On 7/25/21 6:43 PM, Simon Glass wrote:

This file is about 10 years old and the updates have not covered everything that has changed, particularly in the last few years. Update the information and add mention of the u-boot.dtsi files.

Signed-off-by: Simon Glass sjg@chromium.org

doc/develop/devicetree/control.rst | 107 ++++++++++++++++++++--------- 1 file changed, 74 insertions(+), 33 deletions(-)

diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst index 1289b6156fe..ac5f7989cc6 100644 --- a/doc/develop/devicetree/control.rst +++ b/doc/develop/devicetree/control.rst @@ -1,26 +1,27 @@ .. SPDX-License-Identifier: GPL-2.0+ .. sectionauthor:: Copyright 2011 The Chromium OS Authors

-Device Tree Control in U-Boot

+Devicetree Control in U-Boot +============================

This feature provides for run-time configuration of U-Boot via a flat

%s/flag/flattened/

-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. +device tree (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 flat device tree (fdt). This is the approach recently taken by Linux (e.g.

More than a decade ago is not recent in the software world.

+for PowerPC and ARM).

and RISC-V

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. +for three reasons:

-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. +- Firstly it is easy to use, being a simple text file

Nay. The flattened devicetree is a binary format.

Assume that the reader has no knowledge of devicetrees. Please, describe that there is a text format compiled to the binary format before referring to it.

+- It is extensible since it consists of nodes and properties in a nice

• hierarchical format

+- 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 fairly efficient to read incrementally

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 @@ -38,14 +39,8 @@ 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 (dtspec_).

-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

+There is also a mailing list (dtlist_) for the compiler and associated +tools.

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

But why would we use OF in symbol CONFIG_OF_CONTROL if we mean devicetree?

@@ -82,8 +77,9 @@ Then run the compiler (your version will vary)::

• 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. +You will also find a useful `fdtdump` utility for decoding a binary file, as

fdtdump is deprecated. Use 'dtc -I dtb -O dts' instead.

+well as `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 an fdt file for my board? @@ -109,10 +105,11 @@ Use:: to set the filename of the device tree source. Then put your device tree file into::

• board/<vendor>/dts/<name>.dts

• arch/<arch>/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. +`arch/<arch>/dts`, and then make any adjustments required using a u-boot-dtsi +file for your board.

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 @@ -122,11 +119,10 @@ 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

• cat u-boot-nodtb.bin u-boot.dtb >u-boot.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 +u-boot.bin so does the above step for you automatically. If you are using CONFIG_SPL_FRAMEWORK, then u-boot.img will be built to include the device tree binary.

@@ -136,7 +132,8 @@ 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. +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.

@@ -177,6 +174,35 @@ ways: $ make DEVICE_TREE=<dts-file-name>

+Adding tweaks for U-Boot +------------------------

+It is strongly recommended that devicetree files in U-Boot be an exact copy of +those in Linux, so that it is easy to sync them up from time to time.

+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 different is that U-Boot includes different phases. For SPL,

%s/different/difference/

+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 incluces a file with updates to the standard

%s/incluces/includes/

+devicetree for your board, searching for them in the same directory as the +main file, 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.

• Relocation, SPL and TPL

@@ -201,7 +227,7 @@ 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 +platform may have to re-initiliaze the DM subsystem using dm_uninit() and

%s/re-initiliaze/re-initialize/

dm_init_and_scan().

@@ -227,4 +253,19 @@ 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.

-.. _dtspec: https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.0.pdf +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 devicetree around the same time as linux and early boards

%s/linux/Linux/ Linux is a proper name.

+used it before linux (e.g. snow). The two projects developed in parallel

ditto

+and there are still some differences in the bindings with certain boards. +While there has beeen dicussion of having a separate repo for devicetree

%s/dicussion/discussion/

%s/repo/repository/

+files, in practice the kernel's repo has become the place where these are

%/kernel's repo/the Linux kernel Git repository/

Best regards

Heinrich

+stored, with U-Boot taking copies and adding tweaks with u-boot.dtsi files.

+.. _dtspec: https://www.devicetree.org/specifications/ +.. _dtlist: https://www.spinics.net/lists/devicetree-compiler/