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Grant Likely31134ef2011-11-04 11:51:22 -04001Linux 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/devicetree.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.
114
115The 'compatible' property contains a sorted list of strings starting
116with the exact name of the machine, followed by an optional list of
117boards it is compatible with sorted from most compatible to least. For
118example, the root compatible properties for the TI BeagleBoard and its
119successor, the BeagleBoard xM board might look like:
120
121 compatible = "ti,omap3-beagleboard", "ti,omap3450", "ti,omap3";
122 compatible = "ti,omap3-beagleboard-xm", "ti,omap3450", "ti,omap3";
123
124Where "ti,omap3-beagleboard-xm" specifies the exact model, it also
125claims that it compatible with the OMAP 3450 SoC, and the omap3 family
126of SoCs in general. You'll notice that the list is sorted from most
127specific (exact board) to least specific (SoC family).
128
129Astute readers might point out that the Beagle xM could also claim
130compatibility with the original Beagle board. However, one should be
131cautioned about doing so at the board level since there is typically a
132high level of change from one board to another, even within the same
133product line, and it is hard to nail down exactly what is meant when one
134board claims to be compatible with another. For the top level, it is
135better to err on the side of caution and not claim one board is
136compatible with another. The notable exception would be when one
137board is a carrier for another, such as a CPU module attached to a
138carrier board.
139
140One more note on compatible values. Any string used in a compatible
141property must be documented as to what it indicates. Add
142documentation for compatible strings in Documentation/devicetree/bindings.
143
144Again on ARM, for each machine_desc, the kernel looks to see if
145any of the dt_compat list entries appear in the compatible property.
146If one does, then that machine_desc is a candidate for driving the
147machine. After searching the entire table of machine_descs,
148setup_machine_fdt() returns the 'most compatible' machine_desc based
149on which entry in the compatible property each machine_desc matches
150against. If no matching machine_desc is found, then it returns NULL.
151
152The reasoning behind this scheme is the observation that in the majority
153of cases, a single machine_desc can support a large number of boards
154if they all use the same SoC, or same family of SoCs. However,
155invariably there will be some exceptions where a specific board will
156require special setup code that is not useful in the generic case.
157Special cases could be handled by explicitly checking for the
158troublesome board(s) in generic setup code, but doing so very quickly
159becomes ugly and/or unmaintainable if it is more than just a couple of
160cases.
161
162Instead, the compatible list allows a generic machine_desc to provide
163support for a wide common set of boards by specifying "less
164compatible" value in the dt_compat list. In the example above,
165generic board support can claim compatibility with "ti,omap3" or
166"ti,omap3450". If a bug was discovered on the original beagleboard
167that required special workaround code during early boot, then a new
168machine_desc could be added which implements the workarounds and only
169matches on "ti,omap3-beagleboard".
170
171PowerPC uses a slightly different scheme where it calls the .probe()
172hook from each machine_desc, and the first one returning TRUE is used.
173However, this approach does not take into account the priority of the
174compatible list, and probably should be avoided for new architecture
175support.
176
1772.3 Runtime configuration
178-------------------------
179In most cases, a DT will be the sole method of communicating data from
180firmware to the kernel, so also gets used to pass in runtime and
181configuration data like the kernel parameters string and the location
182of an initrd image.
183
184Most of this data is contained in the /chosen node, and when booting
185Linux it will look something like this:
186
187 chosen {
188 bootargs = "console=ttyS0,115200 loglevel=8";
189 initrd-start = <0xc8000000>;
190 initrd-end = <0xc8200000>;
191 };
192
193The bootargs property contains the kernel arguments, and the initrd-*
194properties define the address and size of an initrd blob. The
195chosen node may also optionally contain an arbitrary number of
196additional properties for platform-specific configuration data.
197
198During early boot, the architecture setup code calls of_scan_flat_dt()
199several times with different helper callbacks to parse device tree
200data before paging is setup. The of_scan_flat_dt() code scans through
201the device tree and uses the helpers to extract information required
202during early boot. Typically the early_init_dt_scan_chosen() helper
203is used to parse the chosen node including kernel parameters,
204early_init_dt_scan_root() to initialize the DT address space model,
205and early_init_dt_scan_memory() to determine the size and
206location of usable RAM.
207
208On ARM, the function setup_machine_fdt() is responsible for early
209scanning of the device tree after selecting the correct machine_desc
210that supports the board.
211
2122.4 Device population
213---------------------
214After the board has been identified, and after the early configuration data
215has been parsed, then kernel initialization can proceed in the normal
216way. At some point in this process, unflatten_device_tree() is called
217to convert the data into a more efficient runtime representation.
218This is also when machine-specific setup hooks will get called, like
219the machine_desc .init_early(), .init_irq() and .init_machine() hooks
220on ARM. The remainder of this section uses examples from the ARM
221implementation, but all architectures will do pretty much the same
222thing when using a DT.
223
224As can be guessed by the names, .init_early() is used for any machine-
225specific setup that needs to be executed early in the boot process,
226and .init_irq() is used to set up interrupt handling. Using a DT
227doesn't materially change the behaviour of either of these functions.
228If a DT is provided, then both .init_early() and .init_irq() are able
229to call any of the DT query functions (of_* in include/linux/of*.h) to
230get additional data about the platform.
231
232The most interesting hook in the DT context is .init_machine() which
233is primarily responsible for populating the Linux device model with
234data about the platform. Historically this has been implemented on
235embedded platforms by defining a set of static clock structures,
236platform_devices, and other data in the board support .c file, and
237registering it en-masse in .init_machine(). When DT is used, then
238instead of hard coding static devices for each platform, the list of
239devices can be obtained by parsing the DT, and allocating device
240structures dynamically.
241
242The simplest case is when .init_machine() is only responsible for
243registering a block of platform_devices. A platform_device is a concept
244used by Linux for memory or I/O mapped devices which cannot be detected
245by hardware, and for 'composite' or 'virtual' devices (more on those
246later). While there is no 'platform device' terminology for the DT,
247platform devices roughly correspond to device nodes at the root of the
248tree and children of simple memory mapped bus nodes.
249
250About now is a good time to lay out an example. Here is part of the
251device tree for the NVIDIA Tegra board.
252
253/{
254 compatible = "nvidia,harmony", "nvidia,tegra20";
255 #address-cells = <1>;
256 #size-cells = <1>;
257 interrupt-parent = <&intc>;
258
259 chosen { };
260 aliases { };
261
262 memory {
263 device_type = "memory";
264 reg = <0x00000000 0x40000000>;
265 };
266
267 soc {
268 compatible = "nvidia,tegra20-soc", "simple-bus";
269 #address-cells = <1>;
270 #size-cells = <1>;
271 ranges;
272
273 intc: interrupt-controller@50041000 {
274 compatible = "nvidia,tegra20-gic";
275 interrupt-controller;
276 #interrupt-cells = <1>;
277 reg = <0x50041000 0x1000>, < 0x50040100 0x0100 >;
278 };
279
280 serial@70006300 {
281 compatible = "nvidia,tegra20-uart";
282 reg = <0x70006300 0x100>;
283 interrupts = <122>;
284 };
285
286 i2s1: i2s@70002800 {
287 compatible = "nvidia,tegra20-i2s";
288 reg = <0x70002800 0x100>;
289 interrupts = <77>;
290 codec = <&wm8903>;
291 };
292
293 i2c@7000c000 {
294 compatible = "nvidia,tegra20-i2c";
295 #address-cells = <1>;
296 #size-cells = <0>;
297 reg = <0x7000c000 0x100>;
298 interrupts = <70>;
299
300 wm8903: codec@1a {
301 compatible = "wlf,wm8903";
302 reg = <0x1a>;
303 interrupts = <347>;
304 };
305 };
306 };
307
308 sound {
309 compatible = "nvidia,harmony-sound";
310 i2s-controller = <&i2s1>;
311 i2s-codec = <&wm8903>;
312 };
313};
314
315At .machine_init() time, Tegra board support code will need to look at
316this DT and decide which nodes to create platform_devices for.
317However, looking at the tree, it is not immediately obvious what kind
318of device each node represents, or even if a node represents a device
319at all. The /chosen, /aliases, and /memory nodes are informational
320nodes that don't describe devices (although arguably memory could be
321considered a device). The children of the /soc node are memory mapped
322devices, but the codec@1a is an i2c device, and the sound node
323represents not a device, but rather how other devices are connected
324together to create the audio subsystem. I know what each device is
325because I'm familiar with the board design, but how does the kernel
326know what to do with each node?
327
328The trick is that the kernel starts at the root of the tree and looks
329for nodes that have a 'compatible' property. First, it is generally
330assumed that any node with a 'compatible' property represents a device
331of some kind, and second, it can be assumed that any node at the root
332of the tree is either directly attached to the processor bus, or is a
333miscellaneous system device that cannot be described any other way.
334For each of these nodes, Linux allocates and registers a
335platform_device, which in turn may get bound to a platform_driver.
336
337Why is using a platform_device for these nodes a safe assumption?
338Well, for the way that Linux models devices, just about all bus_types
339assume that its devices are children of a bus controller. For
340example, each i2c_client is a child of an i2c_master. Each spi_device
341is a child of an SPI bus. Similarly for USB, PCI, MDIO, etc. The
342same hierarchy is also found in the DT, where I2C device nodes only
343ever appear as children of an I2C bus node. Ditto for SPI, MDIO, USB,
344etc. The only devices which do not require a specific type of parent
345device are platform_devices (and amba_devices, but more on that
346later), which will happily live at the base of the Linux /sys/devices
347tree. Therefore, if a DT node is at the root of the tree, then it
348really probably is best registered as a platform_device.
349
350Linux board support code calls of_platform_populate(NULL, NULL, NULL)
351to kick off discovery of devices at the root of the tree. The
352parameters are all NULL because when starting from the root of the
353tree, there is no need to provide a starting node (the first NULL), a
354parent struct device (the last NULL), and we're not using a match
355table (yet). For a board that only needs to register devices,
356.init_machine() can be completely empty except for the
357of_platform_populate() call.
358
359In the Tegra example, this accounts for the /soc and /sound nodes, but
360what about the children of the SoC node? Shouldn't they be registered
361as platform devices too? For Linux DT support, the generic behaviour
362is for child devices to be registered by the parent's device driver at
363driver .probe() time. So, an i2c bus device driver will register a
364i2c_client for each child node, an SPI bus driver will register
365its spi_device children, and similarly for other bus_types.
366According to that model, a driver could be written that binds to the
367SoC node and simply registers platform_devices for each of its
368children. The board support code would allocate and register an SoC
369device, a (theoretical) SoC device driver could bind to the SoC device,
370and register platform_devices for /soc/interrupt-controller, /soc/serial,
371/soc/i2s, and /soc/i2c in its .probe() hook. Easy, right?
372
373Actually, it turns out that registering children of some
374platform_devices as more platform_devices is a common pattern, and the
375device tree support code reflects that and makes the above example
376simpler. The second argument to of_platform_populate() is an
377of_device_id table, and any node that matches an entry in that table
378will also get its child nodes registered. In the tegra case, the code
379can look something like this:
380
381static void __init harmony_init_machine(void)
382{
383 /* ... */
384 of_platform_populate(NULL, of_default_bus_match_table, NULL, NULL);
385}
386
387"simple-bus" is defined in the ePAPR 1.0 specification as a property
388meaning a simple memory mapped bus, so the of_platform_populate() code
389could be written to just assume simple-bus compatible nodes will
390always be traversed. However, we pass it in as an argument so that
391board support code can always override the default behaviour.
392
393[Need to add discussion of adding i2c/spi/etc child devices]
394
395Appendix A: AMBA devices
396------------------------
397
398ARM Primecells are a certain kind of device attached to the ARM AMBA
399bus which include some support for hardware detection and power
400management. In Linux, struct amba_device and the amba_bus_type is
401used to represent Primecell devices. However, the fiddly bit is that
402not all devices on an AMBA bus are Primecells, and for Linux it is
403typical for both amba_device and platform_device instances to be
404siblings of the same bus segment.
405
406When using the DT, this creates problems for of_platform_populate()
407because it must decide whether to register each node as either a
408platform_device or an amba_device. This unfortunately complicates the
409device creation model a little bit, but the solution turns out not to
410be too invasive. If a node is compatible with "arm,amba-primecell", then
411of_platform_populate() will register it as an amba_device instead of a
412platform_device.