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When U-Boot started using SPDX tags we were among the early adopters and there weren't a lot of other examples to borrow from. So we picked the area of the file that usually had a full license text and replaced it with an appropriate SPDX-License-Identifier: entry. Since then, the Linux Kernel has adopted SPDX tags and they place it as the very first line in a file (except where shebangs are used, then it's second line) and with slightly different comment styles than us. In part due to community overlap, in part due to better tag visibility and in part for other minor reasons, switch over to that style. This commit changes all instances where we have a single declared license in the tag as both the before and after are identical in tag contents. There's also a few places where I found we did not have a tag and have introduced one. Signed-off-by: Tom Rini <trini@konsulko.com>
197 lines
6.8 KiB
Text
197 lines
6.8 KiB
Text
# SPDX-License-Identifier: GPL-2.0+
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#
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# Copyright (c) 2011 The Chromium OS Authors.
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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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