Hi Heinrich, On Mon, 6 Dec 2021 at 17:58, Heinrich Schuchardt <[email protected]> wrote: > > On 12/6/21 16:11, Simon Glass 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 <[email protected]> > > Reviewed-by: Marcel Ziswiler <[email protected]> > > --- > > This patch attracted quite a bit of discussion here: > > > > https://patchwork.ozlabs.org/project/uboot/patch/[email protected]/ > > > > 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. > > As you already remarked part of this text has not been agreed upon. > > Please, either remove this part of the text or mark it as your private > opinion.
Do you mean the comment after the patch? It won't appear in the commit message. Regards, SImon > > Best regards > > Heinrich > > > > > Changes in v7: > > - Fix 'linst' typo > > > > Changes in v6: > > - Fix description of OF_BOARD so it refers just to the current state > > - Explain that the 'two devicetrees' refers to two *control* devicetrees > > > > Changes in v5: > > - Bring into the OF_BOARD series > > - Rebase to master and drop mention of OF_PRIOR_STAGE, since removed > > - Refer to the 'control' DTB in the first paragraph > > - Use QEMU instead of qemu > > > > Changes in v3: > > - Clarify the 'bug' refered to at the top > > - Reword 'This means that there' paragraph to explain U-Boot-specific things > > - Move to doc/develop/devicetree now that OF_CONTROL is in the docs > > > > Changes in v2: > > - Fix typos per Sean (thank you!) and a few others > > - Add a 'Use of U-Boot /config node' section > > - Drop mention of dm-verity since that actually uses the kernel cmdline > > - Explain that OF_BOARD will still work after these changes (in > > 'Once this bug is fixed...' paragraph) > > - Expand a bit on the reason why the 'Current situation' is bad > > - Clarify in a second place that Linux and U-Boot use the same devicetree > > in 'To be clear, while U-Boot...' > > - Expand on why we should have rules for other projects in > > 'Devicetree in another project' > > - Add a comment as to why devicetree in U-Boot is not 'bad design' > > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot' > > - Rewrite 'Devicetree generated on-the-fly in another project' to cover > > points raised on v1 > > - Add 'Why does U-Boot have its nodes and properties?' > > - Add 'Why not have two devicetrees?' > > > > doc/develop/devicetree/dt_update.rst | 555 +++++++++++++++++++++++++++ > > doc/develop/devicetree/index.rst | 1 + > > 2 files changed, 556 insertions(+) > > create mode 100644 doc/develop/devicetree/dt_update.rst > > > > diff --git a/doc/develop/devicetree/dt_update.rst > > b/doc/develop/devicetree/dt_update.rst > > new file mode 100644 > > index 00000000000..e3b65f6fa66 > > --- /dev/null > > +++ b/doc/develop/devicetree/dt_update.rst > > @@ -0,0 +1,555 @@ > > +.. SPDX-License-Identifier: GPL-2.0+ > > + > > +Updating the devicetree > > +======================= > > + > > +U-Boot uses devicetree for runtime configuration and storing required > > blobs or > > +any other information it needs to operate. This is called the 'control' > > +devicetree since it controls U-Boot. It is possible to update the control > > +devicetree separately from actually building U-Boot. This provides a good > > degree > > +of control and flexibility for firmware that uses U-Boot in conjunction > > with > > +other project. > > + > > +There are many reasons why it is useful to modify the devicetree after > > building > > +it: > > + > > +- Configuration can be changed, e.g. which UART to use > > +- A serial number can be added > > +- Public keys can be added to allow image verification > > +- Console output can be changed (e.g. to select serial or vidconsole) > > + > > +This section describes how to work with devicetree to accomplish your > > goals. > > + > > +See also :doc:`../devicetree/control` for a basic summary of the available > > +features. > > + > > + > > +Devicetree source > > +----------------- > > + > > +Every board in U-Boot must include a devicetree sufficient to build and > > boot > > +that board on suitable hardware (or emulation). This is specified using the > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > + > > + > > +Current situation (October 2021) > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > + > > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be > > empty, > > +e.g. if `CONFIG_OF_BOARD` is used. This has unfortunately created an > > enormous > > +amount of confusion and some wasted effort. This was not intended. Support > > for > > +an empty `CONFIG_DEFAULT_DEVICE_TREE` will be dropped soon. > > + > > +Some of the problems created are: > > + > > +- It is not obvious that the devicetree is coming from another project > > + > > +- There is no way to see even a sample devicetree for these platform in > > U-Boot, > > + so it is hard to know what is going on, e.g. which devices are typically > > + present > > + > > +- The other project may not provide a way to support U-Boot's requirements > > for > > + devicetree, such as the /config node. Note: On the U-Boot mailing list, > > this > > + was only discovered after weeks of discussion and confusion > > + > > +- For QEMU specifically, consulting two QEMU source files is required, for > > which > > + there are no references in U-Boot documentation. The code is generating a > > + devicetree, but it is not clear what controls affect this generation. > > + > > +Specifically on the changes in U-Bootm `CONFIG_OF_BOARD` was added in > > +rpi_patch_ for Raspberry Pi, which does have an in-tree devicetree, but > > this > > +feature has since been used for boards that don't > > + > > +Once this bug is fixed, CONFIG_OF_BOARD will override (at runtime) the > > +evicetree suppled with U-Boot, but will otherwise use CONFIG_OF_SEPARATE > > for the > > +in-tree build. So these two will become options, moving out of the > > 'choice' in > > +`dts/Kconfig`. > > + > > +This means that there is a basic devicetree build in the U-Boot tree, for > > +build-testing, consistency and documentation purposes, but at runtime > > U-Boot can > > +accept its devicetree from another source. The in-tree devicetree may > > contain > > +U-Boot-specific features (in u-boot*.dtsi files) and this may prove useful > > for > > +the other project, so it can ensure that U-Boot functions correctly and > > supports > > +all its expected features. > > + > > +To be clear, while U-Boot has its own copy of the devicetree source for > > each > > +board, this must match the Linux source, perhaps with some u-boot.dtsi > > +additions. The intent here is not to create a separate binding, just to > > provide > > +a representative devicetree in U-Boot. > > + > > +Offending boards are: > > + > > +- rpi_4 and rpi_4_32b (other rpi boards do have an in-tree devicetree) > > +- qemu_arm64 > > +- qemu_arm > > +- qemu-ppce500 > > +- qemu-riscv32 > > +- qemu-riscv32_smode > > +- qemu-riscv64 > > +- qemu-riscv64_smode > > + > > +All of these need to have a devicetree added in-tree. This is targeted to > > be > > +fixed in the 2022.01 release. > > + > > + > > +Building the devicetree > > +----------------------- > > + > > +U-Boot automatically builds the devicetree for a board, from the > > +`arch/<arch>/dts` directory. The Makefile in those directories has rules > > for > > +building devicetree files. It is preferable to avoid target-specific rules > > in > > +those files: i.e. all boards for a particular SoC should be built at once, > > +where practical. Apart from simplifying the Makefile, this helps to > > efficiently > > +(and immediately) ensure that changes in one board's DT do not break > > others that > > +are related. Building devicetrees is fast, so performance is seldom a > > concern > > +here. > > + > > + > > +Overriding the default devicetree > > +--------------------------------- > > + > > +When building U-Boot, the `DEVICE_TREE` environment variable allows the > > +default devicetree file to be overridden at build time. This can be useful > > if > > +modifications have to be made to the in-tree devicetree file, for the > > benefit > > +of a downstream build system. Note that the in-tree devicetree must be > > +sufficient to build and boot, so this is not a way to bypass that > > requirement. > > + > > + > > +Modifying the devicetree after building > > +--------------------------------------- > > + > > +While it is generally painful and hacky to modify the code or rodata of a > > +program after it is built, in many cases it is useful to do so, e.g. to add > > +configuration information like serial numbers, enabling/disabling > > features, etc. > > + > > +Devicetree provides a very nice solution to these problems since it is > > +structured data and it is relatively easy to change it, even in binary form > > +(see fdtput). > > + > > +U-Boot takes care that the devicetree is easily accessible after the build > > +process. In fact it is placed in a separate file called `u-boot.dtb`. If > > the > > +build system wants to modify or replace that file, it can do so. Then all > > that > > +is needed is to run `binman update` to update the file inside the image. If > > +binman is not used, then `u-boot-nodtb.bin` and the new `u-boot.dtb` can > > simply > > +be concatenated to achieve the desired result. U-Boot happily copes with > > the > > +devicetree growing or shrinking. > > + > > +The `u-boot.bin` image contains both pieces. While it is possible to > > locate the > > +devicetree within the image using the signature at the start of the file, > > this > > +is a bit messy. > > + > > +This is why `CONFIG_OF_SEPARATE` should always be used when building > > U-Boot. > > +The `CONFIG_OF_EMBED` option embeds the devicetree somewhere in the U-Boot > > ELF > > +image as rodata, meaning that it is hard to find it and it cannot increase > > in > > +size. > > + > > +When modifying the devicetree, the different cases to consider are as > > follows: > > + > > +- CONFIG_OF_SEPARATE > > + This is easy, described above. Just change, replace or rebuild the > > + devicetree so it suits your needs, then rerun binman or redo the `cat` > > + operation to join `u-boot-nodtb.bin` and the new `u-boot.dtb` > > + > > +- CONFIG_OF_EMBED > > + This is tricky, since the devicetree cannot easily be located. If the > > EFL > > + file is available, then the _dtb_dt_begin and __dtb_dt_end symbols can > > be > > + examined to find it. While it is possible to contract the file, it is > > not > > + possible to expand the file since that would involve re-linking > > + > > +- CONFIG_OF_BOARD > > + This is a board-specific situation, so needs to be considered on a > > + case-by-case base. > > + > > + > > +Use of U-Boot /config node > > +-------------------------- > > + > > +A common problem with firmware is that many builds are needed to deal with > > the > > +slight variations between different, related models. For example, one > > model may > > +have a TPM and another may not. Devicetree provides an excellent solution > > to > > +this problem, in that the devicetree to actually use on a platform can be > > +injected in the factory based on which model is being manufactured at the > > time. > > + > > +A related problem causing build proliferation is dealing with the > > differences > > +between development firmware, developer-friendly firmware (e.g. with all > > +security features present but with the ability to access the command line), > > +test firmware (which runs tests used in the factory), final production > > firmware > > +(before signing), signed firmware (where the signatures have been > > inserted) and > > +the like. Ideally all or most of these should use the same U-Boot build, > > with > > +just some options to determine the features available. For example, being > > able > > +to control whether the UART console or JTAG are available, on any image, > > is a > > +great debugging aid. > > + > > +When the firmware consists of multiple parts, it is helpful that all > > operate > > +the same way at runtime, regardless of how they were built. This can be > > achieved > > +by passing the runtime configuration (e.g. 'enable UART console) along the > > chain > > +through each firmware stage. It is frustrating to have to replicate a bug > > on > > +production firmware which does happen on developer firmware, because they > > are > > +completely different builds. > > + > > +The /config node provides useful functionality for this. It allows the > > different > > +controls to be 'factored out' of the U-Boot binary, so they can be > > controlled > > +separately from the initial source-code build. The node can be easily > > updated by > > +a build or factory tool and can control various features in U-Boot. It is > > +similar in concept to a Kconfig option, except that it can be changed after > > +U-Boot is built. > > + > > +The /config node is similar in concept to the `/chosen node`_ except that > > it is > > +for passing information *into* firmware instead of from firmware to the > > +Operating System. Also, while Linux has a (sometimes extremely long) > > command > > +line, U-Boot does not support this. The devicetree provides a more > > structured > > +approach in any case. > > + > > + > > +Devicetree in another project > > +----------------------------- > > + > > +In some cases U-Boot receive its devicetree at runtime from a program that > > calls > > +it. For example ARM's Trusted Firmware A (`TF-A`_) may have a devicetree > > that it > > +passes to U-Boot. This overrides any devicetree build by U-Boot. When > > packaging > > +the firmware, the U-Boot devicetree may in fact be left out if it can be > > +guaranteed that it will receive one from another project. > > + > > +In this case, the devicetree in the other project must track U-Boot's use > > of > > +device tree, for the following reasons: > > + > > +- U-Boot only has one devicetree. See `Why not have two devicetrees?`_. > > +- For a consistent firmware build, decisions made in early stages should be > > + communicated to later ones at runtime. For example, if the serial > > console is > > + enabled in an early stage, it should be enabled in U-Boot too. > > +- U-Boot is quite capable of managing its own copy of the devicetree. If > > + another project wants to bypass this (often for good reason), it is > > reasonable > > + that it should take on the (fairly small) requirements that U-Boot > > features > > + that rely on devicetree are still available > > +- The point here is not that *U-Boot needs this extra node*, or *U-Boot > > needs > > + to have this public key*. These features are present in U-Boot in > > service of > > + the entire firmware system. If the U-Boot features are used, but cannot > > be > > + supported in the normal way, then there is pressure to implement these > > + features in other ways. In the end, we would have a different mechanism > > for > > + every other project that uses U-Boot. This introduces duplicate ways of > > doing > > + the same thing, needlessly increases the complexity of the U-Boot source > > code, > > + forces authors to consider parallel implementations when writing new > > features, > > + makes U-Boot harder to test, complicates documentation and confuses the > > + runtime flow of U-Boot. If every board did things its own way rather than > > + contributing to the common code, U-Boot would lose a lot of its > > cross-platform > > + value. > > + > > +The above does not indicate *bad design* within U-Boot. Devicetree is a > > core > > +component of U-Boot and U-Boot makes use of it to the full. It solves a > > myriad > > +of problems that would otherwise need their own special C struct, binary > > format, > > +special property, tooling for viewing and updating, etc. > > + > > +Specifically, the other project must provide a way to add configuration and > > +other information to the devicetree for use by U-Boot, such as the /config > > node. > > +Note that the U-Boot in-tree devicetree source must be sufficient to build > > and > > +boot, so this is not a way to bypass that requirement. > > + > > +If binman is used, the devicetree source in U-Boot must contain the binman > > +definition so that a valid image can be build. This helps people discover > > what > > +other firmware components are needed and seek out appropriate > > documentation. > > + > > +If verified boot is used, the project must provide a way to inject a > > public key, > > +certificate or other material into the U-Boot devicetree so that it is > > available > > +to U-Boot at runtime. See `Signing with U-Boot devicetree`_. This may be > > +through tooling in the project itself or by making use of U-Boot's tooling. > > + > > + > > +Devicetree generated on-the-fly in another project > > +-------------------------------------------------- > > + > > +In some rare cases, another project may wish to create a devicetree for > > U-Boot > > +entirely on-the-fly, then pass it to U-Boot at runtime. The only known > > example > > +of this at the time of writing (2021) is QEMU, for ARM (`QEMU ARM`_) and > > +RISC-V (`QEMU RISC-V`_). > > + > > +In effect, when the board boots, U-Boot is *downstream* of the other > > project. > > +It is entirely reliant on that project for its correct operation. > > + > > +This does not mean to imply that the other project is creating its own, > > +incompatible devicetree. In fact QEMU generates a valid devicetree which is > > +suitable for both U-Boot and Linux. It is quite normal for a devicetree to > > be > > +present in flash and be made available to U-Boot at runtime. What matters > > is > > +where the devicetree comes from. If the other project builds a devicetree > > for > > +U-Boot then it needs to support adding the things needed by U-Boot > > features. > > +Without them, for example: > > + > > +- U-Boot may not boot because too many devices are enabled before > > relocation > > +- U-Boot may not have access to the developer or production public keys > > used for > > + signing > > +- U-Boot may not support controlling whether the console is enabled > > +- U-Boot may not be know which MMC device to boot from > > +- U-Boot may not be able to find other firmware components that it needs > > to load > > + > > +Normally, supporting U-Boot's features is trivial, since the devicetree > > compiler > > +(dtc) can compile the source, including any U-Boot pieces. So the burden is > > +extremely low. > > + > > +In this case, the devicetree in the other project must track U-Boot's use > > of > > +device tree, so that it remains compatible. See `Devicetree in another > > project`_ > > +for reasons why. > > + > > +If a particular version of the project is needed for a particular version > > of > > +U-Boot, that must be documented in both projects. > > + > > +Further, it must provide a way to add configuration and other information > > to > > +the devicetree for use by U-Boot, such as the `/config` node and the tags > > used > > +by driver model. Note that the U-Boot in-tree devicetree must be > > sufficient to > > +build and boot, so this is not a way to bypass that requirement. > > + > > +More specifically, tooling or command-line arguments must provide a way to > > +add a `/config` node or items within that node, so that U-Boot can receive > > a > > +suitable configuration. It must provide a way of adding `u-boot,dm-...` > > tags for > > +correct operation of driver model. These options can then be used as part > > of the > > +build process, which puts the firmware image together. For binman, a way > > must be > > +provided to add the binman definition into the devicetree in the same way. > > + > > +One way to do this is to allow a .dtsi file to be merged in with the > > generated > > +devicetree. > > + > > +Note that the burden goes both ways. If a new feature is added to U-Boot > > which > > +needs support in another project, then the author of the U-Boot patch must > > add > > +any required support to the other project. > > + > > + > > +Passing the devicetree through to Linux > > +--------------------------------------- > > + > > +Ideally U-Boot and Linux use the same devicetree source, even though it is > > +hosted in separate projects. U-Boot adds some extra pieces, such as the > > +`config/` node and tags like `u-boot,dm-spl`. Linux adds some extra > > pieces, such > > +as `linux,default-trigger` and `linux,code`. This should not interfere with > > +each other. > > + > > +In principle it is possible for U-Boot's control devicetree to be passed to > > +Linux. This is, after all, one of the goals of devicetree and the original > > +Open Firmware project, to have the firmware provide the hardware > > description to > > +the Operating System. > > + > > +For boards where this approach is used, care must be taken. U-Boot > > typically > > +needs to 'fix up' the devicetree before passing it to Linux, e.g. to add > > +information about the memory map, about which serial console is used, > > provide > > +the kernel address space layout randomization (KASLR) seed or select > > whether the > > +console should be silenced for a faster boot. > > + > > +Fix-ups involve modifying the devicetree. If the control devicetree is > > used, > > +that means the control devicetree could be modified, while U-Boot is using > > it. > > +Removing a device and reinserting it can cause problems if the devicetree > > offset > > +has changed, for example, since the device will be unable to locates its > > +devicetree properties at the expected devicetree offset, which is a fixed > > +integer. > > + > > +To deal with this, it is recommended to employ one or more of the following > > +approaches: > > + > > +- Make a copy of the devicetree and 'fix up' the copy, leaving the control > > + devicetree alone > > +- Enable `CONFIG_OF_LIVE` so that U-Boot makes its own copy of the > > devicetree > > + during relocation; fixups then happen on the original flat tree > > +- Ensure that fix-ups happen after all loading has happened and U-Boot has > > + completed image verification > > + > > +In practice,the last point is typically observed, since boot_prep_linux() > > is > > +called just before jumping to Linux, long after signature verification, for > > +example. But it is important to make sure that this line is not blurred, > > +particularly if untrusted user data is involved. > > + > > + > > +Devicetree use cases that must be supported > > +------------------------------------------- > > + > > +Regardless of how the devicetree is provided to U-Boot at runtime, various > > +U-Boot features must be fully supported. This section describes some of > > these > > +features and the implications for other projects. > > + > > +If U-Boot uses its own in-tree devicetree these features are supported > > +automatically. > > + > > + > > +Signing with U-Boot devicetree > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > + > > +U-Boot supports signing a payload so that it can be verified to have been > > +created by a party owning a private key. This is called verified boot in > > U-Boot > > +(see doc/uImage.FIT/verified-boot.txt). > > + > > +Typically this works by creating a FIT and then running the `mkimage` tool > > to > > +add signatures for particular images. As part of this process, `mkimage` > > writes > > +a public key to the U-Boot devicetree, although this can be done > > separately. > > +See fdt_add_pubkey_ for patches for a suitable tool, for example. > > + > > +As with all configuration information, if another project is providing the > > +devicetree to U-Boot, it must provide a way to add this public key into the > > +devicetree it passes to U-Boot. This could be via a tooling option, making > > use > > +of `mkimage`, or allowing a .dtsi file to be merged in with what is > > generated in > > +the other project. > > + > > + > > +Providing the binman image definition > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > + > > +In complex systems U-Boot must locate and make use of other firmware > > components, > > +such as images for the user interface, files containing peripheral > > firmware, > > +multiple copies of U-Boot for use with A/B boot, etc. U-Boot uses > > +:doc:`Binman <../package/binman>` as a standard way of putting an image > > +together. > > + > > +Typically this works by running binman with the devicetree as an input, to > > +create the file image. Binman then outputs an updated devicetree which is > > +packed in the firmware image, so U-Boot can access the binman definition > > and > > +locate all the components. > > + > > +As with all configuration information, if another project is providing the > > +devicetree to U-Boot, it must provide a way to add this binman definition > > into > > +the devicetree it passes to U-Boot. This could be via a tooling option, > > making > > +use of `binman`, or alowing a .dtsi file to be merged in with what is > > generated > > +in the other project. > > + > > + > > +Protecting the devicetree > > +------------------------- > > + > > +U-Boot relies heavily on devicetree for correct operation. A corrupt or > > invalid > > +device can cause U-Boot to fail to start, behave incorrectly, crash (e.g. > > if > > +`CONFIG_OF_LIBFDT_ASSUME_MASK` is adjusted, or fail to boot an Operating > > System. > > +Within U-Boot, the devicetree is as important as any other part of the > > source > > +code. At ruuntime, the devicetree can be considered to be structured > > rodata. > > + > > +With secure systems, care must be taken that the devicetree is valid: > > + > > +- If the code / rodata has a hash or signature, the devicetree should > > also, if > > + they are packaged separately. > > +- If the code / rodata is write-protected when running, the devicetree > > should be > > + also. Note that U-Boot relocates its code and devicetree, so this is not > > as > > + simple as it sounds. U-Boot must write-protect these items after > > relocating. > > + > > + > > +Why does U-Boot have its nodes and properties? > > +---------------------------------------------- > > + > > +See also :doc:`../devicetree/intro`. > > + > > +There has been pushback at the concept that U-Boot dares have its own > > nodes and > > +properties in the devicetree. > > + > > +Apart from these nodes and properties, U-Boot uses the same bindings as > > Linux. > > +A `u-boot.dtsi` file helps to keep U-Boot-specific changes in separate > > files, > > +making it easier to keep devicetree source files in U-Boot in sync with > > Linux. > > + > > +As a counter-example, the Zephyr OS project takes a different approach. It > > uses > > +entirely different bindings, in general, making no effort to sync > > devicetree > > +source files with Linux. U-Boot strives to be compatible with Linux in a > > number > > +of ways, such as source code style and common APIs, to aid porting of code > > +between the projects. Devicetree is another way where U-Boot and Linux > > follow a > > +similar approach. > > + > > +Fundamentally, the idea that U-Boot cannot have its own tags flies in the > > face > > +of the devicetree specification (see dtspec_), which says: > > + > > + Nonstandard property names should specify a **unique string prefix**, > > such as > > + a stock ticker symbol, identifying the name of the company **or > > organization** > > + that defined the property. Examples: > > + > > + - fsl,channel-fifo-len > > + - ibm,ppc-interrupt-server#s > > + - **linux**,network-index > > + > > +It is also fundamentally unbalanced. Linux has many tags of its own (some > > 36 in > > +version 5.13) and at least one Linux-specific node, even if you ignore > > things > > +like flash partitions which clearly provide configuration information to > > Linux. > > + > > +Practically speaking there are many reasons why U-Boot has its own nodes > > and > > +properties. Some examples: > > + > > +- Binding every device before relocation even if it won't be used, > > consumes time > > + and memory: tags on each node can specify which are needed in SPL or > > before > > + relocation. Linux has no such constraints. > > + > > +- Requiring the full clock tree to be up and running just to get the debug > > UART > > + running is inefficient. It is also and self-defeating, since if that much > > + code is working properly, you probably don't need the debug UART. A > > devicetree > > + property to provide the UART input-clock frequency is a simple solution. > > + > > +- U-Boot does not have a user space to provide policy and configuration. It > > + cannot do what Linux does and run programs and look up filesystems to > > figure > > + out how to boot. > > + > > + > > +Why not have two devicetrees? > > +----------------------------- > > + > > +Setting aside the argument for restricting U-Boot from having its own > > nodes and > > +properties, another idea proposed is to have two devicetrees, one for the > > +U-Boot-specific bits (here called `special`) and one for everything else > > (here > > +called `linux`). This would mean that U-Boot would be controlled by two > > +devicetrees, i.e. OF_CONTROL would require/allow two devicetrees in order > > to > > +work. > > + > > +On the positive side, it might quieten the discussion alluded to in the > > section > > +above. But there are many negatives to consider and many open questions to > > +resolve. > > + > > +- **Bindings** - Presumably the special devicetree would have its own > > bindings. > > + It would not be necessary to put a `u-boot,` prefix on anything. People > > coming > > + across the devicetree source would wonder how it fits in with the Linux > > + devicetree. > > + > > +- **Access** - U-Boot has a nice `ofnode` API for accessing the > > devicetree. This > > + would need to be expanded to support two trees. Features which need to > > access > > + both (such as a device driver which reads the special devicetree to get > > some > > + configuration info) could become quite confusing to read and write. > > + > > +- **Merging** - Can the two devicetree be merged if a platform desires it? > > If > > + so, how is this managed in tooling? Does it happen during the build, in > > which > > + case they are not really separate at all. Or does U-Boot merge them at > > + runtime, in which case this adds time and memory? > > + > > +- **Efficiency** - A second device tree adds more code and more code > > paths. It > > + requires that both be made available to the code in U-Boot, e.g. via a > > + separate pointer or argument or API. Overall the separation would > > certainly > > + not speed up U-Boot, nor decrease its size. > > + > > +- **Source code** - At present `u-boot.dtsi` files provide the pieces > > needed for > > + U-Boot for a particular board. Would we use these same files for the > > special > > + devicetree? > > + > > +- **Complexity** - Two devicetrees complicates the build system since it > > must > > + build and package them both. Errors must be reported in such a way that > > it > > + is obvious which one is failing. > > + > > +- **Referencing each other** - The `u-boot,dm-xxx` tags used by driver > > model > > + are currently placed in the nodes they relate to. How would these tags > > + reference a node that is in a separate devicetree? What extra validation > > would > > + be needed? > > + > > +- **Storage** - How would the two devicetrees be stored in the image? At > > present > > + we simply concatenate the U-Boot binary and the devicetree. We could add > > the > > + special devicetree before the Linux one, so two are concatenated, but it > > is > > + not pretty. We could use binman to support more complex arrangements, > > but only > > + some boards use this at present, so it would be a big change. > > + > > +- **API** - How would another project provide two devicetree files to > > U-Boot at > > + runtime? Presumably this would just be too painful. But if it doesn't, it > > + would be unable to configure run-time features of U-Boot during the boot. > > + > > +- **Confusion** - No other project has two devicetrees used for > > controlling its > > + operation (although having multiple devicetrees to pass on to the OS is > > + common). U-Boot would be in the unfortunate position of having to > > describe > > + the purpose of the two control devicetrees fact to new users, along with > > the > > + (arguably contrived) reason for the arrangement. > > + > > +- **Signing flow** - The current signing flow is simple as it involves > > running > > + `mkimage` with the U-Boot devicetree. This would have to be updated to > > use the > > + special devicetree. Some way of telling the user that they have done it > > wrong > > + would have to be invented. > > + > > +Overall, adding a second devicetree would create enormous confusion and > > +complexity. It seems a lot cheaper to solve this by a change of attitude. > > + > > + > > +.. _rpi_patch: > > https://patchwork.ozlabs.org/project/uboot/patch/[email protected]/ > > +.. _`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 > > >
