings, it is easy to extend existing bindings

or create new ones by defining new nodes and properties.  Be wary,

however, of creating a new binding without first doing some homework

about what already exists.  There are currently two different,

incompatible, bindings for i2c busses that came about because the new

binding was created without first investigating how i2c devices were

already being enumerated in existing systems.

1. History

----------

The DT was originally created by Open Firmware as part of the

communication method for passing data from Open Firmware to a client

program (like to an operating system).  An operating system used the

Device Tree to discover the topology of the hardware at runtime, and

thereby support a majority of available hardware without hard coded

information (assuming drivers were available for all devices).

Since Open Firmware is commonly used on PowerPC and SPARC platforms,

the Linux support for those architectures has for a long time used the

Device Tree.

In 2005, when PowerPC Linux began a major cleanup and to merge 32-bit

and 64-bit support, the decision was made to require DT support on all

powerpc platforms, regardless of whether or not they used Open

Firmware.  To do this, a DT representation called the Flattened Device

Tree (FDT) was created which could be passed to the kernel as a binary

blob without requiring a real Open Firmware implementation.  U-Boot,

kexec, and other bootloaders were modified to support both passing a

Device Tree Binary (dtb) and to modify a dtb at boot time.  DT was

also added to the PowerPC boot wrapper (arch/powerpc/boot/*) so that

a dtb could be wrapped up with the kernel image to support booting

existing non-DT aware firmware.

Some time later, FDT infrastructure was generalized to be usable by

all architectures.  At the time of this writing, 6 mainlined

architectures (arm, microblaze, mips, powerpc, sparc, and x86) and 1

out of mainline (nios) have some level of DT support.

2. Data Model

-------------

If you haven't already read the Device Tree Usage[1] page,

then go read it now.  It's okay, I'll wait....

2.1 High Level View

-------------------

The most important thing to understand is that the DT is simply a data

structure that describes the hardware.  There is nothing magical about

it, and it doesn't magically make all hardware configuration problems

go away.  What it does do is provide a language for decoupling the

hardware configuration from the board and device driver support in the

Linux kernel (or any other operating system for that matter).  Using

it allows board and device support to become data driven; to make

setup decisions based on data passed into the kernel instead of on

per-machine hard coded selections.

Ideally, data driven platform setup should result in less code

duplication and make it easier to support a wide range of hardware

with a single kernel image.

Linux uses DT data for three major purposes:

1) platform identification,

2) runtime configuration, and

3) device population.

2.2 Platform Identification

---------------------------

First and foremost, the kernel will use data in the DT to identify the

specific machine.  In a perfect world, the specific platform shouldn't

matter to the kernel because all platform details would be described

perfectly by the device tree in a consistent and reliable manner.

Hardware is not perfect though, and so the kernel must identify the

machine during early boot so that it has the opportunity to run

machine-specific fixups.

In the majority of cases, the machine identity is irrelevant, and the

kernel will instead select setup code based on the machine's core

CPU or SoC.  On ARM for example, setup_arch() in

arch/arm/kernel/setup.c will call setup_machine_fdt() in

arch/arm/kernel/devicetree.c which searches through the machine_desc

table and selects the machine_desc which best matches the device tree

data.  It determines the best match by looking at the 'compatible'

property in the root device tree node, and comparing it with the

dt_compat list in struct machine_desc.

The 'compatible' property contains a sorted list of strings starting

with the exact name of the machine, followed by an optional list of

boards it is compatible with sorted from most compatible to least.  For

example, the root compatible properties for the TI BeagleBoard and its

successor, the BeagleBoard xM board might look like:

	compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3";

	compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3";

Where "ti,omap3-beagleboard-xm" specifies the exact model, it also

claims that it compatible with the OMAP 3450 SoC, and the omap3 family

of SoCs in general.  You'll notice that the list is sorted from most

specific (exact board) to least specific (SoC family).

Astute readers might point out that the Beagle xM could also claim

compatibility with the original Beagle board.  However, one should be

cautioned about doing so at the board level since there is typically a

high level of change from one board to another, even within the same

product line, and it is hard to nail down exactly what is meant when one

board claims to be compatible with another.  For the top level, it is

better to err on the side of caution and not claim one board is

compatible with another.  The notable exception would be when one

board is a carrier for another, such as a CPU module attached to a

carrier board.

One more note on compatible values.  Any string used in a compatible

property must be documented as to what it indicates.  Add

documentation for compatible strings in Documentation/devicetree/bindings.

Again on ARM, for each machine_desc, the kernel looks to see if

any of the dt_compat list entries appear in the compatible property.

If one does, then that machine_desc is a candidate for driving the

machine.  After searching the entire table of machine_descs,

setup_machine_fdt() returns the 'most compatible' machine_desc based

on which entry in the compatible property each machine_desc matches

against.  If no matching machine_desc is found, then it returns NULL.

The reasoning behind this scheme is the observation that in the majority

of cases, a single machine_desc can support a large number of boards

if they all use the same SoC, or same family of SoCs.  However,

invariably there will be some exceptions where a specific board will

require special setup code that is not useful in the generic case.

Special cases could be handled by explicitly checking for the

troublesome board(s) in generic setup code, but doing so very quickly

becomes ugly and/or unmaintainable if it is more than just a couple of

cases.

Instead, the compatible list allows a generic machine_desc to provide

support for a wide common set of boards by specifying "less

compatible" value in the dt_compat list.  In the example above,

generic board support can claim compatibility with "ti,omap3" or

"ti,omap3450".  If a bug was discovered on the original beagleboard

that required special workaround code during early boot, then a new

machine_desc could be added which implements the workarounds and only

matches on "ti,omap3-beagleboard".

PowerPC uses a slightly different scheme where it calls the .probe()

hook from each machine_desc, and the first one returning TRUE is used.

However, this approach does not take into account the priority of the

compatible list, and probably should be avoided for new architecture

support.

2.3 Runtime configuration

-------------------------

In most cases, a DT will be the sole method of communicating data from

firmware to the kernel, so also gets used to pass in runtime and

configuration data like the kernel parameters string and the location

of an initrd image.

Most of this data is contained in the /chosen node, and when booting

Linux it will look something like this:

	chosen {

		bootargs = "console=ttyS0,115200 loglevel=8";

		initrd-start = <0xc8000000>;

		initrd-end = <0xc8200000>;

	};

The bootargs property contains the kernel arguments, and the initrd-*

properties define the address and size of an initrd blob.  The

chosen node may also optionally contain an arbitrary number of

additional properties for platform-specific configuration data.

During early boot, the architecture setup code calls of_scan_flat_dt()

several times with different helper callbacks to parse device tree

data before paging is setup.  The of_scan_flat_dt() code scans through

the device tree and uses the helpers to extract information required

during early boot.  Typically the early_init_dt_scan_chosen() helper

is used to parse the chosen node including kernel parameters,

early_init_dt_scan_root() to initialize the DT address space model,

and early_init_dt_scan_memory() to determine the size and

location of usable RAM.

On ARM, the function setup_machine_fdt() is responsible for early

scanning of the device tree after selecting the correct machine_desc

that supports the board.

2.4 Device population

---------------------

After the board has been identified, and after the early configuration data

has been parsed, then kernel initialization can proceed in the normal

way.  At some point in this process, unflatten_device_tree() is called

to convert the data into a more efficient runtime representation.

This is also when machine-specific setup hooks will get called, like

the machine_desc .init_early(), .init_irq() and .init_machine() hooks

on ARM.  The remainder of this section uses examples from the ARM

implementation, but all architectures will do pretty much the same

thing when using a DT.

As can be guessed by the names, .init_early() is used for any machine-

specific setup that needs to be executed early in the boot process,

and .init_irq() is used to set up interrupt handling.  Using a DT

doesn't materially change the behaviour of either of these functions.

If a DT is provided, then both .init_early() and .init_irq() are able

to call any of the DT query functions (of_* in include/linux/of*.h) to

get additional data about the platform.

The most interesting hook in the DT context is .init_machine() which

is primarily responsible for populating the Linux device model with

data about the platform.  Historically this has been implemented on

embedded platforms by defining a set of static clock structures,

platform_devices, and other data in the board support .c file, and

registering it en-masse in .init_machine().  When DT is used, then

instead of hard coding static devices for each platform, the list of

devices can be obtained by parsing the DT, and allocating device

structures dynamically.

The simplest case is when .init_machine() is only responsible for

registering a block of platform_devices.  A platform_device is a concept

used by Linux for memory or I/O mapped devices which cannot be detected

by hardware, and for 'composite' or 'virtual' devices (more on those

later).  While there is no 'platform device' terminology for the DT,

platform devices roughly correspond to device nodes at the root of the

tree and children of simple memory mapped bus nodes.

About now is a good time to lay out an example.  Here is part of the

device tree for the NVIDIA Tegra board.

/{

	compatible = "nvidia,harmony", "nvidia,tegra20";

	#address-cells = <1>;

	#size-cells = <1>;

	interrupt-parent = <&intc>;

	chosen { };

	aliases { };

	memory {

		device_type = "memory";

		reg = <0x00000000 0x40000000>;

	};

	soc {

		compatible = "nvidia,tegra20-soc", "simple-bus";

		#address-cells = <1>;

		#size-cells = <1>;

		ranges;

		intc: interrupt-controller@50041000 {

			compatible = "nvidia,tegra20-gic";

			interrupt-controller;

			#interrupt-cells = <1>;

			reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >;

		};

		serial@70006300 {

			compatible = "nvidia,tegra20-uart";

			reg = <0x70006300 0x100>;

			interrupts = <122>;

		};

		i2s1: i2s@70002800 {

			compatible = "nvidia,tegra20-i2s";

			reg = <0x70002800 0x100>;

			interrupts = <77>;

			codec = <&wm8903>;

		};

		i2c@7000c000 {

			compatible = "nvidia,tegra20-i2c";

			#address-cells = <1>;

			#size-cells = <0>;

			reg = <0x7000c000 0x100>;

			interrupts = <70>;

			wm8903: codec@1a {

				compatible = "wlf,wm8903";

				reg = <0x1a>;

				interrupts = <347>;

			};

		};

	};

	sound {

		compatible = "nvidia,harmony-sound";

		i2s-controller = <&i2s1>;

		i2s-codec = <&wm8903>;

	};

};

At .machine_init() time, Tegra board support code will need to look at

this DT and decide which nodes to create platform_devices for.

However, looking at the tree, it is not immediately obvious what kind

of device each node represents, or even if a node represents a device

at all.  The /chosen, /aliases, and /memory nodes are informational

nodes that don't describe devices (although arguably memory could be

considered a device).  The children of the /soc node are memory mapped

devices, but the codec@1a is an i2c device, and the sound node

represents not a device, but rather how other devices are connected

together to create the audio subsystem.  I know what each device is

because I'm familiar with the board design, but how does the kernel

know what to do with each node?

The trick is that the kernel starts at the root of the tree and looks

for nodes that have a 'compatible' property.  First, it is generally

assumed that any node with a 'compatible' property represents a device

of some kind, and second, it can be assumed that any node at the root

of the tree is either directly attached to the processor bus, or is a

miscellaneous system device that cannot be described any other way.

For each of these nodes, Linux allocates and registers a

platform_device, which in turn may get bound to a platform_driver.

Why is using a platform_device for these nodes a safe assumption?

Well, for the way that Linux models devices, just about all bus_types

assume that its devices are children of a bus controller.  For

example, each i2c_client is a child of an i2c_master.  Each spi_device

is a child of an SPI bus.  Similarly for USB, PCI, MDIO, etc.  The

same hierarchy is also found in the DT, where I2C device nodes only

ever appear as children of an I2C bus node.  Ditto for SPI, MDIO, USB,

etc.  The only devices which do not require a specific type of parent

device are platform_devices (and amba_devices, but more on that

later), which will happily live at the base of the Linux /sys/devices

tree.  Therefore, if a DT node is at the root of the tree, then it

really probably is best registered as a platform_device.

Linux board support code calls of_platform_populate(NULL, NULL, NULL)

to kick off discovery of devices at the root of the tree.  The

parameters are all NULL because when starting from the root of the

tree, there is no need to provide a starting node (the first NULL), a

parent struct device (the last NULL), and we're not using a match

table (yet).  For a board that only needs to register devices,

.init_machine() can be completely empty except for the

of_platform_populate() call.

In the Tegra example, this accounts for the /soc and /sound nodes, but

what about the children of the SoC node?  Shouldn't they be registered

as platform devices too?  For Linux DT support, the generic behaviour

is for child devices to be registered by the parent's device driver at

driver .probe() time.  So, an i2c bus device driver will register a

i2c_client for each child node, an SPI bus driver will register

its spi_device children, and similarly for other bus_types.

According to that model, a driver could be written that binds to the

SoC node and simply registers platform_devices for each of its

children.  The board support code would allocate and register an SoC

device, a (theoretical) SoC device driver could bind to the SoC device,

and register platform_devices for /soc/interrupt-controller, /soc/serial,

/soc/i2s, and /soc/i2c in its .probe() hook.  Easy, right?

Actually, it turns out that registering children of some

platform_devices as more platform_devices is a common pattern, and the

device tree support code reflects that and makes the above example

simpler.  The second argument to of_platform_populate() is an

of_device_id table, and any node that matches an entry in that table

will also get its child nodes registered.  In the tegra case, the code

can look something like this:

static void __init harmony_init_machine(void)

{

	/* ... */

	of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);

}

"simple-bus" is defined in the ePAPR 1.0 specification as a property

meaning a simple memory mapped bus, so the of_platform_populate() code

could be written to just assume simple-bus compatible nodes will

always be traversed.  However, we pass it in as an argument so that

board support code can always override the default behaviour.

[Need to add discussion of adding i2c/spi/etc child devices]

Appendix A: AMBA devices

------------------------

ARM Primecells are a certain kind of device attached to the ARM AMBA

bus which include some support for hardware detection and power

management.  In Linux, struct amba_device and the amba_bus_type is

used to represent Primecell devices.  However, the fiddly bit is that

not all devices on an AMBA bus are Primecells, and for Linux it is

typical for both amba_device and platform_device instances to be

siblings of the same bus segment.

When using the DT, this creates problems for of_platform_populate()

because it must decide whether to register each node as either a

platform_device or an amba_device.  This unfortunately complicates the

device creation model a little bit, but the solution turns out not to

be too invasive.  If a node is compatible with "arm,amba-primecell", then

of_platform_populate() will register it as an amba_device instead of a

platform_device.