#
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# Copyright (c) 2011 The Chromium OS Authors.
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#
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# SPDX-License-Identifier: GPL-2.0+
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#
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Device Tree Control in U-Boot
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=============================
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This feature provides for run-time configuration of U-Boot via a flat
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device tree (fdt). U-Boot configuration has traditionally been done
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using CONFIG options in the board config file. This feature aims to
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make it possible for a single U-Boot binary to support multiple boards,
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with the exact configuration of each board controlled by a flat device
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tree (fdt). This is the approach recently taken by the ARM Linux kernel
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and has been used by PowerPC for some time.
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The fdt is a convenient vehicle for implementing run-time configuration
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for three reasons. Firstly it is easy to use, being a simple text file.
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It is extensible since it consists of nodes and properties in a nice
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hierarchical format.
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Finally, there is already excellent infrastructure for the fdt: a
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compiler checks the text file and converts it to a compact binary
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format, and a library is already available in U-Boot (libfdt) for
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handling this format.
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The dts directory contains a Makefile for building the device tree blob
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and embedding it in your U-Boot image. This is useful since it allows
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U-Boot to configure itself according to what it finds there. If you have
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a number of similar boards with different peripherals, you can describe
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the features of each board in the device tree file, and have a single
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generic source base.
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To enable this feature, add CONFIG_OF_CONTROL to your board config file.
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What is a Flat Device Tree?
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---------------------------
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An fdt can be specified in source format as a text file. To read about
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the fdt syntax, take a look at the specification here:
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https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf
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You also might find this section of the Linux kernel documentation
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useful: (access this in the Linux kernel source code)
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Documentation/devicetree/booting-without-of.txt
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There is also a mailing list:
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http://lists.ozlabs.org/listinfo/devicetree-discuss
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In case you are wondering, OF stands for Open Firmware.
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Tools
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-----
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To use this feature you will need to get the device tree compiler here:
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git://git.kernel.org/pub/scm/utils/dtc/dtc.git
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For example:
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$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
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$ cd dtc
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$ make
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$ sudo make install
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Then run the compiler (your version will vary):
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$ dtc -v
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Version: DTC 1.2.0-g2cb4b51f
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$ make tests
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$ cd tests
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$ ./run_tests.sh
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********** TEST SUMMARY
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* Total testcases: 1371
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* PASS: 1371
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* FAIL: 0
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* Bad configuration: 0
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* Strange test result: 0
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You will also find a useful fdtdump utility for decoding a binary file, as
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well as fdtget/fdtput for reading and writing properties in a binary file.
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Where do I get an fdt file for my board?
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----------------------------------------
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You may find that the Linux kernel has a suitable file. Look in the
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kernel source in arch/<arch>/boot/dts.
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If not you might find other boards with suitable files that you can
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modify to your needs. Look in the board directories for files with a
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.dts extension.
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Failing that, you could write one from scratch yourself!
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Configuration
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-------------
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Use:
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#define CONFIG_DEFAULT_DEVICE_TREE "<name>"
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to set the filename of the device tree source. Then put your device tree
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file into
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board/<vendor>/dts/<name>.dts
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This should include your CPU or SOC's device tree file, placed in
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arch/<arch>/dts, and then make any adjustments required.
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If CONFIG_OF_EMBED is defined, then it will be picked up and built into
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the U-Boot image (including u-boot.bin). This is suitable for debugging
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and development only and is not recommended for production devices.
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If CONFIG_OF_SEPARATE is defined, then it will be built and placed in
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a u-boot.dtb file alongside u-boot.bin. A common approach is then to
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join the two:
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cat u-boot.bin u-boot.dtb >image.bin
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and then flash image.bin onto your board. Note that U-Boot creates
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u-boot-dtb.bin which does the above step for you also. If you are using
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CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
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tree binary.
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If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
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device tree at runtime, for example if an earlier bootloader stage creates
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it and passes it to U-Boot.
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If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
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startup. This is only useful for sandbox. Use the -d flag to U-Boot to
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specify the file to read.
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You cannot use more than one of these options at the same time.
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To use a device tree file that you have compiled yourself, pass
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EXT_DTB=<filename> to 'make', as in:
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make EXT_DTB=boot/am335x-boneblack-pubkey.dtb
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Then U-Boot will copy that file to u-boot.dtb, put it in the .img file
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if used, and u-boot-dtb.bin.
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If you wish to put the fdt at a different address in memory, you can
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define the "fdtcontroladdr" environment variable. This is the hex
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address of the fdt binary blob, and will override either of the options.
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Be aware that this environment variable is checked prior to relocation,
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when only the compiled-in environment is available. Therefore it is not
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possible to define this variable in the saved SPI/NAND flash
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environment, for example (it will be ignored). After relocation, this
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variable will be set to the address of the newly relocated fdt blob.
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It is read-only and cannot be changed. It can optionally be used to
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control the boot process of Linux with bootm/bootz commands.
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To use this, put something like this in your board header file:
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#define CONFIG_EXTRA_ENV_SETTINGS "fdtcontroladdr=10000\0"
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Build:
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After board configuration is done, fdt supported u-boot can be build in two ways:
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1) build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
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$ make
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2) build the user specified dts file
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$ make DEVICE_TREE=<dts-file-name>
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Limitations
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-----------
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U-Boot is designed to build with a single architecture type and CPU
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type. So for example it is not possible to build a single ARM binary
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which runs on your AT91 and OMAP boards, relying on an fdt to configure
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the various features. This is because you must select one of
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the CPU families within arch/arm/cpu/arm926ejs (omap or at91) at build
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time. Similarly you cannot build for multiple cpu types or
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architectures.
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That said the complexity reduction by using fdt to support variants of
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boards which use the same SOC / CPU can be substantial.
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It is important to understand that the fdt only selects options
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available in the platform / drivers. It cannot add new drivers (yet). So
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you must still have the CONFIG option to enable the driver. For example,
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you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
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but can use the fdt to specific the UART clock, peripheral address, etc.
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In very broad terms, the CONFIG options in general control *what* driver
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files are pulled in, and the fdt controls *how* those files work.
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--
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Simon Glass <sjg@chromium.org>
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1-Sep-11
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