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17 January 2021

An Introduction to Devicetree specification

by Mike Krinkin

Devicetree is a configuration commonly used to describe hardware present in various platforms. In Linux Devicetree is used for ARMs, MIPSes, RISC-V, XTensa and PowerPC (and probably others).

In this post I’m going to cover the problem that Devicetree is trying to solve, briefly touch on the available alternatives and finally show some code for parsing the binary representation of the Devicetree (a. k. a. Flattened Device Tree or DTB).

All the sources are available on GitHub.

Introduction

When you boot your PC it somehow figures out how many CPUs the system has, how much memory is installed in the system, it figures out what HDD or SSD connected to the system, that there is a keyboard and a mouse, etc.

Likely you don’t need to install a new OS when you add more memory to the system or when you change a mouse. So the same OS can serve multiple different variants of hardware. To put it another way OS doesn’t always know in advance what hardware it runs on and has in some way discover that information.

There are multiple ways OS can discover that information. For example, most of external devices directly or indirectly connect to the system via PCI and USB. Those two support means to automatically enumerate connected devices and find out some basic information about them. Using this basic information OS can figure out if it has a driver that supports the connected devices and loads it.

However, CPUs and memory are not some kind of PCI or USB devices. PCI and USB host controllers cannot depend on PCI and USB enumeration mechanisms either. So for those a different mechanism have to be used.

In simplest cases OS and drivers can just probe for the connected devices. That basically means that a driver can try to interract with the device in hope that it’s there. If the device responds, then it must be there, and it doesn’t respond, then it’s not there. It doesn’t work though with things like memory.

When the simplest method doesn’t work, in the PC world the problem can be addressed via BIOS, UEFI, ACPI or a combination of those. They provide various interfaces to explore the available memory and basic hardware.

However hardware platforms in the PC world are more or less standartized. In the embedded systems worlds there is a bit more variety and there an alternative configuration interface became popular: Devicetree Specification.

Historically Devicetree Specification originates from Open Firmware which for most intents and purposes is long gone now, but the Devicetree Specification is alive and well.

Devicetree Overview

Essentially, the Devicetree Specification describes a data description format, like XML or json, but designed for the purporse of describing hardware. As you could have figured out from the name, the description forms a sort of tree structure.

You can think of a root node as a node that describes the hardware platform as a whole. It must have at least one memory children node that describe the available memory in the system and one cpus node that enumerates the CPUs available in the system as the children.

For other types of hardware the tree structure like this also generally works well. For example, for buses that do not support discovery, like I2C, you can have a node for an I2C controller and describe the devices connected to the I2C controller as children nodes of the controller node and so on.

DTS

The Devicetree Specification provides two formats for the description: a human readable/writable text format (DTS) and binary format (DTB or FDT). The idea is that when you create a tree description you use the DTS, then you compile DTS to DTB and pass the DTB to the OS.

Here is how you can install the DeviceTree Compiler (dtc):

sudo apt-get install device-tree-compiler

After successful installation you should have dtc binary available. This binary can be used to convert between DTS and DTB formats.

Let’s take a look at an example. We can ask QEMU to dump the tree for the hardware it simulates in DTB format like this:

qemu-system-aarch64 -machine virt,dumpdtb=virt.dtb -cpu max

In the command above -machine virt part specifies a particular virtual hardware configuration that QEMU supports. And the dumpdtb=virt.dtb asks QEMU to store the description of that hardware configuration to the virt.dtb file.

We can then convert this file from DTB to DTS format using the compiler:

dtc -I dtb -O dts virt.dtb

This command will parse virt.dtb as DTB and output the same data in DTS format to the stdout. The flag -I is used to specify the input format and -O is used to specify the output format.

The QEMU generates a reasonably useful hardware platform, so the resulting Devicetree is quite large, so let’s create our own Devicetree for testing and to get familiar with the DTS format a little bit.

Before we start, DTS format appear to be versioned and the current version is

  1. There was only one version of DTS ever, but we still have to start DTS file with a version clause:
/dts-v1/;

The version clause can follow with a few memory reservation clauses. I will cover memory reservations a bit further down the post, but for now it’s sufficient to note that memory reservations just describe a range of memory addresses. I will introduce three memory reservation just for testing:

/dts-v1/;
/memreserve/ 0x40000000 0x1000;
/memreserve/ 0x40002000 0x1000;
/memreserve/ 0x40004000 0x1000;

Then goes the root node of the tree. The root node uses a format slightly different compared to other nodes because it doesn’t have a name:

/dts-v1/;
/memreserve/ 0x40000000 0x1000;
/memreserve/ 0x40002000 0x1000;
/memreserve/ 0x40004000 0x1000;
/ {
}

The rest of the description should go inside the root node. The Devicetree Specification mandate a few required properties for the root node:

I’m only going to cover #address-cells and #size-cells for now, because dtc cannot work without them and at the same time is fine when the other two are missing.

Essentially, in the [Devicetree Specficiation] property values can have a few possible types. For example, they can be arrays of 32-bit values (cells) or zero-terminiated strings (though you cannot see zero-terminal in DTS format). For addresses and sizes cells are commonly used, and for text properties zero-terminated strings are used.

For example, if you have some memory mapped device, then registers of that device are mapped to some place in memory. To communicate with the device, drivers and OS kernels want to know where exactly the device registers are mapped. So the node describing the device inside the tree would need to have a property that contains the address of the memory where the device registers are mapped.

Here you might ask, but what if my device has more than 4GiB of memory? 32-bit value is not enough to describe that, right?

That’s where the #size-cells and #address-cells properties come into play. They basically describe how many 32-bit cells do we need to describe size and addresses. For 64-bit systems you’d need two cells to describe an address in memory.

To take it a bit further, addresses and sizes don’t necessary have to refer to addresses in memory. For example, I2C devices have addresses in context of I2C bus. Those are not memory addresses, but addresses nontheless. So what address means in a particular case depends on the node and its place in the tree. Consequently, you can have #address-cells and #size-cells in multiple nodes. Those parameters apply to all the children nodes of the node that contains them, unless overriden somewhere down the tree.

Let’s return to the example. For now we are working with memory addresses, so I will use two cells to specify addresses and sizes to support 64-bit memory:

/dts-v1/;
/memreserve/ 0x40000000 0x1000;
/memreserve/ 0x40002000 0x1000;
/memreserve/ 0x40004000 0x1000;
/ {
	#address-cells = <0x02>;
	#size-cells = <0x02>;
};

Now, let’s move to the first child node: memory. According to the Devicetree Specification we must have at least one memory node in the tree:

/dts-v1/;
/memreserve/ 0x40000000 0x1000;
/memreserve/ 0x40002000 0x1000;
/memreserve/ 0x40004000 0x1000;
/ {
	#address-cells = <0x02>;
	#size-cells = <0x02>;

	memory@40000000 {
		reg = <0x00 0x40000000 0x00 0x8000000>;
		device_type = "memory";
	};
};

The node names generally contain two parts: name and the unit address. Those two parts are separated by @ character. The full name is known as unit name. In the example above we have memory node with unit name memory@40000000. As far as I can tell, the unit address isn’t actually required and normally isn’t used for anything. In other words, unit address seem to be there solely for the purpopse of disambiguating node names when you have more than one node of the same type.

memory nodes have two required parameters:

As you can see in the example, the value of reg property contains four cells. It’s because the reg property value should contain a pair: address and size. And according to the values of the #address-cells and #size-cells properties we use two cells to describe both. All-in-all, we have four cells.

When putting everything together, the memory node above describes an address range that starts at the address 0x40000000 that is 0x8000000 bytes long.

Another mandatory node is cpus. I will describe a system with just one ARM cpu, so let’s take a look at the example:

/dts-v1/;
/memreserve/ 0x40000000 0x1000;
/memreserve/ 0x40002000 0x1000;
/memreserve/ 0x40004000 0x1000;
/ {
	#address-cells = <0x02>;
	#size-cells = <0x02>;

	memory@40000000 {
		reg = <0x00 0x40000000 0x00 0x8000000>;
		device_type = "memory";
	};

	cpus {
		#address-cells = <0x01>;
		#size-cells = <0x00>;

		cpu@0 {
			reg = <0x00>;
			compatible = "arm,cortex-a57";
			device_type = "cpu";
		};
	};
};

#address-cells and #size-cells are mandatory properties for the cpus node. To understand what values we assign to them we need to understand what address space we are working with.

In this address space address is just an identifier of a CPU. And each CPU occupies just one unit of the address space, so the size is always 1 and we don’t need to specify it explicitly.

That’s why the Devicetree Specification tells that the #size-cells shall contain 0, as in the example above. And one 32-bit cell is plenty enough to contain the CPU identifier, thus #address-cells contains 1.

Each CPU is described by a child cpu node. As with memory node it has a few required properties:

Even though, the Devicetree Specification says that clock-frequency and timebase-frequency are required properties, the compiler eats the DTS even without them, so I’ve skipped them for now.

DTB/FDT

The name DTB stands for DeviceTree Blob. FDT stands for Flattened DeviceTree and it’s just another name for the same binary format.

The binary format is what I care about the most. Normally the OS kernels work with DTB and not DTS. I want to use DTB to pass information to my toy hypervisor.

In the previous section we created a simple and not particularly useful DTS that has one memory node, one CPU and three reserved memory ranges (even though they are not part of the tree itself).

I assume that the DTS is saved in file test.dts. Let’s first compile this DTS into a DTB and store the result to test.dtb:

dtc -I dts -O dtb test.dts > test.dtb

Each DTB basically contains four parts in the following order:

The DTB must start with a header:

struct FDTHeader {
    magic: u32,
    totalsize: u32,
    off_dt_struct: u32,
    off_dt_strings: u32,
    off_mem_rsvmap: u32,
    version: u32,
    last_comp_version: u32,
    boot_cpuid_phys: u32,
    size_dt_strings: u32,
    size_dt_struct: u32,
}

NOTE: I will use Rust in the examples and Rust doesn’t specify the internal layout of the structures, but as will be shown below, I don’t really depend on any particular internal layout of Rust structures.

The header describes the overall layout of DTB. Fields off_dt_struct, off_dt_strings and off_mem_rsvmap contains offsets from the beginning of DTB to the memory reservation, structure and strings blocks. size_dt_strings and size_dt_struct contain sizes of structure and strings blocks in bytes.

There might be gaps between the blocks and some systems use those gaps to dynamically manipulate the structure. For example, to add more reserved memory ranges.

magic field must contain 0xd00dfeed (big-endian) to indicate that the data is indeed in DTB format. As for the rest of the fields, I will refer you to the Devicetree Specification.

Memory Reservations

We saw above how we can describe memory available on the platform in DTS format. However for various purposes we may want to reserve parts of that memory and tell the OS that to not use those.

Imagine a situation when firmware needs some memory to store firmware related data. If OS need to call the firmware we need to keep the firmware data in the working state. In such case firmware can mark the memory it needs inside the devicetree as reserved to tell the OS to stay away from it.

The block describing the reserved memory ranges contains an array of structures like this:

struct ReservedMemory {
    addr: u64,
    size: u64,
}

The array begins at the offset off_mem_rsvmap from the beginning of DTB. The array must have a zero-filled entry as a terminating entry - that’s how you can find the end of the array.

NOTE: the header doesn’t contain enough information to find out where the array ends, so we have to use other means for that.

Structure Block

The structure block contains a flattened representation of the tree of nodes starting with the root node. Logically, it can be viewed as a sequency of pieces. Each piece starts with a 32-bit token value that tells us what kind of piece it is. After the token there might be some additional data, depending on the token.

In total there are five different tokens:

Each token must be aligned on the 4-byte aligned boundary and, when necessary data is padded with \0 symbols to enforce the alignment.

The last two tokens are more or less self explanatory, so I’m not going to spend time on them (see the implementation below for details of the Devicetree Specification).

FDT_BEGIN_NODE marks the beginning of a node description. Each node except the root has a name (unit name). The name of the node is stored as a null-terminated string right after the FDT_BEGIN_NODE token. Since the root node doesn’t have a name, the token is followed by just \0 symbol. After the node name there might be zero or more \0 symbols to make sure that the next token is aligned on the 4-bytes boundary.

As you saw above inside the node we can have properties or other nodes. Description of node children and properties goes after the FDT_BEGIN_NODE token and its data.

Children nodes are described in a recursive way, each of the children starts with the FDT_BEGIN_NODE token and ends with the FDT_END_NODE, potentially containing properties and other nodes between them.

Unlike FDT_BEGIN_NODE, FDT_END_NODE doesn’t have any data.

Since FDT_BEGIN_NODE and FDT_END_NODE mark the boundaries of a node, FDT_BEGIN_NODE and FDT_END_NODE must always go in pairs, like parenthesis in a correct arithmetic expression.

Each property starts with FDT_PROP token. The FDT_PROP token is then followed by two 32-bit values: name offset and length of the property value.

The name offset is the offset of the null-terminated string that contains the property name inside the strings block. The value of the property follows right after and, again, padded with \0 symbols until the next 4-bytes aligned boundary.

The description is somewhat wordy and it might be hard to figure out what’s going on from just reading it. So let’s move to implement the DTB parser to see how the pieces fit together.

DTB Scanner

Before we start with the actual DTB parsing let’s create a helper utility to extract values from bytes. We need to be able to extract 32-bit values, 64-bit values, null-terminated strings, data blobs of fixed size. And additionaly we need to align the position inside the data stream on the 4-bytes boundary. So that’s exactly what I’m going to implement here.

I’ll call the tool that does it Scanner, let’s take a look:

struct Scanner<'a> {
    data: &'a [u8],
    offset: usize,
}

impl<'a> Scanner<'a> {
    pub fn new(data: &'a [u8]) -> Scanner<'a> {
        Scanner{ data, offset: 0 }
    }
}

In the snippet above data is our data stream and it’s represented as a slice of u8 values. The offset is the position in the stream - that’s the current state of the Scanner.

Before we move to the actual implementation one thing to keep in mind is that all integer values in DTB use Big-endian according to the Devicetree Specification.

With that in mind, let’s move to the implementation starting with parsing 32-bit and 64-bit integer values from the stream:

use core::convert::TryFrom;
use core::result::Result;

impl<'a> Scanner<'a> {
    ...
    pub fn consume_be32(&mut self) -> Result<u32, &'static str> {
        if self.offset + 4 > self.data.len() {
            return Err("Not enough data in the stream for be32.");
        }

        let value = &self.data[self.offset..self.offset + 4];
        match <[u8; 4]>::try_from(value) {
            Ok(v) => {
                self.offset += value.len();
                Ok(u32::from_be_bytes(v))
            },
            Err(_) => Err("Error while parsing be32."),
        }
    }

    pub fn consume_be64(&mut self) -> Result<u64, &'static str> {
        if self.offset + 8 > self.data.len() {
            return Err("Not enough data in the stream for be64.");
        }

        let value = &self.data[self.offset..self.offset + 8];
        match <[u8; 8]>::try_from(value) {
            Ok(v) => {
                self.offset += value.len();
                Ok(u64::from_be_bytes(v))
            },
            Err(_) => Err("Error while parsing be64."),
        }
    }
    ...
}

The core of the snippet above are u64::from_be_bytes and u32::from_be_bytes functions that take as input arrays of u8 values and parse them as 64-bit or 32-bit Big-endian numbers.

The try_from function basically serves as type cast to convert from slices to arrays.

The rest of the code is error checking and updating the offset field to move further in the byte stream.

The next on the line is parsing null-terminated string:

use core::str;

impl<'a> Scanner<'a> {
    ...
    pub fn consume_cstr(&mut self) -> Result<&'a str, &'static str> {
        for i in self.offset.. {
            if i >= self.data.len() {
                return Err("Failed to find terminating '\0' in the stream.");
            }

            if self.data[i] != 0 {
                continue;
            }

            match str::from_utf8(&self.data[self.offset..i]) {
                Ok(s) => {
                    self.offset = i + 1;
                    return Ok(s);
                },
                Err(_) => return Err("Not a valid UTF8 string."),
            }
        }
	Err("Unreachable")
    }
    ...
}

In the snippet above we just go through the data until we find a zero. Then we take all the data up to the zero character we found and try to interpret it as a utf8 string and convert to str using std::from_utf8. The Devicetree Specification is rather restrictive on the characters possible in the property names, so all of them should be possible to interpret as utf8 strings.

Now let’s take a look at consuming fixed-size data. It’s much simpler than consume_cstr because we don’t need to find the null-terminator or convert data into a string:

impl<'a> Scanner<'a> {
    ...
    pub fn consume_data(&mut self, size: usize) -> Result<&'a [u8], &'static str> {
        if self.offset + size > self.data.len() {
            return Err("Not enough data in the stream.");
        }

        let begin = self.offset;
        let end = begin + size;
        self.offset += size;

        Ok(&self.data[begin..end])
    }
    ...
}

And finally we need to be able to align the position in the stream on a 4-bytes boundary. I will pass the alignment as a parameter instead of hardcoding the 4 bytes value:

impl<'a> Scanner<'a> {
    ...
    pub fn align_forward(&mut self, alignment: usize) -> Result<(), &'static str> {
        if alignment == 0 || self.offset % alignment == 0 {
            return Ok(());
        }

        let shift = alignment - self.offset % alignment;

        if self.offset + shift >= self.data.len() {
            return Err("Not enough data in the stream.");
        }

        self.offset += shift;
        Ok(())
    }
    ...
}

That’s quite a bit of code, but all of that more or less straightforward.

Device Tree Representation

Before moving forward with the parsing of DTB I need to describe what result the parser will return. What I want to do is to create a tree-like structure that represents the tree encoded in DTB and make parser return it.

Let’s start with the tree node. Each node of the tree contains properties and other nodes:

use alloc::collections::btree_map::BTreeMap;
use alloc::string::String;
use alloc::vec::Vec;

struct DeviceTreeNode {
    children: BTreeMap<String, DeviceTreeNode>,
    properties: BTreeMap<String, Vec<u8>>,
}

The data structure by itself is not super useful. We also need to have some interface to modify and explore the nodes:

use core::options::Options;

impl DeviceTreeNode {
    pub fn child(&self, name: &str) -> Option<&DeviceTreeNode> {
        self.children.get(name)
    }

    pub fn children(&self) -> Children {
        Children{ inner: self.children.iter() }
    }

    pub fn property(&self, name: &str) -> Option<&[u8]> {
        self.properties.get(name).map(|v| v.as_slice())
    }

    pub fn properties(&self) -> Properties {
        Properties{ inner: self.properties.iter() }
    }

    fn new() -> DeviceTreeNode {
        DeviceTreeNode{
            children: BTreeMap::new(),
            properties: BTreeMap::new(),
        }
    }

    fn add_child(&mut self, name: String, child: DeviceTreeNode) {
        self.children.insert(name, child);
    }

    fn remove_child(&mut self, name: &str) -> Option<DeviceTreeNode> {
        self.children.remove(name)
    }

    fn add_property(&mut self, name: String, value: Vec<u8>) {
        self.properties.insert(name, value);
    }
}

NOTE: I made non-modifing functions public as they will serve as the interface for the users of the library. All the modifing function will be used during parsing only and not intended for use by anybody else, so they are not public.

The snippet above omits the definition of Children and Properties. Those are just simple iterator wrapped around the BTreeMap iterator. I’m not totally sure that wrapping will serve any purpose yet, so I’m not going to cover that part here. You can find the complete code on GitHub.

Another piece that we need to represent parsed DTB is reserved memory ranges. I already provided the structure for the reserved memory above, so let’s not spend any more time on that.

Finally, I want to introduce another piece that combines all things together and provides functions to lookup nodes in the tree. Let’s take a look:

pub struct DeviceTree {
    reserved: Vec<ReservedMemory>,
    root: DeviceTreeNode,
    last_comp_version: u32,
    boot_cpuid_phys: u32,
}

impl DeviceTree {
    pub fn new(
            reserved: Vec<ReservedMemory>,
            root: DeviceTreeNode,
            last_comp_version: u32,
            boot_cpuid_phys: u32) -> DeviceTree {
        DeviceTree{ reserved, root, last_comp_version, boot_cpuid_phys }
    }
}

It’s simple so far. I’ve just put a few piecies that we can find in the DTB header, the vector with reserved memory ranges and the root node. At the moment we probably only care about the root node and reserved memory ranges, but when I wrote the code I thought that the other two might become useful in the future.

Let’s take a look at the helper functions that would make this structure useful. Those functions will provide access to various pieces and I’ll start with the simplest one - reserved memory ranges:

impl DeviceTree {
    ...
    pub fn reserved_memory(&self) -> &[ReservedMemory] {
        self.reserved.as_slice()
    }
    ...
}

There isn’t much to explain here - it just returns the reserved ranges in a slice.

Let’s take a look at a function that is more interesting:

impl DeviceTree
    ...
    pub fn follow(&self, path: &str) -> Option<&DeviceTreeNode> {
        let mut current = &self.root;

        if path == "/" {
            return Some(current);
        }

        for name in path[1..].split("/") {
            if let Some(node) = current.child(name) {
                current = node;
            } else {
                return None;
            }
        }

        Some(current)
    }
    ...
}

This function takes a string representing a path in a tree and returns the node corresponding to this path. For example, for the CPU in our test DTB, that we created above, the path will be /cpus/cpu@0 and for the memory node in the same DTB it will be /memory@40000000. To get access to the root node we should pass just /. That’s just a convient way to identify a node in the tree.

DTB Parser

With all the preparations out of the way we can actually parse the DTB now. And I’ll start from the beginning of the DTB - it’s header:

fn parse_header(fdt: &[u8]) -> Result<FDTHeader, &'static str> {
    let mut scanner = Scanner::new(fdt);
    let magic = scanner.consume_be32()?;
    let totalsize = scanner.consume_be32()?;
    let off_dt_struct = scanner.consume_be32()?;
    let off_dt_strings = scanner.consume_be32()?;
    let off_mem_rsvmap = scanner.consume_be32()?;
    let version = scanner.consume_be32()?;
    let last_comp_version = scanner.consume_be32()?;
    let boot_cpuid_phys = scanner.consume_be32()?;
    let size_dt_strings = scanner.consume_be32()?;
    let size_dt_struct = scanner.consume_be32()?;

    Ok(FDTHeader{
        magic,
        totalsize,
        off_dt_struct,
        off_dt_strings,
        off_mem_rsvmap,
        version,
        last_comp_version,
        boot_cpuid_phys,
        size_dt_strings,
        size_dt_struct,
    })
}

NOTE: above I mentioned that I don’t need to depend on any particular internal layout of Rust structures. Scanner is the tool that allows to do that, all we need is to call the Scanner methods in the right order.

Parsing the list of reserved ranges is also quite straighforward:

fn parse_reserved(data: &[u8]) -> Result<Vec<ReservedMemory>, &'static str> {
    let mut scanner = Scanner::new(data);
    let mut reserved = Vec::new();

    loop {
        let addr = scanner.consume_be64()?;
        let size = scanner.consume_be64()?;

        if addr == 0 && size == 0 {
            break;
        }
        reserved.push(ReservedMemory{ addr, size });
    }

    Ok(reserved)
}

The function above assumes that the list of reserved memory ranges starts right at the beginning of the data parameter, so we cannot just pass it the whole DTB - we need to find the offset of the reserved memory ranges list in the DTB and pass it only that part.

Now let’s move to the fun part - parsing the actual tree from the DTB. Since the DTB encodes a tree some kind of recursive algorithm or additional memory is required.

I will use additional memory instead of recursion here, so let me introduce a structure that I will use to keep track of the current state:

struct State<'a> {
    parents: Vec<(&'a str, DeviceTreeNode)>,
    current: DeviceTreeNode,
}

The current field will store the DeviceTreeNode strcture for the latest node we discovered. The parents field is a stack of all the parents of the current. Every time we will discover a new node, we will push the current node on the stack and create a new node. Every time a node ends, we will take the parent from the stack and make it the current node.

Let’s take a look:

impl<'a> State<'a> {
    fn new() -> State<'a> {
        State{
            parents: Vec::new(),
            current: DeviceTreeNode::new(),
        }
    }
    ...
}

In the initial state is the parents stack is empty. I create a dummy node and store it in current to avoid handling corner case of the first node - we will have to keep that in mind for later.

When the code discovers a new node it should push the current node on stack and create a new node and store it as current:

impl<'a> State<'a> {
    ...
    fn begin_node(&mut self, name: &'a str) {
        let node = mem::replace(&mut self.current, DeviceTreeNode::new());
        self.parents.push((name, node));
    }
    ...
}

NOTE: mem::replace replaces the value of self.current with a new value and return the old one to the caller.

In the code above I also store the name of the current node on stack together with the parent. We will use this name later.

When we discover the end of node token we need to do a somewhat opposite operation:

impl<'a> State<'a> {
    ...
    fn end_node(&mut self, name: &'a str) -> Result<(), &'static str> {
        if let Some((name, parent)) = self.parents.pop() {
            let node = mem::replace(&mut self.current, parent);
            self.current.add_child(String::from(name), node);
            return Ok(());
        }
        Err("Unmatched end of node token found in FDT.")
    }
    ...
}

The end_node is slightly more complicated than begin_node for two reasons:

We we discover a property in DTB we just need to add it to the current node:

impl<'a> State<'a> {
    ...
    fn new_property(&mut self, name: &str, value: &[u8]) {
        self.current.add_property(String::from(name), Vec::from(value));
    }
    ...
}

Finally, remember that we started with a fake node in current? When parsing is done we need to take that into account and remove that fake node:

impl<'a> State<'a> {
    ...
    fn finish(&mut self) -> Result<DeviceTreeNode, &'static str> {
        if !self.parents.is_empty() {
            return Err("Parsed FDT contains unclosed nodes.");
        }
        if let Some(root) = self.current.remove_child("") {
            return Ok(root);
        }
        Err("FDT root node name is not empty.")
    }
    ...
}

The finish function also does some error checking. Specifically, when we finished parsing the parents stack must be empty as each FDT_BEGIN_NODE token must have a matching FDT_END_NODE token. So if the parents stack is not empty, then some of the FDT_BEGIN_NODE tokens didn’t have a matching FDT_END_NODE token.

The code we’ve covered so far just describes how the state should be updated when we discover different tokens in the DTB, let’s actually take a look at the code that reads the DTB.

The code will essentially go in a loop, one token at a time, updating the state. The loop will end when we hit an error or discover the FDT_END token. When we discover FDT_END token we just need to extract the result from the State and return it:

fn parse_node(structs: &[u8], strings: &[u8]) -> Result<DeviceTreeNode, &'static str> {
    const FDT_BEGIN_NODE: u32 = 0x01;
    const FDT_END_NODE: u32 = 0x02;
    const FDT_PROP: u32 = 0x03;
    const FDT_NOP: u32 = 0x04;
    const FDT_END: u32 = 0x09;

    let mut scanner = Scanner::new(structs);
    let mut state = State::new();

    loop {
        match scanner.consume_be32() {
            ...
            Ok(token) if token == FDT_END => return state.finish(),
            Err(msg) => return Err(msg),
            _ => return Err("Unknown FDT token."),
        }
    }
}

The FDT_NOP token is not very interesting, since we don’t need to do anything:

fn parse_node(structs: &[u8], strings: &[u8]) -> Result<DeviceTreeNode, &'static str> {
    const FDT_BEGIN_NODE: u32 = 0x01;
    const FDT_END_NODE: u32 = 0x02;
    const FDT_PROP: u32 = 0x03;
    const FDT_NOP: u32 = 0x04;
    const FDT_END: u32 = 0x09;

    let mut scanner = Scanner::new(structs);
    let mut state = State::new();

    loop {
        match scanner.consume_be32() {
            ...
            Ok(token) if token == FDT_NOP => {},
            Ok(token) if token == FDT_END => return state.finish(),
            Err(msg) => return Err(msg),
            _ => return Err("Unknown FDT token."),
        }
    }
}

The interesting tokens are FDT_BEGIN_NODE, FDT_END_NODE and FDT_PROP. Let’s start with the first two:

fn parse_node(structs: &[u8], strings: &[u8]) -> Result<DeviceTreeNode, &'static str> {
    const FDT_BEGIN_NODE: u32 = 0x01;
    const FDT_END_NODE: u32 = 0x02;
    const FDT_PROP: u32 = 0x03;
    const FDT_NOP: u32 = 0x04;
    const FDT_END: u32 = 0x09;

    let mut scanner = Scanner::new(structs);
    let mut state = State::new();

    loop {
        match scanner.consume_be32() {
            Ok(token) if token == FDT_BEGIN_NODE => {
                state.begin_node(scanner.consume_cstr()?);
                scanner.align_forward(4)?;
            },
            Ok(token) if token == FDT_END_NODE => {
                state.end_node()?;
            },
            ...
            Ok(token) if token == FDT_NOP => {},
            Ok(token) if token == FDT_END => return state.finish(),
            Err(msg) => return Err(msg),
            _ => return Err("Unknown FDT token."),
        }
    }
}

Since after the FDT_BEGIN_NODE goes a null-terminated string that contains the name of the node we extract it and also make sure that the current position in the stream is aligned on the 4-bytes boundary to be ready to read the next token.

The FDT_END_NODE doesn’t have any data after, so all we have to do is to update the state.

And now the last token - FDT_PROP:

fn parse_node(structs: &[u8], strings: &[u8]) -> Result<DeviceTreeNode, &'static str> {
    const FDT_BEGIN_NODE: u32 = 0x01;
    const FDT_END_NODE: u32 = 0x02;
    const FDT_PROP: u32 = 0x03;
    const FDT_NOP: u32 = 0x04;
    const FDT_END: u32 = 0x09;

    let mut scanner = Scanner::new(structs);
    let mut state = State::new();

    loop {
        match scanner.consume_be32() {
            Ok(token) if token == FDT_BEGIN_NODE => {
                state.begin_node(scanner.consume_cstr()?);
                scanner.align_forward(4)?;
            },
            Ok(token) if token == FDT_END_NODE => {
                state.end_node()?;
            },
            Ok(token) if token == FDT_PROP => {
                let len = scanner.consume_be32()? as usize;
                let off = scanner.consume_be32()? as usize;
                let value = scanner.consume_data(len)?;
                let name = Scanner::new(&strings[off..]).consume_cstr()?;
                state.new_property(name, value);
                scanner.align_forward(4)?;
            },
            Ok(token) if token == FDT_NOP => {},
            Ok(token) if token == FDT_END => return state.finish(),
            Err(msg) => return Err(msg),
            _ => return Err("Unknown FDT token."),
        }
    }
}

When we discover an FDT_PROP token we need to extract the size of the property value and the offset of the property name in the strings block. Then, knowing the size, we can extract from the same stream the actual value of the property.

The name of the property is stored inside the strings block. I think the reason for that is that different nodes actually have the same property names and storing property names separately allows to avoid storing duplicates and save some space.

Anywyas, we still can use Scanner to extract the property name from the strings block, but we need to create a new Scanner all the time.

Now we have different pieces that parse different parts of the DTB. It’s time for the last effort - we need to call those in the right order:

pub fn parse(fdt: &[u8]) -> Result<DeviceTree, &'static str> {
    let header = parse_header(fdt)?;

    if header.magic != 0xd00dfeed {
        return Err("Incorrect FDT magic value.");
    }

    if header.totalsize as usize > fdt.len() {
        return Err("The FDT size is too small to fit the content.");
    }

    let reserved = parse_reserved(&fdt[header.off_mem_rsvmap as usize..])?;

    let begin = header.off_dt_struct as usize;
    let end = begin + header.size_dt_struct as usize;
    let structs = &fdt[begin..end];

    let begin = header.off_dt_strings as usize;
    let end = begin + header.size_dt_strings as usize;
    let strings = &fdt[begin..end];

    let root = parse_node(structs, strings)?;

    Ok(DeviceTree::new(
        reserved, root, header.last_comp_version, header.boot_cpuid_phys))
}

And that’s it - a functional DTB parser is ready to be used. We can test the code on the test DTB that we created earlier. If you put the test.dtb in the same directory with the code you can access it in tests like this:

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_parse() {
        let dtb = include_bytes!("test.dtb");
        let dt = parse(dtb).unwrap();

        assert_eq!(
            dt.reserved_memory(),
            vec![
                ReservedMemory{ addr: 0x40000000, size: 0x1000 },
                ReservedMemory{ addr: 0x40002000, size: 0x1000 },
                ReservedMemory{ addr: 0x40004000, size: 0x1000 }]);
        assert_eq!(
            dt.follow("/").unwrap().property("#size-cells"),
            Some(&[...][..]));
        ...
    }
}

Instead of conclusion

That was a long post, but since it mostly covers the specification and doesn’t introduce any complicated ideas it wasn’t too hard to understad.

I used Rust for the examples in the post and ommitted some of the details, for example, implementation of some interfaces and #[derive(...)] as well as how the code is split between files.

Omitted parts should not be that big of a problem since the complete version is available on GitHub. For the code to this post you need to look inside the kernel/devicetree directory.

Any suggestions and questions are welcome, as always.

tags: devicetree - system-programming