416 lines
19 KiB
Plaintext
416 lines
19 KiB
Plaintext
Linux and the Device Tree
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-------------------------
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The Linux usage model for device tree data
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Author: Grant Likely <grant.likely@secretlab.ca>
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This article describes how Linux uses the device tree. An overview of
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the device tree data format can be found on the device tree usage page
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at devicetree.org[1].
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[1] http://devicetree.org/Device_Tree_Usage
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The "Open Firmware Device Tree", or simply Device Tree (DT), is a data
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structure and language for describing hardware. More specifically, it
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is a description of hardware that is readable by an operating system
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so that the operating system doesn't need to hard code details of the
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machine.
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Structurally, the DT is a tree, or acyclic graph with named nodes, and
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nodes may have an arbitrary number of named properties encapsulating
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arbitrary data. A mechanism also exists to create arbitrary
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links from one node to another outside of the natural tree structure.
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Conceptually, a common set of usage conventions, called 'bindings',
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is defined for how data should appear in the tree to describe typical
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hardware characteristics including data busses, interrupt lines, GPIO
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connections, and peripheral devices.
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As much as possible, hardware is described using existing bindings to
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maximize use of existing support code, but since property and node
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names are simply text strings, it is easy to extend existing bindings
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or create new ones by defining new nodes and properties. Be wary,
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however, of creating a new binding without first doing some homework
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about what already exists. There are currently two different,
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incompatible, bindings for i2c busses that came about because the new
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binding was created without first investigating how i2c devices were
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already being enumerated in existing systems.
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1. History
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----------
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The DT was originally created by Open Firmware as part of the
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communication method for passing data from Open Firmware to a client
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program (like to an operating system). An operating system used the
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Device Tree to discover the topology of the hardware at runtime, and
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thereby support a majority of available hardware without hard coded
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information (assuming drivers were available for all devices).
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Since Open Firmware is commonly used on PowerPC and SPARC platforms,
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the Linux support for those architectures has for a long time used the
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Device Tree.
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In 2005, when PowerPC Linux began a major cleanup and to merge 32-bit
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and 64-bit support, the decision was made to require DT support on all
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powerpc platforms, regardless of whether or not they used Open
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Firmware. To do this, a DT representation called the Flattened Device
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Tree (FDT) was created which could be passed to the kernel as a binary
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blob without requiring a real Open Firmware implementation. U-Boot,
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kexec, and other bootloaders were modified to support both passing a
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Device Tree Binary (dtb) and to modify a dtb at boot time. DT was
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also added to the PowerPC boot wrapper (arch/powerpc/boot/*) so that
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a dtb could be wrapped up with the kernel image to support booting
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existing non-DT aware firmware.
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Some time later, FDT infrastructure was generalized to be usable by
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all architectures. At the time of this writing, 6 mainlined
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architectures (arm, microblaze, mips, powerpc, sparc, and x86) and 1
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out of mainline (nios) have some level of DT support.
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2. Data Model
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-------------
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If you haven't already read the Device Tree Usage[1] page,
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then go read it now. It's okay, I'll wait....
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2.1 High Level View
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-------------------
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The most important thing to understand is that the DT is simply a data
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structure that describes the hardware. There is nothing magical about
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it, and it doesn't magically make all hardware configuration problems
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go away. What it does do is provide a language for decoupling the
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hardware configuration from the board and device driver support in the
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Linux kernel (or any other operating system for that matter). Using
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it allows board and device support to become data driven; to make
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setup decisions based on data passed into the kernel instead of on
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per-machine hard coded selections.
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Ideally, data driven platform setup should result in less code
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duplication and make it easier to support a wide range of hardware
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with a single kernel image.
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Linux uses DT data for three major purposes:
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1) platform identification,
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2) runtime configuration, and
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3) device population.
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2.2 Platform Identification
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---------------------------
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First and foremost, the kernel will use data in the DT to identify the
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specific machine. In a perfect world, the specific platform shouldn't
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matter to the kernel because all platform details would be described
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perfectly by the device tree in a consistent and reliable manner.
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Hardware is not perfect though, and so the kernel must identify the
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machine during early boot so that it has the opportunity to run
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machine-specific fixups.
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In the majority of cases, the machine identity is irrelevant, and the
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kernel will instead select setup code based on the machine's core
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CPU or SoC. On ARM for example, setup_arch() in
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arch/arm/kernel/setup.c will call setup_machine_fdt() in
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arch/arm/kernel/devtree.c which searches through the machine_desc
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table and selects the machine_desc which best matches the device tree
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data. It determines the best match by looking at the 'compatible'
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property in the root device tree node, and comparing it with the
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dt_compat list in struct machine_desc (which is defined in
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arch/arm/include/asm/mach/arch.h if you're curious).
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The 'compatible' property contains a sorted list of strings starting
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with the exact name of the machine, followed by an optional list of
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boards it is compatible with sorted from most compatible to least. For
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example, the root compatible properties for the TI BeagleBoard and its
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successor, the BeagleBoard xM board might look like, respectively:
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compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3";
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compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3";
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Where "ti,omap3-beagleboard-xm" specifies the exact model, it also
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claims that it compatible with the OMAP 3450 SoC, and the omap3 family
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of SoCs in general. You'll notice that the list is sorted from most
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specific (exact board) to least specific (SoC family).
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Astute readers might point out that the Beagle xM could also claim
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compatibility with the original Beagle board. However, one should be
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cautioned about doing so at the board level since there is typically a
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high level of change from one board to another, even within the same
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product line, and it is hard to nail down exactly what is meant when one
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board claims to be compatible with another. For the top level, it is
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better to err on the side of caution and not claim one board is
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compatible with another. The notable exception would be when one
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board is a carrier for another, such as a CPU module attached to a
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carrier board.
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One more note on compatible values. Any string used in a compatible
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property must be documented as to what it indicates. Add
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documentation for compatible strings in Documentation/devicetree/bindings.
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Again on ARM, for each machine_desc, the kernel looks to see if
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any of the dt_compat list entries appear in the compatible property.
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If one does, then that machine_desc is a candidate for driving the
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machine. After searching the entire table of machine_descs,
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setup_machine_fdt() returns the 'most compatible' machine_desc based
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on which entry in the compatible property each machine_desc matches
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against. If no matching machine_desc is found, then it returns NULL.
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The reasoning behind this scheme is the observation that in the majority
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of cases, a single machine_desc can support a large number of boards
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if they all use the same SoC, or same family of SoCs. However,
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invariably there will be some exceptions where a specific board will
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require special setup code that is not useful in the generic case.
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Special cases could be handled by explicitly checking for the
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troublesome board(s) in generic setup code, but doing so very quickly
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becomes ugly and/or unmaintainable if it is more than just a couple of
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cases.
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Instead, the compatible list allows a generic machine_desc to provide
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support for a wide common set of boards by specifying "less
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compatible" values in the dt_compat list. In the example above,
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generic board support can claim compatibility with "ti,omap3" or
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"ti,omap3450". If a bug was discovered on the original beagleboard
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that required special workaround code during early boot, then a new
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machine_desc could be added which implements the workarounds and only
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matches on "ti,omap3-beagleboard".
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PowerPC uses a slightly different scheme where it calls the .probe()
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hook from each machine_desc, and the first one returning TRUE is used.
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However, this approach does not take into account the priority of the
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compatible list, and probably should be avoided for new architecture
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support.
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2.3 Runtime configuration
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-------------------------
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In most cases, a DT will be the sole method of communicating data from
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firmware to the kernel, so also gets used to pass in runtime and
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configuration data like the kernel parameters string and the location
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of an initrd image.
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Most of this data is contained in the /chosen node, and when booting
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Linux it will look something like this:
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chosen {
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bootargs = "console=ttyS0,115200 loglevel=8";
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initrd-start = <0xc8000000>;
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initrd-end = <0xc8200000>;
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};
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The bootargs property contains the kernel arguments, and the initrd-*
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properties define the address and size of an initrd blob. Note that
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initrd-end is the first address after the initrd image, so this doesn't
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match the usual semantic of struct resource. The chosen node may also
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optionally contain an arbitrary number of additional properties for
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platform-specific configuration data.
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During early boot, the architecture setup code calls of_scan_flat_dt()
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several times with different helper callbacks to parse device tree
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data before paging is setup. The of_scan_flat_dt() code scans through
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the device tree and uses the helpers to extract information required
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during early boot. Typically the early_init_dt_scan_chosen() helper
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is used to parse the chosen node including kernel parameters,
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early_init_dt_scan_root() to initialize the DT address space model,
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and early_init_dt_scan_memory() to determine the size and
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location of usable RAM.
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On ARM, the function setup_machine_fdt() is responsible for early
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scanning of the device tree after selecting the correct machine_desc
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that supports the board.
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2.4 Device population
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---------------------
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After the board has been identified, and after the early configuration data
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has been parsed, then kernel initialization can proceed in the normal
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way. At some point in this process, unflatten_device_tree() is called
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to convert the data into a more efficient runtime representation.
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This is also when machine-specific setup hooks will get called, like
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the machine_desc .init_early(), .init_irq() and .init_machine() hooks
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on ARM. The remainder of this section uses examples from the ARM
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implementation, but all architectures will do pretty much the same
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thing when using a DT.
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As can be guessed by the names, .init_early() is used for any machine-
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specific setup that needs to be executed early in the boot process,
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and .init_irq() is used to set up interrupt handling. Using a DT
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doesn't materially change the behaviour of either of these functions.
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If a DT is provided, then both .init_early() and .init_irq() are able
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to call any of the DT query functions (of_* in include/linux/of*.h) to
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get additional data about the platform.
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The most interesting hook in the DT context is .init_machine() which
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is primarily responsible for populating the Linux device model with
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data about the platform. Historically this has been implemented on
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embedded platforms by defining a set of static clock structures,
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platform_devices, and other data in the board support .c file, and
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registering it en-masse in .init_machine(). When DT is used, then
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instead of hard coding static devices for each platform, the list of
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devices can be obtained by parsing the DT, and allocating device
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structures dynamically.
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The simplest case is when .init_machine() is only responsible for
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registering a block of platform_devices. A platform_device is a concept
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used by Linux for memory or I/O mapped devices which cannot be detected
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by hardware, and for 'composite' or 'virtual' devices (more on those
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later). While there is no 'platform device' terminology for the DT,
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platform devices roughly correspond to device nodes at the root of the
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tree and children of simple memory mapped bus nodes.
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About now is a good time to lay out an example. Here is part of the
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device tree for the NVIDIA Tegra board.
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/{
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compatible = "nvidia,harmony", "nvidia,tegra20";
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#address-cells = <1>;
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#size-cells = <1>;
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interrupt-parent = <&intc>;
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chosen { };
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aliases { };
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memory {
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device_type = "memory";
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reg = <0x00000000 0x40000000>;
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};
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soc {
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compatible = "nvidia,tegra20-soc", "simple-bus";
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#address-cells = <1>;
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#size-cells = <1>;
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ranges;
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intc: interrupt-controller@50041000 {
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compatible = "nvidia,tegra20-gic";
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interrupt-controller;
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#interrupt-cells = <1>;
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reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >;
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};
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serial@70006300 {
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compatible = "nvidia,tegra20-uart";
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reg = <0x70006300 0x100>;
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interrupts = <122>;
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};
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i2s1: i2s@70002800 {
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compatible = "nvidia,tegra20-i2s";
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reg = <0x70002800 0x100>;
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interrupts = <77>;
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codec = <&wm8903>;
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};
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i2c@7000c000 {
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compatible = "nvidia,tegra20-i2c";
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#address-cells = <1>;
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#size-cells = <0>;
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reg = <0x7000c000 0x100>;
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interrupts = <70>;
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wm8903: codec@1a {
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compatible = "wlf,wm8903";
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reg = <0x1a>;
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interrupts = <347>;
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};
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};
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};
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sound {
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compatible = "nvidia,harmony-sound";
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i2s-controller = <&i2s1>;
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i2s-codec = <&wm8903>;
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};
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};
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At .init_machine() time, Tegra board support code will need to look at
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this DT and decide which nodes to create platform_devices for.
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However, looking at the tree, it is not immediately obvious what kind
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of device each node represents, or even if a node represents a device
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at all. The /chosen, /aliases, and /memory nodes are informational
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nodes that don't describe devices (although arguably memory could be
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considered a device). The children of the /soc node are memory mapped
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devices, but the codec@1a is an i2c device, and the sound node
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represents not a device, but rather how other devices are connected
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together to create the audio subsystem. I know what each device is
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because I'm familiar with the board design, but how does the kernel
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know what to do with each node?
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The trick is that the kernel starts at the root of the tree and looks
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for nodes that have a 'compatible' property. First, it is generally
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assumed that any node with a 'compatible' property represents a device
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of some kind, and second, it can be assumed that any node at the root
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of the tree is either directly attached to the processor bus, or is a
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miscellaneous system device that cannot be described any other way.
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For each of these nodes, Linux allocates and registers a
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platform_device, which in turn may get bound to a platform_driver.
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Why is using a platform_device for these nodes a safe assumption?
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Well, for the way that Linux models devices, just about all bus_types
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assume that its devices are children of a bus controller. For
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example, each i2c_client is a child of an i2c_master. Each spi_device
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is a child of an SPI bus. Similarly for USB, PCI, MDIO, etc. The
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same hierarchy is also found in the DT, where I2C device nodes only
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ever appear as children of an I2C bus node. Ditto for SPI, MDIO, USB,
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etc. The only devices which do not require a specific type of parent
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device are platform_devices (and amba_devices, but more on that
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later), which will happily live at the base of the Linux /sys/devices
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tree. Therefore, if a DT node is at the root of the tree, then it
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really probably is best registered as a platform_device.
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Linux board support code calls of_platform_populate(NULL, NULL, NULL, NULL)
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to kick off discovery of devices at the root of the tree. The
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parameters are all NULL because when starting from the root of the
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tree, there is no need to provide a starting node (the first NULL), a
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parent struct device (the last NULL), and we're not using a match
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table (yet). For a board that only needs to register devices,
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.init_machine() can be completely empty except for the
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of_platform_populate() call.
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In the Tegra example, this accounts for the /soc and /sound nodes, but
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what about the children of the SoC node? Shouldn't they be registered
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as platform devices too? For Linux DT support, the generic behaviour
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is for child devices to be registered by the parent's device driver at
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driver .probe() time. So, an i2c bus device driver will register a
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i2c_client for each child node, an SPI bus driver will register
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its spi_device children, and similarly for other bus_types.
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According to that model, a driver could be written that binds to the
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SoC node and simply registers platform_devices for each of its
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children. The board support code would allocate and register an SoC
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device, a (theoretical) SoC device driver could bind to the SoC device,
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and register platform_devices for /soc/interrupt-controller, /soc/serial,
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/soc/i2s, and /soc/i2c in its .probe() hook. Easy, right?
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Actually, it turns out that registering children of some
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platform_devices as more platform_devices is a common pattern, and the
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device tree support code reflects that and makes the above example
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simpler. The second argument to of_platform_populate() is an
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of_device_id table, and any node that matches an entry in that table
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will also get its child nodes registered. In the Tegra case, the code
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can look something like this:
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static void __init harmony_init_machine(void)
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{
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/* ... */
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of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
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}
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"simple-bus" is defined in the ePAPR 1.0 specification as a property
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meaning a simple memory mapped bus, so the of_platform_populate() code
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could be written to just assume simple-bus compatible nodes will
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always be traversed. However, we pass it in as an argument so that
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board support code can always override the default behaviour.
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[Need to add discussion of adding i2c/spi/etc child devices]
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Appendix A: AMBA devices
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------------------------
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ARM Primecells are a certain kind of device attached to the ARM AMBA
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bus which include some support for hardware detection and power
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management. In Linux, struct amba_device and the amba_bus_type is
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used to represent Primecell devices. However, the fiddly bit is that
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not all devices on an AMBA bus are Primecells, and for Linux it is
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typical for both amba_device and platform_device instances to be
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siblings of the same bus segment.
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When using the DT, this creates problems for of_platform_populate()
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because it must decide whether to register each node as either a
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platform_device or an amba_device. This unfortunately complicates the
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device creation model a little bit, but the solution turns out not to
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be too invasive. If a node is compatible with "arm,amba-primecell", then
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of_platform_populate() will register it as an amba_device instead of a
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platform_device.
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