Documentation: Add coding style

This is the old wiki page https://www.coreboot.org/Coding_Style
coverted from mediawiki to markdown.

Change-Id: Id56a8b7500121c4d9c18bc0b6bbc2c05402268dc
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/30834
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

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+# Coding Style
+
+This is a short document describing the preferred coding style for the
+coreboot project. It is in many ways exactly the same as the Linux
+kernel coding style. In fact, most of this document has been copied from
+the [Linux kernel coding style](http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/CodingStyle?id=HEAD)
+
+Please at least consider the points made here.
+
+First off, I'd suggest printing out a copy of the GNU coding standards,
+and NOT read it. Burn them, it's a great symbolic gesture.
+
+Anyway, here goes:
+
+## Indentation
+
+Tabs are 8 characters, and thus indentations are also 8 characters.
+There are heretic movements that try to make indentations 4 (or even 2!)
+characters deep, and that is akin to trying to define the value of PI to
+be 3.
+
+Rationale: The whole idea behind indentation is to clearly define where
+a block of control starts and ends. Especially when you've been looking
+at your screen for 20 straight hours, you'll find it a lot easier to
+see how the indentation works if you have large indentations.
+
+Now, some people will claim that having 8-character indentations makes
+the code move too far to the right, and makes it hard to read on a
+80-character terminal screen. The answer to that is that if you need
+more than 3 levels of indentation, you're screwed anyway, and should
+fix your program.
+
+In short, 8-char indents make things easier to read, and have the added
+benefit of warning you when you're nesting your functions too deep.
+Heed that warning.
+
+The preferred way to ease multiple indentation levels in a switch
+statement is to align the "switch" and its subordinate "case" labels
+in the same column instead of "double-indenting" the "case" labels.
+E.g.:
+
+```c
+switch (suffix) {
+case 'G':
+case 'g':
+	mem <<= 30;
+	break;
+case 'M':
+case 'm':
+	mem <<= 20;
+	break;
+case 'K':
+case 'k':
+	mem <<= 10;
+	/* fall through */
+default:
+	break;
+}
+```
+
+Don't put multiple statements on a single line unless you have
+something to hide:
+
+```c
+if (condition) do_this;
+  do_something_everytime;
+```
+
+Don't put multiple assignments on a single line either. Kernel coding
+style is super simple. Avoid tricky expressions.
+
+Outside of comments, documentation and except in Kconfig, spaces are
+never used for indentation, and the above example is deliberately
+broken.
+
+Get a decent editor and don't leave whitespace at the end of lines.
+
+## Breaking long lines and strings
+
+Coding style is all about readability and maintainability using commonly
+available tools.
+
+The limit on the length of lines is 80 columns and this is a strongly
+preferred limit.
+
+Statements longer than 80 columns will be broken into sensible chunks,
+unless exceeding 80 columns significantly increases readability and does
+not hide information. Descendants are always substantially shorter than
+the parent and are placed substantially to the right. The same applies
+to function headers with a long argument list. However, never break
+user-visible strings such as printk messages, because that breaks the
+ability to grep for them.
+
+## Placing Braces and Spaces
+
+The other issue that always comes up in C styling is the placement of
+braces. Unlike the indent size, there are few technical reasons to
+choose one placement strategy over the other, but the preferred way, as
+shown to us by the prophets Kernighan and Ritchie, is to put the opening
+brace last on the line, and put the closing brace first, thusly:
+
+```c
+if (x is true) {
+	we do y
+}
+```
+
+This applies to all non-function statement blocks (if, switch, for,
+while, do). E.g.:
+
+```c
+switch (action) {
+case KOBJ_ADD:
+	return "add";
+case KOBJ_REMOVE:
+	return "remove";
+case KOBJ_CHANGE:
+	return "change";
+default:
+	return NULL;
+}
+```
+
+However, there is one special case, namely functions: they have the
+opening brace at the beginning of the next line, thus:
+
+```c
+int function(int x)
+{
+	body of function
+}
+```
+
+Heretic people all over the world have claimed that this inconsistency
+is ... well ... inconsistent, but all right-thinking people know that
+(a) K&R are _right_ and (b) K&R are right. Besides, functions are
+special anyway (you can't nest them in C).
+
+Note that the closing brace is empty on a line of its own, _except_ in
+the cases where it is followed by a continuation of the same statement,
+ie a "while" in a do-statement or an "else" in an if-statement, like
+this:
+
+```c
+do {
+	body of do-loop
+} while (condition);
+```
+
+and
+
+```c
+if (x == y) {
+	..
+} else if (x > y) {
+	...
+} else {
+	....
+}
+```
+
+Rationale: K&R.
+
+Also, note that this brace-placement also minimizes the number of empty
+(or almost empty) lines, without any loss of readability. Thus, as the
+supply of new-lines on your screen is not a renewable resource (think
+25-line terminal screens here), you have more empty lines to put
+comments on.
+
+Do not unnecessarily use braces where a single statement will do.
+
+```c
+if (condition)
+	action();
+```
+
+and
+
+```c
+if (condition)
+	do_this();
+else
+	do_that();
+```
+
+This does not apply if only one branch of a conditional statement is a
+single statement; in the latter case use braces in both branches:
+
+```c
+if (condition) {
+	do_this();
+	do_that();
+} else {
+	otherwise();
+}
+```
+
+### Spaces
+
+Linux kernel style for use of spaces depends (mostly) on
+function-versus-keyword usage. Use a space after (most) keywords. The
+notable exceptions are sizeof, typeof, alignof, and __attribute__,
+which look somewhat like functions (and are usually used with
+parentheses in Linux, although they are not required in the language, as
+in: "sizeof info" after "struct fileinfo info;" is declared).
+
+So use a space after these keywords:
+
+```
+if, switch, case, for, do, while
+```
+
+but not with sizeof, typeof, alignof, or __attribute__. E.g.,
+
+```c
+s = sizeof(struct file);
+```
+
+Do not add spaces around (inside) parenthesized expressions. This
+example is
+
+-   bad*:
+
+```c
+s = sizeof( struct file );
+```
+
+When declaring pointer data or a function that returns a pointer type,
+the preferred use of '*' is adjacent to the data name or function
+name and not adjacent to the type name. Examples:
+
+```c
+char *linux_banner;
+unsigned long long memparse(char *ptr, char **retptr);
+char *match_strdup(substring_t *s);
+```
+
+Use one space around (on each side of) most binary and ternary
+operators, such as any of these:
+
+```
+=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :
+```
+
+but no space after unary operators:
+
+```
+&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined
+```
+
+no space before the postfix increment & decrement unary operators:
+
+```
+++  --
+```
+
+no space after the prefix increment & decrement unary operators:
+
+```
+++  --
+```
+
+and no space around the '.' and "->" structure member operators.
+
+Do not leave trailing whitespace at the ends of lines. Some editors with
+"smart" indentation will insert whitespace at the beginning of new
+lines as appropriate, so you can start typing the next line of code
+right away. However, some such editors do not remove the whitespace if
+you end up not putting a line of code there, such as if you leave a
+blank line. As a result, you end up with lines containing trailing
+whitespace.
+
+Git will warn you about patches that introduce trailing whitespace, and
+can optionally strip the trailing whitespace for you; however, if
+applying a series of patches, this may make later patches in the series
+fail by changing their context lines.
+
+### Naming
+
+C is a Spartan language, and so should your naming be. Unlike Modula-2
+and Pascal programmers, C programmers do not use cute names like
+ThisVariableIsATemporaryCounter. A C programmer would call that variable
+"tmp", which is much easier to write, and not the least more difficult
+to understand.
+
+HOWEVER, while mixed-case names are frowned upon, descriptive names for
+global variables are a must. To call a global function "foo" is a
+shooting offense.
+
+GLOBAL variables (to be used only if you _really_ need them) need to
+have descriptive names, as do global functions. If you have a function
+that counts the number of active users, you should call that
+"count_active_users()" or similar, you should _not_ call it
+"cntusr()".
+
+Encoding the type of a function into the name (so-called Hungarian
+notation) is brain damaged - the compiler knows the types anyway and can
+check those, and it only confuses the programmer. No wonder MicroSoft
+makes buggy programs.
+
+LOCAL variable names should be short, and to the point. If you have some
+random integer loop counter, it should probably be called "i". Calling
+it "loop_counter" is non-productive, if there is no chance of it
+being mis-understood. Similarly, "tmp" can be just about any type of
+variable that is used to hold a temporary value.
+
+If you are afraid to mix up your local variable names, you have another
+problem, which is called the function-growth-hormone-imbalance syndrome.
+See chapter 6 (Functions).
+
+## Typedefs
+
+Please don't use things like "vps_t".
+
+It's a _mistake_ to use typedef for structures and pointers. When you
+see a
+
+```c
+vps_t a;
+```
+
+in the source, what does it mean?
+
+In contrast, if it says
+
+```c
+struct virtual_container *a;
+```
+
+you can actually tell what "a" is.
+
+Lots of people think that typedefs "help readability". Not so. They
+are useful only for:
+
+(a) totally opaque objects (where the typedef is actively used to
+_hide_ what the object is).
+
+Example: "pte_t" etc. opaque objects that you can only access using
+the proper accessor functions.
+
+NOTE! Opaqueness and "accessor functions" are not good in themselves.
+The reason we have them for things like pte_t etc. is that there really
+is absolutely _zero_ portably accessible information there.
+
+(b) Clear integer types, where the abstraction _helps_ avoid confusion
+whether it is "int" or "long".
+
+u8/u16/u32 are perfectly fine typedefs, although they fit into category
+(d) better than here.
+
+NOTE! Again - there needs to be a _reason_ for this. If something is
+"unsigned long", then there's no reason to do
+
+```c
+typedef unsigned long myflags_t;
+```
+
+but if there is a clear reason for why it under certain circumstances
+might be an "unsigned int" and under other configurations might be
+"unsigned long", then by all means go ahead and use a typedef.
+
+(c) when you use sparse to literally create a _new_ type for
+type-checking.
+
+(d) New types which are identical to standard C99 types, in certain
+exceptional circumstances.
+
+Although it would only take a short amount of time for the eyes and
+brain to become accustomed to the standard types like 'uint32_t',
+some people object to their use anyway.
+
+Therefore, the Linux-specific 'u8/u16/u32/u64' types and their signed
+equivalents which are identical to standard types are permitted --
+although they are not mandatory in new code of your own.
+
+When editing existing code which already uses one or the other set of
+types, you should conform to the existing choices in that code.
+
+(e) Types safe for use in userspace.
+
+In certain structures which are visible to userspace, we cannot require
+C99 types and cannot use the 'u32' form above. Thus, we use __u32
+and similar types in all structures which are shared with userspace.
+
+Maybe there are other cases too, but the rule should basically be to
+NEVER EVER use a typedef unless you can clearly match one of those
+rules.
+
+In general, a pointer, or a struct that has elements that can reasonably
+be directly accessed should _never_ be a typedef.
+
+## Functions
+
+Functions should be short and sweet, and do just one thing. They should
+fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
+as we all know), and do one thing and do that well.
+
+The maximum length of a function is inversely proportional to the
+complexity and indentation level of that function. So, if you have a
+conceptually simple function that is just one long (but simple)
+case-statement, where you have to do lots of small things for a lot of
+different cases, it's OK to have a longer function.
+
+However, if you have a complex function, and you suspect that a
+less-than-gifted first-year high-school student might not even
+understand what the function is all about, you should adhere to the
+maximum limits all the more closely. Use helper functions with
+descriptive names (you can ask the compiler to in-line them if you think
+it's performance-critical, and it will probably do a better job of it
+than you would have done).
+
+Another measure of the function is the number of local variables. They
+shouldn't exceed 5-10, or you're doing something wrong. Re-think the
+function, and split it into smaller pieces. A human brain can generally
+easily keep track of about 7 different things, anything more and it gets
+confused. You know you're brilliant, but maybe you'd like to
+understand what you did 2 weeks from now.
+
+In source files, separate functions with one blank line. If the function
+is exported, the EXPORT* macro for it should follow immediately after
+the closing function brace line. E.g.:
+
+```c
+int system_is_up(void)
+{
+	return system_state == SYSTEM_RUNNING;
+}
+EXPORT_SYMBOL(system_is_up);
+```
+
+In function prototypes, include parameter names with their data types.
+Although this is not required by the C language, it is preferred in
+Linux because it is a simple way to add valuable information for the
+reader.
+
+## Centralized exiting of functions
+
+Albeit deprecated by some people, the equivalent of the goto statement
+is used frequently by compilers in form of the unconditional jump
+instruction.
+
+The goto statement comes in handy when a function exits from multiple
+locations and some common work such as cleanup has to be done. If there
+is no cleanup needed then just return directly.
+
+The rationale is:
+
+-   unconditional statements are easier to understand and follow
+-   nesting is reduced
+-   errors by not updating individual exit points when making
+    modifications are prevented
+-   saves the compiler work to optimize redundant code away ;)
+
+```c
+int fun(int a)
+{
+	int result = 0;
+	char *buffer = kmalloc(SIZE);
+
+	if (buffer == NULL)
+		return -ENOMEM;
+
+	if (condition1) {
+		while (loop1) {
+			...
+		}
+		result = 1;
+		goto out;
+	}
+	...
+	out:
+	kfree(buffer);
+	return result;
+}
+```
+
+## Commenting
+
+Comments are good, but there is also a danger of over-commenting. NEVER
+try to explain HOW your code works in a comment: it's much better to
+write the code so that the _working_ is obvious, and it's a waste of
+time to explain badly written code.
+
+Generally, you want your comments to tell WHAT your code does, not HOW.
+Also, try to avoid putting comments inside a function body: if the
+function is so complex that you need to separately comment parts of it,
+you should probably go back to chapter 6 for a while. You can make small
+comments to note or warn about something particularly clever (or ugly),
+but try to avoid excess. Instead, put the comments at the head of the
+function, telling people what it does, and possibly WHY it does it.
+
+When commenting the kernel API functions, please use the kernel-doc
+format. See the files Documentation/kernel-doc-nano-HOWTO.txt and
+scripts/kernel-doc for details.
+
+coreboot style for comments is the C89 "/* ... */" style. You may
+use C99-style "// ..." comments.
+
+The preferred style for *short* (multi-line) comments is:
+
+```c
+/* This is the preferred style for short multi-line
+   comments in the Linux kernel source code.
+   Please use it consistently. */
+```
+
+The preferred style for *long* (multi-line) comments is:
+
+```c
+/*
+ * This is the preferred style for multi-line
+ * comments in the Linux kernel source code.
+ * Please use it consistently.
+ *
+ * Description:  A column of asterisks on the left side,
+ * with beginning and ending almost-blank lines.
+ */
+```
+
+It's also important to comment data, whether they are basic types or
+derived types. To this end, use just one data declaration per line (no
+commas for multiple data declarations). This leaves you room for a small
+comment on each item, explaining its use.
+
+## You've made a mess of it
+That's OK, we all do. You've probably been told by your long-time Unix user
+helper that "GNU emacs" automatically formats the C sources for you, and
+you've noticed that yes, it does do that, but the defaults it uses are less
+than desirable (in fact, they are worse than random typing - an infinite
+number of monkeys typing into GNU emacs would never make a good program).
+
+So, you can either get rid of GNU emacs, or change it to use saner values.
+To do the latter, you can stick the following in your .emacs file: 
+
+```lisp
+(defun c-lineup-arglist-tabs-only (ignored)
+  "Line up argument lists by tabs, not spaces"
+  (let* ((anchor (c-langelem-pos c-syntactic-element))
+	 (column (c-langelem-2nd-pos c-syntactic-element))
+	 (offset (- (1+ column) anchor))
+	 (steps (floor offset c-basic-offset)))
+    (* (max steps 1)
+       c-basic-offset)))
+
+(add-hook 'c-mode-common-hook
+          (lambda ()
+            ;; Add kernel style
+            (c-add-style
+             "linux-tabs-only"
+             '("linux" (c-offsets-alist
+                        (arglist-cont-nonempty
+                         c-lineup-gcc-asm-reg
+                         c-lineup-arglist-tabs-only))))))
+
+(add-hook 'c-mode-hook
+          (lambda ()
+            (let ((filename (buffer-file-name)))
+              ;; Enable kernel mode for the appropriate files
+              (when (and filename
+                         (string-match (expand-file-name "~/src/linux-trees")
+                                        filename))
+                (setq indent-tabs-mode t)
+                (c-set-style "linux-tabs-only")))))
+```
+
+This will make emacs go better with the kernel coding style for C files
+below ~/src/linux-trees.
+
+But even if you fail in getting emacs to do sane formatting, not
+everything is lost: use "indent".
+
+Now, again, GNU indent has the same brain-dead settings that GNU emacs
+has, which is why you need to give it a few command line options.
+However, that's not too bad, because even the makers of GNU indent
+recognize the authority of K&R (the GNU people aren't evil, they are
+just severely misguided in this matter), so you just give indent the
+options "-kr -i8" (stands for "K&R, 8 character indents"), or use
+"scripts/Lindent", which indents in the latest style.
+
+"indent" has a lot of options, and especially when it comes to comment
+re-formatting you may want to take a look at the man page. But remember:
+"indent" is not a fix for bad programming.
+
+## Kconfig configuration files
+
+For all of the Kconfig* configuration files throughout the source tree,
+the indentation is somewhat different. Lines under a "config"
+definition are indented with one tab, while help text is indented an
+additional two spaces. Example:
+
+```kconfig
+config AUDIT
+	bool "Auditing support"
+	depends on NET
+	help
+	  Enable auditing infrastructure that can be used with another
+	  kernel subsystem, such as SELinux (which requires this for
+	  logging of avc messages output).  Does not do system-call
+	  auditing without CONFIG_AUDITSYSCALL.
+```
+
+Seriously dangerous features (such as write support for certain
+filesystems) should advertise this prominently in their prompt string:
+
+```kconfig
+config ADFS_FS_RW
+	bool "ADFS write support (DANGEROUS)"
+	depends on ADFS_FS
+	...
+```
+
+For full documentation on the configuration files, see the file
+Documentation/kbuild/kconfig-language.txt.
+
+Data structures
+---------------
+
+Data structures that have visibility outside the single-threaded
+environment they are created and destroyed in should always have
+reference counts. In the kernel, garbage collection doesn't exist (and
+outside the kernel garbage collection is slow and inefficient), which
+means that you absolutely _have_ to reference count all your uses.
+
+Reference counting means that you can avoid locking, and allows multiple
+users to have access to the data structure in parallel - and not having
+to worry about the structure suddenly going away from under them just
+because they slept or did something else for a while.
+
+Note that locking is _not_ a replacement for reference counting.
+Locking is used to keep data structures coherent, while reference
+counting is a memory management technique. Usually both are needed, and
+they are not to be confused with each other.
+
+Many data structures can indeed have two levels of reference counting,
+when there are users of different "classes". The subclass count counts
+the number of subclass users, and decrements the global count just once
+when the subclass count goes to zero.
+
+Examples of this kind of "multi-level-reference-counting" can be found
+in memory management ("struct mm_struct": mm_users and mm_count),
+and in filesystem code ("struct super_block": s_count and
+s_active).
+
+Remember: if another thread can find your data structure, and you don't
+have a reference count on it, you almost certainly have a bug.
+
+Macros, Enums and RTL
+---------------------
+
+Names of macros defining constants and labels in enums are capitalized.
+
+```c
+#define CONSTANT 0x12345
+```
+
+Enums are preferred when defining several related constants.
+
+CAPITALIZED macro names are appreciated but macros resembling functions
+may be named in lower case.
+
+Generally, inline functions are preferable to macros resembling
+functions.
+
+Macros with multiple statements should be enclosed in a do - while
+block:
+
+```c
+#define macrofun(a, b, c)   \
+    do {                    \
+        if (a == 5)         \
+            do_this(b, c);  \
+    } while (0)
+```
+
+Things to avoid when using macros:
+
+1) macros that affect control flow:
+
+```c
+#define FOO(x)              \
+    do {                        \
+	    if (blah(x) < 0)        \
+		    return -EBUGGERED;  \
+    } while(0)
+```
+
+is a *very* bad idea. It looks like a function call but exits the
+"calling" function; don't break the internal parsers of those who
+will read the code.
+
+2) macros that depend on having a local variable with a magic name:
+
+```c
+#define FOO(val) bar(index, val)
+```
+
+might look like a good thing, but it's confusing as hell when one reads
+the code and it's prone to breakage from seemingly innocent changes.
+
+3) macros with arguments that are used as l-values: FOO(x) = y; will
+bite you if somebody e.g. turns FOO into an inline function.
+
+4) forgetting about precedence: macros defining constants using
+expressions must enclose the expression in parentheses. Beware of
+similar issues with macros using parameters.
+
+```c
+#define CONSTANT 0x4000
+#define CONSTEXP (CONSTANT | 3)
+```
+
+The cpp manual deals with macros exhaustively. The gcc internals manual
+also covers RTL which is used frequently with assembly language in the
+kernel.
+
+Printing kernel messages
+------------------------
+
+Kernel developers like to be seen as literate. Do mind the spelling of
+kernel messages to make a good impression. Do not use crippled words
+like "dont"; use "do not" or "don't" instead. Make the messages
+concise, clear, and unambiguous.
+
+Kernel messages do not have to be terminated with a period.
+
+Printing numbers in parentheses (%d) adds no value and should be
+avoided.
+
+There are a number of driver model diagnostic macros in
+<linux/device.h> which you should use to make sure messages are
+matched to the right device and driver, and are tagged with the right
+level: dev_err(), dev_warn(), dev_info(), and so forth. For messages
+that aren't associated with a particular device, <linux/printk.h>
+defines pr_debug() and pr_info().
+
+Coming up with good debugging messages can be quite a challenge; and
+once you have them, they can be a huge help for remote troubleshooting.
+Such messages should be compiled out when the DEBUG symbol is not
+defined (that is, by default they are not included). When you use
+dev_dbg() or pr_debug(), that's automatic. Many subsystems have
+Kconfig options to turn on -DDEBUG. A related convention uses
+VERBOSE_DEBUG to add dev_vdbg() messages to the ones already enabled
+by DEBUG.
+
+Allocating memory
+-----------------
+
+coreboot provides a single general purpose memory allocator: malloc()
+
+The preferred form for passing a size of a struct is the following:
+
+```c
+p = malloc(sizeof(*p));
+```
+
+The alternative form where struct name is spelled out hurts readability
+and introduces an opportunity for a bug when the pointer variable type
+is changed but the corresponding sizeof that is passed to a memory
+allocator is not.
+
+Casting the return value which is a void pointer is redundant. The
+conversion from void pointer to any other pointer type is guaranteed by
+the C programming language.
+
+You should contain your memory usage to stack variables whenever
+possible. Only use malloc() as a last resort. In ramstage, you may also
+be able to get away with using static variables. Never use malloc()
+outside of ramstage.
+
+Since coreboot only runs for a very short time, there is no memory
+deallocator, although a corresponding free() is offered. It is a no-op.
+Use of free() is not required though it is accepted. It is useful when
+sharing code with other codebases that make use of free().
+
+The inline disease
+------------------
+
+There appears to be a common misperception that gcc has a magic "make
+me faster" speedup option called "inline". While the use of inlines
+can be appropriate (for example as a means of replacing macros, see
+Chapter 12), it very often is not. Abundant use of the inline keyword
+leads to a much bigger kernel, which in turn slows the system as a whole
+down, due to a bigger icache footprint for the CPU and simply because
+there is less memory available for the pagecache. Just think about it; a
+pagecache miss causes a disk seek, which easily takes 5 milliseconds.
+There are a LOT of cpu cycles that can go into these 5 milliseconds.
+
+A reasonable rule of thumb is to not put inline at functions that have
+more than 3 lines of code in them. An exception to this rule are the
+cases where a parameter is known to be a compiletime constant, and as a
+result of this constantness you *know* the compiler will be able to
+optimize most of your function away at compile time. For a good example
+of this later case, see the kmalloc() inline function.
+
+Often people argue that adding inline to functions that are static and
+used only once is always a win since there is no space tradeoff. While
+this is technically correct, gcc is capable of inlining these
+automatically without help, and the maintenance issue of removing the
+inline when a second user appears outweighs the potential value of the
+hint that tells gcc to do something it would have done anyway.
+
+Function return values and names
+--------------------------------
+
+Functions can return values of many different kinds, and one of the most
+common is a value indicating whether the function succeeded or failed.
+Such a value can be represented as an error-code integer (-Exxx =
+failure, 0 = success) or a "succeeded" boolean (0 = failure, non-zero
+= success).
+
+Mixing up these two sorts of representations is a fertile source of
+difficult-to-find bugs. If the C language included a strong distinction
+between integers and booleans then the compiler would find these
+mistakes for us... but it doesn't. To help prevent such bugs, always
+follow this convention:
+
+If the name of a function is an action or an imperative command,
+the function should return an error-code integer.  If the name
+is a predicate, the function should return a "succeeded" boolean.
+
+For example, "add work" is a command, and the add_work() function
+returns 0 for success or -EBUSY for failure. In the same way, "PCI
+device present" is a predicate, and the pci_dev_present() function
+returns 1 if it succeeds in finding a matching device or 0 if it
+doesn't.
+
+All EXPORTed functions must respect this convention, and so should all
+public functions. Private (static) functions need not, but it is
+recommended that they do.
+
+Functions whose return value is the actual result of a computation,
+rather than an indication of whether the computation succeeded, are not
+subject to this rule. Generally they indicate failure by returning some
+out-of-range result. Typical examples would be functions that return
+pointers; they use NULL or the ERR_PTR mechanism to report failure.
+
+Don't re-invent the kernel macros
+----------------------------------
+
+The header file include/linux/kernel.h contains a number of macros that
+you should use, rather than explicitly coding some variant of them
+yourself. For example, if you need to calculate the length of an array,
+take advantage of the macro
+
+```c
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+```
+
+There are also min() and max() macros that do strict type checking if
+you need them. Feel free to peruse that header file to see what else is
+already defined that you shouldn't reproduce in your code.
+
+Editor modelines and other cruft
+--------------------------------
+
+Some editors can interpret configuration information embedded in source
+files, indicated with special markers. For example, emacs interprets
+lines marked like this:
+
+```
+-*- mode: c -*-
+```
+
+Or like this:
+
+```
+/*
+Local Variables:
+compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
+End:
+*/
+```
+
+Vim interprets markers that look like this:
+
+```
+/* vim:set sw=8 noet */
+```
+
+Do not include any of these in source files. People have their own
+personal editor configurations, and your source files should not
+override them. This includes markers for indentation and mode
+configuration. People may use their own custom mode, or may have some
+other magic method for making indentation work correctly.
+
+Inline assembly
+---------------
+
+In architecture-specific code, you may need to use inline assembly to
+interface with CPU or platform functionality. Don't hesitate to do so
+when necessary. However, don't use inline assembly gratuitously when C
+can do the job. You can and should poke hardware from C when possible.
+
+Consider writing simple helper functions that wrap common bits of inline
+assembly, rather than repeatedly writing them with slight variations.
+Remember that inline assembly can use C parameters.
+
+Large, non-trivial assembly functions should go in .S files, with
+corresponding C prototypes defined in C header files. The C prototypes
+for assembly functions should use "asmlinkage".
+
+You may need to mark your asm statement as volatile, to prevent GCC from
+removing it if GCC doesn't notice any side effects. You don't always
+need to do so, though, and doing so unnecessarily can limit
+optimization.
+
+When writing a single inline assembly statement containing multiple
+instructions, put each instruction on a separate line in a separate
+quoted string, and end each string except the last with nt to
+properly indent the next instruction in the assembly output:
+
+```c
+asm ("magic %reg1, #42nt"
+	"more_magic %reg2, %reg3"
+	: /* outputs */ : /* inputs */ : /* clobbers */);
+```
+
+References
+----------
+
+The C Programming Language, Second Edition by Brian W. Kernighan and
+Dennis M. Ritchie. Prentice Hall, Inc., 1988. ISBN 0-13-110362-8
+(paperback), 0-13-110370-9 (hardback). URL:
+<http://cm.bell-labs.com/cm/cs/cbook/>
+
+The Practice of Programming by Brian W. Kernighan and Rob Pike.
+Addison-Wesley, Inc., 1999. ISBN 0-201-61586-X. URL:
+<http://cm.bell-labs.com/cm/cs/tpop/>
+
+GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
+gcc internals and indent, all available from
+<http://www.gnu.org/manual/>
+
+WG14 is the international standardization working group for the
+programming language C, URL: <http://www.open-std.org/JTC1/SC22/WG14/>
+
+Kernel CodingStyle, by greg@kroah.com at OLS 2002:
+<http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/>
diff --git a/Documentation/index.md b/Documentation/index.md
index 9fdb0dca8c..5c389097ce 100644
--- a/Documentation/index.md
+++ b/Documentation/index.md
@@ -9,6 +9,7 @@ Contents:
 
 * [Getting Started](getting_started/index.md)
 * [Rookie Guide](lessons/index.md)
+* [Coding Style](coding_style.md)
 * [Code of Conduct](code_of_conduct.md)
 * [Timestamps](timestamp.md)
 * [Intel IFD Binary Extraction](Binary_Extraction.md)