Hi Simon, On Tue, 26 Oct 2021 at 02:25, Simon Glass <s...@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 <s...@chromium.org> > Reviewed-by: Marcel Ziswiler <marcel.ziswi...@toradex.com> > --- > This patch attracted quite a bit of discussion here: > > > https://patchwork.ozlabs.org/project/uboot/patch/20210909201033.755713-4-...@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 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 | 556 +++++++++++++++++++++++++++ > doc/develop/devicetree/index.rst | 1 + > 2 files changed, 557 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..3d4902e3592 > --- /dev/null > +++ b/doc/develop/devicetree/dt_update.rst > @@ -0,0 +1,556 @@ > +.. 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 > + 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. 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 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`). > + > +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. > + > False: 1) projects in trustedfirmware.org are built to have multiple FDT objects, some for "dynamic" configuration purposes. 2) STM32MP1 can have dedicated DTBs for TF-A, OP-TEE and U-Boot in addition to operating system As Ilias said, this is not about documentation about the current use of DT in U-Boot, but justification of your views on DT. If taken by the letter, I feel (may be wrong though) that your views prevent establish the DT lifecycle and usage as per the desire of vendors, partners and customers that supports Arm SystemReady standards. +- **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-de...@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 > -- > 2.33.0.1079.g6e70778dc9-goog > > -- François-Frédéric Ozog | *Director Business Development* T: +33.67221.6485 francois.o...@linaro.org | Skype: ffozog
