--- 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 Defining shared libraries
+ --- 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
=== 6 Architecture Makefiles
--- 6.1 Set variables to tweak the build to the architecture
- --- 6.2 Add prerequisites to prepare:
+ --- 6.2 Add prerequisites to archprepare:
--- 6.3 List directories to visit when descending
- --- 6.4 Architecture specific boot images
+ --- 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
+ === 7 Kbuild syntax for exported headers
+ --- 7.1 header-y
+ --- 7.2 objhdr-y
+ --- 7.3 destination-y
+ --- 7.4 unifdef-y (deprecated)
+
+ === 8 Kbuild Variables
+ === 9 Makefile language
+ === 10 Credits
+ === 11 TODO
=== 1 Overview
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
+.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
*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
+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.
=== 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 infrastructure. This chapter introduces 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.
+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.
Example:
obj-y += foo.o
- This tell kbuild that there is one object in that directory named
+ 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.
--- 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
+ in the $(obj-y) lists. These lists depend on the kernel
configuration.
Kbuild compiles all the $(obj-y) files. It then calls
Link order is significant, because certain functions
(module_init() / __initcall) will be called during boot in the
order they appear. So keep in mind that changing the link
- order may e.g. change the order in which your SCSI
- controllers are detected, and thus you disks are renumbered.
+ order may e.g. change the order in which your SCSI
+ controllers are detected, and thus your disks are renumbered.
Example:
#drivers/isdn/i4l/Makefile
Example:
#fs/ext2/Makefile
obj-$(CONFIG_EXT2_FS) += ext2.o
- ext2-y := balloc.o bitmap.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'.
+
+ 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,
--- 3.5 Library file goals - lib-y
- Objects listed with obj-* are used for modules or
+ 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.
+ 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
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
+ For kbuild to actually recognize that there is a lib.a being built,
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.
+
+ Use of lib-y is normally restricted to lib/ and arch/*/lib.
--- 3.6 Descending down in directories
invoke make recursively in subdirectories, provided you let it know of
them.
- To do so obj-y and obj-m are used.
+ 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.
--- 3.7 Compilation flags
- EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS
+ ccflags-y, asflags-y and ldflags-y
+ The three flags listed above applies only to the kbuild makefile
+ where they are assigned. They are used for all the normal
+ cc, as and ld invocation happenign during a recursive build.
+ Note: Flags with the same behaviour were previously named:
+ EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
+ They are yet supported but their use are deprecated.
- 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).
+ ccflags-y specifies options for compiling C files with $(CC).
Example:
# drivers/sound/emu10k1/Makefile
- EXTRA_CFLAGS += -I$(obj)
- ifdef DEBUG
- EXTRA_CFLAGS += -DEMU10K1_DEBUG
- endif
+ ccflags-y += -I$(obj)
+ ccflags-$(DEBUG) += -DEMU10K1_DEBUG
This variable is necessary because the top Makefile owns the
- variable $(CFLAGS) and uses it for compilation flags for the
+ variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
entire tree.
- $(EXTRA_AFLAGS) is a similar string for per-directory options
+ asflags-y is a similar string for per-directory options
when compiling assembly language source.
Example:
#arch/x86_64/kernel/Makefile
- EXTRA_AFLAGS := -traditional
+ asflags-y := -traditional
- $(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
- per-directory options to $(LD) and $(AR).
+ ldflags-y is a string for per-directory options to $(LD).
Example:
#arch/m68k/fpsp040/Makefile
- EXTRA_LDFLAGS := -x
+ ldflags-y := -x
+
+ subdir-ccflags-y, subdir-asflags-y
+ The two flags listed above are similar to ccflags-y and as-falgs-y.
+ The difference is that the subdir- variants has effect for the kbuild
+ file where tey are present and all subdirectories.
+ Options specified using subdir-* are added to the commandline before
+ the options specified using the non-subdir variants.
+
+ Example:
+ subdir-ccflags-y := -Werror
CFLAGS_$@, AFLAGS_$@
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.
+ 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
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 define $(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 provide 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/i386/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 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.
+ 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-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/i386/Makefile
+ cflags-y += $(shell \
+ if [ $(call 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 last argument if
+ version expression is true.
+
+ 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
+ $(Q)if test "$(call 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 really-ld-option, -X)
+
=== 4 Host Program support
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,
+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.
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>-objs) lists all objects used to link the final
executable.
Example:
#scripts/lxdialog/Makefile
- hostprogs-y := lxdialog
+ 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
+ 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.
+ 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
-
+--- 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
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.
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.
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 $(HOSTCFLAGS).
To set flags that will take effect for all host programs created
- in that Makefile use the variable HOST_EXTRACFLAGS.
+ in that Makefile, use the variable HOST_EXTRACFLAGS.
Example:
#scripts/lxdialog/Makefile
HOST_EXTRACFLAGS += -I/usr/include/ncurses
-
+
To set specific flags for a single file the following construction
is used:
Example:
#arch/ppc64/boot/Makefile
HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(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".
-
+ 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
$(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
( cd $(obj); ./gen-devlist ) < $<
- The target $(obj)/devlist.h will not be built before
+ 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 $(obj).
--- 4.7 Using hostprogs-$(CONFIG_FOO)
- A typcal pattern in a Kbuild file lok like this:
+ A typical pattern in a Kbuild file looks like this:
Example:
#scripts/Makefile
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.
+ 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 src tree where the kernel
+"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".
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:
+To delete a directory hierarchy use:
+
Example:
#scripts/package/Makefile
clean-dirs := $(objtree)/debian/
The top level Makefile sets up the environment and does the preparation,
before starting to descend down in the individual directories.
-The top level makefile contains the generic part, whereas the
-arch/$(ARCH)/Makefile contains what is required to set-up kbuild
-to the said architecture.
-To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
+The top level makefile contains the generic part, whereas
+arch/$(ARCH)/Makefile contains what is required to set up kbuild
+for said architecture.
+To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
a few targets.
-When kbuild executes the following steps are followed (roughly):
-1) Configuration of the kernel => produced .config
+When kbuild executes, the following steps are followed (roughly):
+1) Configuration of the kernel => produce .config
2) Store kernel version in include/linux/version.h
3) Symlink include/asm to include/asm-$(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 values of the above variables are expanded in arch/$(ARCH)/Makefile.
+6) All object files are then linked and the resulting file vmlinux is
+ located at the root of the obj tree.
The very first objects linked are listed in head-y, assigned by
arch/$(ARCH)/Makefile.
-7) Finally the architecture specific part does any required post processing
+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
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.
-
+ 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
LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
LDFLAGS_vmlinux is used to specify additional flags to pass to
- the linker when linking the final vmlinux.
+ the linker when linking the final vmlinux image.
LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
Example:
OBJCOPYFLAGS objcopy flags
When $(call if_changed,objcopy) is used to translate a .o file,
- then the flags specified in OBJCOPYFLAGS will be used.
+ the flags specified in OBJCOPYFLAGS will be used.
$(call if_changed,objcopy) is often used to generate raw binaries on
vmlinux.
$(obj)/image: vmlinux FORCE
$(call if_changed,objcopy)
- In this example the binary $(obj)/image is a binary version of
+ 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
+ KBUILD_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
+ KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
- CFLAGS $(CC) compiler flags
+ KBUILD_CFLAGS $(CC) compiler flags
Default value - see top level Makefile
Append or modify as required per architecture.
- Often the CFLAGS variable depends on the configuration.
+ Often, the KBUILD_CFLAGS variable depends on the configuration.
Example:
#arch/i386/Makefile
cflags-$(CONFIG_M386) += -march=i386
- CFLAGS += $(cflags-y)
+ KBUILD_CFLAGS += $(cflags-y)
Many arch Makefiles dynamically run the target C compiler to
probe supported options:
-march=pentium2,-march=i686)
...
# Disable unit-at-a-time mode ...
- CFLAGS += $(call cc-option,-fno-unit-at-a-time)
+ KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
...
- The first examples utilises the trick that a config option expands
+ The first example utilises the trick that a config option expands
to 'y' when selected.
CFLAGS_KERNEL $(CC) options specific for built-in
$(CFLAGS_MODULE) contains extra C compiler flags used to compile code
for loadable kernel modules.
-
---- 6.2 Add prerequisites to prepare:
- The prepare: rule is used to list prerequisites that needs to be
+--- 6.2 Add prerequisites to archprepare:
+
+ The archprepare: rule is used to list prerequisites that need to be
built before starting to descend down in the subdirectories.
- This is usual header files containing assembler constants.
+ This is usually used for header files containing assembler constants.
Example:
- #arch/s390/Makefile
- prepare: include/asm-$(ARCH)/offsets.h
+ #arch/arm/Makefile
+ archprepare: maketools
- In this example the file include/asm-$(ARCH)/offsets.h will
- be built before descending down in the subdirectories.
+ 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.
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.
+ $(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 $(net-y).
- The top level Makefile define values for all generic directories,
- and arch/$(ARCH)/Makefile only adds architecture specific directories.
+ The top level Makefile defines values for all generic directories,
+ and arch/$(ARCH)/Makefile only adds architecture-specific directories.
Example:
#arch/sparc64/Makefile
drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
---- 6.4 Architecture specific boot images
+--- 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
"$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
make in a subdirectory.
- There are no rules for naming of the architecture specific targets,
+ 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.
+ To support this, $(archhelp) must be defined.
Example:
#arch/i386/Makefile
define archhelp
echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
- endef
+ 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 '*'.
+ 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
+ all: bzImage
When "make" is executed without arguments, bzImage will be built.
#arch/i386/kernel/Makefile
extra-y := head.o init_task.o
- In this example extra-y is used to list object files that
+ 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
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
+ 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),
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.
+ 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.
-
+ Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
+
objcopy
Copy binary. Uses OBJCOPYFLAGS usually specified in
arch/$(ARCH)/Makefile.
$(obj)/setup $(obj)/bootsect: %: %.o FORCE
$(call if_changed,ld)
- In this example there are two possible targets, requiring different
- options to the linker. the linker options are specified using the
+ In this example, there are two possible targets, requiring different
+ options to the linker. The linker options are specified using the
LDFLAGS_$@ syntax - one for each potential target.
- $(targets) are assinged all potential targets, herby kbuild knows
+ $(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
--- 6.7 Custom kbuild commands
- When kbuild is executing with KBUILD_VERBOSE=0 then only a shorthand
+ 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:
$(call if_changed,image)
@echo 'Kernel: $@ is ready'
- When updating the $(obj)/bzImage target the line:
+ When updating the $(obj)/bzImage target, the line
BUILD arch/i386/boot/bzImage
will be displayed with "make KBUILD_VERBOSE=0".
-
+
--- 6.8 Preprocessing linker scripts
- When the vmlinux image is build the linker script:
+ When the vmlinux image is built, the linker script
arch/$(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 file and includes a rule *lds.S -> *lds.
-
+ kbuild knows .lds files 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
+
+ The assignment to $(always) is used to tell kbuild to build the
+ target vmlinux.lds.
+ The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
specified options when building the target vmlinux.lds.
-
- When building the *.lds target kbuild used the variakles:
- CPPFLAGS : Set in top-level Makefile
- EXTRA_CPPFLAGS : May be set in the kbuild makefile
+
+ When building the *.lds target, kbuild uses the variables:
+ KBUILD_CPPFLAGS : Set in top-level Makefile
+ cppflags-y : May be set in the kbuild makefile
CPPFLAGS_$(@F) : Target specific flags.
Note that the full filename is used in this
assignment.
The kbuild infrastructure for *lds file are used in several
- architecture specific files.
+ architecture-specific files.
+=== 7 Kbuild syntax for exported headers
---- 6.9 $(CC) support functions
+The kernel include a set of headers that is exported to userspace.
+Many headers can be exported as-is but other headers requires 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 is kernel internat (guarded by ifdef __KERNEL__)
- 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.
+Each relevant directory contain a file name "Kbuild" which specify the
+headers to be exported.
+See subsequent chapter for the syntax of the Kbuild file.
- cc-option
- cc-option is used to check if $(CC) support a given option, and not
- supported to use an optional second option.
+ --- 7.1 header-y
- Example:
- #arch/i386/Makefile
- cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
+ header-y specify header files to be exported.
- 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.
+ Example:
+ #include/linux/Kbuild
+ header-y += usb/
+ header-y += aio_abi.h
- cc-option-yn
- cc-option-yn is used to check if gcc supports a given option
- and return 'y' if supported, otherwise 'n'.
+ The convention is to list one file per line and
+ preferably in alphabetic order.
- 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.
+ header-y also specify which subdirectories to visit.
+ A subdirectory is identified by a trailing '/' which
+ can be seen in the example above for the usb subdirectory.
- 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
+ Subdirectories are visited before their parent directories.
- 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.
+ --- 7.2 objhdr-y
- Example:
- #arch/i386/Makefile
- GCC_VERSION := $(call cc-version)
- cflags-y += $(shell \
- if [ $(GCC_VERSION) -ge 0300 ] ; then echo "-mregparm=3"; fi ;)
+ objhdr-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.
- In the above example -mregparm=3 is only used for gcc version greater
- than or equal to gcc 3.0.
-
+ Example:
+ #include/linux/Kbuild
+ objhdr-y += version.h
+
+ --- 7.3 destination-y
+
+ When an architecture have a set of exported headers that needs to be
+ exported to a different directory destination-y is used.
+ destination-y specify 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 Kbuild Variables
+
+ --- 7.4 unifdef-y (deprecated)
+
+ unifdef-y is deprecated. A direct replacement is header-y.
+
+
+=== 8 Kbuild Variables
The top Makefile exports the following variables:
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.
+ Use this for architecture-specific install targets.
INSTALL_MOD_PATH, MODLIB
$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
override this value on the command line if desired.
-=== 8 Makefile language
+ INSTALL_MOD_STRIP
+
+ If this variable is specified, 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 will used as the option(s) to the strip command.
-The kernel Makefiles are designed to run with GNU Make. The Makefiles
+
+=== 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.
There are some cases where "=" is appropriate. Usually, though, ":="
is the right choice.
-=== 9 Credits
+=== 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>
-=== 10 TODO
+=== 11 TODO
-- Describe how kbuild support shipped files with _shipped.
+- Describe how kbuild supports shipped files with _shipped.
- Generating offset header files.
- Add more variables to section 7?
+
+
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff