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-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.