[PATCH v2 2/3] doc: Update devicedocs including how to add tweaks

[Heinrich Schuchardt]

On 8/2/21 2:57 AM, 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 at chromium.org>

I will fix a few typos when merging, e.g.

Risc-V -> RISC-V
beeen -> been
repo -> repository

Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>

> ---
>
> Changes in v2:
> - Update many, many things as suggested by Heinrich
>
>   doc/develop/devicetree/control.rst | 181 ++++++++++++++++-------------
>   1 file changed, 100 insertions(+), 81 deletions(-)
>
> diff --git a/doc/develop/devicetree/control.rst b/doc/develop/devicetree/control.rst
> index 1289b6156fe..9f2c21034d0 100644
> --- a/doc/develop/devicetree/control.rst
> +++ b/doc/develop/devicetree/control.rst
> @@ -1,93 +1,72 @@
>   .. 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
> -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.
> +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. 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.
> +- 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 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
> +The arch/<arch>/dts directories contains a Makefile 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 device tree file, and have a single
> +the features of each board in the devicetree 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?
> ----------------------------
> +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 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:
> +There is also a mailing list (dtlist_) for the compiler and associated
> +tools.
>
> -	http://lists.ozlabs.org/listinfo/devicetree-discuss
> -
> -In case you are wondering, OF stands for Open Firmware.
> +In case you are wondering, OF stands for Open Firmware. This follows the
> +convention used in Linux.
>
>
>   Tools
>   -----
>
> -To use this feature you will need to get the device tree compiler. This is
> +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), it is currently not used.
> +(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
> -
> -For example::
> +    git://git.kernel.org/pub/scm/utils/dtc/dtc.git
>
> -	$ git clone git://git.kernel.org/pub/scm/utils/dtc/dtc.git
> -	$ cd dtc
> -	$ make
> -	$ sudo make install
> +You can decode a binary file with::
>
> -Then run the compiler (your version will vary)::
> +    dtc -I dtb -O dts <filename.dtb>
>
> -	$ 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
> +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.
>
> -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?
> -----------------------------------------
> +Where do I get an devicetree 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.
> @@ -106,41 +85,37 @@ Use::
>
>      #define CONFIG_DEFAULT_DEVICE_TREE	"<name>"
>
> -to set the filename of the device tree source. Then put your device tree
> +to set the filename of the devicetree source. Then put your devicetree
>   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.
> +This should include your CPU or SOC's devicetree file, placed in
> +`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
>   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
> +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 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
> +devicetree 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.
> +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 device tree file that you have compiled yourself, pass
> +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
> @@ -165,7 +140,7 @@ To use this, put something like this in your board header file::
>
>   Build:
>
> -After board configuration is done, fdt supported u-boot can be build in two
> +After the board configuration is done, fdt supported u-boot can be built in two
>   ways:
>
>   #  build the default dts which is defined from CONFIG_DEFAULT_DEVICE_TREE::
> @@ -177,12 +152,41 @@ 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 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 incluces a file with updates to the standard
> +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
>   -----------------------
>
>   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
> +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.
>
> @@ -201,24 +205,24 @@ 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-initialise the DM subsystem using dm_uninit() and
>   dm_init_and_scan().
>
>
>   Limitations
>   -----------
>
> -U-Boot is designed to build with a single architecture type and CPU
> +Devicetree 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 you cannot build for multiple cpu types or
> +time. Similarly U-Boot cannot be built 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,
> @@ -227,4 +231,19 @@ 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
> +used it before Linux (e.g. snow). The two projects developed in parallel
> +and there are still some differences in the bindings with certain boards.
> +While there has beeen discussion of having a separate repo for devicetree
> +files, in practice the Linux kernel Git repository has become the place where
> +these are 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/
>