u-boot/doc/README.fdt-control

# SPDX-License-Identifier: GPL-2.0+
#
# Copyright (c) 2011 The Chromium OS Authors.

Device Tree Control in U-Boot
=============================

This feature provides for run-time configuration of U-Boot via a flat
device tree (fdt). U-Boot configuration has traditionally been done
using CONFIG options in the board config file. 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 flat device
tree (fdt). This is the approach recently taken by the ARM Linux kernel
and has been used by PowerPC for some time.

The fdt is a convenient vehicle for implementing run-time configuration
for three reasons. Firstly it is easy to use, being a simple text file.
It is extensible since it consists of nodes and properties in a nice
hierarchical format.

Finally, 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.

The dts directory contains a Makefile for building the device tree blob
and embedding it in your 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 device tree file, and have a single
generic source base.

To enable this feature, add CONFIG_OF_CONTROL to your board config file.


What is a Flat Device Tree?
---------------------------

An fdt can be specified in source format as a text file. To read about
the fdt syntax, take a look at the specification here:

https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf

You also might find this section of the Linux kernel documentation
useful: (access this in the Linux kernel source code)

	Documentation/devicetree/booting-without-of.txt

There is also a mailing list:

	http://lists.ozlabs.org/listinfo/devicetree-discuss

In case you are wondering, OF stands for Open Firmware.


Tools
-----

To use this feature you will need to get the device tree compiler. This is
provided by U-Boot automatically. If you have a system version of dtc
(typically in the 'device-tree-compiler' package), it 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

For example:

	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
	$ cd dtc
	$ make
	$ sudo make install

Then run the compiler (your version will vary):

	$ dtc -v
	Version: DTC 1.2.0-g2cb4b51f
	$ make tests
	$ cd tests
	$ ./run_tests.sh
	********** TEST SUMMARY
	*     Total testcases:	1371
	*                PASS:	1371
	*                FAIL:	0
	*   Bad configuration:	0
	* Strange test result:	0

You will also find a useful fdtdump utility for decoding a binary file, as
well as fdtget/fdtput for reading and writing properties in a binary file.


Where do I get an fdt file for my board?
----------------------------------------

You may find that the Linux kernel has a suitable file. Look in the
kernel source in arch/<arch>/boot/dts.

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!


Configuration
-------------

Use:

#define CONFIG_DEFAULT_DEVICE_TREE	"<name>"

to set the filename of the device tree source. Then put your device tree
file into

	board/<vendor>/dts/<name>.dts

This should include your CPU or SOC's device tree file, placed in
arch/<arch>/dts, and then make any adjustments required.

If CONFIG_OF_EMBED is defined, 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 CONFIG_OF_SEPARATE is defined, then it will be built and placed in
a u-boot.dtb file alongside u-boot-nodtb.bin. A common approach is then to
join the two:

	cat u-boot-nodtb.bin u-boot.dtb >image.bin

and then flash image.bin onto your board. Note that U-Boot creates
u-boot-dtb.bin which does the above step for you also. Resulting
u-boot.bin is a copy of u-boot-dtb.bin in this case. If you are using
CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device
tree binary.

If CONFIG_OF_BOARD is defined, a board-specific routine will provide the
device tree at runtime, for example if an earlier bootloader stage creates
it and passes it to U-Boot.

If CONFIG_OF_HOSTFILE is defined, then it will be read from a file on
startup. This is only useful for sandbox. Use the -d flag to U-Boot to
specify the file to read.

You cannot use more than one of these options at the same time.

To use a device tree 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 CONFIG_EXTRA_ENV_SETTINGS	"fdtcontroladdr=10000\0"

Build:

After board configuration is done, fdt supported u-boot can be build in two ways:
1)  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE
    $ make
2)  build the user specified dts file
    $ make DEVICE_TREE=<dts-file-name>


Relocation, SPL and TPL
-----------------------

U-Boot can be divided into three phases: TPL, SPL and U-Boot proper.

The full device tree 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 (CONFIG_SPL_MULTI_DTB)
----------------------------------------------------
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 CONFIG_SPL_MULTI_DTB. 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-initiliaze the DM subusystem using dm_uninit() and
dm_init_and_scan().


Limitations
-----------

U-Boot is designed to build with 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 you cannot build for multiple cpu types or
architectures.

That said the complexity reduction by using fdt to support variants of
boards which use the same SOC / CPU can be substantial.

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 CONFIG option to enable the driver. For example,
you need to define CONFIG_SYS_NS16550 to bring in the NS16550 driver,
but can use the fdt to specific the UART clock, peripheral address, etc.
In very broad terms, the CONFIG options in general control *what* driver
files are pulled in, and the fdt controls *how* those files work.

--
Simon Glass <sjg@chromium.org>
1-Sep-11