linux/Documentation/devicetree/usage-model.txt
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   1Linux and the Device Tree
   2-------------------------
   3The Linux usage model for device tree data
   4
   5Author: Grant Likely <grant.likely@secretlab.ca>
   6
   7This article describes how Linux uses the device tree.  An overview of
   8the device tree data format can be found on the device tree usage page
   9at devicetree.org[1].
  10
  11[1] http://devicetree.org/Device_Tree_Usage
  12
  13The "Open Firmware Device Tree", or simply Device Tree (DT), is a data
  14structure and language for describing hardware.  More specifically, it
  15is a description of hardware that is readable by an operating system
  16so that the operating system doesn't need to hard code details of the
  17machine.
  18
  19Structurally, the DT is a tree, or acyclic graph with named nodes, and
  20nodes may have an arbitrary number of named properties encapsulating
  21arbitrary data.  A mechanism also exists to create arbitrary
  22links from one node to another outside of the natural tree structure.
  23
  24Conceptually, a common set of usage conventions, called 'bindings',
  25is defined for how data should appear in the tree to describe typical
  26hardware characteristics including data busses, interrupt lines, GPIO
  27connections, and peripheral devices.
  28
  29As much as possible, hardware is described using existing bindings to
  30maximize use of existing support code, but since property and node
  31names are simply text strings, it is easy to extend existing bindings
  32or create new ones by defining new nodes and properties.  Be wary,
  33however, of creating a new binding without first doing some homework
  34about what already exists.  There are currently two different,
  35incompatible, bindings for i2c busses that came about because the new
  36binding was created without first investigating how i2c devices were
  37already being enumerated in existing systems.
  38
  391. History
  40----------
  41The DT was originally created by Open Firmware as part of the
  42communication method for passing data from Open Firmware to a client
  43program (like to an operating system).  An operating system used the
  44Device Tree to discover the topology of the hardware at runtime, and
  45thereby support a majority of available hardware without hard coded
  46information (assuming drivers were available for all devices).
  47
  48Since Open Firmware is commonly used on PowerPC and SPARC platforms,
  49the Linux support for those architectures has for a long time used the
  50Device Tree.
  51
  52In 2005, when PowerPC Linux began a major cleanup and to merge 32-bit
  53and 64-bit support, the decision was made to require DT support on all
  54powerpc platforms, regardless of whether or not they used Open
  55Firmware.  To do this, a DT representation called the Flattened Device
  56Tree (FDT) was created which could be passed to the kernel as a binary
  57blob without requiring a real Open Firmware implementation.  U-Boot,
  58kexec, and other bootloaders were modified to support both passing a
  59Device Tree Binary (dtb) and to modify a dtb at boot time.  DT was
  60also added to the PowerPC boot wrapper (arch/powerpc/boot/*) so that
  61a dtb could be wrapped up with the kernel image to support booting
  62existing non-DT aware firmware.
  63
  64Some time later, FDT infrastructure was generalized to be usable by
  65all architectures.  At the time of this writing, 6 mainlined
  66architectures (arm, microblaze, mips, powerpc, sparc, and x86) and 1
  67out of mainline (nios) have some level of DT support.
  68
  692. Data Model
  70-------------
  71If you haven't already read the Device Tree Usage[1] page,
  72then go read it now.  It's okay, I'll wait....
  73
  742.1 High Level View
  75-------------------
  76The most important thing to understand is that the DT is simply a data
  77structure that describes the hardware.  There is nothing magical about
  78it, and it doesn't magically make all hardware configuration problems
  79go away.  What it does do is provide a language for decoupling the
  80hardware configuration from the board and device driver support in the
  81Linux kernel (or any other operating system for that matter).  Using
  82it allows board and device support to become data driven; to make
  83setup decisions based on data passed into the kernel instead of on
  84per-machine hard coded selections.
  85
  86Ideally, data driven platform setup should result in less code
  87duplication and make it easier to support a wide range of hardware
  88with a single kernel image.
  89
  90Linux uses DT data for three major purposes:
  911) platform identification,
  922) runtime configuration, and
  933) device population.
  94
  952.2 Platform Identification
  96---------------------------
  97First and foremost, the kernel will use data in the DT to identify the
  98specific machine.  In a perfect world, the specific platform shouldn't
  99matter to the kernel because all platform details would be described
 100perfectly by the device tree in a consistent and reliable manner.
 101Hardware is not perfect though, and so the kernel must identify the
 102machine during early boot so that it has the opportunity to run
 103machine-specific fixups.
 104
 105In the majority of cases, the machine identity is irrelevant, and the
 106kernel will instead select setup code based on the machine's core
 107CPU or SoC.  On ARM for example, setup_arch() in
 108arch/arm/kernel/setup.c will call setup_machine_fdt() in
 109arch/arm/kernel/devtree.c which searches through the machine_desc
 110table and selects the machine_desc which best matches the device tree
 111data.  It determines the best match by looking at the 'compatible'
 112property in the root device tree node, and comparing it with the
 113dt_compat list in struct machine_desc (which is defined in
 114arch/arm/include/asm/mach/arch.h if you're curious).
 115
 116The 'compatible' property contains a sorted list of strings starting
 117with the exact name of the machine, followed by an optional list of
 118boards it is compatible with sorted from most compatible to least.  For
 119example, the root compatible properties for the TI BeagleBoard and its
 120successor, the BeagleBoard xM board might look like, respectively:
 121
 122        compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3";
 123        compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3";
 124
 125Where "ti,omap3-beagleboard-xm" specifies the exact model, it also
 126claims that it compatible with the OMAP 3450 SoC, and the omap3 family
 127of SoCs in general.  You'll notice that the list is sorted from most
 128specific (exact board) to least specific (SoC family).
 129
 130Astute readers might point out that the Beagle xM could also claim
 131compatibility with the original Beagle board.  However, one should be
 132cautioned about doing so at the board level since there is typically a
 133high level of change from one board to another, even within the same
 134product line, and it is hard to nail down exactly what is meant when one
 135board claims to be compatible with another.  For the top level, it is
 136better to err on the side of caution and not claim one board is
 137compatible with another.  The notable exception would be when one
 138board is a carrier for another, such as a CPU module attached to a
 139carrier board.
 140
 141One more note on compatible values.  Any string used in a compatible
 142property must be documented as to what it indicates.  Add
 143documentation for compatible strings in Documentation/devicetree/bindings.
 144
 145Again on ARM, for each machine_desc, the kernel looks to see if
 146any of the dt_compat list entries appear in the compatible property.
 147If one does, then that machine_desc is a candidate for driving the
 148machine.  After searching the entire table of machine_descs,
 149setup_machine_fdt() returns the 'most compatible' machine_desc based
 150on which entry in the compatible property each machine_desc matches
 151against.  If no matching machine_desc is found, then it returns NULL.
 152
 153The reasoning behind this scheme is the observation that in the majority
 154of cases, a single machine_desc can support a large number of boards
 155if they all use the same SoC, or same family of SoCs.  However,
 156invariably there will be some exceptions where a specific board will
 157require special setup code that is not useful in the generic case.
 158Special cases could be handled by explicitly checking for the
 159troublesome board(s) in generic setup code, but doing so very quickly
 160becomes ugly and/or unmaintainable if it is more than just a couple of
 161cases.
 162
 163Instead, the compatible list allows a generic machine_desc to provide
 164support for a wide common set of boards by specifying "less
 165compatible" values in the dt_compat list.  In the example above,
 166generic board support can claim compatibility with "ti,omap3" or
 167"ti,omap3450".  If a bug was discovered on the original beagleboard
 168that required special workaround code during early boot, then a new
 169machine_desc could be added which implements the workarounds and only
 170matches on "ti,omap3-beagleboard".
 171
 172PowerPC uses a slightly different scheme where it calls the .probe()
 173hook from each machine_desc, and the first one returning TRUE is used.
 174However, this approach does not take into account the priority of the
 175compatible list, and probably should be avoided for new architecture
 176support.
 177
 1782.3 Runtime configuration
 179-------------------------
 180In most cases, a DT will be the sole method of communicating data from
 181firmware to the kernel, so also gets used to pass in runtime and
 182configuration data like the kernel parameters string and the location
 183of an initrd image.
 184
 185Most of this data is contained in the /chosen node, and when booting
 186Linux it will look something like this:
 187
 188        chosen {
 189                bootargs = "console=ttyS0,115200 loglevel=8";
 190                initrd-start = <0xc8000000>;
 191                initrd-end = <0xc8200000>;
 192        };
 193
 194The bootargs property contains the kernel arguments, and the initrd-*
 195properties define the address and size of an initrd blob.  Note that
 196initrd-end is the first address after the initrd image, so this doesn't
 197match the usual semantic of struct resource.  The chosen node may also
 198optionally contain an arbitrary number of additional properties for
 199platform-specific configuration data.
 200
 201During early boot, the architecture setup code calls of_scan_flat_dt()
 202several times with different helper callbacks to parse device tree
 203data before paging is setup.  The of_scan_flat_dt() code scans through
 204the device tree and uses the helpers to extract information required
 205during early boot.  Typically the early_init_dt_scan_chosen() helper
 206is used to parse the chosen node including kernel parameters,
 207early_init_dt_scan_root() to initialize the DT address space model,
 208and early_init_dt_scan_memory() to determine the size and
 209location of usable RAM.
 210
 211On ARM, the function setup_machine_fdt() is responsible for early
 212scanning of the device tree after selecting the correct machine_desc
 213that supports the board.
 214
 2152.4 Device population
 216---------------------
 217After the board has been identified, and after the early configuration data
 218has been parsed, then kernel initialization can proceed in the normal
 219way.  At some point in this process, unflatten_device_tree() is called
 220to convert the data into a more efficient runtime representation.
 221This is also when machine-specific setup hooks will get called, like
 222the machine_desc .init_early(), .init_irq() and .init_machine() hooks
 223on ARM.  The remainder of this section uses examples from the ARM
 224implementation, but all architectures will do pretty much the same
 225thing when using a DT.
 226
 227As can be guessed by the names, .init_early() is used for any machine-
 228specific setup that needs to be executed early in the boot process,
 229and .init_irq() is used to set up interrupt handling.  Using a DT
 230doesn't materially change the behaviour of either of these functions.
 231If a DT is provided, then both .init_early() and .init_irq() are able
 232to call any of the DT query functions (of_* in include/linux/of*.h) to
 233get additional data about the platform.
 234
 235The most interesting hook in the DT context is .init_machine() which
 236is primarily responsible for populating the Linux device model with
 237data about the platform.  Historically this has been implemented on
 238embedded platforms by defining a set of static clock structures,
 239platform_devices, and other data in the board support .c file, and
 240registering it en-masse in .init_machine().  When DT is used, then
 241instead of hard coding static devices for each platform, the list of
 242devices can be obtained by parsing the DT, and allocating device
 243structures dynamically.
 244
 245The simplest case is when .init_machine() is only responsible for
 246registering a block of platform_devices.  A platform_device is a concept
 247used by Linux for memory or I/O mapped devices which cannot be detected
 248by hardware, and for 'composite' or 'virtual' devices (more on those
 249later).  While there is no 'platform device' terminology for the DT,
 250platform devices roughly correspond to device nodes at the root of the
 251tree and children of simple memory mapped bus nodes.
 252
 253About now is a good time to lay out an example.  Here is part of the
 254device tree for the NVIDIA Tegra board.
 255
 256/{
 257        compatible = "nvidia,harmony", "nvidia,tegra20";
 258        #address-cells = <1>;
 259        #size-cells = <1>;
 260        interrupt-parent = <&intc>;
 261
 262        chosen { };
 263        aliases { };
 264
 265        memory {
 266                device_type = "memory";
 267                reg = <0x00000000 0x40000000>;
 268        };
 269
 270        soc {
 271                compatible = "nvidia,tegra20-soc", "simple-bus";
 272                #address-cells = <1>;
 273                #size-cells = <1>;
 274                ranges;
 275
 276                intc: interrupt-controller@50041000 {
 277                        compatible = "nvidia,tegra20-gic";
 278                        interrupt-controller;
 279                        #interrupt-cells = <1>;
 280                        reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >;
 281                };
 282
 283                serial@70006300 {
 284                        compatible = "nvidia,tegra20-uart";
 285                        reg = <0x70006300 0x100>;
 286                        interrupts = <122>;
 287                };
 288
 289                i2s1: i2s@70002800 {
 290                        compatible = "nvidia,tegra20-i2s";
 291                        reg = <0x70002800 0x100>;
 292                        interrupts = <77>;
 293                        codec = <&wm8903>;
 294                };
 295
 296                i2c@7000c000 {
 297                        compatible = "nvidia,tegra20-i2c";
 298                        #address-cells = <1>;
 299                        #size-cells = <0>;
 300                        reg = <0x7000c000 0x100>;
 301                        interrupts = <70>;
 302
 303                        wm8903: codec@1a {
 304                                compatible = "wlf,wm8903";
 305                                reg = <0x1a>;
 306                                interrupts = <347>;
 307                        };
 308                };
 309        };
 310
 311        sound {
 312                compatible = "nvidia,harmony-sound";
 313                i2s-controller = <&i2s1>;
 314                i2s-codec = <&wm8903>;
 315        };
 316};
 317
 318At .init_machine() time, Tegra board support code will need to look at
 319this DT and decide which nodes to create platform_devices for.
 320However, looking at the tree, it is not immediately obvious what kind
 321of device each node represents, or even if a node represents a device
 322at all.  The /chosen, /aliases, and /memory nodes are informational
 323nodes that don't describe devices (although arguably memory could be
 324considered a device).  The children of the /soc node are memory mapped
 325devices, but the codec@1a is an i2c device, and the sound node
 326represents not a device, but rather how other devices are connected
 327together to create the audio subsystem.  I know what each device is
 328because I'm familiar with the board design, but how does the kernel
 329know what to do with each node?
 330
 331The trick is that the kernel starts at the root of the tree and looks
 332for nodes that have a 'compatible' property.  First, it is generally
 333assumed that any node with a 'compatible' property represents a device
 334of some kind, and second, it can be assumed that any node at the root
 335of the tree is either directly attached to the processor bus, or is a
 336miscellaneous system device that cannot be described any other way.
 337For each of these nodes, Linux allocates and registers a
 338platform_device, which in turn may get bound to a platform_driver.
 339
 340Why is using a platform_device for these nodes a safe assumption?
 341Well, for the way that Linux models devices, just about all bus_types
 342assume that its devices are children of a bus controller.  For
 343example, each i2c_client is a child of an i2c_master.  Each spi_device
 344is a child of an SPI bus.  Similarly for USB, PCI, MDIO, etc.  The
 345same hierarchy is also found in the DT, where I2C device nodes only
 346ever appear as children of an I2C bus node.  Ditto for SPI, MDIO, USB,
 347etc.  The only devices which do not require a specific type of parent
 348device are platform_devices (and amba_devices, but more on that
 349later), which will happily live at the base of the Linux /sys/devices
 350tree.  Therefore, if a DT node is at the root of the tree, then it
 351really probably is best registered as a platform_device.
 352
 353Linux board support code calls of_platform_populate(NULL, NULL, NULL, NULL)
 354to kick off discovery of devices at the root of the tree.  The
 355parameters are all NULL because when starting from the root of the
 356tree, there is no need to provide a starting node (the first NULL), a
 357parent struct device (the last NULL), and we're not using a match
 358table (yet).  For a board that only needs to register devices,
 359.init_machine() can be completely empty except for the
 360of_platform_populate() call.
 361
 362In the Tegra example, this accounts for the /soc and /sound nodes, but
 363what about the children of the SoC node?  Shouldn't they be registered
 364as platform devices too?  For Linux DT support, the generic behaviour
 365is for child devices to be registered by the parent's device driver at
 366driver .probe() time.  So, an i2c bus device driver will register a
 367i2c_client for each child node, an SPI bus driver will register
 368its spi_device children, and similarly for other bus_types.
 369According to that model, a driver could be written that binds to the
 370SoC node and simply registers platform_devices for each of its
 371children.  The board support code would allocate and register an SoC
 372device, a (theoretical) SoC device driver could bind to the SoC device,
 373and register platform_devices for /soc/interrupt-controller, /soc/serial,
 374/soc/i2s, and /soc/i2c in its .probe() hook.  Easy, right?
 375
 376Actually, it turns out that registering children of some
 377platform_devices as more platform_devices is a common pattern, and the
 378device tree support code reflects that and makes the above example
 379simpler.  The second argument to of_platform_populate() is an
 380of_device_id table, and any node that matches an entry in that table
 381will also get its child nodes registered.  In the Tegra case, the code
 382can look something like this:
 383
 384static void __init harmony_init_machine(void)
 385{
 386        /* ... */
 387        of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
 388}
 389
 390"simple-bus" is defined in the ePAPR 1.0 specification as a property
 391meaning a simple memory mapped bus, so the of_platform_populate() code
 392could be written to just assume simple-bus compatible nodes will
 393always be traversed.  However, we pass it in as an argument so that
 394board support code can always override the default behaviour.
 395
 396[Need to add discussion of adding i2c/spi/etc child devices]
 397
 398Appendix A: AMBA devices
 399------------------------
 400
 401ARM Primecells are a certain kind of device attached to the ARM AMBA
 402bus which include some support for hardware detection and power
 403management.  In Linux, struct amba_device and the amba_bus_type is
 404used to represent Primecell devices.  However, the fiddly bit is that
 405not all devices on an AMBA bus are Primecells, and for Linux it is
 406typical for both amba_device and platform_device instances to be
 407siblings of the same bus segment.
 408
 409When using the DT, this creates problems for of_platform_populate()
 410because it must decide whether to register each node as either a
 411platform_device or an amba_device.  This unfortunately complicates the
 412device creation model a little bit, but the solution turns out not to
 413be too invasive.  If a node is compatible with "arm,amba-primecell", then
 414of_platform_populate() will register it as an amba_device instead of a
 415platform_device.
 416