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diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt new file mode 100644 index 000000000000..2616a58a5a4b --- /dev/null +++ b/Documentation/kbuild/makefiles.txt @@ -0,0 +1,1122 @@ +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 + + === 4 Host Program support + --- 4.1 Simple Host Program + --- 4.2 Composite Host Programs + --- 4.3 Defining shared libraries + --- 4.4 Using C++ for host programs + --- 4.5 Controlling compiler options for host programs + --- 4.6 When host programs are actually built + --- 4.7 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 prepare: + --- 6.3 List directories to visit when descending + --- 6.4 Architecture specific boot images + --- 6.5 Building non-kbuild targets + --- 6.6 Commands useful for building a boot image + --- 6.7 Custom kbuild commands + --- 6.8 Preprocessing linker scripts + --- 6.9 $(CC) support functions + + === 7 Kbuild Variables + === 8 Makefile language + === 9 Credits + === 10 TODO + +=== 1 Overview + +The Makefiles have five parts: + + Makefile the top Makefile. + .config the kernel configuration file. + arch/$(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 that 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 introduce the syntax used in the +kbuild makefiles. +The preferred name for the kbuild files is 'Kbuild' but 'Makefile' will +continue to be supported. All new developmen is expected to use the +Kbuild filename. + +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 tell 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 lists $(obj-y). 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 $(obj-y) is significant. Duplicates in + the lists are allowed: the first instance will be linked into + built-in.o and succeeding instances will be ignored. + + Link order is significant, because certain functions + (module_init() / __initcall) will be called during boot in the + order they appear. So keep in mind that changing the link + order may e.g. change the order in which your SCSI + controllers are detected, and thus you disks are renumbered. + + Example: + #drivers/isdn/i4l/Makefile + # Makefile for the kernel ISDN subsystem and device drivers. + # Each configuration option enables a list of files. + obj-$(CONFIG_ISDN) += isdn.o + obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o + +--- 3.3 Loadable module goals - obj-m + + $(obj-m) specify 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. + + Kbuild needs to know which the parts that you want to build your + module from, so you have to tell it by setting an + $(<module_name>-objs) variable. + + Example: + #drivers/isdn/i4l/Makefile + obj-$(CONFIG_ISDN) += isdn.o + isdn-objs := 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 $(isdn-objs) and then run + "$(LD) -r" on the list of these files to generate isdn.o. + + Kbuild recognises objects used for composite objects by the suffix + -objs, and the suffix -y. This allows the Makefiles to use + the value of a CONFIG_ symbol to determine if an object is part + of a composite object. + + Example: + #fs/ext2/Makefile + obj-$(CONFIG_EXT2_FS) += ext2.o + ext2-y := balloc.o bitmap.o + ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o + + In this example xattr.o is 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 additional listed in + lib-y will not be included in the library, since they will anyway + be accessible. + 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/i386/lib/Makefile + lib-y := checksum.o delay.o + + This will create a library lib.a based on checksum.o and delay.o. + For kbuild to actually recognize that there is a lib.a being build + the directory shall be listed in libs-y. + See also "6.3 List directories to visit when descending". + + Usage 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 modules 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 + + EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS + + All the EXTRA_ variables apply only to the kbuild makefile + where they are assigned. The EXTRA_ variables apply to all + commands executed in the kbuild makefile. + + $(EXTRA_CFLAGS) specifies options for compiling C files with + $(CC). + + Example: + # drivers/sound/emu10k1/Makefile + EXTRA_CFLAGS += -I$(obj) + ifdef DEBUG + EXTRA_CFLAGS += -DEMU10K1_DEBUG + endif + + + This variable is necessary because the top Makefile owns the + variable $(CFLAGS) and uses it for compilation flags for the + entire tree. + + $(EXTRA_AFLAGS) is a similar string for per-directory options + when compiling assembly language source. + + Example: + #arch/x86_64/kernel/Makefile + EXTRA_AFLAGS := -traditional + + + $(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for + per-directory options to $(LD) and $(AR). + + Example: + #arch/m68k/fpsp040/Makefile + EXTRA_LDFLAGS := -x + + CFLAGS_$@, 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 + CFLAGS_seagate.o = -DARBITRATE -DPARITY -DSEAGATE_USE_ASM + + These three lines specify compilation flags for aha152x.o, + gdth.o, and seagate.o + + $(AFLAGS_$@) is a similar feature for source files in assembly + languages. + + Example: + # arch/arm/kernel/Makefile + AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional + AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional + +--- 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 is the architecture specific Makefiles which + needs 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). + + +=== 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. + $(<executeable>-objs) list 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 Defining shared libraries + + Objects with extension .so are considered shared libraries, and + will be compiled as position independent objects. + Kbuild provides support for shared libraries, but the usage + shall be restricted. + In the following example the libkconfig.so shared library is used + to link the executable conf. + + Example: + #scripts/kconfig/Makefile + hostprogs-y := conf + conf-objs := conf.o libkconfig.so + libkconfig-objs := expr.o type.o + + Shared libraries always require a corresponding -objs line, and + in the example above the shared library libkconfig is composed by + the two objects expr.o and type.o. + expr.o and type.o will be built as position independent code and + linked as a shared library libkconfig.so. C++ is not supported for + shared libraries. + +--- 4.4 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 by 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 + +--- 4.5 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". + +--- 4.6 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 := $(hostprogs-y) + + This will tell kbuild to build lxdialog even if not referenced in + any rule. + +--- 4.7 Using hostprogs-$(CONFIG_FOO) + + A typcal pattern in a Kbuild file lok 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 evaluate to 'm', kbuild will still build + the binary. In other words Kbuild handle hostprogs-m exactly + like hostprogs-y. But only hostprogs-y is recommend used + when no CONFIG symbol are involved. + +=== 5 Kbuild clean infrastructure + +"make clean" deletes most generated files in the src 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: + #drivers/pci/Makefile + clean-files := devlist.h classlist.h + +When executing "make clean", the two files "devlist.h classlist.h" will +be deleted. Kbuild will assume files to be in same relative directory as the +Makefile except if an absolute path is specified (path starting with '/'). + +To delete a directory hirachy use: + Example: + #scripts/package/Makefile + clean-dirs := $(objtree)/debian/ + +This will delete the directory debian, including all subdirectories. +Kbuild will assume the directories to be in the same relative path as the +Makefile if no absolute path is specified (path does not start with '/'). + +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/i386/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 builds the +final bootimage there is an optional target named archclean: + + Example: + #arch/i386/Makefile + archclean: + $(Q)$(MAKE) $(clean)=arch/i386/boot + +When "make clean" is executed, make will descend down in arch/i386/boot, +and clean as usual. The Makefile located in arch/i386/boot/ may use +the subdir- trick to descend further down. + +Note 1: arch/$(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 the +arch/$(ARCH)/Makefile contains what is required to set-up kbuild +to the said architecture. +To do so arch/$(ARCH)/Makefile sets a number of variables, and defines +a few targets. + +When kbuild executes the following steps are followed (roughly): +1) Configuration of the kernel => produced .config +2) Store kernel version in include/linux/version.h +3) Symlink include/asm to include/asm-$(ARCH) +4) Updating all other prerequisites to the target prepare: + - Additional prerequisites are specified in arch/$(ARCH)/Makefile +5) Recursively descend down in all directories listed in + init-* core* drivers-* net-* libs-* and build all targets. + - The value of the above variables are extended in arch/$(ARCH)/Makefile. +6) All object files are then linked and the resulting file vmlinux is + located at the root of the src tree. + The very first objects linked are listed in head-y, assigned by + arch/$(ARCH)/Makefile. +7) 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 $(LD) options + + Flags used for all invocations of the linker. + Often specifying the emulation is sufficient. + + Example: + #arch/s390/Makefile + LDFLAGS := -m elf_s390 + Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise + the flags used. See chapter 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 $(LD) when linking vmlinux + + LDFLAGS_vmlinux is used to specify additional flags to pass to + the linker when linking the final vmlinux. + LDFLAGS_vmlinux uses the LDFLAGS_$@ support. + + Example: + #arch/i386/Makefile + LDFLAGS_vmlinux := -e stext + + OBJCOPYFLAGS objcopy flags + + When $(call if_changed,objcopy) is used to translate a .o file, + then 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. + + AFLAGS $(AS) assembler flags + + Default value - see top level Makefile + Append or modify as required per architecture. + + Example: + #arch/sparc64/Makefile + AFLAGS += -m64 -mcpu=ultrasparc + + CFLAGS $(CC) compiler flags + + Default value - see top level Makefile + Append or modify as required per architecture. + + Often the CFLAGS variable depends on the configuration. + + Example: + #arch/i386/Makefile + cflags-$(CONFIG_M386) += -march=i386 + CFLAGS += $(cflags-y) + + Many arch Makefiles dynamically run the target C compiler to + probe supported options: + + #arch/i386/Makefile + + ... + cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\ + -march=pentium2,-march=i686) + ... + # Disable unit-at-a-time mode ... + CFLAGS += $(call cc-option,-fno-unit-at-a-time) + ... + + + The first examples utilises the trick that a config option expands + to 'y' when selected. + + CFLAGS_KERNEL $(CC) options specific for built-in + + $(CFLAGS_KERNEL) contains extra C compiler flags used to compile + resident kernel code. + + CFLAGS_MODULE $(CC) options specific for modules + + $(CFLAGS_MODULE) contains extra C compiler flags used to compile code + for loadable kernel modules. + + +--- 6.2 Add prerequisites to prepare: + + The prepare: rule is used to list prerequisites that needs to be + built before starting to descend down in the subdirectories. + This is usual header files containing assembler constants. + + Example: + #arch/s390/Makefile + prepare: include/asm-$(ARCH)/offsets.h + + In this example the file include/asm-$(ARCH)/offsets.h will + be built before descending down in the subdirectories. + See also chapter XXX-TODO that describe how kbuild supports + generating offset header files. + + +--- 6.3 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) list objects to be linked first in vmlinux. + $(libs-y) list 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 $(net-y). + + The top level Makefile define values for all generic directories, + and arch/$(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-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/ + + +--- 6.4 Architecture specific boot images + + An arch Makefile specifies goals that take the vmlinux file, compress + it, wrap it in bootstrapping code, and copy the resulting files + somewhere. This includes various kinds of installation commands. + The actual goals are not standardized across architectures. + + It is common to locate any additional processing in a boot/ + directory below arch/$(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/i386/Makefile + boot := arch/i386/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 of the architecture specific targets, + but executing "make help" will list all relevant targets. + To support this $(archhelp) must be defined. + + Example: + #arch/i386/Makefile + define archhelp + echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)' + endef + + 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/i386/Makefile + all: bzImage + + When "make" is executed without arguments, bzImage will be built. + +--- 6.5 Building non-kbuild targets + + extra-y + + extra-y specify 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/i386/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. + + +--- 6.6 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 + needs an update, or the commandline has changed since 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 $(obj)/ prefix. + if_changed may be used in conjunction with custom commands as + defined in 6.7 "Custom kbuild commands". + Note: It is a typical mistake to forget the FORCE prerequisite. + + ld + Link target. Often LDFLAGS_$@ 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/i386/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 assinged all potential targets, herby 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 + free us from listing the setup.o and bootsect.o files. + Note: It is a common mistake to forget the "target :=" assignment, + resulting in the target file being recompiled for no + obvious reason. + + +--- 6.7 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 $(obj)/bzImage target the line: + + BUILD arch/i386/boot/bzImage + + will be displayed with "make KBUILD_VERBOSE=0". + + +--- 6.8 Preprocessing linker scripts + + When the vmlinux image is build the linker script: + arch/$(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 file and includes a rule *lds.S -> *lds. + + Example: + #arch/i386/kernel/Makefile + always := vmlinux.lds + + #Makefile + export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) + + The assigment to $(always) is used to tell kbuild to build the + target: vmlinux.lds. + The assignment to $(CPPFLAGS_vmlinux.lds) tell kbuild to use the + specified options when building the target vmlinux.lds. + + When building the *.lds target kbuild used the variakles: + CPPFLAGS : Set in top-level Makefile + EXTRA_CPPFLAGS : May be set in the kbuild makefile + CPPFLAGS_$(@F) : Target specific flags. + Note that the full filename is used in this + assignment. + + The kbuild infrastructure for *lds file are used in several + architecture specific files. + + +--- 6.9 $(CC) support functions + + The kernel may be build with several different versions of + $(CC), each supporting a unique set of features and options. + kbuild provide basic support to check for valid options for $(CC). + $(CC) is useally the gcc compiler, but other alternatives are + available. + + cc-option + cc-option is used to check if $(CC) support a given option, and not + supported to use an optional second option. + + Example: + #arch/i386/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. + + 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 to y the expanded variables $(aflags-y) + and $(cflags-y) will be assigned the values -a32 and -m32. + + cc-option-align + gcc version >= 3.0 shifted type of options used to speify + alignment of functions, loops etc. $(cc-option-align) whrn 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: + 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. + + cc-version + cc-version return 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 the -mregparm=3 were broken in some gcc version + even though the option was accepted by gcc. + + Example: + #arch/i386/Makefile + GCC_VERSION := $(call cc-version) + cflags-y += $(shell \ + if [ $(GCC_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. + + +=== 7 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. + +=== 8 Makefile language + +The kernel Makefiles are designed to 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. + +=== 9 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> + +=== 10 TODO + +- Describe how kbuild support shipped files with _shipped. +- Generating offset header files. +- Add more variables to section 7? + |