[PATCH v6 01/25] doc: Add documentation about devicetree usage

[Heinrich Schuchardt]

On 12/2/21 16:58, 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.
>
> 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..a1429d6e21a
> --- /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 linst, this

Even if you remove these lines in 17/25 I would prefer not to introduce
typos here:

%s/linst/list/

> +  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.

Did you ever ask the QEMU community what they think about your ideas?
What was the reply?

Best regards

Heinrich

> +
> +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
>