[PATCH v3 3/3] RFC: doc: Add documentation about devicetree usage
Heinrich Schuchardt
xypron.glpk at gmx.de
Fri Sep 10 18:44:12 CEST 2021
On 9/10/21 12:08 PM, François Ozog wrote:
>
>
> On Thu, 9 Sept 2021 at 22:10, Simon Glass <sjg at chromium.org
> <mailto:sjg at chromium.org>> 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 <mailto:sjg at chromium.org>>
> Reviewed-by: Marcel Ziswiler <marcel.ziswiler at toradex.com
> <mailto:marcel.ziswiler at toradex.com>>
> ---
>
> Changes in v3:
> - Fix typos linst suppled receive EFL
> - Drop 'and' before 'self-defeating'
> - Reword mention of control of QEMU's devicetree generation
> - Add mention of dropping CONFIG_OF_BOARD
> - Clarify the 'Once this bug is fixed' paragraph a bit
> - Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot
> devicetree
> - Add a note at the top explaining that his patch covers 'now', not
> 'future'
> - Add note 'Note: Some boards use a devicetree in U-Boot which does
> not match'
>
> 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/index.rst | 1 +
> doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++
> doc/develop/package/index.rst | 1 +
> 3 files changed, 585 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..b1bd310d906
> --- /dev/null
> +++ b/doc/develop/package/devicetree.rst
> @@ -0,0 +1,583 @@
> +.. SPDX-License-Identifier: GPL-2.0+
> +
> +Updating the devicetree
> +=======================
> +
> +Note: This documentation describes how things are today, mostly,
> with some
> +mention of things that need to be fixed. It is not intended to
> point the way to
> +what might be done in the future. That should be the subject of
> discussions on
> +the mailing list.
> +
>
> Some text about device tree usage and standardization may be good.
> What about something along the lines:
>
> Device Tree defines a source language, a serialized binary format and
> bindings.
> The power of device tree is that it can be leveraged by applications
> to store self describing rich data out of the context of hardware
> description.
> This power led to abuse in hardware description and device drivers
> started to
> leverage that to store parameters instead of using the operating system
> provided
> capabilities to do so.
> The current state of standardization is such that source and binary
> formats are
> fully defined but unfortunately bindings standardization is split
> between devicetree.org <http://devicetree.org>,
> IEEE1475 and Linux kernel Documentation tree.
> A platform may have multiple device trees attached to it:
> - there can be a System Device Tree covering compute elements outside
> scope of
> U-Boot and subsequent payloads
> - pre-U-Boot firmware components may have their specific ones to deal
> with power
> and clock distribution infrastructure
> - U-Boot may have a richer/different vision of the hardware than the
> booted payload,
> for instance U-Boot may have to view SerDes hardware to configure lines
> into PCI
> or MDIO lanes to match actual board physical routing. The booted payload
> (say Linux)
> should only see the PCI ports or Ethernet ports, not the Serdes. For
> boot-time control
> one may want to limit the hardware vision of U-Boot to just what is
> necessary to boot
> - U-Boot is in the best position to assemble the full device view that
> can result from
> connected capes or hats.
> - The booted payload should just consume what U-Boot hands over
>
> +U-Boot uses devicetree for runtime configuration and storing
> required blobs or
> +any other information it needs to operate.
>
> Those elements shall not be visible to the booted payload. They may be
> stored in a
> separate device tree binary. The bindings for this tree are out of scope
> for projects other
> than U-Boot. U-Boot shall define its own bindings, even if it means
> referring to external
> ones.
>
> 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)
>
> Console control is still in its infancy and has many problems. For
> instance, Linux leverages
> stdout *only* with earlyconsole, there is no way to state console=dt/stdout
> in the command line so that the console is directed to firmware provided
> source.
>
> +
> +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).
>
>
> That should be noted as current situation, not a *must* that will last
> forever
> U-Boot plays a central role in "massaging" the device tree that is
> handed over
> to the booted payload. But it is certainly not the "authoritative"
> entity that defines
> the board. It may even control only a portion of the entire board
> (covered by System
> Device Tree).
>
>
> 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.
> +
> +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, with some control from command-line args, but it is
> not clear
> + how to add properties required by U-Boot.
> +
> +Specifically on the changes in U-Boot:
> +
> +- `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.
> +
> +Note: It is not clear that we actually need both of these. Possibly
> +`CONFIG_OF_BOARD` can be dropped.
> +
> +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE
> will override
> +(at runtime) the devicetree supplied 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`. To be clear, the
> devicetree in the
> +U-Boot tree may be largely for documentation and build-testing
> purposes, if at
> +runtime the devicetree if provided by another project. But the in-tree
> +devicetree is packaged with U-Boot as a fallback if it does not get
> one from a
> +prior stage at runtime. This does not create two devicetrees that
> need to be
> +merged, or anything like that. If the prior stage provides one, it
> is used as
> +is, with the one provided by U-Boot being ignored.
> +
> +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.
> +
> +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.
> +
>
> +Note: Some boards use a devicetree in U-Boot which does not match
> that in Linux.
> +This is a significant problem which needs to be fixed.
>
>
> That may not be a problem. The requirement is that the generated Device
> Tree is
> conformant to the spec and complete and accurate. U-Boot may further
> amend it.
> Should Qemu change PSCI implementation (new version of API...), U-Boot
> shall
> not have a different vision of this in a separately maintained device tree,
> that just make no sense. For CI/CD, you just use different Qemu runtime
> parameters rather than use fake static device trees.
>
> +
> +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 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 ELF
> + 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. This can be achieved by copying the
> devicetree from
> + the U-Boot tree, for example, or by providing an option to (at
> build-time)
> + merge U-Boot's version with the one provided by the project.
> +
> +- 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.
> +
> +
> +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 receives 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:
>
>
> STRONG NO. there shall be a platform device tree that is "massaged" by
> different
> firmware components before reaching the booted payload.
> TF-A shall ignore *entirely* if the non secure firmware is U-Boot or
> LinuxBoot or EDK2.
> It shall do what it has authority to do on the platform DT and that's all.
>
> +
> +- U-Boot only has one devicetree.
>
>
> U-Boot has vision on hardware that the booted payload shall not even see,
> U-Boot builder for the platform may decide to only consider a hardware
> subset
> for booting purposes. The implementation may be either a single
Hello Francois,
some part of your the message got lost here. Could you, please resend
with the missing part.
Best regards
Heinrich
>
> 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 <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`).
> +
> +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. U-Boot
> would be in the
> + unfortunate position of having to describe this 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/
> <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/
> <https://patchwork.ozlabs.org/project/uboot/patch/16fc0901f4521d3c399eac950c52a634b2f9473b.1528485916.git.fitzsim@fitzsim.org/>
> +.. _`TF-A`: https://www.trustedfirmware.org/projects/tf-a
> <https://www.trustedfirmware.org/projects/tf-a>
> +.. _`QEMU ARM`:
> https://github.com/qemu/qemu/blob/master/hw/arm/virt.c
> <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
> <https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c>
> +.. _`/chosen node`:
> https://www.kernel.org/doc/Documentation/devicetree/bindings/chosen.txt
> <https://www.kernel.org/doc/Documentation/devicetree/bindings/chosen.txt>
> +.. _fdt_add_pubkey:
> https://patchwork.ozlabs.org/project/uboot/list/?series=157843&state=*
> <https://patchwork.ozlabs.org/project/uboot/list/?series=157843&state=*>
> +.. _dtspec: https://www.devicetree.org/specifications/
> <https://www.devicetree.org/specifications/>
> 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
> --
> 2.33.0.309.g3052b89438-goog
>
>
>
> --
>
> François-Frédéric Ozog | /Director Business Development/
> T: +33.67221.6485
> francois.ozog at linaro.org <mailto:francois.ozog at linaro.org> | Skype: ffozog
>
>
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