[PATCH v7 01/31] doc: Add documentation about devicetree usage

Simon Glass sjg at chromium.org
Tue Dec 7 16:09:03 CET 2021


Hi Heinrich,

On Mon, 6 Dec 2021 at 17:58, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> On 12/6/21 16:11, Simon Glass wrote:
> > At present some of the ideas and techniques behind devicetree in U-Boot
> > are assumed, implied or unsaid. Add some documentation to cover how
> > devicetree is build, how it can be modified and the rules about using
> > the various CONFIG_OF_... options.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > Reviewed-by: Marcel Ziswiler <marcel.ziswiler at toradex.com>
> > ---
> > This patch attracted quite a bit of discussion here:
> >
> > https://patchwork.ozlabs.org/project/uboot/patch/20210909201033.755713-4-sjg@chromium.org/
> >
> > I have not included the text suggested by François. While I agree that
> > it would be useful to have an introduction in this space, I do not agree
> > that we should have two devicetrees or that U-Boot should not have its own
> > things in the devicetree, so it is not clear to me what we should actually
> > write.
> >
> > The 'Devicetree Control in U-Boot' docs were recently merged and these
> > provide some base info, for now.
>
> As you already remarked part of this text has not been agreed upon.
>
> Please, either remove this part of the text or mark it as your private
> opinion.

Do you mean the comment after the patch? It won't appear in the commit message.

Regards,
SImon

>
> Best regards
>
> Heinrich
>
> >
> > Changes in v7:
> > - Fix 'linst' typo
> >
> > Changes in v6:
> > - Fix description of OF_BOARD so it refers just to the current state
> > - Explain that the 'two devicetrees' refers to two *control* devicetrees
> >
> > Changes in v5:
> > - Bring into the OF_BOARD series
> > - Rebase to master and drop mention of OF_PRIOR_STAGE, since removed
> > - Refer to the 'control' DTB in the first paragraph
> > - Use QEMU instead of qemu
> >
> > Changes in v3:
> > - Clarify the 'bug' refered to at the top
> > - Reword 'This means that there' paragraph to explain U-Boot-specific things
> > - Move to doc/develop/devicetree now that OF_CONTROL is in the docs
> >
> > Changes in v2:
> > - Fix typos per Sean (thank you!) and a few others
> > - Add a 'Use of U-Boot /config node' section
> > - Drop mention of dm-verity since that actually uses the kernel cmdline
> > - Explain that OF_BOARD will still work after these changes (in
> >    'Once this bug is fixed...' paragraph)
> > - Expand a bit on the reason why the 'Current situation' is bad
> > - Clarify in a second place that Linux and U-Boot use the same devicetree
> >    in 'To be clear, while U-Boot...'
> > - Expand on why we should have rules for other projects in
> >    'Devicetree in another project'
> > - Add a comment as to why devicetree in U-Boot is not 'bad design'
> > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'
> > - Rewrite 'Devicetree generated on-the-fly in another project' to cover
> >    points raised on v1
> > - Add 'Why does U-Boot have its nodes and properties?'
> > - Add 'Why not have two devicetrees?'
> >
> >   doc/develop/devicetree/dt_update.rst | 555 +++++++++++++++++++++++++++
> >   doc/develop/devicetree/index.rst     |   1 +
> >   2 files changed, 556 insertions(+)
> >   create mode 100644 doc/develop/devicetree/dt_update.rst
> >
> > diff --git a/doc/develop/devicetree/dt_update.rst b/doc/develop/devicetree/dt_update.rst
> > new file mode 100644
> > index 00000000000..e3b65f6fa66
> > --- /dev/null
> > +++ b/doc/develop/devicetree/dt_update.rst
> > @@ -0,0 +1,555 @@
> > +.. SPDX-License-Identifier: GPL-2.0+
> > +
> > +Updating the devicetree
> > +=======================
> > +
> > +U-Boot uses devicetree for runtime configuration and storing required blobs or
> > +any other information it needs to operate. This is called the 'control'
> > +devicetree since it controls U-Boot. It is possible to update the control
> > +devicetree separately from actually building U-Boot. This provides a good degree
> > +of control and flexibility for firmware that uses U-Boot in conjunction with
> > +other project.
> > +
> > +There are many reasons why it is useful to modify the devicetree after building
> > +it:
> > +
> > +- Configuration can be changed, e.g. which UART to use
> > +- A serial number can be added
> > +- Public keys can be added to allow image verification
> > +- Console output can be changed (e.g. to select serial or vidconsole)
> > +
> > +This section describes how to work with devicetree to accomplish your goals.
> > +
> > +See also :doc:`../devicetree/control` for a basic summary of the available
> > +features.
> > +
> > +
> > +Devicetree source
> > +-----------------
> > +
> > +Every board in U-Boot must include a devicetree sufficient to build and boot
> > +that board on suitable hardware (or emulation). This is specified using the
> > +`CONFIG DEFAULT_DEVICE_TREE` option.
> > +
> > +
> > +Current situation (October 2021)
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty,
> > +e.g. if `CONFIG_OF_BOARD` is used. This has unfortunately created an enormous
> > +amount of confusion and some wasted effort. This was not intended. Support for
> > +an empty `CONFIG_DEFAULT_DEVICE_TREE` will be dropped soon.
> > +
> > +Some of the problems created are:
> > +
> > +- It is not obvious that the devicetree is coming from another project
> > +
> > +- There is no way to see even a sample devicetree for these platform in U-Boot,
> > +  so it is hard to know what is going on, e.g. which devices are typically
> > +  present
> > +
> > +- The other project may not provide a way to support U-Boot's requirements for
> > +  devicetree, such as the /config node. Note: On the U-Boot mailing list, this
> > +  was only discovered after weeks of discussion and confusion
> > +
> > +- For QEMU specifically, consulting two QEMU source files is required, for which
> > +  there are no references in U-Boot documentation. The code is generating a
> > +  devicetree, but it is not clear what controls affect this generation.
> > +
> > +Specifically on the changes in U-Bootm `CONFIG_OF_BOARD` was added in
> > +rpi_patch_ for Raspberry Pi, which does have an in-tree devicetree, but this
> > +feature has since been used for boards that don't
> > +
> > +Once this bug is fixed, CONFIG_OF_BOARD will override (at runtime) the
> > +evicetree suppled with U-Boot, but will otherwise use CONFIG_OF_SEPARATE for the
> > +in-tree build. So these two will become options, moving out of the 'choice' in
> > +`dts/Kconfig`.
> > +
> > +This means that there is a basic devicetree build in the U-Boot tree, for
> > +build-testing, consistency and documentation purposes, but at runtime U-Boot can
> > +accept its devicetree from another source. The in-tree devicetree may contain
> > +U-Boot-specific features (in u-boot*.dtsi files) and this may prove useful for
> > +the other project, so it can ensure that U-Boot functions correctly and supports
> > +all its expected features.
> > +
> > +To be clear, while U-Boot has its own copy of the devicetree source for each
> > +board, this must match the Linux source, perhaps with some u-boot.dtsi
> > +additions. The intent here is not to create a separate binding, just to provide
> > +a representative devicetree in U-Boot.
> > +
> > +Offending boards are:
> > +
> > +- rpi_4 and rpi_4_32b (other rpi boards do have an in-tree devicetree)
> > +- qemu_arm64
> > +- qemu_arm
> > +- qemu-ppce500
> > +- qemu-riscv32
> > +- qemu-riscv32_smode
> > +- qemu-riscv64
> > +- qemu-riscv64_smode
> > +
> > +All of these need to have a devicetree added in-tree. This is targeted to be
> > +fixed in the 2022.01 release.
> > +
> > +
> > +Building the devicetree
> > +-----------------------
> > +
> > +U-Boot automatically builds the devicetree for a board, from the
> > +`arch/<arch>/dts` directory. The Makefile in those directories has rules for
> > +building devicetree files. It is preferable to avoid target-specific rules in
> > +those files: i.e. all boards for a particular SoC should be built at once,
> > +where practical. Apart from simplifying the Makefile, this helps to efficiently
> > +(and immediately) ensure that changes in one board's DT do not break others that
> > +are related. Building devicetrees is fast, so performance is seldom a concern
> > +here.
> > +
> > +
> > +Overriding the default devicetree
> > +---------------------------------
> > +
> > +When building U-Boot, the `DEVICE_TREE` environment variable allows the
> > +default devicetree file to be overridden at build time. This can be useful if
> > +modifications have to be made to the in-tree devicetree file, for the benefit
> > +of a downstream build system. Note that the in-tree devicetree must be
> > +sufficient to build and boot, so this is not a way to bypass that requirement.
> > +
> > +
> > +Modifying the devicetree after building
> > +---------------------------------------
> > +
> > +While it is generally painful and hacky to modify the code or rodata of a
> > +program after it is built, in many cases it is useful to do so, e.g. to add
> > +configuration information like serial numbers, enabling/disabling features, etc.
> > +
> > +Devicetree provides a very nice solution to these problems since it is
> > +structured data and it is relatively easy to change it, even in binary form
> > +(see fdtput).
> > +
> > +U-Boot takes care that the devicetree is easily accessible after the build
> > +process. In fact it is placed in a separate file called `u-boot.dtb`. If the
> > +build system wants to modify or replace that file, it can do so. Then all that
> > +is needed is to run `binman update` to update the file inside the image. If
> > +binman is not used, then `u-boot-nodtb.bin` and the new `u-boot.dtb` can simply
> > +be concatenated to achieve the desired result. U-Boot happily copes with the
> > +devicetree growing or shrinking.
> > +
> > +The `u-boot.bin` image contains both pieces. While it is possible to locate the
> > +devicetree within the image using the signature at the start of the file, this
> > +is a bit messy.
> > +
> > +This is why `CONFIG_OF_SEPARATE` should always be used when building U-Boot.
> > +The `CONFIG_OF_EMBED` option embeds the devicetree somewhere in the U-Boot ELF
> > +image as rodata, meaning that it is hard to find it and it cannot increase in
> > +size.
> > +
> > +When modifying the devicetree, the different cases to consider are as follows:
> > +
> > +- CONFIG_OF_SEPARATE
> > +    This is easy, described above. Just change, replace or rebuild the
> > +    devicetree so it suits your needs, then rerun binman or redo the `cat`
> > +    operation to join `u-boot-nodtb.bin` and the new `u-boot.dtb`
> > +
> > +- CONFIG_OF_EMBED
> > +    This is tricky, since the devicetree cannot easily be located. If the EFL
> > +    file is available, then the _dtb_dt_begin and __dtb_dt_end symbols can be
> > +    examined to find it. While it is possible to contract the file, it is not
> > +    possible to expand the file since that would involve re-linking
> > +
> > +- CONFIG_OF_BOARD
> > +    This is a board-specific situation, so needs to be considered on a
> > +    case-by-case base.
> > +
> > +
> > +Use of U-Boot /config node
> > +--------------------------
> > +
> > +A common problem with firmware is that many builds are needed to deal with the
> > +slight variations between different, related models. For example, one model may
> > +have a TPM and another may not. Devicetree provides an excellent solution to
> > +this problem, in that the devicetree to actually use on a platform can be
> > +injected in the factory based on which model is being manufactured at the time.
> > +
> > +A related problem causing build proliferation is dealing with the differences
> > +between development firmware, developer-friendly firmware (e.g. with all
> > +security features present but with the ability to access the command line),
> > +test firmware (which runs tests used in the factory), final production firmware
> > +(before signing), signed firmware (where the signatures have been inserted) and
> > +the like. Ideally all or most of these should use the same U-Boot build, with
> > +just some options to determine the features available. For example, being able
> > +to control whether the UART console or JTAG are available, on any image, is a
> > +great debugging aid.
> > +
> > +When the firmware consists of multiple parts, it is helpful that all operate
> > +the same way at runtime, regardless of how they were built. This can be achieved
> > +by passing the runtime configuration (e.g. 'enable UART console) along the chain
> > +through each firmware stage. It is frustrating to have to replicate a bug on
> > +production firmware which does happen on developer firmware, because they are
> > +completely different builds.
> > +
> > +The /config node provides useful functionality for this. It allows the different
> > +controls to be 'factored out' of the U-Boot binary, so they can be controlled
> > +separately from the initial source-code build. The node can be easily updated by
> > +a build or factory tool and can control various features in U-Boot. It is
> > +similar in concept to a Kconfig option, except that it can be changed after
> > +U-Boot is built.
> > +
> > +The /config node is similar in concept to the `/chosen node`_ except that it is
> > +for passing information *into* firmware instead of from firmware to the
> > +Operating System. Also, while Linux has a (sometimes extremely long) command
> > +line, U-Boot does not support this. The devicetree provides a more structured
> > +approach in any case.
> > +
> > +
> > +Devicetree in another project
> > +-----------------------------
> > +
> > +In some cases U-Boot receive its devicetree at runtime from a program that calls
> > +it. For example ARM's Trusted Firmware A (`TF-A`_) may have a devicetree that it
> > +passes to U-Boot. This overrides any devicetree build by U-Boot. When packaging
> > +the firmware, the U-Boot devicetree may in fact be left out if it can be
> > +guaranteed that it will receive one from another project.
> > +
> > +In this case, the devicetree in the other project must track U-Boot's use of
> > +device tree, for the following reasons:
> > +
> > +- U-Boot only has one devicetree. See `Why not have two devicetrees?`_.
> > +- For a consistent firmware build, decisions made in early stages should be
> > +  communicated to later ones at runtime. For example, if the serial console is
> > +  enabled in an early stage, it should be enabled in U-Boot too.
> > +- U-Boot is quite capable of managing its own copy of the devicetree. If
> > +  another project wants to bypass this (often for good reason), it is reasonable
> > +  that it should take on the (fairly small) requirements that U-Boot features
> > +  that rely on devicetree are still available
> > +- The point here is not that *U-Boot needs this extra node*, or *U-Boot needs
> > +  to have this public key*. These features are present in U-Boot in service of
> > +  the entire firmware system. If the U-Boot features are used, but cannot be
> > +  supported in the normal way, then there is pressure to implement these
> > +  features in other ways. In the end, we would have a different mechanism for
> > +  every other project that uses U-Boot. This introduces duplicate ways of doing
> > +  the same thing, needlessly increases the complexity of the U-Boot source code,
> > +  forces authors to consider parallel implementations when writing new features,
> > +  makes U-Boot harder to test, complicates documentation and confuses the
> > +  runtime flow of U-Boot. If every board did things its own way rather than
> > +  contributing to the common code, U-Boot would lose a lot of its cross-platform
> > +  value.
> > +
> > +The above does not indicate *bad design* within U-Boot. Devicetree is a core
> > +component of U-Boot and U-Boot makes use of it to the full. It solves a myriad
> > +of problems that would otherwise need their own special C struct, binary format,
> > +special property, tooling for viewing and updating, etc.
> > +
> > +Specifically, the other project must provide a way to add configuration and
> > +other information to the devicetree for use by U-Boot, such as the /config node.
> > +Note that the U-Boot in-tree devicetree source must be sufficient to build and
> > +boot, so this is not a way to bypass that requirement.
> > +
> > +If binman is used, the devicetree source in U-Boot must contain the binman
> > +definition so that a valid image can be build. This helps people discover what
> > +other firmware components are needed and seek out appropriate documentation.
> > +
> > +If verified boot is used, the project must provide a way to inject a public key,
> > +certificate or other material into the U-Boot devicetree so that it is available
> > +to U-Boot at runtime. See `Signing with U-Boot devicetree`_. This may be
> > +through tooling in the project itself or by making use of U-Boot's tooling.
> > +
> > +
> > +Devicetree generated on-the-fly in another project
> > +--------------------------------------------------
> > +
> > +In some rare cases, another project may wish to create a devicetree for U-Boot
> > +entirely on-the-fly, then pass it to U-Boot at runtime. The only known example
> > +of this at the time of writing (2021) is QEMU, for ARM (`QEMU ARM`_) and
> > +RISC-V (`QEMU RISC-V`_).
> > +
> > +In effect, when the board boots, U-Boot is *downstream* of the other project.
> > +It is entirely reliant on that project for its correct operation.
> > +
> > +This does not mean to imply that the other project is creating its own,
> > +incompatible devicetree. In fact QEMU generates a valid devicetree which is
> > +suitable for both U-Boot and Linux. It is quite normal for a devicetree to be
> > +present in flash and be made available to U-Boot at runtime. What matters is
> > +where the devicetree comes from. If the other project builds a devicetree for
> > +U-Boot then it needs to support adding the things needed by U-Boot features.
> > +Without them, for example:
> > +
> > +- U-Boot may not boot because too many devices are enabled before relocation
> > +- U-Boot may not have access to the developer or production public keys used for
> > +  signing
> > +- U-Boot may not support controlling whether the console is enabled
> > +- U-Boot may not be know which MMC device to boot from
> > +- U-Boot may not be able to find other firmware components that it needs to load
> > +
> > +Normally, supporting U-Boot's features is trivial, since the devicetree compiler
> > +(dtc) can compile the source, including any U-Boot pieces. So the burden is
> > +extremely low.
> > +
> > +In this case, the devicetree in the other project must track U-Boot's use of
> > +device tree, so that it remains compatible. See `Devicetree in another project`_
> > +for reasons why.
> > +
> > +If a particular version of the project is needed for a particular version of
> > +U-Boot, that must be documented in both projects.
> > +
> > +Further, it must provide a way to add configuration and other information to
> > +the devicetree for use by U-Boot, such as the `/config` node and the tags used
> > +by driver model. Note that the U-Boot in-tree devicetree must be sufficient to
> > +build and boot, so this is not a way to bypass that requirement.
> > +
> > +More specifically, tooling or command-line arguments must provide a way to
> > +add a `/config` node or items within that node, so that U-Boot can receive a
> > +suitable configuration. It must provide a way of adding `u-boot,dm-...` tags for
> > +correct operation of driver model. These options can then be used as part of the
> > +build process, which puts the firmware image together. For binman, a way must be
> > +provided to add the binman definition into the devicetree in the same way.
> > +
> > +One way to do this is to allow a .dtsi file to be merged in with the generated
> > +devicetree.
> > +
> > +Note that the burden goes both ways. If a new feature is added to U-Boot which
> > +needs support in another project, then the author of the U-Boot patch must add
> > +any required support to the other project.
> > +
> > +
> > +Passing the devicetree through to Linux
> > +---------------------------------------
> > +
> > +Ideally U-Boot and Linux use the same devicetree source, even though it is
> > +hosted in separate projects. U-Boot adds some extra pieces, such as the
> > +`config/` node and tags like `u-boot,dm-spl`. Linux adds some extra pieces, such
> > +as `linux,default-trigger` and `linux,code`. This should not interfere with
> > +each other.
> > +
> > +In principle it is possible for U-Boot's control devicetree to be passed to
> > +Linux. This is, after all, one of the goals of devicetree and the original
> > +Open Firmware project, to have the firmware provide the hardware description to
> > +the Operating System.
> > +
> > +For boards where this approach is used, care must be taken. U-Boot typically
> > +needs to 'fix up' the devicetree before passing it to Linux, e.g. to add
> > +information about the memory map, about which serial console is used, provide
> > +the kernel address space layout randomization (KASLR) seed or select whether the
> > +console should be silenced for a faster boot.
> > +
> > +Fix-ups involve modifying the devicetree. If the control devicetree is used,
> > +that means the control devicetree could be modified, while U-Boot is using it.
> > +Removing a device and reinserting it can cause problems if the devicetree offset
> > +has changed, for example, since the device will be unable to locates its
> > +devicetree properties at the expected devicetree offset, which is a fixed
> > +integer.
> > +
> > +To deal with this, it is recommended to employ one or more of the following
> > +approaches:
> > +
> > +- Make a copy of the devicetree and 'fix up' the copy, leaving the control
> > +  devicetree alone
> > +- Enable `CONFIG_OF_LIVE` so that U-Boot makes its own copy of the devicetree
> > +  during relocation; fixups then happen on the original flat tree
> > +- Ensure that fix-ups happen after all loading has happened and U-Boot has
> > +  completed image verification
> > +
> > +In practice,the last point is typically observed, since boot_prep_linux() is
> > +called just before jumping to Linux, long after signature verification, for
> > +example. But it is important to make sure that this line is not blurred,
> > +particularly if untrusted user data is involved.
> > +
> > +
> > +Devicetree use cases that must be supported
> > +-------------------------------------------
> > +
> > +Regardless of how the devicetree is provided to U-Boot at runtime, various
> > +U-Boot features must be fully supported. This section describes some of these
> > +features and the implications for other projects.
> > +
> > +If U-Boot uses its own in-tree devicetree these features are supported
> > +automatically.
> > +
> > +
> > +Signing with U-Boot devicetree
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +U-Boot supports signing a payload so that it can be verified to have been
> > +created by a party owning a private key. This is called verified boot in U-Boot
> > +(see doc/uImage.FIT/verified-boot.txt).
> > +
> > +Typically this works by creating a FIT and then running the `mkimage` tool to
> > +add signatures for particular images. As part of this process, `mkimage` writes
> > +a public key to the U-Boot devicetree, although this can be done separately.
> > +See fdt_add_pubkey_ for patches for a suitable tool, for example.
> > +
> > +As with all configuration information, if another project is providing the
> > +devicetree to U-Boot, it must provide a way to add this public key into the
> > +devicetree it passes to U-Boot. This could be via a tooling option, making use
> > +of `mkimage`, or allowing a .dtsi file to be merged in with what is generated in
> > +the other project.
> > +
> > +
> > +Providing the binman image definition
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +In complex systems U-Boot must locate and make use of other firmware components,
> > +such as images for the user interface, files containing peripheral firmware,
> > +multiple copies of U-Boot for use with A/B boot, etc. U-Boot uses
> > +:doc:`Binman <../package/binman>` as a standard way of putting an image
> > +together.
> > +
> > +Typically this works by running binman with the devicetree as an input, to
> > +create the file image. Binman then outputs an updated devicetree which is
> > +packed in the firmware image, so U-Boot can access the binman definition and
> > +locate all the components.
> > +
> > +As with all configuration information, if another project is providing the
> > +devicetree to U-Boot, it must provide a way to add this binman definition into
> > +the devicetree it passes to U-Boot. This could be via a tooling option, making
> > +use of `binman`, or alowing a .dtsi file to be merged in with what is generated
> > +in the other project.
> > +
> > +
> > +Protecting the devicetree
> > +-------------------------
> > +
> > +U-Boot relies heavily on devicetree for correct operation. A corrupt or invalid
> > +device can cause U-Boot to fail to start, behave incorrectly, crash (e.g. if
> > +`CONFIG_OF_LIBFDT_ASSUME_MASK` is adjusted, or fail to boot an Operating System.
> > +Within U-Boot, the devicetree is as important as any other part of the source
> > +code. At ruuntime, the devicetree can be considered to be structured rodata.
> > +
> > +With secure systems, care must be taken that the devicetree is valid:
> > +
> > +- If the code / rodata has a hash or signature, the devicetree should also, if
> > +  they are packaged separately.
> > +- If the code / rodata is write-protected when running, the devicetree should be
> > +  also. Note that U-Boot relocates its code and devicetree, so this is not as
> > +  simple as it sounds. U-Boot must write-protect these items after relocating.
> > +
> > +
> > +Why does U-Boot have its nodes and properties?
> > +----------------------------------------------
> > +
> > +See also :doc:`../devicetree/intro`.
> > +
> > +There has been pushback at the concept that U-Boot dares have its own nodes and
> > +properties in the devicetree.
> > +
> > +Apart from these nodes and properties, U-Boot uses the same bindings as Linux.
> > +A `u-boot.dtsi` file helps to keep U-Boot-specific changes in separate files,
> > +making it easier to keep devicetree source files in U-Boot in sync with Linux.
> > +
> > +As a counter-example, the Zephyr OS project takes a different approach. It uses
> > +entirely different bindings, in general, making no effort to sync devicetree
> > +source files with Linux. U-Boot strives to be compatible with Linux in a number
> > +of ways, such as source code style and common APIs, to aid porting of code
> > +between the projects. Devicetree is another way where U-Boot and Linux follow a
> > +similar approach.
> > +
> > +Fundamentally, the idea that U-Boot cannot have its own tags flies in the face
> > +of the devicetree specification (see dtspec_), which says:
> > +
> > +  Nonstandard property names should specify a **unique string prefix**, such as
> > +  a stock ticker symbol, identifying the name of the company **or organization**
> > +  that defined the property. Examples:
> > +
> > +  - fsl,channel-fifo-len
> > +  - ibm,ppc-interrupt-server#s
> > +  - **linux**,network-index
> > +
> > +It is also fundamentally unbalanced. Linux has many tags of its own (some 36 in
> > +version 5.13) and at least one Linux-specific node, even if you ignore things
> > +like flash partitions which clearly provide configuration information to Linux.
> > +
> > +Practically speaking there are many reasons why U-Boot has its own nodes and
> > +properties. Some examples:
> > +
> > +- Binding every device before relocation even if it won't be used, consumes time
> > +  and memory: tags on each node can specify which are needed in SPL or before
> > +  relocation. Linux has no such constraints.
> > +
> > +- Requiring the full clock tree to be up and running just to get the debug UART
> > +  running is inefficient. It is also and self-defeating, since if that much
> > +  code is working properly, you probably don't need the debug UART. A devicetree
> > +  property to provide the UART input-clock frequency is a simple solution.
> > +
> > +- U-Boot does not have a user space to provide policy and configuration. It
> > +  cannot do what Linux does and run programs and look up filesystems to figure
> > +  out how to boot.
> > +
> > +
> > +Why not have two devicetrees?
> > +-----------------------------
> > +
> > +Setting aside the argument for restricting U-Boot from having its own nodes and
> > +properties, another idea proposed is to have two devicetrees, one for the
> > +U-Boot-specific bits (here called `special`) and one for everything else (here
> > +called `linux`). This would mean that U-Boot would be controlled by two
> > +devicetrees, i.e. OF_CONTROL would require/allow two devicetrees in order to
> > +work.
> > +
> > +On the positive side, it might quieten the discussion alluded to in the section
> > +above. But there are many negatives to consider and many open questions to
> > +resolve.
> > +
> > +- **Bindings** - Presumably the special devicetree would have its own bindings.
> > +  It would not be necessary to put a `u-boot,` prefix on anything. People coming
> > +  across the devicetree source would wonder how it fits in with the Linux
> > +  devicetree.
> > +
> > +- **Access** - U-Boot has a nice `ofnode` API for accessing the devicetree. This
> > +  would need to be expanded to support two trees. Features which need to access
> > +  both (such as a device driver which reads the special devicetree to get some
> > +  configuration info) could become quite confusing to read and write.
> > +
> > +- **Merging** - Can the two devicetree be merged if a platform desires it? If
> > +  so, how is this managed in tooling? Does it happen during the build, in which
> > +  case they are not really separate at all. Or does U-Boot merge them at
> > +  runtime, in which case this adds time and memory?
> > +
> > +- **Efficiency** - A second device tree adds more code and more code paths. It
> > +  requires that both be made available to the code in U-Boot, e.g. via a
> > +  separate pointer or argument or API. Overall the separation would certainly
> > +  not speed up U-Boot, nor decrease its size.
> > +
> > +- **Source code** - At present `u-boot.dtsi` files provide the pieces needed for
> > +  U-Boot for a particular board. Would we use these same files for the special
> > +  devicetree?
> > +
> > +- **Complexity** - Two devicetrees complicates the build system since it must
> > +  build and package them both. Errors must be reported in such a way that it
> > +  is obvious which one is failing.
> > +
> > +- **Referencing each other** - The `u-boot,dm-xxx` tags used by driver model
> > +  are currently placed in the nodes they relate to. How would these tags
> > +  reference a node that is in a separate devicetree? What extra validation would
> > +  be needed?
> > +
> > +- **Storage** - How would the two devicetrees be stored in the image? At present
> > +  we simply concatenate the U-Boot binary and the devicetree. We could add the
> > +  special devicetree before the Linux one, so two are concatenated, but it is
> > +  not pretty. We could use binman to support more complex arrangements, but only
> > +  some boards use this at present, so it would be a big change.
> > +
> > +- **API** - How would another project provide two devicetree files to U-Boot at
> > +  runtime? Presumably this would just be too painful. But if it doesn't, it
> > +  would be unable to configure run-time features of U-Boot during the boot.
> > +
> > +- **Confusion** - No other project has two devicetrees used for controlling its
> > +  operation (although having multiple devicetrees to pass on to the OS is
> > +  common). U-Boot would be in the unfortunate position of having to describe
> > +  the purpose of the two control devicetrees fact to new users, along with the
> > +  (arguably contrived) reason for the arrangement.
> > +
> > +- **Signing flow** - The current signing flow is simple as it involves running
> > +  `mkimage` with the U-Boot devicetree. This would have to be updated to use the
> > +  special devicetree. Some way of telling the user that they have done it wrong
> > +  would have to be invented.
> > +
> > +Overall, adding a second devicetree would create enormous confusion and
> > +complexity. It seems a lot cheaper to solve this by a change of attitude.
> > +
> > +
> > +.. _rpi_patch: https://patchwork.ozlabs.org/project/uboot/patch/20170402082520.32546-1-deymo@google.com/
> > +.. _`TF-A`: https://www.trustedfirmware.org/projects/tf-a
> > +.. _`QEMU ARM`: https://github.com/qemu/qemu/blob/master/hw/arm/virt.c
> > +.. _`QEMU RISC-V`: https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c
> > +.. _`/chosen node`: https://www.kernel.org/doc/Documentation/devicetree/bindings/chosen.txt
> > +.. _fdt_add_pubkey: https://patchwork.ozlabs.org/project/uboot/list/?series=157843&state=*
> > +.. _dtspec: https://www.devicetree.org/specifications/
> > diff --git a/doc/develop/devicetree/index.rst b/doc/develop/devicetree/index.rst
> > index fa5db3eb76e..b5b33dfea0f 100644
> > --- a/doc/develop/devicetree/index.rst
> > +++ b/doc/develop/devicetree/index.rst
> > @@ -11,3 +11,4 @@ build-time and runtime configuration.
> >
> >      intro
> >      control
> > +   dt_update
> >
>


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