Devicetree Control in U-Boot
This feature provides for run-time configuration of U-Boot via a flattened devicetree (fdt).
This feature aims to make it possible for a single U-Boot binary to support multiple boards, with the exact configuration of each board controlled by a flattened devicetree (fdt). This is the approach taken by Linux kernel for ARM and RISC-V and has been used by PowerPC for some time.
The fdt is a convenient vehicle for implementing run-time configuration for three reasons:
There is already excellent infrastructure for the fdt: a compiler checks the text file and converts it to a compact binary format, and a library is already available in U-Boot (libfdt) for handling this format
It is extensible since it consists of nodes and properties in a nice hierarchical format
It is fairly efficient to read incrementally
The U-Boot Makefile infrastructure allows for building the devicetree blob and embedding it in the U-Boot image. This is useful since it allows U-Boot to configure itself according to what it finds there. If you have a number of similar boards with different peripherals, you can describe the features of each board in the devicetree file, and have a single generic source base.
To enable this feature, select OF_CONTROL via Kconfig.
What is a Flattened Devicetree?
An fdt can be specified in source format as a text file. To read about the fdt syntax, take a look at the devicetree specification.
There is also a devicetree compiler mailing list for the compiler and associated tools.
In case you are wondering, OF stands for Open Firmware. This follows the convention used in Linux.
Tools
To create flattened device trees the device tree compiler is used. This is provided by U-Boot automatically. If you have a system version of dtc (typically in the ‘device-tree-compiler’ package), that system version is currently not used.
If you want to build your own dtc, it is kept here:
git://git.kernel.org/pub/scm/utils/dtc/dtc.git
You can decode a binary file with:
dtc -I dtb -O dts <filename.dtb>
That repo also includes fdtget/fdtput for reading and writing properties in a binary file. U-Boot adds its own fdtgrep for creating subsets of the file.
Where do I get a devicetree file for my board?
The devicetree files and devicetree bindings are maintained as part of the Linux kernel git repository. Traditionally, U-Boot placed copies of devicetree source files from the Linux kernel into arch/<arch>/dts/<name>.dts. However, this required each board maintainer to manually keep their devicetree in sync with the Linux kernel and often led to divergence between these copies.
U-Boot rather maintains a Git subtree as dts/upstream/ sub-directory. It is regularly synced with the Linux kernel and hence no need for manual devicetree sync. You may find that the dts/upstream/ already has a suitable devicetree file for your board. Look in dts/upstream/src/<arch>/<vendor>.
If not you might find other boards with suitable files that you can modify to your needs. Look in the board directories for files with a .dts extension.
Failing that, you could write one from scratch yourself!
Resyncing with devicetree-rebasing
The devicetree-rebasing repository maintains a fork cum mirror copy of devicetree files along with the bindings synced at every Linux kernel major release or intermediate release candidates. The U-Boot maintainers regularly sync the dts/upstream/ subtree from the devicetree-rebasing repo whenever the next branch opens (refer: Release Cycle) with the latest mainline Linux kernel release. To sync the dts/upstream/ subtree, run:
./dts/update-dts-subtree.sh pull <devicetree-rebasing-release-tag>
If required it is also possible to cherry-pick fixes from the devicetree-rebasing repository prior to next sync, usage:
./dts/update-dts-subtree.sh pick <devicetree-rebasing-commit-id>
Configuration
SoC/board maintainers are encouraged to migrate to use synced copies from dts/upstream/src/<arch>/<vendor>. To do that add imply OF_UPSTREAM for the SoC being used via Kconfig and set DEFAULT_DEVICE_TREE=<vendor>/<name> when prompted by Kconfig.
However, if dts/upstream/ hasn’t yet received devicetree source file for your newly added board support then you can add corresponding devicetree source file as arch/<arch>/dts/<name>.dts. To select that add # CONFIG_OF_UPSTREAM is not set and set DEFAULT_DEVICE_TREE=<name> when prompted by Kconfig.
This should include your CPU or SoC’s devicetree file. On top of that any U-Boot specific tweaks (see: Adding tweaks for U-Boot) can be made for your board.
If OF_EMBED is selected by Kconfig, then it will be picked up and built into the U-Boot image (including u-boot.bin). This is suitable for debugging and development only and is not recommended for production devices.
If OF_SEPARATE is selected by Kconfig, then it will be built and placed in a u-boot.dtb file alongside u-boot-nodtb.bin with the combined result placed in u-boot.bin so you can still just flash u-boot.bin onto your board. If Kconfig option SPL_FRAMEWORK is enabled, then u-boot.img will be built to include the device tree binary.
If OF_BOARD is selected by Kconfig, a board-specific routine will provide the devicetree at runtime, for example if an earlier bootloader stage creates it and passes it to U-Boot.
If BLOBLIST is selected by Kconfig, the devicetree may come from a bloblist passed from a previous stage, if present.
If SANDBOX is selected by Kconfig, then it will be read from a file on startup. Use the -d flag to U-Boot to specify the file to read, -D for the default and -T for the test devicetree, used to run sandbox unit tests.
You cannot use more than one of these options at the same time.
To use a devicetree file that you have compiled yourself, pass EXT_DTB=<filename> to ‘make’, as in:
make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
Then U-Boot will copy that file to u-boot.dtb, put it in the .img file if used, and u-boot-dtb.bin.
If you wish to put the fdt at a different address in memory, you can define the “fdtcontroladdr” environment variable. This is the hex address of the fdt binary blob, and will override either of the options. Be aware that this environment variable is checked prior to relocation, when only the compiled-in environment is available. Therefore it is not possible to define this variable in the saved SPI/NAND flash environment, for example (it will be ignored). After relocation, this variable will be set to the address of the newly relocated fdt blob. It is read-only and cannot be changed. It can optionally be used to control the boot process of Linux with bootm/bootz commands.
To use this, put something like this in your board header file:
#define CFG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
Build:
After the board configuration is done, fdt supported u-boot can be built in two ways:
# build the default dts which is selected by DEFAULT_DEVICE_TREE Kconfig:
$ make
# build the user specified dts file:
$ make DEVICE_TREE=<dts-file-name>
Adding tweaks for U-Boot
With dts/upstream Git subtree, it is ensured that devicetree files in U-Boot are an exact copy of those in Linux kernel available under dts/upstream/src/<arch>/<vendor>.
U-Boot is of course a very different project from Linux, e.g. it operates under much more restrictive memory and code-size constraints. Where Linux may use a full clock driver with Common Clock Format (CCF) to find the input clock to the UART, U-Boot typically wants to output a banner as early as possible before too much code has run.
A second difference is that U-Boot includes different phases. For SPL, constraints are even more extreme and the devicetree is shrunk to remove unwanted nodes, or even turned into C code to avoid access overhead.
U-Boot automatically looks for and includes a file with updates to the standard devicetree for your board, searching for them in arch/<arch>/dts/ in this order:
<orig_filename>-u-boot.dtsi
<CONFIG_SYS_SOC>-u-boot.dtsi
<CONFIG_SYS_CPU>-u-boot.dtsi
<CONFIG_SYS_VENDOR>-u-boot.dtsi
u-boot.dtsi
Only one of these is selected but of course you can #include another one within that file, to create a hierarchy of shared files.
External .dtsi fragments
Apart from describing the hardware present, U-Boot also uses its control dtb for various configuration purposes. For example, the public key(s) used for Verified Boot are embedded in a specific format in a /signature node.
As mentioned above, the U-Boot build system automatically includes a *-u-boot.dtsi file, if found, containing U-Boot specific quirks. However, some data, such as the mentioned public keys, are not appropriate for upstream U-Boot but are better kept and maintained outside the U-Boot repository. You can use DEVICE_TREE_INCLUDES Kconfig option to specify a list of .dtsi files that will also be included when building .dtb files.
Devicetree bindings schema checks
With devicetee-rebasing Git subtree, the devicetree bindings are also regularly synced with Linux kernel as dts/upstream/Bindings/ sub-directory. This allows U-Boot to run devicetree bindings schema checks which will bring compliance to U-Boot core/drivers regarding usage of devicetree.
Dependencies
The DT schema project must be installed in order to validate the DT schema binding documents and validate DTS files using the DT schema. For installation instructions, refer to the DT schema project page.
Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be installed. Ensure they are in your PATH (~/.local/bin by default).
You should also install yamllint (used by dtschema when present). On Debian/ Ubuntu systems:
apt install yamllint
Running checks
In order to perform validation of DTB files, use the dtbs_check target:
make dtbs_check
It is also possible to run checks with a subset of matching schema files by
setting the DT_SCHEMA_FILES variable to 1 or more specific schema files or
patterns (partial match of a fixed string). Each file or pattern should be
separated by ‘:’.
make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
make dtbs_check DT_SCHEMA_FILES=/gpio/
make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml
Relocation, SPL and TPL
U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.
The full devicetree is available to U-Boot proper, but normally only a subset (or none at all) is available to TPL and SPL. See ‘Pre-Relocation Support’ and ‘SPL Support’ in doc/driver-model/design.rst for more details.
Using several DTBs in the SPL (SPL_MULTI_DTB_FIT Kconfig option)
In some rare cases it is desirable to let SPL be able to select one DTB among many. This usually not very useful as the DTB for the SPL is small and usually fits several platforms. However the DTB sometimes include information that do work on several platforms (like IO tuning parameters). In this case it is possible to use SPL_MULTI_DTB_FIT Kconfig option. This option appends to the SPL a FIT image containing several DTBs listed in SPL_OF_LIST. board_fit_config_name_match() is called to select the right DTB.
If board_fit_config_name_match() relies on DM (DM driver to access an EEPROM containing the board ID for example), it possible to start with a generic DTB and then switch over to the right DTB after the detection. For this purpose, the platform code must call fdtdec_resetup(). Based on the returned flag, the platform may have to re-initialise the DM subsystem using dm_uninit() and dm_init_and_scan().
Limitations
Devicetrees can help reduce the complexity of supporting variants of boards which use the same SOC / CPU.
However U-Boot is designed to build for a single architecture type and CPU type. So for example it is not possible to build a single ARM binary which runs on your AT91 and OMAP boards, relying on an fdt to configure the various features. This is because you must select one of the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build time. Similarly U-Boot cannot be built for multiple cpu types or architectures.
It is important to understand that the fdt only selects options available in the platform / drivers. It cannot add new drivers (yet). So you must still have the Kconfig option to enable the driver. For example, you need to enable SYS_NS16550 Kconfig option to bring in the NS16550 driver, but can use the fdt to specific the UART clock, peripheral address, etc. In very broad terms, the Kconfig options in general control what driver files are pulled in, and the fdt controls how those files work.
History
U-Boot configuration was previous done using CONFIG options in the board config file. This eventually got out of hand with nearly 10,000 options.
U-Boot adopted devicetrees around the same time as Linux and early boards used it before Linux (e.g. snow). The two projects developed in parallel and there are still some differences in the bindings for certain boards. While there has been discussion of having a separate repository for devicetree files, in practice the Linux kernel Git repository has become the place where these are stored, with U-Boot taking copies via devicetree-rebasing repository and adding tweaks with u-boot.dtsi files.