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

Simon Glass sjg at chromium.org
Tue Dec 7 01:11:39 CET 2021 

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 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
-- 
2.34.1.400.ga245620fadb-goog

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