.. SPDX-License-Identifier: GPL-2.0+: U-Boot Coding Style =================== The following Coding Style requirements shall be mandatory for all code contributed to the U-Boot project. Exceptions are only allowed if code from other projects is integrated with no or only minimal changes. The following rules apply: * All contributions to U-Boot should conform to the `Linux kernel coding style `_. U-Boot includes a `.clang-format` configuration file that can be used to automatically format code according to these standards. * The exception for net files to the `multi-line comment `_ applies only to Linux, not to U-Boot. Only large hunks which are copied unchanged from Linux may retain that comment format. * Python code shall conform to `PEP8 (Style Guide for Python Code) `_. Use `pylint `_ for checking the code. * Use the `b4 `_ tool to prepare and send your patches. b4 has become the preferred tool to sending patches for many Linux kernel contributors, and U-Boot ships with a ready-to-use ``.b4-config`` that targets ``u-boot@lists.denx.de`` and integrates with ``scripts/get_maintainer.pl`` for recipient discovery. Start a topical series with ``b4 prep`` and keep the commits organised with ``git rebase -i``. ``b4 prep --edit-cover`` opens an editor for the cover letter, while ``b4 prep --auto-to-cc`` collects reviewers and maintainers from both the configuration file and ``scripts/get_maintainer.pl``. .. code-block:: bash b4 prep -n mmc-fixes git rebase -i origin/master b4 prep --edit-cover b4 prep --auto-to-cc Run the style checks before sending. ``b4 prep --check`` wraps the existing tooling so you see the output from ``scripts/checkpatch.pl`` alongside b4's own validation. You can always invoke ``scripts/checkpatch.pl`` directly for additional runs. .. code-block:: bash b4 prep --check When the series is ready, use ``b4 send``. Begin with ``--dry-run`` to review the generated emails and ``--reflect`` to copy yourself for records before dispatching to ``u-boot@lists.denx.de``. .. code-block:: bash b4 send --dry-run b4 send --reflect b4 send After reviews arrive, collect Acked-by/Tested-by tags with ``b4 trailers -u`` and fold them into your commits before resending the updated series. .. code-block:: bash b4 trailers -u git rebase -i origin/master b4 send * Run ``scripts/checkpatch.pl`` directly or via ``b4 prep --check`` so that all issues are resolved *before* posting on the mailing list. For more information, read :doc:`checkpatch`. Code Formatting with clang-format ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ U-Boot provides a `.clang-format` configuration file that was copied directly from the Linux kernel without any modifications, ensuring complete compatibility with kernel coding standards. Here are common ways to use clang-format: **Basic usage for single files:** .. code-block:: bash clang-format -style=file -i **Format multiple files:** .. code-block:: bash find . -name '*.c' -o -name '*.h' | xargs clang-format -style=file -i **Integration with git (format only staged changes):** .. code-block:: bash git clang-format **Editor integration examples:** * **Vim/Neovim:** Install vim-clang-format plugin * **Emacs:** Use clang-format.el * **VSCode:** Install "Clang-Format" extension * **Most IDEs:** Have built-in or plugin support for clang-format The `.clang-format` file in the repository root ensures consistent formatting across the entire codebase and aligns with Linux kernel coding standards. **Disabling clang-format for specific code blocks:** In some cases, you may want to disable automatic formatting for specific code sections, such as carefully formatted tables, assembly code, or imported code from other projects. Use the following comments to control formatting: .. code-block:: c // clang-format off static const struct register_config regs[] = { { 0x1000, 0x12345678 }, // Base address register { 0x1004, 0xabcdef00 }, // Control register { 0x1008, 0x00000001 }, // Status register }; // clang-format on **Controversial aspects of coding style enforcement:** Coding style enforcement can be controversial, and it's difficult to have one configuration that satisfies everyone's personal preferences. The goal of using clang-format is consistency across the codebase rather than accommodating individual preferences. While some developers may disagree with specific formatting choices, maintaining a uniform style throughout the project makes code more readable and maintainable for the entire development community. **Best practices for formatting:** When using clang-format to format code, consider these best practices: * **Format only changed blocks:** It's preferred to format only the blocks of code that have been modified rather than entire files. This keeps diffs focused on actual changes and makes code reviews easier. * **Separate formatting commits:** If you need to format entire files, create a separate commit containing only formatting changes. This allows reviewers to easily distinguish between functional changes and pure formatting updates. * **Use git clang-format:** The ``git clang-format`` command is particularly useful as it formats only the lines that have been modified in your current changes, avoiding unnecessary formatting of unchanged code. * Source files originating from different projects (for example the MTD subsystem or the hush shell code from the BusyBox project) may, after careful consideration, be exempted from these rules. For such files, the original coding style may be kept to ease subsequent migration to newer versions of those sources. * Please also stick to the following formatting rules: * Remove any trailing white space * Use TAB characters for indentation and vertical alignment, not spaces * The exception here is Python which requires 4 spaces instead. * All source files need to be in "Unix" and not "DOS" or "Windows" format, with respect to line ends. * Do not add more than 2 consecutive empty lines to source files * Do not add trailing empty lines to source files * Using the option ``git config --global color.diff auto`` will help to visually see whitespace problems in ``diff`` output from ``git``. * In Emacs one can use ``=M-x whitespace-global-mode=`` to get visual feedback on the nasty details. ``=M-x whitespace-cleanup=`` does The Right Thing (tm) Submissions of new code or patches that do not conform to these requirements shall be rejected with a request to reformat the changes. U-Boot Code Documentation ------------------------- U-Boot adopted the kernel-doc annotation style, this is the only exception from multi-line comment rule of Coding Style. While not mandatory, adding documentation is strongly advised. The Linux kernel `kernel-doc `_ documentation applies with no changes. Our Python code documentation follows `PEP257 (Docstring Conventions) `_. Use structures for I/O access ----------------------------- U-Boot typically uses a C structure to map out the registers in an I/O region, rather than offsets. The reasons for this are: * It dissociates the register location (offset) from the register type, which means the developer has to make sure the type is right for each access, whereas with the struct method, this is checked by the compiler; * It avoids actually writing all offsets, which is (more) error-prone; * It allows for better compile time sanity-checking of values we write to registers. Some reasons why you might not use C structures: * Where the registers appear at different offsets in different hardware revisions supported by the same driver * Where the driver only uses a small subset of registers and it is not worth defining a struct to cover them all, with large empty regions * Where the offset of a register might be hard to figure out when buried a long way down a structure, possibly with embedded sub-structures * This may need to change to the kernel model if we allow for more run-time detection of what drivers are appropriate for what we're running on. Please use the check_member() macro to verify that your structure is the expected size, or that particular members appear at the right offset. Include files ------------- You should follow this ordering in U-Boot. In all cases, they should be listed in alphabetical order. First comes headers which are located directly in our top-level include diretory. Second are headers within subdirectories, Finally directory-local includes should be listed. See this example: .. code-block:: C #include #include #include #include #include #include #include #include "local.h" For files that need to be compiled for the host (e.g. tools), you need to use ``#ifndef USE_HOSTCC`` to avoid including U-Boot specific include files. See common/image.c for an example. If your file uses driver model, include in the C file. Do not include dm.h in a header file. Try to use forward declarations (e.g. ``struct udevice``) instead. Filenames --------- For .c and .h files try to use underscore rather than hyphen unless you want the file to stand out (e.g. driver-model uclasses should be named xxx-uclass.h. Avoid upper case and keep the names fairly short. Function and struct comments ---------------------------- Non-trivial functions should have a comment which describes what they do. If it is an exported function, put the comment in the header file so the API is in one place. If it is a static function, put it in the C file. If the function returns errors, mention that and list the different errors that are returned. If it is merely passing errors back from a function it calls, then you can skip that. See `here `_ for style. Conditional Compilation ----------------------- Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c files; doing so makes code harder to read and logic harder to follow. Instead, use such conditionals in a header file defining functions for use in those .c files, providing no-op stub versions in the #else case, and then call those functions unconditionally from .c files. The compiler will avoid generating any code for the stub calls, producing identical results, but the logic will remain easy to follow. Prefer to compile out entire functions, rather than portions of functions or portions of expressions. Rather than putting an ifdef in an expression, factor out part or all of the expression into a separate helper function and apply the conditional to that function. If you have a function or variable which may potentially go unused in a particular configuration, and the compiler would warn about its definition going unused, mark the definition as __maybe_unused rather than wrapping it in a preprocessor conditional. (However, if a function or variable *always* goes unused, delete it.) Within code, where possible, use the IS_ENABLED macro to convert a Kconfig symbol into a C boolean expression, and use it in a normal C conditional: .. code-block:: c if (IS_ENABLED(CONFIG_SOMETHING)) { ... } The compiler will constant-fold the conditional away, and include or exclude the block of code just as with an #ifdef, so this will not add any runtime overhead. However, this approach still allows the C compiler to see the code inside the block, and check it for correctness (syntax, types, symbol references, etc). Thus, you still have to use an #ifdef if the code inside the block references symbols that will not exist if the condition is not met. When working with xPL (see :doc:`spl` for more information) we need to take further care to use the right macro. In the case where a symbol may be referenced with an xPL-specific Kconfig symbol, use the CONFIG_IS_ENABLED macro instead, in a similar manner: .. code-block:: c if (CONIG_IS_ENABLED(SOMETHING)) { ... } When dealing with a Kconfig symbol that has both a normal name and one or more xPL-prefixed names, the Makefile needs special consideration as well. The PHASE\_ macro helps us in this situation thusly: .. code-block:: make obj-$(CONFIG_$(PHASE_)SOMETHING) += something.o At the end of any non-trivial #if or #ifdef block (more than a few lines), place a comment after the #endif on the same line, noting the conditional expression used. For instance: .. code-block:: c #ifdef CONFIG_SOMETHING ... #endif /* CONFIG_SOMETHING */ Driver model ------------ When declaring a device, try to use ``struct udevice *dev``, i.e. ``dev`` as the name: .. code-block:: C struct udevice *dev; Use ``ret`` as the return value: .. code-block:: C struct udevice *dev; int ret; ret = uclass_first_device_err(UCLASS_ACPI_PMC, &dev); if (ret) return log_msg_ret("pmc", dev); Consider using log_ret() or log_msg_ret() to return a value (see above). Add a ``p`` suffix on return arguments: .. code-block:: C int dm_pci_find_class(uint find_class, int index, struct udevice **devp) { ... *devp = dev; return 0; } There are standard variable names that you should use in drivers: * ``struct xxx_priv`` and ``priv`` for dev_get_priv() * ``struct xxx_plat`` and ``plat`` for dev_get_platdata() For example: .. code-block:: C struct simple_bus_plat { u32 base; u32 size; u32 target; }; /* Davinci MMC board definitions */ struct davinci_mmc_priv { struct davinci_mmc_regs *reg_base; /* Register base address */ uint input_clk; /* Input clock to MMC controller */ struct gpio_desc cd_gpio; /* Card Detect GPIO */ struct gpio_desc wp_gpio; /* Write Protect GPIO */ }; struct rcar_gpio_priv *priv = dev_get_priv(dev); struct pl01x_serial_platdata *plat = dev_get_platdata(dev); Other ----- Some minor things: * Put a blank line before the last ``return`` in a function unless it is the only line: .. code-block:: C struct udevice *pci_get_controller(struct udevice *dev) { while (device_is_on_pci_bus(dev)) dev = dev->parent; return dev; } Tests ----- Please add tests when you add code. Please change or expand tests when you change code. Run the tests with:: make check make qcheck (skips some tests) Python tests are in test/py/tests - see the docs in test/py for info. Try to write your tests in C if you can. For example, tests to check a command will be much faster (10-100x or more) if they can directly call run_command() and ut_check_console_line() instead of using Python to send commands over a pipe to U-Boot. Tests run all supported CI systems (GitLab, Azure) using scripts in the root of the U-Boot tree.