Hi !
Here's revision 3 of the spec for the booting of linux/ppc64 with a flattened device-tree. The novelty is that I added a new more compact format. A followup mail will have the kernel patches to add support to this new format, I'll submit them upstream for after 2.6.12 I think.
David and I are still working on sample code & tools. We have a prototype of a device-tree "compiler" that can build the flattened blob from a textual representation. We'll release that soon, hopefully this week.
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Booting the Linux/ppc64 kernel without Open Firmware ----------------------------------------------------
(c) 2005 Benjamin Herrenschmidt benh@kernel.crashing.org, IBM Corp.
May 18, 2005: Rev 0.1 - Initial draft, no chapter III yet.
May 19, 2005: Rev 0.2 - Add chapter III and bits & pieces here or clarifies the fact that a lot of things are optional, the kernel only requires a very small device tree, though it is encouraged to provide an as complete one as possible.
May 24, 2005: Rev 0.3 - Precise that DT block has to be in RAM - Misc fixes - Define version 3 and new format version 16 for the DT block (version 16 needs kernel patches, will be fwd separately). String block now has a size, and full path is replaced by unit name for more compactness. linux,phandle is made optional, only nodes that are referenced by other nodes need it. "name" property is now automatically deduced from the unit name
ToDo:
- Add some definitions of interrupt tree (simple/complex) - Add some definitions for pci host bridges
I- Introduction ===============
During the recent developpements of the Linux/ppc64 kernel, and more specifically, the addition of new platform types outside of the old IBM pSeries/iSeries pair, it was decided to enforce some strict rules regarding the kernel entry and bootloader <-> kernel interfaces, in order to avoid the degeneration that has become the ppc32 kernel entry point and the way a new platform should be added to the kernel. The legacy iSeries platform breaks those rules as it predates this scheme, but no new board support will be accepted in the main tree that doesn't follows them properly.
The main requirement that will be defined in more details below is the presence of a device-tree whose format is defined after Open Firmware specification. However, in order to make life easier to embedded board vendors, the kernel doesn't require the device-tree to represent every device in the system and only requires some nodes and properties to be present. This will be described in details in section III, but, for example, the kernel does not require you to create a node for every PCI device in the system. It is a requirement to have a node for PCI host bridges in order to provide interrupt routing informations and memory/IO ranges, among others. It is also recommended to define nodes for on chip devices and other busses that doesn't specifically fit in an existing OF specification, like on chip devices, this creates a great flexibility in the way the kernel can them probe those and match drivers to device, without having to hard code all sorts of tables. It also makes it more flexible for board vendors to do minor hardware upgrades without impacting significantly the kernel code or cluttering it with special cases.
1) Entry point --------------
There is one and one single entry point to the kernel, at the start of the kernel image. That entry point support two calling conventions:
a) Boot from Open Firmware. If your firmware is compatible with Open Firmware (IEEE 1275) or provides an OF compatible client interface API (support for "interpret" callback of forth words isn't required), you can enter the kernel with:
r5 : OF callback pointer as defined by IEEE 1275 bindings to powerpc. Only the 32 bits client interface is currently supported
r3, r4 : address & lenght of an initrd if any or 0
MMU is either on or off, the kernel will run the trampoline located in arch/ppc64/kernel/prom_init.c to extract the device-tree and other informations from open firmware and build a flattened device-tree as described in b). prom_init() will then re-enter the kernel using the second method. This trampoline code runs in the context of the firmware, which is supposed to handle all exceptions during that time.
b) Direct entry with a flattened device-tree block. This entry point is called by a) after the OF trampoline and can also be called directly by a bootloader that does not support the Open Firmware client interface. It is also used by "kexec" to implement "hot" booting of a new kernel from a previous running one. This method is what I will describe in more details in this document, as method a) is simply standard Open Firmware, and thus should be implemented according to the various standard documents defining it and it's binding to the PowerPC platform. The entry point definition then becomes:
r3 : physical pointer to the device-tree block (defined in chapter II) in RAM
r4 : physical pointer to the kernel itself. This is used by the assembly code to properly disable the MMU in case you are entering the kernel with MMU enabled and a non-1:1 mapping.
r5 : NULL (as to differenciate with method a)
Note about SMP entry: Either your firmware puts your other CPUs in some sleep loop or spin loop in ROM where you can get them out via a soft reset or some other mean, in which case you don't need to care, or you'll have to enter the kernel with all CPUs. The way to do that with method b) will be described in a later revision of this document.
2) Board support ----------------
Board supports (platforms) are not exclusive config options. An arbitrary set of board supports can be built in a single kernel image. The kernel will "known" what set of functions to use for a given platform based on the content of the device-tree. Thus, you should:
a) add your platform support as a _boolean_ option in arch/ppc64/Kconfig, following the example of PPC_PSERIES, PPC_PMAC and PPC_MAPLE. The later is probably a good example of a board support to start from.
b) create your main platform file as "arch/ppc64/kernel/myboard_setup.c" and add it to the Makefile under the condition of your CONFIG_ option. This file will define a structure of type "ppc_md" containing the various callbacks that the generic code will use to get to your platform specific code
c) Add a reference to your "ppc_md" structure in the "machines" table in arch/ppc64/kernel/setup.c
d) request and get assigned a platform number (see PLATFORM_* constants in include/asm-ppc64/processor.h
I will describe later the boot process and various callbacks that your platform should implement.
II - The DT block format ===========================
This chapter defines the actual format of the flattened device-tree passed to the kernel. The actual content of it and kernel requirements are described later. You can find example of code manipulating that format in various places, including arch/ppc64/kernel/prom_init.c which will generate a flattened device-tree from the Open Firmware representation, or the fs2dt utility which is part of the kexec tools which will generate one from a filesystem representation. It is expected that a bootloader like uboot provides a bit more support, that will be discussed later as well.
Note: The block has to be in main memory. It has to be accessible in both real mode and virtual mode with no other mapping than main memory. If you are writing a simple flash bootloader, it should copy the block to RAM before passing it to the kernel.
1) Header ---------
The kernel is entered with r3 pointing to an area of memory that is roughtly described in include/asm-ppc64/prom.h by the structure boot_param_header:
struct boot_param_header { u32 magic; /* magic word OF_DT_HEADER */ u32 totalsize; /* total size of DT block */ u32 off_dt_struct; /* offset to structure */ u32 off_dt_strings; /* offset to strings */ u32 off_mem_rsvmap; /* offset to memory reserve map */ u32 version; /* format version */ u32 last_comp_version; /* last compatible version */ /* version 2 fields below */ u32 boot_cpuid_phys; /* Which physical CPU id we're booting on */ /* version 3 fields below */ u32 size_dt_strings; /* size of the strings block */ };
Along with the constants:
/* Definitions used by the flattened device tree */ #define OF_DT_HEADER 0xd00dfeed /* 4: version, 4: total size */ #define OF_DT_BEGIN_NODE 0x1 /* Start node: full name */ #define OF_DT_END_NODE 0x2 /* End node */ #define OF_DT_PROP 0x3 /* Property: name off, size, content */ #define OF_DT_END 0x9
All values in this header are in big endian format, the various fields in this header are defined more precisely below. All "offsets" values are in bytes from the start of the header, that is from r3 value.
- magic
This is a magic value that "marks" the beginning of the device-tree block header. It contains the value 0xd00dfeed and is defined by the constant OF_DT_HEADER
- totalsize
This is the total size of the DT block including the header. The "DT" block should enclose all data structures defined in this chapter (who are pointed to by offsets in this header). That is, the device-tree structure, strings, and the memory reserve map.
- off_dt_struct
This is an offset from the beginning of the header to the start of the "structure" part the device tree. (see 2) device tree)
- off_dt_strings
This is an offset from the beginning of the header to the start of the "strings" part of the device-tree
- off_mem_rsvmap
This is an offset from the beginning of the header to the start of the reserved memory map. This map is a list of pairs of 64 bits integers. Each pair is a physical address and a size. The list is terminated by an entry of size 0. This map provides the kernel with a list of physical memory areas that are "reserved" and thus not to be used for memory allocations, especially during early initialisation. The kernel needs to allocate memory during boot for things like un-flattening the device-tree, allocating an MMU hash table, etc... Those allocations must be done in such a way to avoid overriding critical things like, on Open Firmware capable machines, the RTAS instance, or on some pSeries, the TCE tables used for the iommu. Typically, the reserve map should contain _at least_ this DT block itself (header,total_size). If you are passing an initrd to the kernel, you should reserve it as well. You do not need to reserve the kernel image itself. The map should be 64 bits aligned.
- version
This is the version of this structure. Version 1 stops here. Version 2 adds an additional field boot_cpuid_phys. Version 3 adds the size of the strings block, allowing the kernel to reallocate it easily at boot and free up the unused flattened structure after expansion. Version 16 introduces a new more "compact" format for the tree itself that is however not backward compatible. You should always generate a structure of the highest version defined at the time of your implementation. Currently that is version 16, unless you explicitely aim at being backward compatible
- last_comp_version
Last compatible version. This indicates down to what version of the DT block you are backward compatible with. For example, version 2 is backward compatible with version 1 (that is, a kernel build for version 1 will be able to boot with a version 2 format). You should put a 1 in this field if you generate a device tree of version 1 to 3, or 0x10 if you generate a tree of version 0x10 using the new unit name format.
- boot_cpuid_phys
This field only exist on version 2 headers. It indicate which physical CPU ID is calling the kernel entry point. This is used, among others, by kexec. If you are on an SMP system, this value should match the content of the "reg" property of the CPU node in the device-tree corresponding to the CPU calling the kernel entry point (see further chapters for more informations on the required device-tree contents)
So the typical layout of a DT block (though the various parts don't need to be in that order) looks like (addresses go from top to bottom):
------------------------------ r3 -> | struct boot_param_header | ------------------------------ | (alignment gap) (*) | ------------------------------ | memory reserve map | ------------------------------ | (alignment gap) | ------------------------------ | | | device-tree structure | | | ------------------------------ | (alignment gap) | ------------------------------ | | | device-tree strings | | | -----> ------------------------------ | | --- (r3 + totalsize)
(*) The alignment gaps are not necessarily present, their presence and size are dependent on the various alignment requirements of the individual data blocks.
2) Device tree generalities ---------------------------
This device-tree itself is separated in two different blocks, a structure block and a strings block. Both need to be page aligned.
First, let's quickly describe the device-tree concept before detailing the storage format. This chapter does _not_ describe the detail of the required types of nodes & properties for the kernel, this is done later in chapter III.
The device-tree layout is strongly inherited from the definition of the Open Firmware IEEE 1275 device-tree. It's basically a tree of nodes, each node having two or more named properties. A property can have a value or not.
It is a tree, so each node has one and only one parent except for the root node who has no parent.
A node has 2 names. The actual node name is generally contained in a property of type "name" in the node property list whose value is a zero terminated string and is mandatory for version 1 to 3 of the format definition (as it is in Open Firmware). Version 0x10 makes it optional as it can generate it from the unit name defined below.
There is also a "unit name" that is used to differenciate nodes with the same name at the same level, it is usually made of the node name's, the "@" sign, and a "unit address", which definition is specific to the bus type the node sits on.
The unit name doesn't exist as a property per-se but is included in the device-tree structure. It is typically used to represent "path" in the device-tree. More details about the actual format of these will be below.
The kernel ppc64 generic code does not make any formal use of the unit address (though some board support code may do) so the only real requirement here for the unit address is to ensure uniqueness of the node unit name at a given level of the tree. Nodes with no notion of address and no possible sibling of the same name (like /memory or /cpus) may ommit the unit address in the context of this specification, or use the "@0" default unit address. The unit name is used to define a node "full path", which is the concatenation of all parent nodes unit names separated with "/".
The root node doesn't have a defined name, and isn't required to have a name property either if you are using version 3 or earlier of the format. It also has no unit address (no @ symbol followed by a unit address). The root node unit name is thus an empty string. The full path to the root node is "/"
Every node who actually represents an actual device (that is who isn't only a virtual "container" for more nodes, like "/cpus" is) is also required to have a "device_type" property indicating the type of node
Finally, every node that can be referrenced from a property in another node is required to have a "linux,phandle" property. Real open firmware implementations do provide a unique "phandle" value for every node that the "prom_init()" trampoline code turns into "linux,phandle" properties. However, this is made optional if the flattened is used directly. An example of a node referencing another node via "phandle" is when laying out the interrupt tree which will be described in a further version of this document.
This propery is a 32 bits value that uniquely identify a node. You are free to use whatever values or system of values, internal pointers, or whatever to generate these, the only requirement is that every node for which you provide that property has a unique value for it.
Here is an example of a simple device-tree. In this example, a "o" designates a node followed by the node unit name. Properties are presented with their name followed by their content. "content" represent an ASCII string (zero terminated) value, while <content> represent a 32 bits hexadecimal value. The various nodes in this example will be discusse in a later chapter. At this point, it is only meant to give you a idea of what a device-tree looks like. I have on purpose kept the "name" and "linux,phandle" properties which aren't necessary in order to give you a better idea of what the tree looks like in practice.
/ o device-tree |- name = "device-tree" |- model = "MyBoardName" |- compatible = "MyBoardFamilyName" |- #address-cells = <2> |- #size-cells = <2> |- linux,phandle = <0> | o cpus | | - name = "cpus" | | - linux,phandle = <1> | | - #address-cells = <1> | | - #size-cells = <0> | | | o PowerPC,970@0 | |- name = "PowerPC,970" | |- device_type = "cpu" | |- reg = <0> | |- clock-frequency = <5f5e1000> | |- linux,boot-cpu | |- linux,phandle = <2> | o memory@0 | |- name = "memory" | |- device_type = "memory" | |- reg = <00000000 00000000 00000000 20000000> | |- linux,phandle = <3> | o chosen |- name = "chosen" |- bootargs = "root=/dev/sda2" |- linux,platform = <00000600> |- linux,phandle = <4>
This tree is almost a minimal tree. It pretty much contains the minimal set of required nodes and properties to boot a linux kernel, that is some basic model informations at the root, the CPUs, the physical memory layout, and misc informations passed through /chosen like in this example, the platform type (mandatory) and the kernel command line arguments (optional).
The /cpus/PowerPC,970@0/linux,boot-cpu property is an example of a property without a value. All other properties have a value. The signification of the #address-cells and #size-cells properties will be explained in chapter IV which defines precisely the required nodes and properties and their content.
3) Device tree "structure" block
The structure of the device tree is a linearized tree structure. The "OF_DT_BEGIN_NODE" token starts a new node, and the "OF_DT_END" ends that node definition. Child nodes are simply defined before "OF_DT_END" (that is nodes within the node). A 'token' is a 32 bits value.
Here's the basic structure of a single node:
* token OF_DT_BEGIN_NODE (that is 0x00000001) * for version 1 to 3, this is the node full path as a zero terminated string, starting with "/". For version 16 and later, this is the node unit name only (or an empty string for the root node) * [align gap to next 4 bytes boundary] * for each property: * token OF_DT_PROP (that is 0x00000003) * 32 bits value of property value size in bytes (or 0 of no value) * 32 bits value of offset in string block of property name * [align gap to either next 4 bytes boundary if the property value size is less or equal to 4 bytes, or to next 8 bytes boundary if the property value size is larger than 4 bytes] * property value data if any * [align gap to next 4 bytes boundary] * [child nodes if any] * token OF_DT_END (that is 0x00000002)
So the node content can be summmarised as a start token, a full path, a list of properties, a list of child node and an end token. Every child node is a full node structure itself as defined above
4) Device tree 'strings" block
In order to save space, property names, which are generally redundant, are stored separately in the "strings" block. This block is simply the whole bunch of zero terminated strings for all property names concatenated together. The device-tree property definitions in the structure block will contain offset values from the beginning of the strings block.
III - Required content of the device tree =========================================
WARNING: All "linux,*" properties defined in this document apply only to a flattened device-tree. If your platform uses a real implementation of Open Firmware or an implementation compatible with the Open Firmware client interface, those properties will be created by the trampoline code in the kernel's prom_init() file. For example, that's where you'll have to add code to detect your board model and set the platform number. However, when using the flatenned device-tree entry point, there is no prom_init() pass, and thus you have to provide those properties yourself.
1) Note about cells and address representation ----------------------------------------------
The general rule is documented in the various Open Firmware documentations. If you chose to describe a bus with the device-tree and there exist an OF bus binding, then you should follow the specification. However, the kernel does not require every single device or bus to be described by the device tree.
In general, the format of an address for a device is defined by the parent bus type, based on the #address-cells and #size-cells property. In absence of such a property, the parent's parent values are used, etc... The kernel requires the root node to have those properties defining addresses format for devices directly mapped on the processor bus.
Those 2 properties define 'cells' for representing an address and a size. A "cell" is a 32 bits number. For example, if both contain 2 like the example tree given above, then an address and a size are both composed of 2 cells, that is a 64 bits number (cells are concatenated and expected to be in big endian format). Another example is the way Apple firmware define them, that is 2 cells for an address and one cell for a size.
A device IO or MMIO areas on the bus are defined in the "reg" property. The format of this property depends on the bus the device is sitting on. Standard bus types define their "reg" properties format in the various OF bindings for those bus types, you are free to define your own "reg" format for proprietary busses or virtual busses enclosing on-chip devices, though it is recommended that the parts of the "reg" property containing addresses and sizes do respect the defined #address-cells and #size-cells when those make sense.
Later, I will define more precisely some common address formats.
For a new ppc64 board, I recommend to use either the 2/2 format or Apple's 2/1 format which is slightly more compact since sizes usually fit in a single 32 bits word.
2) Note about "compatible" properties -------------------------------------
Those properties are optional, but recommended in devices and the root node. The format of a "compatible" property is a list of concatenated zero terminated strings. They allow a device to express it's compatibility with a family of similar devices, in some cases, allowing a single driver to match against several devices regardless of their actual names
3) Note about "name" properties -------------------------------
While earlier users of Open Firmware like OldWorld macintoshes tended to use the actual device name for the "name" property, it's nowadays considered a good practice to use a name that is closer to the device class (often equal to device_type). For example, nowadays, ethernet controllers are named "ethernet", an additional "model" property defining precisely the chip type/model, and "compatible" property defining the family in case a single driver can driver more than one of these chips. The kernel however doesn't generally put any restriction on the "name" property, it is simply considered good practice to folow the standard and it's evolutions as closely as possible.
Note also that the new format version 16 makes the "name" property optional. If it's absent for a node, then the node's unit name is then used to reconstruct the name. That is, the part of the unit name before the "@" sign is used (or the entire unit name if no "@" sign is present).
4) Note about node and property names and character set -------------------------------------------------------
While open firmware provides more flexibe usage of 8859-1, this specification enforces more strict rules. Nodes and properties should be comprised only of ASCII characters 'a' to 'z', '0' to '9', ',', '.', '_', '+', '#', '?', and '-'. Node names additionally allow uppercase characters 'A' to 'Z' (property names should be lowercase. The fact that vendors like Apple don't respect this rule is irrelevant here). Additionally, node and property names should always begin with a character in the range 'a' to 'z' (or 'A' to 'Z' for node names).
The maximum number of characters for both nodes and property names is 31. In the case of node names, this is only the leftmost part of a unit name (the pure "name" property), it doesn't include the unit address which can extend beyond that limit.
5) Required nodes and properties --------------------------------
a) The root node
The root node requires some properties to be present:
- model : this is your board name/model - #address-cells : address representation for "root" devices - #size-cells: the size representation for "root" devices
Additionally, some recommended properties are:
- compatible : the board "family" generally finds its way here, for example, if you have 2 board models with a similar layout, that typically get driven by the same platform code in the kernel, you would use a different "model" property but put a value in "compatible". The kernel doesn't directly use that value (see /chosen/linux,platform for how the kernel choses a platform type) but it is generally useful.
It's also generally where you add additional properties specific to your board like the serial number if any, that sort of thing. it is recommended that if you add any "custom" property whose name may clash with standard defined ones, you prefix them with your vendor name and a comma.
b) The /cpus node
This node is the parent of all individual CPUs nodes. It doesn't have any specific requirements, though it's generally good practice to have at least:
#address-cells = <00000001> #size-cells = <00000000>
This defines that the "address" for a CPU is a single cell, and has no meaningful size. This is not necessary but the kernel will assume that format when reading the "reg" properties of a CPU node, see below
c) The /cpus/* nodes
So under /cpus, you are supposed to create a node for every CPU on the machine. There is no specific restriction on the name of the CPU, though It's common practice to call it PowerPC,<name>, for example, Apple uses PowerPC,G5 while IBM uses PowerPC,970FX.
Required properties:
- device_type : has to be "cpu" - reg : This is the physical cpu number, it's single 32 bits cell, this is also used as-is as the unit number for constructing the unit name in the full path, for example, with 2 CPUs, you would have the full path: /cpus/PowerPC,970FX@0 /cpus/PowerPC,970FX@1 (unit addresses do not require to have leading zero's) - d-cache-line-size : one cell, L1 data cache line size in bytes - i-cache-line-size : one cell, L1 instruction cache line size in bytes - d-cache-size : one cell, size of L1 data cache in bytes - i-cache-size : one cell, size of L1 instruction cache in bytes
Recommended properties:
- timebase-frequency : a cell indicating the frequency of the timebase in Hz. This is not directly used by the generic code, but you are welcome to copy/paste the pSeries code for setting the kernel timebase/decrementer calibration based on this value. - clock-frequency : a cell indicating the CPU core clock frequency in Hz. A new property will be defined for 64 bits value, but if your frequency is < 4Ghz, one cell is enough. Here as well as for the above, the common code doesn't use that property, but you are welcome to re-use the pSeries or Maple one. A future kernel version might provide a common function for this.
You are welcome to add any property you find relevant to your board, like some informations about mecanism used to soft-reset the CPUs for example (Apple puts the GPIO number for CPU soft reset lines in there as a "soft-reset" property as they start secondary CPUs by soft-resetting them).
d) the /memory node(s)
To define the physical memory layout of your board, you should create one or more memory node(s). You can either create a single node with all memory ranges in it's reg property, or you can create several nodes, as you wishes. The unit address (@ part) used for the full path is the address of the first range of memory defined by a given node. If you use a single memory node, this will typically be @0.
Required properties:
- device_type : has to be "memory" - reg : This property contain all the physical memory ranges of your board. It's a list of addresses/sizes concatenated together, the number of cell of those beeing defined by the #address-cells and #size-cells of the root node. For example, with both of these properties beeing 2 like in the example given earlier, a 970 based machine with 6Gb of RAM could typically have a "reg" property here that looks like:
00000000 00000000 00000000 80000000 00000001 00000000 00000001 00000000
That is a range starting at 0 of 0x80000000 bytes and a range starting at 0x100000000 and of 0x100000000 bytes. You can see that there is no memory covering the IO hold between 2Gb and 4Gb. Some vendors prefer splitting those ranges into smaller segments, the kernel doesn't care.
c) The /chosen node
This node is a bit "special". Normally, that's where open firmware puts some variable environment informations, like the arguments, or phandle pointers to nodes like the main interrupt controller, or the default input/output devices.
This specification makes a few of these mandatory, but also defines some linux specific properties that would be normally constructed by the prom_init() trampoline when booting with an OF client interface, but that you have to provide yourself when using the flattened format.
Required properties:
- linux,platform : This is your platform number as assigned by the architecture maintainers
Recommended properties:
- bootargs : This zero terminated string is passed as the kernel command line - linux,stdout-path : This is the full path to your standard console device if any. Typically, if you have serial devices on your board, you may want to put the full path to the one set as the default console in the firmware here, for the kernel to pick it up as it's own default console. If you look at the funciton set_preferred_console() in arch/ppc64/kernel/setup.c, you'll see that the kernel tries to find out the default console and has knowledge of various types like 8250 serial ports. You may want to extend this function to add your own. - interrupt-controller : This is one cell containing a phandle value that matches the "linux,phandle" property of your main interrupt controller node. May be used for interrupt routing.
This is all that is currently required. However, it is strongly recommended that you expose PCI host bridges as documented in the PCI binding to open firmware, and your interrupt tree as documented in OF interrupt tree specification.
IV - Recommendation for a bootloader ====================================
Here are some various ideas/recommendations that have been proposed while all this has been defined and implemented.
- It should be possible to write a parser that turns an ASCII representation of a device-tree (or even XML though I find that less readable) into a device-tree block. This would allow to basically build the device-tree structure and strings "blobs" at bootloader build time, and have the bootloader just pass-them as-is to the kernel. In fact, the device-tree blob could be then separate from the bootloader itself, an be placed in a separate portion of the flash that can be "personalized" for different board types by flashing a different device-tree
- A very The bootloader may want to be able to use the device-tree itself and may want to manipulate it (to add/edit some properties, like physical memory size or kernel arguments). At this point, 2 choices can be made. Either the bootloader works directly on the flattened format, or the bootloader has it's own internal tree representation with pointers (similar to the kernel one) and re-flattens the tree when booting the kernel. The former is a bit more difficult to edit/modify, the later requires probably a bit more code to handle the tree structure. Note that the structure format has been designed so it's relatively easy to "insert" properties or nodes or delete them by just memmovin'g things around. It contains no internal offsets or pointers for this purpose.
- An example of code for iterating nodes & retreiving properties directly from the flattened tree format can be found in the kernel file arch/ppc64/kernel/prom.c, look at scan_flat_dt() function, it's usage in early_init_devtree(), and the corresponding various early_init_dt_scan_*() callbacks. That code can be re-used in a GPL bootloader, and as the author of that code, I would be happy do discuss possible free licencing to any vendor who wishes to integrate all or part of this code into a non-GPL bootloader.