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diff --git a/Documentation/coding_style.md b/Documentation/coding_style.md new file mode 100644 index 0000000000..048b8e673e --- /dev/null +++ b/Documentation/coding_style.md @@ -0,0 +1,938 @@ +# 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/> |