[PATCH] doc: Add documentation about devicetree usage
Simon Glass
sjg at chromium.org
Sat Aug 28 05:23:48 CEST 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>
---
doc/develop/index.rst | 1 +
doc/develop/package/devicetree.rst | 315 +++++++++++++++++++++++++++++
doc/develop/package/index.rst | 1 +
3 files changed, 317 insertions(+)
create mode 100644 doc/develop/package/devicetree.rst
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 83c929babda..d5ad8f9fe53 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -36,6 +36,7 @@ Packaging
:maxdepth: 1
package/index
+ package/devicetree
Testing
-------
diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst
new file mode 100644
index 00000000000..fccbb182f3e
--- /dev/null
+++ b/doc/develop/package/devicetree.rst
@@ -0,0 +1,315 @@
+.. 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. It is possible to update the
+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 (August 2021)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty,
+e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has
+unfortunately created an enormous amount of confusion and some wasted effort.
+This was not intended and this bug will be fixed soon. Specifically:
+
+- `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
+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom
+ change with a tag indicating it only affected one board, so the change in
+ behaviour was not noticed at the time. It has since been used by RISC-V qemu
+ boards.
+
+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override
+(at runtime) the devicetree 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`
+
+Offending boards are:
+
+- bcm7260
+- bcm7445
+- 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 useul 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_EMBD
+ 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_PRIOR_STAGE
+ In this case the devicetree must be modified in the project which provides
+ it, as described below
+
+- CONFIG_OF_BOARD
+ This is a board-specific situation, so needs to be considered on a
+ case-by-case base. The devicetree must be modified so that the correct
+ one is provided to U-Boot. How this is done depends entirely on the
+ implementation of this option for the board. It might require injecting the
+ changes into a different project somehow using tooling available there, or
+ it might involve merging an overlay file at runtime to obtain the desired
+ result.
+
+
+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. It 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 must be sufficient to build and boot, so this is not a
+way to bypass that requirement.
+
+If binman is used, the in-tree U-Boot devicetree must contain the binman
+definition so that a valid image can be build.
+
+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 this case, the devicetree in the other project must track U-Boot's use of
+device tree, so that it remains compatible. 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. 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. 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.
+
+
+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 root hash for dm-verify 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 in 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.
+
+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 alowing 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 <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.
+
+
+.. _rpi_patch: https://patchwork.ozlabs.org/project/uboot/patch/20170402082520.32546-1-deymo@google.com/
+.. _bcm_patch: https://patchwork.ozlabs.org/project/uboot/patch/16fc0901f4521d3c399eac950c52a634b2f9473b.1528485916.git.fitzsim@fitzsim.org/
+.. _`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
diff --git a/doc/develop/package/index.rst b/doc/develop/package/index.rst
index 9374be2e62c..188c376950e 100644
--- a/doc/develop/package/index.rst
+++ b/doc/develop/package/index.rst
@@ -17,3 +17,4 @@ SPI flash.
:maxdepth: 2
binman
+ devicetree
--
2.33.0.259.gc128427fd7-goog
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