Documentation/devicetree/usage-model.txt - kernel/msm-4.19 - Gitiles

 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 .init_machine() 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, 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.
	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 .init_machine() 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, 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.