Hi François, On Tue, 26 Oct 2021 at 08:31, François Ozog <francois.o...@linaro.org> wrote: > > 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.
Can you provided a link and I can update this. > 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. I have gone to great efforts to document things here, as they work in U-Boot today. As you know, U-Boot supports separate control and active devicetrees. But if you are wanting to change to multiple control trees within U-Boot, I'd say the answer is "no, thank you". If there is a use case for that, please can you be specific about what we cannot do with a combined devicetree? Regards, Simon
