1.. SPDX-License-Identifier: GPL-2.0 2 3General Information 4=================== 5 6This document contains useful information to know when working with 7the Rust support in the kernel. 8 9 10``no_std`` 11---------- 12 13The Rust support in the kernel can link only `core <https://doc.rust-lang.org/core/>`_, 14but not `std <https://doc.rust-lang.org/std/>`_. Crates for use in the 15kernel must opt into this behavior using the ``#![no_std]`` attribute. 16 17 18Code documentation 19------------------ 20 21Rust kernel code is documented using ``rustdoc``, its built-in documentation 22generator. 23 24The generated HTML docs include integrated search, linked items (e.g. types, 25functions, constants), source code, etc. They may be read at (TODO: link when 26in mainline and generated alongside the rest of the documentation): 27 28 http://kernel.org/ 29 30The docs can also be easily generated and read locally. This is quite fast 31(same order as compiling the code itself) and no special tools or environment 32are needed. This has the added advantage that they will be tailored to 33the particular kernel configuration used. To generate them, use the ``rustdoc`` 34target with the same invocation used for compilation, e.g.:: 35 36 make LLVM=1 rustdoc 37 38To read the docs locally in your web browser, run e.g.:: 39 40 xdg-open Documentation/output/rust/rustdoc/kernel/index.html 41 42To learn about how to write the documentation, please see coding-guidelines.rst. 43 44 45Extra lints 46----------- 47 48While ``rustc`` is a very helpful compiler, some extra lints and analyses are 49available via ``clippy``, a Rust linter. To enable it, pass ``CLIPPY=1`` to 50the same invocation used for compilation, e.g.:: 51 52 make LLVM=1 CLIPPY=1 53 54Please note that Clippy may change code generation, thus it should not be 55enabled while building a production kernel. 56 57 58Abstractions vs. bindings 59------------------------- 60 61Abstractions are Rust code wrapping kernel functionality from the C side. 62 63In order to use functions and types from the C side, bindings are created. 64Bindings are the declarations for Rust of those functions and types from 65the C side. 66 67For instance, one may write a ``Mutex`` abstraction in Rust which wraps 68a ``struct mutex`` from the C side and calls its functions through the bindings. 69 70Abstractions are not available for all the kernel internal APIs and concepts, 71but it is intended that coverage is expanded as time goes on. "Leaf" modules 72(e.g. drivers) should not use the C bindings directly. Instead, subsystems 73should provide as-safe-as-possible abstractions as needed. 74 75.. code-block:: 76 77 rust/bindings/ 78 (rust/helpers.c) 79 80 include/ -----+ <-+ 81 | | 82 drivers/ rust/kernel/ +----------+ <-+ | 83 fs/ | bindgen | | 84 .../ +-------------------+ +----------+ --+ | 85 | Abstractions | | | 86 +---------+ | +------+ +------+ | +----------+ | | 87 | my_foo | -----> | | foo | | bar | | -------> | Bindings | <-+ | 88 | driver | Safe | | sub- | | sub- | | Unsafe | | | 89 +---------+ | |system| |system| | | bindings | <-----+ 90 | | +------+ +------+ | | crate | | 91 | | kernel crate | +----------+ | 92 | +-------------------+ | 93 | | 94 +------------------# FORBIDDEN #--------------------------------+ 95 96The main idea is to encapsulate all direct interaction with the kernel's C APIs 97into carefully reviewed and documented abstractions. Then users of these 98abstractions cannot introduce undefined behavior (UB) as long as: 99 100#. The abstractions are correct ("sound"). 101#. Any ``unsafe`` blocks respect the safety contract necessary to call the 102 operations inside the block. Similarly, any ``unsafe impl``\ s respect the 103 safety contract necessary to implement the trait. 104 105Bindings 106~~~~~~~~ 107 108By including a C header from ``include/`` into 109``rust/bindings/bindings_helper.h``, the ``bindgen`` tool will auto-generate the 110bindings for the included subsystem. After building, see the ``*_generated.rs`` 111output files in the ``rust/bindings/`` directory. 112 113For parts of the C header that ``bindgen`` does not auto generate, e.g. C 114``inline`` functions or non-trivial macros, it is acceptable to add a small 115wrapper function to ``rust/helpers.c`` to make it available for the Rust side as 116well. 117 118Abstractions 119~~~~~~~~~~~~ 120 121Abstractions are the layer between the bindings and the in-kernel users. They 122are located in ``rust/kernel/`` and their role is to encapsulate the unsafe 123access to the bindings into an as-safe-as-possible API that they expose to their 124users. Users of the abstractions include things like drivers or file systems 125written in Rust. 126 127Besides the safety aspect, the abstractions are supposed to be "ergonomic", in 128the sense that they turn the C interfaces into "idiomatic" Rust code. Basic 129examples are to turn the C resource acquisition and release into Rust 130constructors and destructors or C integer error codes into Rust's ``Result``\ s. 131 132 133Conditional compilation 134----------------------- 135 136Rust code has access to conditional compilation based on the kernel 137configuration: 138 139.. code-block:: rust 140 141 #[cfg(CONFIG_X)] // Enabled (`y` or `m`) 142 #[cfg(CONFIG_X="y")] // Enabled as a built-in (`y`) 143 #[cfg(CONFIG_X="m")] // Enabled as a module (`m`) 144 #[cfg(not(CONFIG_X))] // Disabled 145