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makefiles.txt
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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 $(LD) support functions
=== 4 Host Program support
--- 4.1 Simple Host Program
--- 4.2 Composite Host Programs
--- 4.3 Defining shared libraries
--- 4.4 Using C++ for host programs
--- 4.5 Controlling compiler options for host programs
--- 4.6 When host programs are actually built
--- 4.7 Using hostprogs-$(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 archprepare:
--- 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
=== 7 Kbuild syntax for exported headers
--- 7.1 header-y
--- 7.2 objhdr-y
--- 7.3 destination-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) 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; 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 $(CONFIG_EXT2_FS_XATTR)
evaluates to 'y'.
Note: Of course, when you are building objects into the kernel,
the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
kbuild will build an ext2.o file for you out of the individual
parts and then link this into built-in.o, as you would expect.
--- 3.4 Objects which export symbols
No special notation is required in the makefiles for
modules exporting symbols.
--- 3.5 Library file goals - lib-y
Objects listed with obj-* are used for modules, or
combined in a built-in.o for that specific directory.
There is also the possibility to list objects that will
be included in a library, lib.a.
All objects listed with lib-y are combined in a single
library for that directory.
Objects that are listed in obj-y and additionally listed in
lib-y will not be included in the library, since they will
be accessible anyway.
For consistency, objects listed in lib-m will be included in lib.a.
Note that the same kbuild makefile may list files to be built-in
and to be part of a library. Therefore the same directory
may contain both a built-in.o and a lib.a file.
Example:
#arch/x86/lib/Makefile
lib-y := delay.o
This will create a library lib.a based on delay.o. For kbuild to
actually recognize that there is a lib.a being built, the directory
shall be listed in libs-y.
See also "6.3 List directories to visit when descending".
Use of lib-y is normally restricted to lib/ and arch/*/lib.
--- 3.6 Descending down in directories
A Makefile is only responsible for building objects in its own
directory. Files in subdirectories should be taken care of by
Makefiles in these subdirs. The build system will automatically
invoke make recursively in subdirectories, provided you let it know of
them.
To do so, obj-y and obj-m are used.
ext2 lives in a separate directory, and the Makefile present in fs/
tells kbuild to descend down using the following assignment.
Example:
#fs/Makefile
obj-$(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 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
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 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
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 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 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 evaluate 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:
#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 hierarchy 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 '/').
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/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
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) 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 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
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/i386/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 $(CC) compiler flags
Default value - see top level Makefile
Append or modify as required per architecture.
Often, the KBUILD_CFLAGS variable depends on the configuration.
Example:
#arch/i386/Makefile
cflags-$(CONFIG_M386) += -march=i386
KBUILD_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 ...
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).
--- 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 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.
--- 6.3 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) 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/