Linux Kernel Makefiles

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

=== 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

--- 7.4 unifdef-y (deprecated)

=== 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)             += 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 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/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 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

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.

ccflags-y specifies options for compiling C files with $(CC).

Example:

# drivers/sound/emu10k1/Makefile

ccflags-y += -I$(obj)

ccflags-$(DEBUG) += -DEMU10K1_DEBUG

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 is a similar string for per-directory options

when compiling assembly language source.

Example:

#arch/x86_64/kernel/Makefile

asflags-y := -traditional

ldflags-y is a string for per-directory options to $(LD).

Example:

#arch/m68k/fpsp040/Makefile

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_$@, AFLAGS_$@

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 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.

    ld-option

ld-option 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 += $(call ld-option, -Wl$(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

=== 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 '/').

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.

    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 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/

--- 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 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)'

endif

When make is executed without arguments, the first goal encountered

will be built. In the top level Makefile the first goal present

is all:.

An architecture shall always, per default, build a bootable image.

In "make help", the default goal is highlighted with a '*'.

Add a new prerequisite to all: to select a default goal different

from vmlinux.

Example:

#arch/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

need an update, or the command line has changed since the last

invocation. The latter will force a rebuild if any options

to the executable have changed.

Any target that utilises if_changed must be listed in $(targets),

otherwise the command line check will fail, and the target will

always be built.

Assignments to $(targets) are without $(obj)/ prefix.

if_changed may be used in conjunction with custom commands as

defined in 6.7 "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.

    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 assigned all potential targets, by which kbuild knows

the targets and will:

1) check for commandline changes

2) delete target during make clean

The ": %: %.o" part of the prerequisite is a shorthand that

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 built, the linker script

arch/$(ARCH)/kernel/vmlinux.lds is used.

The script is a preprocessed variant of the file vmlinux.lds.S

located in the same directory.

kbuild knows .lds files and includes a rule *lds.S -> *lds.

Example:

#arch/i386/kernel/Makefile

always := vmlinux.lds

#Makefile

export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)

The assignment to $(always) is used to tell kbuild to build the

target vmlinux.lds.

The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the

specified options when building the target vmlinux.lds.

When building the *.lds target, kbuild uses the variables:

KBUILD_CPPFLAGS : Set in top-level Makefile

cppflags-y : May be set in the kbuild makefile

CPPFLAGS_$(@F)  : Target specific flags.

                 Note that the full filename is used in this

                 assignment.

The kbuild infrastructure for *lds file are used in several

architecture-specific files.

=== 7 Kbuild syntax for exported headers

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__)

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.

--- 7.1 header-y

header-y specify header files to be exported.

Example:

#include/linux/Kbuild

header-y += usb/

header-y += aio_abi.h

The convention is to list one file per line and

preferably in alphabetic order.

header-y also specify which subdirectories to visit.

A subdirectory is identified by a trailing '/' which

can be seen in the example above for the usb subdirectory.

Subdirectories are visited before their parent directories.

--- 7.2 objhdr-y

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.

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.

--- 7.4 unifdef-y (deprecated)

unifdef-y is deprecated. A direct replacement is header-y.

=== 8 Kbuild Variables

The top Makefile exports the following variables:

    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION

These variables define the current kernel version.  A few arch

Makefiles actually use these values directly; they should use

$(KERNELRELEASE) instead.

$(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic

three-part version number, such as "2", "4", and "0".  These three

values are always numeric.

$(EXTRAVERSION) defines an even tinier sublevel for pre-patches

or additional patches. It is usually some non-numeric string

such as "-pre4", and is often blank.

    KERNELRELEASE

$(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable

for constructing installation directory names or showing in

version strings.  Some arch Makefiles use it for this purpose.

    ARCH

This variable defines the target architecture, such as "i386",

"arm", or "sparc". Some kbuild Makefiles test $(ARCH) to

determine which files to compile.

By default, the top Makefile sets $(ARCH) to be the same as the

host system architecture.  For a cross build, a user may

override the value of $(ARCH) on the command line:

   make ARCH=m68k ...

    INSTALL_PATH

This variable defines a place for the arch Makefiles to install

the resident kernel image and System.map file.

Use this for architecture-specific install targets.

    INSTALL_MOD_PATH, MODLIB

$(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module

installation.  This variable is not defined in the Makefile but

may be passed in by the user if desired.

$(MODLIB) specifies the directory for module installation.

The top Makefile defines $(MODLIB) to

$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may

override this value on the command line if desired.

    INSTALL_MOD_STRIP

If this variable is specified, 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.

=== 9 Makefile language

The kernel Makefiles are designed to be run with GNU Make.  The Makefiles

use only the documented features of GNU Make, but they do use many

GNU extensions.

GNU Make supports elementary list-processing functions.  The kernel

Makefiles use a novel style of list building and manipulation with few

"if" statements.

GNU Make has two assignment operators, ":=" and "=".  ":=" performs

immediate evaluation of the right-hand side and stores an actual string

into the left-hand side.  "=" is like a formula definition; it stores the

right-hand side in an unevaluated form and then evaluates this form each

time the left-hand side is used.

There are some cases where "=" is appropriate.  Usually, though, ":="

is the right choice.

=== 10 Credits

Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>

Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>

Updates by Sam Ravnborg <sam@ravnborg.org>

Language QA by Jan Engelhardt <jengelh@gmx.de>

=== 11 TODO

- Describe how kbuild supports shipped files with _shipped.

- Generating offset header files.

- Add more variables to section 7?