diff options
Diffstat (limited to 'Documentation/kbuild')
| -rw-r--r-- | Documentation/kbuild/00-INDEX | 14 | ||||
| -rw-r--r-- | Documentation/kbuild/headers_install.txt | 47 | ||||
| -rw-r--r-- | Documentation/kbuild/kbuild.txt | 235 | ||||
| -rw-r--r-- | Documentation/kbuild/kconfig-language.txt | 395 | ||||
| -rw-r--r-- | Documentation/kbuild/kconfig.txt | 237 | ||||
| -rw-r--r-- | Documentation/kbuild/makefiles.txt | 1410 | ||||
| -rw-r--r-- | Documentation/kbuild/modules.txt | 541 |
7 files changed, 2879 insertions, 0 deletions
diff --git a/Documentation/kbuild/00-INDEX b/Documentation/kbuild/00-INDEX new file mode 100644 index 000000000..8c5e6aa78 --- /dev/null +++ b/Documentation/kbuild/00-INDEX @@ -0,0 +1,14 @@ +00-INDEX + - this file: info on the kernel build process +headers_install.txt + - how to export Linux headers for use by userspace +kbuild.txt + - developer information on kbuild +kconfig.txt + - usage help for make *config +kconfig-language.txt + - specification of Config Language, the language in Kconfig files +makefiles.txt + - developer information for linux kernel makefiles +modules.txt + - how to build modules and to install them diff --git a/Documentation/kbuild/headers_install.txt b/Documentation/kbuild/headers_install.txt new file mode 100644 index 000000000..951eb9f1e --- /dev/null +++ b/Documentation/kbuild/headers_install.txt @@ -0,0 +1,47 @@ +Exporting kernel headers for use by userspace +============================================= + +The "make headers_install" command exports the kernel's header files in a +form suitable for use by userspace programs. + +The linux kernel's exported header files describe the API for user space +programs attempting to use kernel services. These kernel header files are +used by the system's C library (such as glibc or uClibc) to define available +system calls, as well as constants and structures to be used with these +system calls. The C library's header files include the kernel header files +from the "linux" subdirectory. The system's libc headers are usually +installed at the default location /usr/include and the kernel headers in +subdirectories under that (most notably /usr/include/linux and +/usr/include/asm). + +Kernel headers are backwards compatible, but not forwards compatible. This +means that a program built against a C library using older kernel headers +should run on a newer kernel (although it may not have access to new +features), but a program built against newer kernel headers may not work on an +older kernel. + +The "make headers_install" command can be run in the top level directory of the +kernel source code (or using a standard out-of-tree build). It takes two +optional arguments: + + make headers_install ARCH=i386 INSTALL_HDR_PATH=/usr/include + +ARCH indicates which architecture to produce headers for, and defaults to the +current architecture. The linux/asm directory of the exported kernel headers +is platform-specific, to see a complete list of supported architectures use +the command: + + ls -d include/asm-* | sed 's/.*-//' + +INSTALL_HDR_PATH indicates where to install the headers. It defaults to +"./usr/include". + +The command "make headers_install_all" exports headers for all architectures +simultaneously. (This is mostly of interest to distribution maintainers, +who create an architecture-independent tarball from the resulting include +directory.) You also can use HDR_ARCH_LIST to specify list of architectures. +Remember to provide the appropriate linux/asm directory via "mv" or "ln -s" +before building a C library with headers exported this way. + +The kernel header export infrastructure is maintained by David Woodhouse +<dwmw2@infradead.org>. diff --git a/Documentation/kbuild/kbuild.txt b/Documentation/kbuild/kbuild.txt new file mode 100644 index 000000000..6466704d4 --- /dev/null +++ b/Documentation/kbuild/kbuild.txt @@ -0,0 +1,235 @@ +Output files + +modules.order +-------------------------------------------------- +This file records the order in which modules appear in Makefiles. This +is used by modprobe to deterministically resolve aliases that match +multiple modules. + +modules.builtin +-------------------------------------------------- +This file lists all modules that are built into the kernel. This is used +by modprobe to not fail when trying to load something builtin. + + +Environment variables + +KCPPFLAGS +-------------------------------------------------- +Additional options to pass when preprocessing. The preprocessing options +will be used in all cases where kbuild does preprocessing including +building C files and assembler files. + +KAFLAGS +-------------------------------------------------- +Additional options to the assembler (for built-in and modules). + +AFLAGS_MODULE +-------------------------------------------------- +Additional module specific options to use for $(AS). + +AFLAGS_KERNEL +-------------------------------------------------- +Additional options for $(AS) when used for assembler +code for code that is compiled as built-in. + +KCFLAGS +-------------------------------------------------- +Additional options to the C compiler (for built-in and modules). + +CFLAGS_KERNEL +-------------------------------------------------- +Additional options for $(CC) when used to compile +code that is compiled as built-in. + +CFLAGS_MODULE +-------------------------------------------------- +Additional module specific options to use for $(CC). + +LDFLAGS_MODULE +-------------------------------------------------- +Additional options used for $(LD) when linking modules. + +LDFLAGS_vmlinux +-------------------------------------------------- +Additional options passed to final link of vmlinux. + +KBUILD_VERBOSE +-------------------------------------------------- +Set the kbuild verbosity. Can be assigned same values as "V=...". +See make help for the full list. +Setting "V=..." takes precedence over KBUILD_VERBOSE. + +KBUILD_EXTMOD +-------------------------------------------------- +Set the directory to look for the kernel source when building external +modules. +The directory can be specified in several ways: +1) Use "M=..." on the command line +2) Environment variable KBUILD_EXTMOD +3) Environment variable SUBDIRS +The possibilities are listed in the order they take precedence. +Using "M=..." will always override the others. + +KBUILD_OUTPUT +-------------------------------------------------- +Specify the output directory when building the kernel. +The output directory can also be specified using "O=...". +Setting "O=..." takes precedence over KBUILD_OUTPUT. + +KBUILD_DEBARCH +-------------------------------------------------- +For the deb-pkg target, allows overriding the normal heuristics deployed by +deb-pkg. Normally deb-pkg attempts to guess the right architecture based on +the UTS_MACHINE variable, and on some architectures also the kernel config. +The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian +architecture. + +ARCH +-------------------------------------------------- +Set ARCH to the architecture to be built. +In most cases the name of the architecture is the same as the +directory name found in the arch/ directory. +But some architectures such as x86 and sparc have aliases. +x86: i386 for 32 bit, x86_64 for 64 bit +sparc: sparc for 32 bit, sparc64 for 64 bit + +CROSS_COMPILE +-------------------------------------------------- +Specify an optional fixed part of the binutils filename. +CROSS_COMPILE can be a part of the filename or the full path. + +CROSS_COMPILE is also used for ccache in some setups. + +CF +-------------------------------------------------- +Additional options for sparse. +CF is often used on the command-line like this: + + make CF=-Wbitwise C=2 + +INSTALL_PATH +-------------------------------------------------- +INSTALL_PATH specifies where to place the updated kernel and system map +images. Default is /boot, but you can set it to other values. + +INSTALLKERNEL +-------------------------------------------------- +Install script called when using "make install". +The default name is "installkernel". + +The script will be called with the following arguments: + $1 - kernel version + $2 - kernel image file + $3 - kernel map file + $4 - default install path (use root directory if blank) + +The implementation of "make install" is architecture specific +and it may differ from the above. + +INSTALLKERNEL is provided to enable the possibility to +specify a custom installer when cross compiling a kernel. + +MODLIB +-------------------------------------------------- +Specify where to install modules. +The default value is: + + $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE) + +The value can be overridden in which case the default value is ignored. + +INSTALL_MOD_PATH +-------------------------------------------------- +INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory +relocations required by build roots. This is not defined in the +makefile but the argument can be passed to make if needed. + +INSTALL_MOD_STRIP +-------------------------------------------------- +INSTALL_MOD_STRIP, if defined, will cause modules to be +stripped after they are installed. If INSTALL_MOD_STRIP is '1', then +the default option --strip-debug will be used. Otherwise, +INSTALL_MOD_STRIP value will be used as the options to the strip command. + +INSTALL_FW_PATH +-------------------------------------------------- +INSTALL_FW_PATH specifies where to install the firmware blobs. +The default value is: + + $(INSTALL_MOD_PATH)/lib/firmware + +The value can be overridden in which case the default value is ignored. + +INSTALL_HDR_PATH +-------------------------------------------------- +INSTALL_HDR_PATH specifies where to install user space headers when +executing "make headers_*". +The default value is: + + $(objtree)/usr + +$(objtree) is the directory where output files are saved. +The output directory is often set using "O=..." on the commandline. + +The value can be overridden in which case the default value is ignored. + +KBUILD_MODPOST_WARN +-------------------------------------------------- +KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined +symbols in the final module linking stage. It changes such errors +into warnings. + +KBUILD_MODPOST_NOFINAL +-------------------------------------------------- +KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules. +This is solely useful to speed up test compiles. + +KBUILD_EXTRA_SYMBOLS +-------------------------------------------------- +For modules that use symbols from other modules. +See more details in modules.txt. + +ALLSOURCE_ARCHS +-------------------------------------------------- +For tags/TAGS/cscope targets, you can specify more than one arch +to be included in the databases, separated by blank space. E.g.: + + $ make ALLSOURCE_ARCHS="x86 mips arm" tags + +To get all available archs you can also specify all. E.g.: + + $ make ALLSOURCE_ARCHS=all tags + +KBUILD_ENABLE_EXTRA_GCC_CHECKS +-------------------------------------------------- +If enabled over the make command line with "W=1", it turns on additional +gcc -W... options for more extensive build-time checking. + +KBUILD_BUILD_TIMESTAMP +-------------------------------------------------- +Setting this to a date string overrides the timestamp used in the +UTS_VERSION definition (uname -v in the running kernel). The value has to +be a string that can be passed to date -d. The default value +is the output of the date command at one point during build. + +KBUILD_BUILD_USER, KBUILD_BUILD_HOST +-------------------------------------------------- +These two variables allow to override the user@host string displayed during +boot and in /proc/version. The default value is the output of the commands +whoami and host, respectively. + +KBUILD_LDS +-------------------------------------------------- +The linker script with full path. Assigned by the top-level Makefile. + +KBUILD_VMLINUX_INIT +-------------------------------------------------- +All object files for the init (first) part of vmlinux. +Files specified with KBUILD_VMLINUX_INIT are linked first. + +KBUILD_VMLINUX_MAIN +-------------------------------------------------- +All object files for the main part of vmlinux. +KBUILD_VMLINUX_INIT and KBUILD_VMLINUX_MAIN together specify +all the object files used to link vmlinux. diff --git a/Documentation/kbuild/kconfig-language.txt b/Documentation/kbuild/kconfig-language.txt new file mode 100644 index 000000000..350f733bf --- /dev/null +++ b/Documentation/kbuild/kconfig-language.txt @@ -0,0 +1,395 @@ +Introduction +------------ + +The configuration database is a collection of configuration options +organized in a tree structure: + + +- Code maturity level options + | +- Prompt for development and/or incomplete code/drivers + +- General setup + | +- Networking support + | +- System V IPC + | +- BSD Process Accounting + | +- Sysctl support + +- Loadable module support + | +- Enable loadable module support + | +- Set version information on all module symbols + | +- Kernel module loader + +- ... + +Every entry has its own dependencies. These dependencies are used +to determine the visibility of an entry. Any child entry is only +visible if its parent entry is also visible. + +Menu entries +------------ + +Most entries define a config option; all other entries help to organize +them. A single configuration option is defined like this: + +config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + help + Usually, modules have to be recompiled whenever you switch to a new + kernel. ... + +Every line starts with a key word and can be followed by multiple +arguments. "config" starts a new config entry. The following lines +define attributes for this config option. Attributes can be the type of +the config option, input prompt, dependencies, help text and default +values. A config option can be defined multiple times with the same +name, but every definition can have only a single input prompt and the +type must not conflict. + +Menu attributes +--------------- + +A menu entry can have a number of attributes. Not all of them are +applicable everywhere (see syntax). + +- type definition: "bool"/"tristate"/"string"/"hex"/"int" + Every config option must have a type. There are only two basic types: + tristate and string; the other types are based on these two. The type + definition optionally accepts an input prompt, so these two examples + are equivalent: + + bool "Networking support" + and + bool + prompt "Networking support" + +- input prompt: "prompt" <prompt> ["if" <expr>] + Every menu entry can have at most one prompt, which is used to display + to the user. Optionally dependencies only for this prompt can be added + with "if". + +- default value: "default" <expr> ["if" <expr>] + A config option can have any number of default values. If multiple + default values are visible, only the first defined one is active. + Default values are not limited to the menu entry where they are + defined. This means the default can be defined somewhere else or be + overridden by an earlier definition. + The default value is only assigned to the config symbol if no other + value was set by the user (via the input prompt above). If an input + prompt is visible the default value is presented to the user and can + be overridden by him. + Optionally, dependencies only for this default value can be added with + "if". + +- type definition + default value: + "def_bool"/"def_tristate" <expr> ["if" <expr>] + This is a shorthand notation for a type definition plus a value. + Optionally dependencies for this default value can be added with "if". + +- dependencies: "depends on" <expr> + This defines a dependency for this menu entry. If multiple + dependencies are defined, they are connected with '&&'. Dependencies + are applied to all other options within this menu entry (which also + accept an "if" expression), so these two examples are equivalent: + + bool "foo" if BAR + default y if BAR + and + depends on BAR + bool "foo" + default y + +- reverse dependencies: "select" <symbol> ["if" <expr>] + While normal dependencies reduce the upper limit of a symbol (see + below), reverse dependencies can be used to force a lower limit of + another symbol. The value of the current menu symbol is used as the + minimal value <symbol> can be set to. If <symbol> is selected multiple + times, the limit is set to the largest selection. + Reverse dependencies can only be used with boolean or tristate + symbols. + Note: + select should be used with care. select will force + a symbol to a value without visiting the dependencies. + By abusing select you are able to select a symbol FOO even + if FOO depends on BAR that is not set. + In general use select only for non-visible symbols + (no prompts anywhere) and for symbols with no dependencies. + That will limit the usefulness but on the other hand avoid + the illegal configurations all over. + +- limiting menu display: "visible if" <expr> + This attribute is only applicable to menu blocks, if the condition is + false, the menu block is not displayed to the user (the symbols + contained there can still be selected by other symbols, though). It is + similar to a conditional "prompt" attribute for individual menu + entries. Default value of "visible" is true. + +- numerical ranges: "range" <symbol> <symbol> ["if" <expr>] + This allows to limit the range of possible input values for int + and hex symbols. The user can only input a value which is larger than + or equal to the first symbol and smaller than or equal to the second + symbol. + +- help text: "help" or "---help---" + This defines a help text. The end of the help text is determined by + the indentation level, this means it ends at the first line which has + a smaller indentation than the first line of the help text. + "---help---" and "help" do not differ in behaviour, "---help---" is + used to help visually separate configuration logic from help within + the file as an aid to developers. + +- misc options: "option" <symbol>[=<value>] + Various less common options can be defined via this option syntax, + which can modify the behaviour of the menu entry and its config + symbol. These options are currently possible: + + - "defconfig_list" + This declares a list of default entries which can be used when + looking for the default configuration (which is used when the main + .config doesn't exists yet.) + + - "modules" + This declares the symbol to be used as the MODULES symbol, which + enables the third modular state for all config symbols. + At most one symbol may have the "modules" option set. + + - "env"=<value> + This imports the environment variable into Kconfig. It behaves like + a default, except that the value comes from the environment, this + also means that the behaviour when mixing it with normal defaults is + undefined at this point. The symbol is currently not exported back + to the build environment (if this is desired, it can be done via + another symbol). + + - "allnoconfig_y" + This declares the symbol as one that should have the value y when + using "allnoconfig". Used for symbols that hide other symbols. + +Menu dependencies +----------------- + +Dependencies define the visibility of a menu entry and can also reduce +the input range of tristate symbols. The tristate logic used in the +expressions uses one more state than normal boolean logic to express the +module state. Dependency expressions have the following syntax: + +<expr> ::= <symbol> (1) + <symbol> '=' <symbol> (2) + <symbol> '!=' <symbol> (3) + '(' <expr> ')' (4) + '!' <expr> (5) + <expr> '&&' <expr> (6) + <expr> '||' <expr> (7) + +Expressions are listed in decreasing order of precedence. + +(1) Convert the symbol into an expression. Boolean and tristate symbols + are simply converted into the respective expression values. All + other symbol types result in 'n'. +(2) If the values of both symbols are equal, it returns 'y', + otherwise 'n'. +(3) If the values of both symbols are equal, it returns 'n', + otherwise 'y'. +(4) Returns the value of the expression. Used to override precedence. +(5) Returns the result of (2-/expr/). +(6) Returns the result of min(/expr/, /expr/). +(7) Returns the result of max(/expr/, /expr/). + +An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2 +respectively for calculations). A menu entry becomes visible when its +expression evaluates to 'm' or 'y'. + +There are two types of symbols: constant and non-constant symbols. +Non-constant symbols are the most common ones and are defined with the +'config' statement. Non-constant symbols consist entirely of alphanumeric +characters or underscores. +Constant symbols are only part of expressions. Constant symbols are +always surrounded by single or double quotes. Within the quote, any +other character is allowed and the quotes can be escaped using '\'. + +Menu structure +-------------- + +The position of a menu entry in the tree is determined in two ways. First +it can be specified explicitly: + +menu "Network device support" + depends on NET + +config NETDEVICES + ... + +endmenu + +All entries within the "menu" ... "endmenu" block become a submenu of +"Network device support". All subentries inherit the dependencies from +the menu entry, e.g. this means the dependency "NET" is added to the +dependency list of the config option NETDEVICES. + +The other way to generate the menu structure is done by analyzing the +dependencies. If a menu entry somehow depends on the previous entry, it +can be made a submenu of it. First, the previous (parent) symbol must +be part of the dependency list and then one of these two conditions +must be true: +- the child entry must become invisible, if the parent is set to 'n' +- the child entry must only be visible, if the parent is visible + +config MODULES + bool "Enable loadable module support" + +config MODVERSIONS + bool "Set version information on all module symbols" + depends on MODULES + +comment "module support disabled" + depends on !MODULES + +MODVERSIONS directly depends on MODULES, this means it's only visible if +MODULES is different from 'n'. The comment on the other hand is always +visible when MODULES is visible (the (empty) dependency of MODULES is +also part of the comment dependencies). + + +Kconfig syntax +-------------- + +The configuration file describes a series of menu entries, where every +line starts with a keyword (except help texts). The following keywords +end a menu entry: +- config +- menuconfig +- choice/endchoice +- comment +- menu/endmenu +- if/endif +- source +The first five also start the definition of a menu entry. + +config: + + "config" <symbol> + <config options> + +This defines a config symbol <symbol> and accepts any of above +attributes as options. + +menuconfig: + "menuconfig" <symbol> + <config options> + +This is similar to the simple config entry above, but it also gives a +hint to front ends, that all suboptions should be displayed as a +separate list of options. + +choices: + + "choice" [symbol] + <choice options> + <choice block> + "endchoice" + +This defines a choice group and accepts any of the above attributes as +options. A choice can only be of type bool or tristate, while a boolean +choice only allows a single config entry to be selected, a tristate +choice also allows any number of config entries to be set to 'm'. This +can be used if multiple drivers for a single hardware exists and only a +single driver can be compiled/loaded into the kernel, but all drivers +can be compiled as modules. +A choice accepts another option "optional", which allows to set the +choice to 'n' and no entry needs to be selected. +If no [symbol] is associated with a choice, then you can not have multiple +definitions of that choice. If a [symbol] is associated to the choice, +then you may define the same choice (ie. with the same entries) in another +place. + +comment: + + "comment" <prompt> + <comment options> + +This defines a comment which is displayed to the user during the +configuration process and is also echoed to the output files. The only +possible options are dependencies. + +menu: + + "menu" <prompt> + <menu options> + <menu block> + "endmenu" + +This defines a menu block, see "Menu structure" above for more +information. The only possible options are dependencies and "visible" +attributes. + +if: + + "if" <expr> + <if block> + "endif" + +This defines an if block. The dependency expression <expr> is appended +to all enclosed menu entries. + +source: + + "source" <prompt> + +This reads the specified configuration file. This file is always parsed. + +mainmenu: + + "mainmenu" <prompt> + +This sets the config program's title bar if the config program chooses +to use it. It should be placed at the top of the configuration, before any +other statement. + + +Kconfig hints +------------- +This is a collection of Kconfig tips, most of which aren't obvious at +first glance and most of which have become idioms in several Kconfig +files. + +Adding common features and make the usage configurable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is a common idiom to implement a feature/functionality that are +relevant for some architectures but not all. +The recommended way to do so is to use a config variable named HAVE_* +that is defined in a common Kconfig file and selected by the relevant +architectures. +An example is the generic IOMAP functionality. + +We would in lib/Kconfig see: + +# Generic IOMAP is used to ... +config HAVE_GENERIC_IOMAP + +config GENERIC_IOMAP + depends on HAVE_GENERIC_IOMAP && FOO + +And in lib/Makefile we would see: +obj-$(CONFIG_GENERIC_IOMAP) += iomap.o + +For each architecture using the generic IOMAP functionality we would see: + +config X86 + select ... + select HAVE_GENERIC_IOMAP + select ... + +Note: we use the existing config option and avoid creating a new +config variable to select HAVE_GENERIC_IOMAP. + +Note: the use of the internal config variable HAVE_GENERIC_IOMAP, it is +introduced to overcome the limitation of select which will force a +config option to 'y' no matter the dependencies. +The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the +situation where select forces a symbol equals to 'y'. + +Build as module only +~~~~~~~~~~~~~~~~~~~~ +To restrict a component build to module-only, qualify its config symbol +with "depends on m". E.g.: + +config FOO + depends on BAR && m + +limits FOO to module (=m) or disabled (=n). diff --git a/Documentation/kbuild/kconfig.txt b/Documentation/kbuild/kconfig.txt new file mode 100644 index 000000000..bbc99c0c1 --- /dev/null +++ b/Documentation/kbuild/kconfig.txt @@ -0,0 +1,237 @@ +This file contains some assistance for using "make *config". + +Use "make help" to list all of the possible configuration targets. + +The xconfig ('qconf') and menuconfig ('mconf') programs also +have embedded help text. Be sure to check it for navigation, +search, and other general help text. + +====================================================================== +General +-------------------------------------------------- + +New kernel releases often introduce new config symbols. Often more +important, new kernel releases may rename config symbols. When +this happens, using a previously working .config file and running +"make oldconfig" won't necessarily produce a working new kernel +for you, so you may find that you need to see what NEW kernel +symbols have been introduced. + +To see a list of new config symbols when using "make oldconfig", use + + cp user/some/old.config .config + make listnewconfig + +and the config program will list any new symbols, one per line. + + scripts/diffconfig .config.old .config | less + +______________________________________________________________________ +Environment variables for '*config' + +KCONFIG_CONFIG +-------------------------------------------------- +This environment variable can be used to specify a default kernel config +file name to override the default name of ".config". + +KCONFIG_OVERWRITECONFIG +-------------------------------------------------- +If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not +break symlinks when .config is a symlink to somewhere else. + +CONFIG_ +-------------------------------------------------- +If you set CONFIG_ in the environment, Kconfig will prefix all symbols +with its value when saving the configuration, instead of using the default, +"CONFIG_". + +______________________________________________________________________ +Environment variables for '{allyes/allmod/allno/rand}config' + +KCONFIG_ALLCONFIG +-------------------------------------------------- +(partially based on lkml email from/by Rob Landley, re: miniconfig) +-------------------------------------------------- +The allyesconfig/allmodconfig/allnoconfig/randconfig variants can also +use the environment variable KCONFIG_ALLCONFIG as a flag or a filename +that contains config symbols that the user requires to be set to a +specific value. If KCONFIG_ALLCONFIG is used without a filename where +KCONFIG_ALLCONFIG == "" or KCONFIG_ALLCONFIG == "1", "make *config" +checks for a file named "all{yes/mod/no/def/random}.config" +(corresponding to the *config command that was used) for symbol values +that are to be forced. If this file is not found, it checks for a +file named "all.config" to contain forced values. + +This enables you to create "miniature" config (miniconfig) or custom +config files containing just the config symbols that you are interested +in. Then the kernel config system generates the full .config file, +including symbols of your miniconfig file. + +This 'KCONFIG_ALLCONFIG' file is a config file which contains +(usually a subset of all) preset config symbols. These variable +settings are still subject to normal dependency checks. + +Examples: + KCONFIG_ALLCONFIG=custom-notebook.config make allnoconfig +or + KCONFIG_ALLCONFIG=mini.config make allnoconfig +or + make KCONFIG_ALLCONFIG=mini.config allnoconfig + +These examples will disable most options (allnoconfig) but enable or +disable the options that are explicitly listed in the specified +mini-config files. + +______________________________________________________________________ +Environment variables for 'randconfig' + +KCONFIG_SEED +-------------------------------------------------- +You can set this to the integer value used to seed the RNG, if you want +to somehow debug the behaviour of the kconfig parser/frontends. +If not set, the current time will be used. + +KCONFIG_PROBABILITY +-------------------------------------------------- +This variable can be used to skew the probabilities. This variable can +be unset or empty, or set to three different formats: + KCONFIG_PROBABILITY y:n split y:m:n split + ----------------------------------------------------------------- + unset or empty 50 : 50 33 : 33 : 34 + N N : 100-N N/2 : N/2 : 100-N + [1] N:M N+M : 100-(N+M) N : M : 100-(N+M) + [2] N:M:L N : 100-N M : L : 100-(M+L) + +where N, M and L are integers (in base 10) in the range [0,100], and so +that: + [1] N+M is in the range [0,100] + [2] M+L is in the range [0,100] + +Examples: + KCONFIG_PROBABILITY=10 + 10% of booleans will be set to 'y', 90% to 'n' + 5% of tristates will be set to 'y', 5% to 'm', 90% to 'n' + KCONFIG_PROBABILITY=15:25 + 40% of booleans will be set to 'y', 60% to 'n' + 15% of tristates will be set to 'y', 25% to 'm', 60% to 'n' + KCONFIG_PROBABILITY=10:15:15 + 10% of booleans will be set to 'y', 90% to 'n' + 15% of tristates will be set to 'y', 15% to 'm', 70% to 'n' + +______________________________________________________________________ +Environment variables for 'silentoldconfig' + +KCONFIG_NOSILENTUPDATE +-------------------------------------------------- +If this variable has a non-blank value, it prevents silent kernel +config updates (requires explicit updates). + +KCONFIG_AUTOCONFIG +-------------------------------------------------- +This environment variable can be set to specify the path & name of the +"auto.conf" file. Its default value is "include/config/auto.conf". + +KCONFIG_TRISTATE +-------------------------------------------------- +This environment variable can be set to specify the path & name of the +"tristate.conf" file. Its default value is "include/config/tristate.conf". + +KCONFIG_AUTOHEADER +-------------------------------------------------- +This environment variable can be set to specify the path & name of the +"autoconf.h" (header) file. +Its default value is "include/generated/autoconf.h". + + +====================================================================== +menuconfig +-------------------------------------------------- + +SEARCHING for CONFIG symbols + +Searching in menuconfig: + + The Search function searches for kernel configuration symbol + names, so you have to know something close to what you are + looking for. + + Example: + /hotplug + This lists all config symbols that contain "hotplug", + e.g., HOTPLUG_CPU, MEMORY_HOTPLUG. + + For search help, enter / followed TAB-TAB-TAB (to highlight + <Help>) and Enter. This will tell you that you can also use + regular expressions (regexes) in the search string, so if you + are not interested in MEMORY_HOTPLUG, you could try + + /^hotplug + + When searching, symbols are sorted thus: + - first, exact matches, sorted alphabetically (an exact match + is when the search matches the complete symbol name); + - then, other matches, sorted alphabetically. + For example: ^ATH.K matches: + ATH5K ATH9K ATH5K_AHB ATH5K_DEBUG [...] ATH6KL ATH6KL_DEBUG + [...] ATH9K_AHB ATH9K_BTCOEX_SUPPORT ATH9K_COMMON [...] + of which only ATH5K and ATH9K match exactly and so are sorted + first (and in alphabetical order), then come all other symbols, + sorted in alphabetical order. + +______________________________________________________________________ +User interface options for 'menuconfig' + +MENUCONFIG_COLOR +-------------------------------------------------- +It is possible to select different color themes using the variable +MENUCONFIG_COLOR. To select a theme use: + + make MENUCONFIG_COLOR=<theme> menuconfig + +Available themes are: + mono => selects colors suitable for monochrome displays + blackbg => selects a color scheme with black background + classic => theme with blue background. The classic look + bluetitle => a LCD friendly version of classic. (default) + +MENUCONFIG_MODE +-------------------------------------------------- +This mode shows all sub-menus in one large tree. + +Example: + make MENUCONFIG_MODE=single_menu menuconfig + + +====================================================================== +xconfig +-------------------------------------------------- + +Searching in xconfig: + + The Search function searches for kernel configuration symbol + names, so you have to know something close to what you are + looking for. + + Example: + Ctrl-F hotplug + or + Menu: File, Search, hotplug + + lists all config symbol entries that contain "hotplug" in + the symbol name. In this Search dialog, you may change the + config setting for any of the entries that are not grayed out. + You can also enter a different search string without having + to return to the main menu. + + +====================================================================== +gconfig +-------------------------------------------------- + +Searching in gconfig: + + None (gconfig isn't maintained as well as xconfig or menuconfig); + however, gconfig does have a few more viewing choices than + xconfig does. + +### diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt new file mode 100644 index 000000000..74b6c6d97 --- /dev/null +++ b/Documentation/kbuild/makefiles.txt @@ -0,0 +1,1410 @@ +Linux Kernel Makefiles + +This document describes the Linux kernel Makefiles. + +=== Table of Contents + + === 1 Overview + === 2 Who does what + === 3 The kbuild files + --- 3.1 Goal definitions + --- 3.2 Built-in object goals - obj-y + --- 3.3 Loadable module goals - obj-m + --- 3.4 Objects which export symbols + --- 3.5 Library file goals - lib-y + --- 3.6 Descending down in directories + --- 3.7 Compilation flags + --- 3.8 Command line dependency + --- 3.9 Dependency tracking + --- 3.10 Special Rules + --- 3.11 $(CC) support functions + --- 3.12 $(LD) support functions + + === 4 Host Program support + --- 4.1 Simple Host Program + --- 4.2 Composite Host Programs + --- 4.3 Using C++ for host programs + --- 4.4 Controlling compiler options for host programs + --- 4.5 When host programs are actually built + --- 4.6 Using hostprogs-$(CONFIG_FOO) + + === 5 Kbuild clean infrastructure + + === 6 Architecture Makefiles + --- 6.1 Set variables to tweak the build to the architecture + --- 6.2 Add prerequisites to archheaders: + --- 6.3 Add prerequisites to archprepare: + --- 6.4 List directories to visit when descending + --- 6.5 Architecture-specific boot images + --- 6.6 Building non-kbuild targets + --- 6.7 Commands useful for building a boot image + --- 6.8 Custom kbuild commands + --- 6.9 Preprocessing linker scripts + --- 6.10 Generic header files + + === 7 Kbuild syntax for exported headers + --- 7.1 header-y + --- 7.2 genhdr-y + --- 7.3 destination-y + --- 7.4 generic-y + + === 8 Kbuild Variables + === 9 Makefile language + === 10 Credits + === 11 TODO + +=== 1 Overview + +The Makefiles have five parts: + + Makefile the top Makefile. + .config the kernel configuration file. + arch/$(ARCH)/Makefile the arch Makefile. + scripts/Makefile.* common rules etc. for all kbuild Makefiles. + kbuild Makefiles there are about 500 of these. + +The top Makefile reads the .config file, which comes from the kernel +configuration process. + +The top Makefile is responsible for building two major products: vmlinux +(the resident kernel image) and modules (any module files). +It builds these goals by recursively descending into the subdirectories of +the kernel source tree. +The list of subdirectories which are visited depends upon the kernel +configuration. The top Makefile textually includes an arch Makefile +with the name arch/$(ARCH)/Makefile. The arch Makefile supplies +architecture-specific information to the top Makefile. + +Each subdirectory has a kbuild Makefile which carries out the commands +passed down from above. The kbuild Makefile uses information from the +.config file to construct various file lists used by kbuild to build +any built-in or modular targets. + +scripts/Makefile.* contains all the definitions/rules etc. that +are used to build the kernel based on the kbuild makefiles. + + +=== 2 Who does what + +People have four different relationships with the kernel Makefiles. + +*Users* are people who build kernels. These people type commands such as +"make menuconfig" or "make". They usually do not read or edit +any kernel Makefiles (or any other source files). + +*Normal developers* are people who work on features such as device +drivers, file systems, and network protocols. These people need to +maintain the kbuild Makefiles for the subsystem they are +working on. In order to do this effectively, they need some overall +knowledge about the kernel Makefiles, plus detailed knowledge about the +public interface for kbuild. + +*Arch developers* are people who work on an entire architecture, such +as sparc or ia64. Arch developers need to know about the arch Makefile +as well as kbuild Makefiles. + +*Kbuild developers* are people who work on the kernel build system itself. +These people need to know about all aspects of the kernel Makefiles. + +This document is aimed towards normal developers and arch developers. + + +=== 3 The kbuild files + +Most Makefiles within the kernel are kbuild Makefiles that use the +kbuild infrastructure. This chapter introduces the syntax used in the +kbuild makefiles. +The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can +be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild' +file will be used. + +Section 3.1 "Goal definitions" is a quick intro, further chapters provide +more details, with real examples. + +--- 3.1 Goal definitions + + Goal definitions are the main part (heart) of the kbuild Makefile. + These lines define the files to be built, any special compilation + options, and any subdirectories to be entered recursively. + + The most simple kbuild makefile contains one line: + + Example: + obj-y += foo.o + + This tells kbuild that there is one object in that directory, named + foo.o. foo.o will be built from foo.c or foo.S. + + If foo.o shall be built as a module, the variable obj-m is used. + Therefore the following pattern is often used: + + Example: + obj-$(CONFIG_FOO) += foo.o + + $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module). + If CONFIG_FOO is neither y nor m, then the file will not be compiled + nor linked. + +--- 3.2 Built-in object goals - obj-y + + The kbuild Makefile specifies object files for vmlinux + in the $(obj-y) lists. These lists depend on the kernel + configuration. + + Kbuild compiles all the $(obj-y) files. It then calls + "$(LD) -r" to merge these files into one built-in.o file. + built-in.o is later linked into vmlinux by the parent Makefile. + + The order of files in $(obj-y) is significant. Duplicates in + the lists are allowed: the first instance will be linked into + built-in.o and succeeding instances will be ignored. + + Link order is significant, because certain functions + (module_init() / __initcall) will be called during boot in the + order they appear. So keep in mind that changing the link + order may e.g. change the order in which your SCSI + controllers are detected, and thus your disks are renumbered. + + Example: + #drivers/isdn/i4l/Makefile + # Makefile for the kernel ISDN subsystem and device drivers. + # Each configuration option enables a list of files. + obj-$(CONFIG_ISDN_I4L) += isdn.o + obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o + +--- 3.3 Loadable module goals - obj-m + + $(obj-m) specifies object files which are built as loadable + kernel modules. + + A module may be built from one source file or several source + files. In the case of one source file, the kbuild makefile + simply adds the file to $(obj-m). + + Example: + #drivers/isdn/i4l/Makefile + obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o + + Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm' + + If a kernel module is built from several source files, you specify + that you want to build a module in the same way as above; however, + kbuild needs to know which object files you want to build your + module from, so you have to tell it by setting a $(<module_name>-y) + variable. + + Example: + #drivers/isdn/i4l/Makefile + obj-$(CONFIG_ISDN_I4L) += isdn.o + isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o + + In this example, the module name will be isdn.o. Kbuild will + compile the objects listed in $(isdn-y) and then run + "$(LD) -r" on the list of these files to generate isdn.o. + + Due to kbuild recognizing $(<module_name>-y) for composite objects, + you can use the value of a CONFIG_ symbol to optionally include an + object file as part of a composite object. + + Example: + #fs/ext2/Makefile + obj-$(CONFIG_EXT2_FS) += ext2.o + ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \ + namei.o super.o symlink.o + ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \ + xattr_trusted.o + + In this example, xattr.o, xattr_user.o and xattr_trusted.o are only + part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR) + evaluates to 'y'. + + Note: Of course, when you are building objects into the kernel, + the syntax above will also work. So, if you have CONFIG_EXT2_FS=y, + kbuild will build an ext2.o file for you out of the individual + parts and then link this into built-in.o, as you would expect. + +--- 3.4 Objects which export symbols + + No special notation is required in the makefiles for + modules exporting symbols. + +--- 3.5 Library file goals - lib-y + + Objects listed with obj-* are used for modules, or + combined in a built-in.o for that specific directory. + There is also the possibility to list objects that will + be included in a library, lib.a. + All objects listed with lib-y are combined in a single + library for that directory. + Objects that are listed in obj-y and additionally listed in + lib-y will not be included in the library, since they will + be accessible anyway. + For consistency, objects listed in lib-m will be included in lib.a. + + Note that the same kbuild makefile may list files to be built-in + and to be part of a library. Therefore the same directory + may contain both a built-in.o and a lib.a file. + + Example: + #arch/x86/lib/Makefile + lib-y := delay.o + + This will create a library lib.a based on delay.o. For kbuild to + actually recognize that there is a lib.a being built, the directory + shall be listed in libs-y. + See also "6.4 List directories to visit when descending". + + Use of lib-y is normally restricted to lib/ and arch/*/lib. + +--- 3.6 Descending down in directories + + A Makefile is only responsible for building objects in its own + directory. Files in subdirectories should be taken care of by + Makefiles in these subdirs. The build system will automatically + invoke make recursively in subdirectories, provided you let it know of + them. + + To do so, obj-y and obj-m are used. + ext2 lives in a separate directory, and the Makefile present in fs/ + tells kbuild to descend down using the following assignment. + + Example: + #fs/Makefile + obj-$(CONFIG_EXT2_FS) += ext2/ + + If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular) + the corresponding obj- variable will be set, and kbuild will descend + down in the ext2 directory. + Kbuild only uses this information to decide that it needs to visit + the directory, it is the Makefile in the subdirectory that + specifies what is modular and what is built-in. + + It is good practice to use a CONFIG_ variable when assigning directory + names. This allows kbuild to totally skip the directory if the + corresponding CONFIG_ option is neither 'y' nor 'm'. + +--- 3.7 Compilation flags + + ccflags-y, asflags-y and ldflags-y + These three flags apply only to the kbuild makefile in which they + are assigned. They are used for all the normal cc, as and ld + invocations happening during a recursive build. + Note: Flags with the same behaviour were previously named: + EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS. + They are still supported but their usage is deprecated. + + ccflags-y specifies options for compiling with $(CC). + + Example: + # drivers/acpi/Makefile + ccflags-y := -Os + ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT + + This variable is necessary because the top Makefile owns the + variable $(KBUILD_CFLAGS) and uses it for compilation flags for the + entire tree. + + asflags-y specifies options for assembling with $(AS). + + Example: + #arch/sparc/kernel/Makefile + asflags-y := -ansi + + ldflags-y specifies options for linking with $(LD). + + Example: + #arch/cris/boot/compressed/Makefile + ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds + + subdir-ccflags-y, subdir-asflags-y + The two flags listed above are similar to ccflags-y and asflags-y. + The difference is that the subdir- variants have effect for the kbuild + file where they are present and all subdirectories. + Options specified using subdir-* are added to the commandline before + the options specified using the non-subdir variants. + + Example: + subdir-ccflags-y := -Werror + + CFLAGS_$@, AFLAGS_$@ + + CFLAGS_$@ and AFLAGS_$@ only apply to commands in current + kbuild makefile. + + $(CFLAGS_$@) specifies per-file options for $(CC). The $@ + part has a literal value which specifies the file that it is for. + + Example: + # drivers/scsi/Makefile + CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF + CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \ + -DGDTH_STATISTICS + + These two lines specify compilation flags for aha152x.o and gdth.o. + + $(AFLAGS_$@) is a similar feature for source files in assembly + languages. + + Example: + # arch/arm/kernel/Makefile + AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET) + AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312 + AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt + + +--- 3.9 Dependency tracking + + Kbuild tracks dependencies on the following: + 1) All prerequisite files (both *.c and *.h) + 2) CONFIG_ options used in all prerequisite files + 3) Command-line used to compile target + + Thus, if you change an option to $(CC) all affected files will + be re-compiled. + +--- 3.10 Special Rules + + Special rules are used when the kbuild infrastructure does + not provide the required support. A typical example is + header files generated during the build process. + Another example are the architecture-specific Makefiles which + need special rules to prepare boot images etc. + + Special rules are written as normal Make rules. + Kbuild is not executing in the directory where the Makefile is + located, so all special rules shall provide a relative + path to prerequisite files and target files. + + Two variables are used when defining special rules: + + $(src) + $(src) is a relative path which points to the directory + where the Makefile is located. Always use $(src) when + referring to files located in the src tree. + + $(obj) + $(obj) is a relative path which points to the directory + where the target is saved. Always use $(obj) when + referring to generated files. + + Example: + #drivers/scsi/Makefile + $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl + $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl + + This is a special rule, following the normal syntax + required by make. + The target file depends on two prerequisite files. References + to the target file are prefixed with $(obj), references + to prerequisites are referenced with $(src) (because they are not + generated files). + + $(kecho) + echoing information to user in a rule is often a good practice + but when execution "make -s" one does not expect to see any output + except for warnings/errors. + To support this kbuild defines $(kecho) which will echo out the + text following $(kecho) to stdout except if "make -s" is used. + + Example: + #arch/blackfin/boot/Makefile + $(obj)/vmImage: $(obj)/vmlinux.gz + $(call if_changed,uimage) + @$(kecho) 'Kernel: $@ is ready' + + +--- 3.11 $(CC) support functions + + The kernel may be built with several different versions of + $(CC), each supporting a unique set of features and options. + kbuild provides basic support to check for valid options for $(CC). + $(CC) is usually the gcc compiler, but other alternatives are + available. + + as-option + as-option is used to check if $(CC) -- when used to compile + assembler (*.S) files -- supports the given option. An optional + second option may be specified if the first option is not supported. + + Example: + #arch/sh/Makefile + cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),) + + In the above example, cflags-y will be assigned the option + -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC). + The second argument is optional, and if supplied will be used + if first argument is not supported. + + cc-ldoption + cc-ldoption is used to check if $(CC) when used to link object files + supports the given option. An optional second option may be + specified if first option are not supported. + + Example: + #arch/x86/kernel/Makefile + vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) + + In the above example, vsyscall-flags will be assigned the option + -Wl$(comma)--hash-style=sysv if it is supported by $(CC). + The second argument is optional, and if supplied will be used + if first argument is not supported. + + as-instr + as-instr checks if the assembler reports a specific instruction + and then outputs either option1 or option2 + C escapes are supported in the test instruction + Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options + + cc-option + cc-option is used to check if $(CC) supports a given option, and if + not supported to use an optional second option. + + Example: + #arch/x86/Makefile + cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586) + + In the above example, cflags-y will be assigned the option + -march=pentium-mmx if supported by $(CC), otherwise -march=i586. + The second argument to cc-option is optional, and if omitted, + cflags-y will be assigned no value if first option is not supported. + Note: cc-option uses KBUILD_CFLAGS for $(CC) options + + cc-option-yn + cc-option-yn is used to check if gcc supports a given option + and return 'y' if supported, otherwise 'n'. + + Example: + #arch/ppc/Makefile + biarch := $(call cc-option-yn, -m32) + aflags-$(biarch) += -a32 + cflags-$(biarch) += -m32 + + In the above example, $(biarch) is set to y if $(CC) supports the -m32 + option. When $(biarch) equals 'y', the expanded variables $(aflags-y) + and $(cflags-y) will be assigned the values -a32 and -m32, + respectively. + Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options + + cc-option-align + gcc versions >= 3.0 changed the type of options used to specify + alignment of functions, loops etc. $(cc-option-align), when used + as prefix to the align options, will select the right prefix: + gcc < 3.00 + cc-option-align = -malign + gcc >= 3.00 + cc-option-align = -falign + + Example: + KBUILD_CFLAGS += $(cc-option-align)-functions=4 + + In the above example, the option -falign-functions=4 is used for + gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used. + Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options + + cc-disable-warning + cc-disable-warning checks if gcc supports a given warning and returns + the commandline switch to disable it. This special function is needed, + because gcc 4.4 and later accept any unknown -Wno-* option and only + warn about it if there is another warning in the source file. + + Example: + KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable) + + In the above example, -Wno-unused-but-set-variable will be added to + KBUILD_CFLAGS only if gcc really accepts it. + + cc-version + cc-version returns a numerical version of the $(CC) compiler version. + The format is <major><minor> where both are two digits. So for example + gcc 3.41 would return 0341. + cc-version is useful when a specific $(CC) version is faulty in one + area, for example -mregparm=3 was broken in some gcc versions + even though the option was accepted by gcc. + + Example: + #arch/x86/Makefile + cflags-y += $(shell \ + if [ $(cc-version) -ge 0300 ] ; then \ + echo "-mregparm=3"; fi ;) + + In the above example, -mregparm=3 is only used for gcc version greater + than or equal to gcc 3.0. + + cc-ifversion + cc-ifversion tests the version of $(CC) and equals the fourth parameter + if version expression is true, or the fifth (if given) if the version + expression is false. + + Example: + #fs/reiserfs/Makefile + ccflags-y := $(call cc-ifversion, -lt, 0402, -O1) + + In this example, ccflags-y will be assigned the value -O1 if the + $(CC) version is less than 4.2. + cc-ifversion takes all the shell operators: + -eq, -ne, -lt, -le, -gt, and -ge + The third parameter may be a text as in this example, but it may also + be an expanded variable or a macro. + + cc-fullversion + cc-fullversion is useful when the exact version of gcc is needed. + One typical use-case is when a specific GCC version is broken. + cc-fullversion points out a more specific version than cc-version does. + + Example: + #arch/powerpc/Makefile + $(Q)if test "$(cc-fullversion)" = "040200" ; then \ + echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \ + false ; \ + fi + + In this example for a specific GCC version the build will error out + explaining to the user why it stops. + + cc-cross-prefix + cc-cross-prefix is used to check if there exists a $(CC) in path with + one of the listed prefixes. The first prefix where there exist a + prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found + then nothing is returned. + Additional prefixes are separated by a single space in the + call of cc-cross-prefix. + This functionality is useful for architecture Makefiles that try + to set CROSS_COMPILE to well-known values but may have several + values to select between. + It is recommended only to try to set CROSS_COMPILE if it is a cross + build (host arch is different from target arch). And if CROSS_COMPILE + is already set then leave it with the old value. + + Example: + #arch/m68k/Makefile + ifneq ($(SUBARCH),$(ARCH)) + ifeq ($(CROSS_COMPILE),) + CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-) + endif + endif + +--- 3.12 $(LD) support functions + + ld-option + ld-option is used to check if $(LD) supports the supplied option. + ld-option takes two options as arguments. + The second argument is an optional option that can be used if the + first option is not supported by $(LD). + + Example: + #Makefile + LDFLAGS_vmlinux += $(call ld-option, -X) + + +=== 4 Host Program support + +Kbuild supports building executables on the host for use during the +compilation stage. +Two steps are required in order to use a host executable. + +The first step is to tell kbuild that a host program exists. This is +done utilising the variable hostprogs-y. + +The second step is to add an explicit dependency to the executable. +This can be done in two ways. Either add the dependency in a rule, +or utilise the variable $(always). +Both possibilities are described in the following. + +--- 4.1 Simple Host Program + + In some cases there is a need to compile and run a program on the + computer where the build is running. + The following line tells kbuild that the program bin2hex shall be + built on the build host. + + Example: + hostprogs-y := bin2hex + + Kbuild assumes in the above example that bin2hex is made from a single + c-source file named bin2hex.c located in the same directory as + the Makefile. + +--- 4.2 Composite Host Programs + + Host programs can be made up based on composite objects. + The syntax used to define composite objects for host programs is + similar to the syntax used for kernel objects. + $(<executable>-objs) lists all objects used to link the final + executable. + + Example: + #scripts/lxdialog/Makefile + hostprogs-y := lxdialog + lxdialog-objs := checklist.o lxdialog.o + + Objects with extension .o are compiled from the corresponding .c + files. In the above example, checklist.c is compiled to checklist.o + and lxdialog.c is compiled to lxdialog.o. + Finally, the two .o files are linked to the executable, lxdialog. + Note: The syntax <executable>-y is not permitted for host-programs. + +--- 4.3 Using C++ for host programs + + kbuild offers support for host programs written in C++. This was + introduced solely to support kconfig, and is not recommended + for general use. + + Example: + #scripts/kconfig/Makefile + hostprogs-y := qconf + qconf-cxxobjs := qconf.o + + In the example above the executable is composed of the C++ file + qconf.cc - identified by $(qconf-cxxobjs). + + If qconf is composed of a mixture of .c and .cc files, then an + additional line can be used to identify this. + + Example: + #scripts/kconfig/Makefile + hostprogs-y := qconf + qconf-cxxobjs := qconf.o + qconf-objs := check.o + +--- 4.4 Controlling compiler options for host programs + + When compiling host programs, it is possible to set specific flags. + The programs will always be compiled utilising $(HOSTCC) passed + the options specified in $(HOSTCFLAGS). + To set flags that will take effect for all host programs created + in that Makefile, use the variable HOST_EXTRACFLAGS. + + Example: + #scripts/lxdialog/Makefile + HOST_EXTRACFLAGS += -I/usr/include/ncurses + + To set specific flags for a single file the following construction + is used: + + Example: + #arch/ppc64/boot/Makefile + HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE) + + It is also possible to specify additional options to the linker. + + Example: + #scripts/kconfig/Makefile + HOSTLOADLIBES_qconf := -L$(QTDIR)/lib + + When linking qconf, it will be passed the extra option + "-L$(QTDIR)/lib". + +--- 4.5 When host programs are actually built + + Kbuild will only build host-programs when they are referenced + as a prerequisite. + This is possible in two ways: + + (1) List the prerequisite explicitly in a special rule. + + Example: + #drivers/pci/Makefile + hostprogs-y := gen-devlist + $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist + ( cd $(obj); ./gen-devlist ) < $< + + The target $(obj)/devlist.h will not be built before + $(obj)/gen-devlist is updated. Note that references to + the host programs in special rules must be prefixed with $(obj). + + (2) Use $(always) + When there is no suitable special rule, and the host program + shall be built when a makefile is entered, the $(always) + variable shall be used. + + Example: + #scripts/lxdialog/Makefile + hostprogs-y := lxdialog + always := $(hostprogs-y) + + This will tell kbuild to build lxdialog even if not referenced in + any rule. + +--- 4.6 Using hostprogs-$(CONFIG_FOO) + + A typical pattern in a Kbuild file looks like this: + + Example: + #scripts/Makefile + hostprogs-$(CONFIG_KALLSYMS) += kallsyms + + Kbuild knows about both 'y' for built-in and 'm' for module. + So if a config symbol evaluates to 'm', kbuild will still build + the binary. In other words, Kbuild handles hostprogs-m exactly + like hostprogs-y. But only hostprogs-y is recommended to be used + when no CONFIG symbols are involved. + +=== 5 Kbuild clean infrastructure + +"make clean" deletes most generated files in the obj tree where the kernel +is compiled. This includes generated files such as host programs. +Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always), +$(extra-y) and $(targets). They are all deleted during "make clean". +Files matching the patterns "*.[oas]", "*.ko", plus some additional files +generated by kbuild are deleted all over the kernel src tree when +"make clean" is executed. + +Additional files can be specified in kbuild makefiles by use of $(clean-files). + + Example: + #lib/Makefile + clean-files := crc32table.h + +When executing "make clean", the two files "devlist.h classlist.h" will be +deleted. Kbuild will assume files to be in the same relative directory as the +Makefile, except if prefixed with $(objtree). + +To delete a directory hierarchy use: + + Example: + #scripts/package/Makefile + clean-dirs := $(objtree)/debian/ + +This will delete the directory debian in the toplevel directory, including all +subdirectories. + +To exclude certain files from make clean, use the $(no-clean-files) variable. +This is only a special case used in the top level Kbuild file: + + Example: + #Kbuild + no-clean-files := $(bounds-file) $(offsets-file) + +Usually kbuild descends down in subdirectories due to "obj-* := dir/", +but in the architecture makefiles where the kbuild infrastructure +is not sufficient this sometimes needs to be explicit. + + Example: + #arch/x86/boot/Makefile + subdir- := compressed/ + +The above assignment instructs kbuild to descend down in the +directory compressed/ when "make clean" is executed. + +To support the clean infrastructure in the Makefiles that build the +final bootimage there is an optional target named archclean: + + Example: + #arch/x86/Makefile + archclean: + $(Q)$(MAKE) $(clean)=arch/x86/boot + +When "make clean" is executed, make will descend down in arch/x86/boot, +and clean as usual. The Makefile located in arch/x86/boot/ may use +the subdir- trick to descend further down. + +Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is +included in the top level makefile, and the kbuild infrastructure +is not operational at that point. + +Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will +be visited during "make clean". + +=== 6 Architecture Makefiles + +The top level Makefile sets up the environment and does the preparation, +before starting to descend down in the individual directories. +The top level makefile contains the generic part, whereas +arch/$(ARCH)/Makefile contains what is required to set up kbuild +for said architecture. +To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines +a few targets. + +When kbuild executes, the following steps are followed (roughly): +1) Configuration of the kernel => produce .config +2) Store kernel version in include/linux/version.h +3) Updating all other prerequisites to the target prepare: + - Additional prerequisites are specified in arch/$(ARCH)/Makefile +4) Recursively descend down in all directories listed in + init-* core* drivers-* net-* libs-* and build all targets. + - The values of the above variables are expanded in arch/$(ARCH)/Makefile. +5) All object files are then linked and the resulting file vmlinux is + located at the root of the obj tree. + The very first objects linked are listed in head-y, assigned by + arch/$(ARCH)/Makefile. +6) Finally, the architecture-specific part does any required post processing + and builds the final bootimage. + - This includes building boot records + - Preparing initrd images and the like + + +--- 6.1 Set variables to tweak the build to the architecture + + LDFLAGS Generic $(LD) options + + Flags used for all invocations of the linker. + Often specifying the emulation is sufficient. + + Example: + #arch/s390/Makefile + LDFLAGS := -m elf_s390 + Note: ldflags-y can be used to further customise + the flags used. See chapter 3.7. + + LDFLAGS_MODULE Options for $(LD) when linking modules + + LDFLAGS_MODULE is used to set specific flags for $(LD) when + linking the .ko files used for modules. + Default is "-r", for relocatable output. + + LDFLAGS_vmlinux Options for $(LD) when linking vmlinux + + LDFLAGS_vmlinux is used to specify additional flags to pass to + the linker when linking the final vmlinux image. + LDFLAGS_vmlinux uses the LDFLAGS_$@ support. + + Example: + #arch/x86/Makefile + LDFLAGS_vmlinux := -e stext + + OBJCOPYFLAGS objcopy flags + + When $(call if_changed,objcopy) is used to translate a .o file, + the flags specified in OBJCOPYFLAGS will be used. + $(call if_changed,objcopy) is often used to generate raw binaries on + vmlinux. + + Example: + #arch/s390/Makefile + OBJCOPYFLAGS := -O binary + + #arch/s390/boot/Makefile + $(obj)/image: vmlinux FORCE + $(call if_changed,objcopy) + + In this example, the binary $(obj)/image is a binary version of + vmlinux. The usage of $(call if_changed,xxx) will be described later. + + KBUILD_AFLAGS $(AS) assembler flags + + Default value - see top level Makefile + Append or modify as required per architecture. + + Example: + #arch/sparc64/Makefile + KBUILD_AFLAGS += -m64 -mcpu=ultrasparc + + KBUILD_CFLAGS $(CC) compiler flags + + Default value - see top level Makefile + Append or modify as required per architecture. + + Often, the KBUILD_CFLAGS variable depends on the configuration. + + Example: + #arch/x86/boot/compressed/Makefile + cflags-$(CONFIG_X86_32) := -march=i386 + cflags-$(CONFIG_X86_64) := -mcmodel=small + KBUILD_CFLAGS += $(cflags-y) + + Many arch Makefiles dynamically run the target C compiler to + probe supported options: + + #arch/x86/Makefile + + ... + cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ + -march=pentium2,-march=i686) + ... + # Disable unit-at-a-time mode ... + KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time) + ... + + + The first example utilises the trick that a config option expands + to 'y' when selected. + + KBUILD_AFLAGS_KERNEL $(AS) options specific for built-in + + $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile + resident kernel code. + + KBUILD_AFLAGS_MODULE Options for $(AS) when building modules + + $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that + are used for $(AS). + From commandline AFLAGS_MODULE shall be used (see kbuild.txt). + + KBUILD_CFLAGS_KERNEL $(CC) options specific for built-in + + $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile + resident kernel code. + + KBUILD_CFLAGS_MODULE Options for $(CC) when building modules + + $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that + are used for $(CC). + From commandline CFLAGS_MODULE shall be used (see kbuild.txt). + + KBUILD_LDFLAGS_MODULE Options for $(LD) when linking modules + + $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options + used when linking modules. This is often a linker script. + From commandline LDFLAGS_MODULE shall be used (see kbuild.txt). + + KBUILD_ARFLAGS Options for $(AR) when creating archives + + $(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic + mode) if this option is supported by $(AR). + +--- 6.2 Add prerequisites to archheaders: + + The archheaders: rule is used to generate header files that + may be installed into user space by "make header_install" or + "make headers_install_all". In order to support + "make headers_install_all", this target has to be able to run + on an unconfigured tree, or a tree configured for another + architecture. + + It is run before "make archprepare" when run on the + architecture itself. + + +--- 6.3 Add prerequisites to archprepare: + + The archprepare: rule is used to list prerequisites that need to be + built before starting to descend down in the subdirectories. + This is usually used for header files containing assembler constants. + + Example: + #arch/arm/Makefile + archprepare: maketools + + In this example, the file target maketools will be processed + before descending down in the subdirectories. + See also chapter XXX-TODO that describe how kbuild supports + generating offset header files. + + +--- 6.4 List directories to visit when descending + + An arch Makefile cooperates with the top Makefile to define variables + which specify how to build the vmlinux file. Note that there is no + corresponding arch-specific section for modules; the module-building + machinery is all architecture-independent. + + + head-y, init-y, core-y, libs-y, drivers-y, net-y + + $(head-y) lists objects to be linked first in vmlinux. + $(libs-y) lists directories where a lib.a archive can be located. + The rest list directories where a built-in.o object file can be + located. + + $(init-y) objects will be located after $(head-y). + Then the rest follows in this order: + $(core-y), $(libs-y), $(drivers-y) and $(net-y). + + The top level Makefile defines values for all generic directories, + and arch/$(ARCH)/Makefile only adds architecture-specific directories. + + Example: + #arch/sparc64/Makefile + core-y += arch/sparc64/kernel/ + libs-y += arch/sparc64/prom/ arch/sparc64/lib/ + drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ + + +--- 6.5 Architecture-specific boot images + + An arch Makefile specifies goals that take the vmlinux file, compress + it, wrap it in bootstrapping code, and copy the resulting files + somewhere. This includes various kinds of installation commands. + The actual goals are not standardized across architectures. + + It is common to locate any additional processing in a boot/ + directory below arch/$(ARCH)/. + + Kbuild does not provide any smart way to support building a + target specified in boot/. Therefore arch/$(ARCH)/Makefile shall + call make manually to build a target in boot/. + + The recommended approach is to include shortcuts in + arch/$(ARCH)/Makefile, and use the full path when calling down + into the arch/$(ARCH)/boot/Makefile. + + Example: + #arch/x86/Makefile + boot := arch/x86/boot + bzImage: vmlinux + $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@ + + "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke + make in a subdirectory. + + There are no rules for naming architecture-specific targets, + but executing "make help" will list all relevant targets. + To support this, $(archhelp) must be defined. + + Example: + #arch/x86/Makefile + define archhelp + echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' + endif + + When make is executed without arguments, the first goal encountered + will be built. In the top level Makefile the first goal present + is all:. + An architecture shall always, per default, build a bootable image. + In "make help", the default goal is highlighted with a '*'. + Add a new prerequisite to all: to select a default goal different + from vmlinux. + + Example: + #arch/x86/Makefile + all: bzImage + + When "make" is executed without arguments, bzImage will be built. + +--- 6.6 Building non-kbuild targets + + extra-y + + extra-y specifies additional targets created in the current + directory, in addition to any targets specified by obj-*. + + Listing all targets in extra-y is required for two purposes: + 1) Enable kbuild to check changes in command lines + - When $(call if_changed,xxx) is used + 2) kbuild knows what files to delete during "make clean" + + Example: + #arch/x86/kernel/Makefile + extra-y := head.o init_task.o + + In this example, extra-y is used to list object files that + shall be built, but shall not be linked as part of built-in.o. + + +--- 6.7 Commands useful for building a boot image + + Kbuild provides a few macros that are useful when building a + boot image. + + if_changed + + if_changed is the infrastructure used for the following commands. + + Usage: + target: source(s) FORCE + $(call if_changed,ld/objcopy/gzip/...) + + When the rule is evaluated, it is checked to see if any files + need an update, or the command line has changed since the last + invocation. The latter will force a rebuild if any options + to the executable have changed. + Any target that utilises if_changed must be listed in $(targets), + otherwise the command line check will fail, and the target will + always be built. + Assignments to $(targets) are without $(obj)/ prefix. + if_changed may be used in conjunction with custom commands as + defined in 6.8 "Custom kbuild commands". + + Note: It is a typical mistake to forget the FORCE prerequisite. + Another common pitfall is that whitespace is sometimes + significant; for instance, the below will fail (note the extra space + after the comma): + target: source(s) FORCE + #WRONG!# $(call if_changed, ld/objcopy/gzip/...) + + ld + Link target. Often, LDFLAGS_$@ is used to set specific options to ld. + + objcopy + Copy binary. Uses OBJCOPYFLAGS usually specified in + arch/$(ARCH)/Makefile. + OBJCOPYFLAGS_$@ may be used to set additional options. + + gzip + Compress target. Use maximum compression to compress target. + + Example: + #arch/x86/boot/Makefile + LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary + LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext + + targets += setup setup.o bootsect bootsect.o + $(obj)/setup $(obj)/bootsect: %: %.o FORCE + $(call if_changed,ld) + + In this example, there are two possible targets, requiring different + options to the linker. The linker options are specified using the + LDFLAGS_$@ syntax - one for each potential target. + $(targets) are assigned all potential targets, by which kbuild knows + the targets and will: + 1) check for commandline changes + 2) delete target during make clean + + The ": %: %.o" part of the prerequisite is a shorthand that + frees us from listing the setup.o and bootsect.o files. + Note: It is a common mistake to forget the "targets :=" assignment, + resulting in the target file being recompiled for no + obvious reason. + + dtc + Create flattened device tree blob object suitable for linking + into vmlinux. Device tree blobs linked into vmlinux are placed + in an init section in the image. Platform code *must* copy the + blob to non-init memory prior to calling unflatten_device_tree(). + + To use this command, simply add *.dtb into obj-y or targets, or make + some other target depend on %.dtb + + A central rule exists to create $(obj)/%.dtb from $(src)/%.dts; + architecture Makefiles do no need to explicitly write out that rule. + + Example: + targets += $(dtb-y) + clean-files += *.dtb + DTC_FLAGS ?= -p 1024 + +--- 6.8 Custom kbuild commands + + When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand + of a command is normally displayed. + To enable this behaviour for custom commands kbuild requires + two variables to be set: + quiet_cmd_<command> - what shall be echoed + cmd_<command> - the command to execute + + Example: + # + quiet_cmd_image = BUILD $@ + cmd_image = $(obj)/tools/build $(BUILDFLAGS) \ + $(obj)/vmlinux.bin > $@ + + targets += bzImage + $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE + $(call if_changed,image) + @echo 'Kernel: $@ is ready' + + When updating the $(obj)/bzImage target, the line + + BUILD arch/x86/boot/bzImage + + will be displayed with "make KBUILD_VERBOSE=0". + + +--- 6.9 Preprocessing linker scripts + + When the vmlinux image is built, the linker script + arch/$(ARCH)/kernel/vmlinux.lds is used. + The script is a preprocessed variant of the file vmlinux.lds.S + located in the same directory. + kbuild knows .lds files and includes a rule *lds.S -> *lds. + + Example: + #arch/x86/kernel/Makefile + always := vmlinux.lds + + #Makefile + export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) + + The assignment to $(always) is used to tell kbuild to build the + target vmlinux.lds. + The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the + specified options when building the target vmlinux.lds. + + When building the *.lds target, kbuild uses the variables: + KBUILD_CPPFLAGS : Set in top-level Makefile + cppflags-y : May be set in the kbuild makefile + CPPFLAGS_$(@F) : Target-specific flags. + Note that the full filename is used in this + assignment. + + The kbuild infrastructure for *lds files is used in several + architecture-specific files. + +--- 6.10 Generic header files + + The directory include/asm-generic contains the header files + that may be shared between individual architectures. + The recommended approach how to use a generic header file is + to list the file in the Kbuild file. + See "7.4 generic-y" for further info on syntax etc. + +=== 7 Kbuild syntax for exported headers + +The kernel includes a set of headers that is exported to userspace. +Many headers can be exported as-is but other headers require a +minimal pre-processing before they are ready for user-space. +The pre-processing does: +- drop kernel-specific annotations +- drop include of compiler.h +- drop all sections that are kernel internal (guarded by ifdef __KERNEL__) + +Each relevant directory contains a file name "Kbuild" which specifies the +headers to be exported. +See subsequent chapter for the syntax of the Kbuild file. + + --- 7.1 header-y + + header-y specifies header files to be exported. + + Example: + #include/linux/Kbuild + header-y += usb/ + header-y += aio_abi.h + + The convention is to list one file per line and + preferably in alphabetic order. + + header-y also specifies which subdirectories to visit. + A subdirectory is identified by a trailing '/' which + can be seen in the example above for the usb subdirectory. + + Subdirectories are visited before their parent directories. + + --- 7.2 genhdr-y + + genhdr-y specifies generated files to be exported. + Generated files are special as they need to be looked + up in another directory when doing 'make O=...' builds. + + Example: + #include/linux/Kbuild + genhdr-y += version.h + + --- 7.3 destination-y + + When an architecture has a set of exported headers that needs to be + exported to a different directory destination-y is used. + destination-y specifies the destination directory for all exported + headers in the file where it is present. + + Example: + #arch/xtensa/platforms/s6105/include/platform/Kbuild + destination-y := include/linux + + In the example above all exported headers in the Kbuild file + will be located in the directory "include/linux" when exported. + + --- 7.4 generic-y + + If an architecture uses a verbatim copy of a header from + include/asm-generic then this is listed in the file + arch/$(ARCH)/include/asm/Kbuild like this: + + Example: + #arch/x86/include/asm/Kbuild + generic-y += termios.h + generic-y += rtc.h + + During the prepare phase of the build a wrapper include + file is generated in the directory: + + arch/$(ARCH)/include/generated/asm + + When a header is exported where the architecture uses + the generic header a similar wrapper is generated as part + of the set of exported headers in the directory: + + usr/include/asm + + The generated wrapper will in both cases look like the following: + + Example: termios.h + #include <asm-generic/termios.h> + +=== 8 Kbuild Variables + +The top Makefile exports the following variables: + + VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION + + These variables define the current kernel version. A few arch + Makefiles actually use these values directly; they should use + $(KERNELRELEASE) instead. + + $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic + three-part version number, such as "2", "4", and "0". These three + values are always numeric. + + $(EXTRAVERSION) defines an even tinier sublevel for pre-patches + or additional patches. It is usually some non-numeric string + such as "-pre4", and is often blank. + + KERNELRELEASE + + $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable + for constructing installation directory names or showing in + version strings. Some arch Makefiles use it for this purpose. + + ARCH + + This variable defines the target architecture, such as "i386", + "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to + determine which files to compile. + + By default, the top Makefile sets $(ARCH) to be the same as the + host system architecture. For a cross build, a user may + override the value of $(ARCH) on the command line: + + make ARCH=m68k ... + + + INSTALL_PATH + + This variable defines a place for the arch Makefiles to install + the resident kernel image and System.map file. + Use this for architecture-specific install targets. + + INSTALL_MOD_PATH, MODLIB + + $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module + installation. This variable is not defined in the Makefile but + may be passed in by the user if desired. + + $(MODLIB) specifies the directory for module installation. + The top Makefile defines $(MODLIB) to + $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may + override this value on the command line if desired. + + INSTALL_MOD_STRIP + + If this variable is specified, it will cause modules to be stripped + after they are installed. If INSTALL_MOD_STRIP is '1', then the + default option --strip-debug will be used. Otherwise, the + INSTALL_MOD_STRIP value will be used as the option(s) to the strip + command. + + +=== 9 Makefile language + +The kernel Makefiles are designed to be run with GNU Make. The Makefiles +use only the documented features of GNU Make, but they do use many +GNU extensions. + +GNU Make supports elementary list-processing functions. The kernel +Makefiles use a novel style of list building and manipulation with few +"if" statements. + +GNU Make has two assignment operators, ":=" and "=". ":=" performs +immediate evaluation of the right-hand side and stores an actual string +into the left-hand side. "=" is like a formula definition; it stores the +right-hand side in an unevaluated form and then evaluates this form each +time the left-hand side is used. + +There are some cases where "=" is appropriate. Usually, though, ":=" +is the right choice. + +=== 10 Credits + +Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net> +Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de> +Updates by Sam Ravnborg <sam@ravnborg.org> +Language QA by Jan Engelhardt <jengelh@gmx.de> + +=== 11 TODO + +- Describe how kbuild supports shipped files with _shipped. +- Generating offset header files. +- Add more variables to section 7? + + + diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt new file mode 100644 index 000000000..3fb39e011 --- /dev/null +++ b/Documentation/kbuild/modules.txt @@ -0,0 +1,541 @@ +Building External Modules + +This document describes how to build an out-of-tree kernel module. + +=== Table of Contents + + === 1 Introduction + === 2 How to Build External Modules + --- 2.1 Command Syntax + --- 2.2 Options + --- 2.3 Targets + --- 2.4 Building Separate Files + === 3. Creating a Kbuild File for an External Module + --- 3.1 Shared Makefile + --- 3.2 Separate Kbuild file and Makefile + --- 3.3 Binary Blobs + --- 3.4 Building Multiple Modules + === 4. Include Files + --- 4.1 Kernel Includes + --- 4.2 Single Subdirectory + --- 4.3 Several Subdirectories + === 5. Module Installation + --- 5.1 INSTALL_MOD_PATH + --- 5.2 INSTALL_MOD_DIR + === 6. Module Versioning + --- 6.1 Symbols From the Kernel (vmlinux + modules) + --- 6.2 Symbols and External Modules + --- 6.3 Symbols From Another External Module + === 7. Tips & Tricks + --- 7.1 Testing for CONFIG_FOO_BAR + + + +=== 1. Introduction + +"kbuild" is the build system used by the Linux kernel. Modules must use +kbuild to stay compatible with changes in the build infrastructure and +to pick up the right flags to "gcc." Functionality for building modules +both in-tree and out-of-tree is provided. The method for building +either is similar, and all modules are initially developed and built +out-of-tree. + +Covered in this document is information aimed at developers interested +in building out-of-tree (or "external") modules. The author of an +external module should supply a makefile that hides most of the +complexity, so one only has to type "make" to build the module. This is +easily accomplished, and a complete example will be presented in +section 3. + + +=== 2. How to Build External Modules + +To build external modules, you must have a prebuilt kernel available +that contains the configuration and header files used in the build. +Also, the kernel must have been built with modules enabled. If you are +using a distribution kernel, there will be a package for the kernel you +are running provided by your distribution. + +An alternative is to use the "make" target "modules_prepare." This will +make sure the kernel contains the information required. The target +exists solely as a simple way to prepare a kernel source tree for +building external modules. + +NOTE: "modules_prepare" will not build Module.symvers even if +CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be +executed to make module versioning work. + +--- 2.1 Command Syntax + + The command to build an external module is: + + $ make -C <path_to_kernel_src> M=$PWD + + The kbuild system knows that an external module is being built + due to the "M=<dir>" option given in the command. + + To build against the running kernel use: + + $ make -C /lib/modules/`uname -r`/build M=$PWD + + Then to install the module(s) just built, add the target + "modules_install" to the command: + + $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install + +--- 2.2 Options + + ($KDIR refers to the path of the kernel source directory.) + + make -C $KDIR M=$PWD + + -C $KDIR + The directory where the kernel source is located. + "make" will actually change to the specified directory + when executing and will change back when finished. + + M=$PWD + Informs kbuild that an external module is being built. + The value given to "M" is the absolute path of the + directory where the external module (kbuild file) is + located. + +--- 2.3 Targets + + When building an external module, only a subset of the "make" + targets are available. + + make -C $KDIR M=$PWD [target] + + The default will build the module(s) located in the current + directory, so a target does not need to be specified. All + output files will also be generated in this directory. No + attempts are made to update the kernel source, and it is a + precondition that a successful "make" has been executed for the + kernel. + + modules + The default target for external modules. It has the + same functionality as if no target was specified. See + description above. + + modules_install + Install the external module(s). The default location is + /lib/modules/<kernel_release>/extra/, but a prefix may + be added with INSTALL_MOD_PATH (discussed in section 5). + + clean + Remove all generated files in the module directory only. + + help + List the available targets for external modules. + +--- 2.4 Building Separate Files + + It is possible to build single files that are part of a module. + This works equally well for the kernel, a module, and even for + external modules. + + Example (The module foo.ko, consist of bar.o and baz.o): + make -C $KDIR M=$PWD bar.lst + make -C $KDIR M=$PWD baz.o + make -C $KDIR M=$PWD foo.ko + make -C $KDIR M=$PWD / + + +=== 3. Creating a Kbuild File for an External Module + +In the last section we saw the command to build a module for the +running kernel. The module is not actually built, however, because a +build file is required. Contained in this file will be the name of +the module(s) being built, along with the list of requisite source +files. The file may be as simple as a single line: + + obj-m := <module_name>.o + +The kbuild system will build <module_name>.o from <module_name>.c, +and, after linking, will result in the kernel module <module_name>.ko. +The above line can be put in either a "Kbuild" file or a "Makefile." +When the module is built from multiple sources, an additional line is +needed listing the files: + + <module_name>-y := <src1>.o <src2>.o ... + +NOTE: Further documentation describing the syntax used by kbuild is +located in Documentation/kbuild/makefiles.txt. + +The examples below demonstrate how to create a build file for the +module 8123.ko, which is built from the following files: + + 8123_if.c + 8123_if.h + 8123_pci.c + 8123_bin.o_shipped <= Binary blob + +--- 3.1 Shared Makefile + + An external module always includes a wrapper makefile that + supports building the module using "make" with no arguments. + This target is not used by kbuild; it is only for convenience. + Additional functionality, such as test targets, can be included + but should be filtered out from kbuild due to possible name + clashes. + + Example 1: + --> filename: Makefile + ifneq ($(KERNELRELEASE),) + # kbuild part of makefile + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + else + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build + + default: + $(MAKE) -C $(KDIR) M=$$PWD + + # Module specific targets + genbin: + echo "X" > 8123_bin.o_shipped + + endif + + The check for KERNELRELEASE is used to separate the two parts + of the makefile. In the example, kbuild will only see the two + assignments, whereas "make" will see everything except these + two assignments. This is due to two passes made on the file: + the first pass is by the "make" instance run on the command + line; the second pass is by the kbuild system, which is + initiated by the parameterized "make" in the default target. + +--- 3.2 Separate Kbuild File and Makefile + + In newer versions of the kernel, kbuild will first look for a + file named "Kbuild," and only if that is not found, will it + then look for a makefile. Utilizing a "Kbuild" file allows us + to split up the makefile from example 1 into two files: + + Example 2: + --> filename: Kbuild + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + --> filename: Makefile + KDIR ?= /lib/modules/`uname -r`/build + + default: + $(MAKE) -C $(KDIR) M=$$PWD + + # Module specific targets + genbin: + echo "X" > 8123_bin.o_shipped + + The split in example 2 is questionable due to the simplicity of + each file; however, some external modules use makefiles + consisting of several hundred lines, and here it really pays + off to separate the kbuild part from the rest. + + The next example shows a backward compatible version. + + Example 3: + --> filename: Kbuild + obj-m := 8123.o + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + --> filename: Makefile + ifneq ($(KERNELRELEASE),) + # kbuild part of makefile + include Kbuild + + else + # normal makefile + KDIR ?= /lib/modules/`uname -r`/build + + default: + $(MAKE) -C $(KDIR) M=$$PWD + + # Module specific targets + genbin: + echo "X" > 8123_bin.o_shipped + + endif + + Here the "Kbuild" file is included from the makefile. This + allows an older version of kbuild, which only knows of + makefiles, to be used when the "make" and kbuild parts are + split into separate files. + +--- 3.3 Binary Blobs + + Some external modules need to include an object file as a blob. + kbuild has support for this, but requires the blob file to be + named <filename>_shipped. When the kbuild rules kick in, a copy + of <filename>_shipped is created with _shipped stripped off, + giving us <filename>. This shortened filename can be used in + the assignment to the module. + + Throughout this section, 8123_bin.o_shipped has been used to + build the kernel module 8123.ko; it has been included as + 8123_bin.o. + + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + Although there is no distinction between the ordinary source + files and the binary file, kbuild will pick up different rules + when creating the object file for the module. + +--- 3.4 Building Multiple Modules + + kbuild supports building multiple modules with a single build + file. For example, if you wanted to build two modules, foo.ko + and bar.ko, the kbuild lines would be: + + obj-m := foo.o bar.o + foo-y := <foo_srcs> + bar-y := <bar_srcs> + + It is that simple! + + +=== 4. Include Files + +Within the kernel, header files are kept in standard locations +according to the following rule: + + * If the header file only describes the internal interface of a + module, then the file is placed in the same directory as the + source files. + * If the header file describes an interface used by other parts + of the kernel that are located in different directories, then + the file is placed in include/linux/. + + NOTE: There are two notable exceptions to this rule: larger + subsystems have their own directory under include/, such as + include/scsi; and architecture specific headers are located + under arch/$(ARCH)/include/. + +--- 4.1 Kernel Includes + + To include a header file located under include/linux/, simply + use: + + #include <linux/module.h> + + kbuild will add options to "gcc" so the relevant directories + are searched. + +--- 4.2 Single Subdirectory + + External modules tend to place header files in a separate + include/ directory where their source is located, although this + is not the usual kernel style. To inform kbuild of the + directory, use either ccflags-y or CFLAGS_<filename>.o. + + Using the example from section 3, if we moved 8123_if.h to a + subdirectory named include, the resulting kbuild file would + look like: + + --> filename: Kbuild + obj-m := 8123.o + + ccflags-y := -Iinclude + 8123-y := 8123_if.o 8123_pci.o 8123_bin.o + + Note that in the assignment there is no space between -I and + the path. This is a limitation of kbuild: there must be no + space present. + +--- 4.3 Several Subdirectories + + kbuild can handle files that are spread over several directories. + Consider the following example: + + . + |__ src + | |__ complex_main.c + | |__ hal + | |__ hardwareif.c + | |__ include + | |__ hardwareif.h + |__ include + |__ complex.h + + To build the module complex.ko, we then need the following + kbuild file: + + --> filename: Kbuild + obj-m := complex.o + complex-y := src/complex_main.o + complex-y += src/hal/hardwareif.o + + ccflags-y := -I$(src)/include + ccflags-y += -I$(src)/src/hal/include + + As you can see, kbuild knows how to handle object files located + in other directories. The trick is to specify the directory + relative to the kbuild file's location. That being said, this + is NOT recommended practice. + + For the header files, kbuild must be explicitly told where to + look. When kbuild executes, the current directory is always the + root of the kernel tree (the argument to "-C") and therefore an + absolute path is needed. $(src) provides the absolute path by + pointing to the directory where the currently executing kbuild + file is located. + + +=== 5. Module Installation + +Modules which are included in the kernel are installed in the +directory: + + /lib/modules/$(KERNELRELEASE)/kernel/ + +And external modules are installed in: + + /lib/modules/$(KERNELRELEASE)/extra/ + +--- 5.1 INSTALL_MOD_PATH + + Above are the default directories but as always some level of + customization is possible. A prefix can be added to the + installation path using the variable INSTALL_MOD_PATH: + + $ make INSTALL_MOD_PATH=/frodo modules_install + => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/ + + INSTALL_MOD_PATH may be set as an ordinary shell variable or, + as shown above, can be specified on the command line when + calling "make." This has effect when installing both in-tree + and out-of-tree modules. + +--- 5.2 INSTALL_MOD_DIR + + External modules are by default installed to a directory under + /lib/modules/$(KERNELRELEASE)/extra/, but you may wish to + locate modules for a specific functionality in a separate + directory. For this purpose, use INSTALL_MOD_DIR to specify an + alternative name to "extra." + + $ make INSTALL_MOD_DIR=gandalf -C $KDIR \ + M=$PWD modules_install + => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/ + + +=== 6. Module Versioning + +Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used +as a simple ABI consistency check. A CRC value of the full prototype +for an exported symbol is created. When a module is loaded/used, the +CRC values contained in the kernel are compared with similar values in +the module; if they are not equal, the kernel refuses to load the +module. + +Module.symvers contains a list of all exported symbols from a kernel +build. + +--- 6.1 Symbols From the Kernel (vmlinux + modules) + + During a kernel build, a file named Module.symvers will be + generated. Module.symvers contains all exported symbols from + the kernel and compiled modules. For each symbol, the + corresponding CRC value is also stored. + + The syntax of the Module.symvers file is: + <CRC> <Symbol> <module> + + 0x2d036834 scsi_remove_host drivers/scsi/scsi_mod + + For a kernel build without CONFIG_MODVERSIONS enabled, the CRC + would read 0x00000000. + + Module.symvers serves two purposes: + 1) It lists all exported symbols from vmlinux and all modules. + 2) It lists the CRC if CONFIG_MODVERSIONS is enabled. + +--- 6.2 Symbols and External Modules + + When building an external module, the build system needs access + to the symbols from the kernel to check if all external symbols + are defined. This is done in the MODPOST step. modpost obtains + the symbols by reading Module.symvers from the kernel source + tree. If a Module.symvers file is present in the directory + where the external module is being built, this file will be + read too. During the MODPOST step, a new Module.symvers file + will be written containing all exported symbols that were not + defined in the kernel. + +--- 6.3 Symbols From Another External Module + + Sometimes, an external module uses exported symbols from + another external module. kbuild needs to have full knowledge of + all symbols to avoid spitting out warnings about undefined + symbols. Three solutions exist for this situation. + + NOTE: The method with a top-level kbuild file is recommended + but may be impractical in certain situations. + + Use a top-level kbuild file + If you have two modules, foo.ko and bar.ko, where + foo.ko needs symbols from bar.ko, you can use a + common top-level kbuild file so both modules are + compiled in the same build. Consider the following + directory layout: + + ./foo/ <= contains foo.ko + ./bar/ <= contains bar.ko + + The top-level kbuild file would then look like: + + #./Kbuild (or ./Makefile): + obj-y := foo/ bar/ + + And executing + + $ make -C $KDIR M=$PWD + + will then do the expected and compile both modules with + full knowledge of symbols from either module. + + Use an extra Module.symvers file + When an external module is built, a Module.symvers file + is generated containing all exported symbols which are + not defined in the kernel. To get access to symbols + from bar.ko, copy the Module.symvers file from the + compilation of bar.ko to the directory where foo.ko is + built. During the module build, kbuild will read the + Module.symvers file in the directory of the external + module, and when the build is finished, a new + Module.symvers file is created containing the sum of + all symbols defined and not part of the kernel. + + Use "make" variable KBUILD_EXTRA_SYMBOLS + If it is impractical to copy Module.symvers from + another module, you can assign a space separated list + of files to KBUILD_EXTRA_SYMBOLS in your build file. + These files will be loaded by modpost during the + initialization of its symbol tables. + + +=== 7. Tips & Tricks + +--- 7.1 Testing for CONFIG_FOO_BAR + + Modules often need to check for certain CONFIG_ options to + decide if a specific feature is included in the module. In + kbuild this is done by referencing the CONFIG_ variable + directly. + + #fs/ext2/Makefile + obj-$(CONFIG_EXT2_FS) += ext2.o + + ext2-y := balloc.o bitmap.o dir.o + ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o + + External modules have traditionally used "grep" to check for + specific CONFIG_ settings directly in .config. This usage is + broken. As introduced before, external modules should use + kbuild for building and can therefore use the same methods as + in-tree modules when testing for CONFIG_ definitions. + |