doc/develop/codingstyle.rst: Add a section on conditional compilation

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 <trini@konsulko.com>

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
 ------------