dt: Linux DT usage model documentation

v2: 2nd draft
 - Editorial cleanups (Randy Dunlap and Stephen Warren)
 - Added missing Microblaze reference (Stephen Neuendorffer)
 - Make example of platform_device creation clearer (Shawn Guo)
 - Expand on PowerPC history and mention i2c mess (David Gibson)
 - convert to plain text (remove bits of html formating)

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

1 files changed, 412 insertions, 0 deletions

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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.