In order to make a start on explaining how and when to use certain macros, we need to document their usage somewhere. As a first step, take section 21 of the v6.13 Linux Kernel coding-style document on conditional compilation, verbatim, and add it to our documentation. Further rewording to be clearer about U-Boot will be done next.
Signed-off-by: Tom Rini <tr...@konsulko.com> --- Changes in v3: - Reword the commit message to be clearer about where this comes from. Changes in v2: - New patch. --- doc/develop/codingstyle.rst | 48 +++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/doc/develop/codingstyle.rst b/doc/develop/codingstyle.rst index fa3cd6aec82e..7211e4e4eed1 100644 --- a/doc/develop/codingstyle.rst +++ b/doc/develop/codingstyle.rst @@ -154,6 +154,54 @@ See `here <https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#function-documentation>`_ 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. + +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 ------------ -- 2.43.0
