Linux Kernel Makefiles
2000-September-14
Michael Elizabeth Chastain, <mec@shout.net>



=== Table of Contents

This document describes the Linux kernel Makefiles.

  1  Overview
  2  Who does what
  3  Makefile language
  4  Variables passed down from the top
  5  The structure of an arch Makefile
     5.1  Architecture-specific variables
     5.2  Vmlinux build variables
     5.3  Post-vmlinux goals
     5.4  Mandatory arch-specific goals
  6  The structure of a subdirectory Makefile
     6.1  Comments
     6.2  Goal definitions
     6.3  Adapter section
     6.4  Rules.make section
     6.5  Special rules
  7  Rules.make variables
     7.1  Subdirectories
     7.2  Object file goals
     7.3  Library file goals
     7.4  Loadable module goals
     7.5  Multi-part modules
     7.6  Compilation flags
     7.7  Miscellaneous variables
  8  New-style variables
     8.1  New variables
     8.2  Converting to old-style
  9  Compatibility with Linux Kernel 2.2
 10  Credits


=== 1 Overview

The Makefiles have five parts:

    Makefile: the top Makefile.
    .config: the kernel configuration file.
    arch/*/Makefile: the arch Makefiles.
    Subdirectory Makefiles: there are about 300 of these.
    Rules.make: the common rules for all subdirectory Makefiles.

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 Makefile which carries out the commands passed
down from above.  The subdirectory Makefile uses information from the
.config file to construct various file lists, and then it textually
includes the common rules in Rules.make.

Rules.make defines rules which are common to all the subdirectory
Makefiles.  It has a public interface in the form of certain variable
lists.  It then declares rules based on those lists.



=== 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 bzImage".  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 subdirectory Makefiles for the subsystem that they are
working on.  In order to do this effectively, they need some overall
knowledge about the kernel Makefiles, plus detailed knowledge about the
public interface for Rules.make.

*Arch developers* are people who work on an entire architecture, such
as sparc or ia64.  Arch developers need to know about the arch Makefiles
as well as subdirectory 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 Makefile language

The kernel Makefiles are designed to run with Gnu Make.  The Makefiles
use only the documented features of Gnu Make, but they do use many
Gnu extensions.

Gnu Make supports elementary list-processing functions.  The kernel
Makefiles use a novel style of list building and manipulation with few
"if" statements.

Gnu Make has two assignment operators, ":=" and "=".  ":=" performs
immediate evaluation of the right-hand side and stores an actual string
into the left-hand side.  "=" is like a formula definition; it stores the
right-hand side in an unevaluated form and then evaluates this form each
time the left-hand side is used.

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

All of the examples in this document were drawn from actual kernel
sources.  The examples have been reformatted (white space changed, lines
split), but are otherwise exactly the same.



=== 4 Variables passed down from the top

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".   Many subdirectory Makefiles test $(ARCH)
	to determine which files to compile.

	By default, the top Makefile sets $(ARCH) to be the same as the
	host system system architecture.  For a cross build, a user may
	override the value of $(ARCH) on the command line:

	    make ARCH=m68k ...

    TOPDIR, HPATH
	
	$(TOPDIR) is the path to the top of the kernel source tree.
	Subdirectory Makefiles need this so that they can include
	$(TOPDIR)/Rules.make.

	$(HPATH) is equal to $(TOPDIR)/include.  A few arch Makefiles
	need to use this to do special things using include files.

    SUBDIRS

	$(SUBDIRS) is a list of directories which the top Makefile
	enters in order to build either vmlinux or modules.  The actual
	directories in $(SUBDIRS) depend on the kernel configuration.
	The top Makefile defines this variable, and the arch Makefile
	extends it.

    HEAD, CORE_FILES, NETWORKS, DRIVERS, LIBS
    LINKFLAGS

	$(HEAD), $(CORE_FILES), $(NETWORKS), $(DRIVERS), and $(LIBS)
	specify lists of object files and libraries to be linked into
	vmlinux.
	
	The files in $(HEAD) are linked first in vmlinux.

	$(LINKFLAGS) specifies the flags to build vmlinux.

	The top Makefile and the arch Makefile jointly define these
	variables.  The top Makefile defines $(CORE_FILES), $(NETWORKS),
	$(DRIVERS), and $(LIBS).  The arch Makefile defines $(HEAD)
	and $(LINKFLAGS), and extends $(CORE_FILES) and $(LIBS).

	Note: there are more names here than necessary.  $(NETWORKS),
	$(DRIVERS), and even $(LIBS) could be subsumed into $(CORE_FILES).

    CPP, CC, AS, LD, AR, NM, STRIP, OBJCOPY, OBJDUMP
    CPPFLAGS, CFLAGS, CFLAGS_KERNEL, MODFLAGS, AFLAGS, LDFLAGS
    PERL
    GENKSYMS

	These variables specify the commands and flags that Rules.make
	uses to build goal files from source files.

	$(CFLAGS_KERNEL) contains extra C compiler flags used to compile
	resident kernel code.

	$(MODFLAGS) contains extra C compiler flags used to compile code
	for loadable kernel modules.  In the future, this flag may be
	renamed to the more regular name $(CFLAGS_MODULE).

	$(AFLAGS) contains assembler flags.

	$(GENKSYMS) contains the command used to generate kernel symbol
	signatures when CONFIG_MODVERSIONS is enabled.	The genksyms
	command comes from the modutils package.

    CROSS_COMPILE

	This variable is a prefix path for other variables such as $(CC),
	$(AS), and $(LD).  The arch Makefiles sometimes use and set this
	variable explicitly.  Subdirectory Makefiles don't need to worry
	about it.

	The user may override $(CROSS_COMPILE) on the command line if
	desired.

    HOSTCC, HOSTCFLAGS

	These variables define the C compiler and C compiler flags to
	be used for compiling host side programs.  These are separate
	variables because the target architecture can be different from
	the host architecture.

	If your Makefile compiles and runs a program that is executed
	during the course of building the kernel, then it should use
	$(HOSTCC) and $(HOSTCFLAGS).

	For example, the subdirectory drivers/pci has a helper program
	named gen-devlist.c.  This program reads a list of PCI ID's and
	generates C code in the output files classlist.h and devlist.h.

	Suppose that a user has an i386 computer and wants to build a
	kernel for an ia64 machine.  Then the user would use an ia64
	cross-compiler for most of the compilation, but would use a
	native i386 host compiler to compile drivers/pci/gen-devlist.c.

	For another example, kbuild helper programs such as
	scripts/mkdep.c and scripts/lxdialog/*.c are compiled with
	$(HOSTCC) rather than $(CC).

    ROOT_DEV, SVGA_MODE, RAMDISK

	End users edit these variables to specify certain information
	about the configuration of their kernel.  These variables
	are ancient!  They are also specific to the i386 architecture.
	They really should be replaced with CONFIG_* options.

    MAKEBOOT

	This variable is defined and used only inside the main arch
	Makefiles.  The top Makefile should not export it.

    INSTALL_PATH

	This variable defines a place for the arch Makefiles to install
	the resident kernel image and System.map file.

    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.

    CONFIG_SHELL

	This variable is private between Makefile and Rules.make.
	Arch makefiles and subdirectory Makefiles should never use this.

    MODVERFILE

	An internal variable.  This doesn't need to be exported, as it
	is never used outside of the top Makefile.

    MAKE, MAKEFILES

	Some variables internal to Gnu Make.

	$(MAKEFILES) in particular is used to force the arch Makefiles
	and subdirectory Makefiles to read $(TOPDIR)/.config without
	including it explicitly.  (This was an implementation hack and
	could be fixed).



=== 5 The structure of an arch Makefile



--- 5.1 Architecture-specific variables

The top Makefile includes one arch Makefile file, arch/$(ARCH)/Makefile.
This section describes the functions of the arch Makefile.

An arch Makefile extends some of the top Makefile's variables with
architecture-specific values.

    SUBDIRS

	The top Makefile defines $(SUBDIRS).  The arch Makefile extends
	$(SUBDIRS) with a list of architecture-specific directories.

	Example:

		# arch/alpha/Makefile

		SUBDIRS := $(SUBDIRS) arch/alpha/kernel arch/alpha/mm \
		           arch/alpha/lib arch/alpha/math-emu

	This list may depend on the configuration:

		# arch/arm/Makefile

		ifeq ($(CONFIG_ARCH_ACORN),y)
		SUBDIRS         += drivers/acorn
		...
		endif

    CPP, CC, AS, LD, AR, NM, STRIP, OBJCOPY, OBJDUMP
    CPPFLAGS, CFLAGS, CFLAGS_KERNEL, MODFLAGS, AFLAGS, LDFLAGS

	The top Makefile defines these variables, and the arch Makefile
	extends them.

	Many arch Makefiles dynamically run the target C compiler to
	probe what options it supports:

		# arch/i386/Makefile

		# prevent gcc from keeping the stack 16 byte aligned
		CFLAGS += $(shell if $(CC) -mpreferred-stack-boundary=2 \
			  -S -o /dev/null -xc /dev/null >/dev/null 2>&1; \
			  then echo "-mpreferred-stack-boundary=2"; fi)

	And, of course, $(CFLAGS) can depend on the configuration:

		# arch/i386/Makefile

		ifdef CONFIG_M386
		CFLAGS += -march=i386
		endif

		ifdef CONFIG_M486
		CFLAGS += -march=i486
		endif

		ifdef CONFIG_M586
		CFLAGS += -march=i586
		endif

	Some arch Makefiles redefine the compilation commands in order
	to add architecture-specific flags:

		# arch/s390/Makefile

		LD=$(CROSS_COMPILE)ld -m elf_s390
		OBJCOPY=$(CROSS_COMPILE)objcopy -O binary -R .note -R .comment -S



--- 5.2 Vmlinux build variables

An arch Makefile co-operates 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, CORE_FILES, LIBS
    LINKFLAGS

	The top Makefile defines the architecture-independent core of
	thse variables, and the arch Makefile extends them.  Note that the
	arch Makefile defines (not just extends) $(HEAD) and $(LINKFLAGS).

	Example:

		# arch/m68k/Makefile

		ifndef CONFIG_SUN3
		LINKFLAGS = -T $(TOPDIR)/arch/m68k/vmlinux.lds
		else
		LINKFLAGS = -T $(TOPDIR)/arch/m68k/vmlinux-sun3.lds -N
		endif

		...

		ifndef CONFIG_SUN3
		HEAD := arch/m68k/kernel/head.o
		else
		HEAD := arch/m68k/kernel/sun3-head.o
		endif

		SUBDIRS += arch/m68k/kernel arch/m68k/mm arch/m68k/lib
		CORE_FILES := arch/m68k/kernel/kernel.o arch/m68k/mm/mm.o $(CORE_FILES)
		LIBS += arch/m68k/lib/lib.a



--- 5.3 Post-vmlinux goals

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.

These post-vmlinux goals are not standardized across different
architectures.  Here is a list of these goals and the architectures
that support each of them (as of kernel version 2.4.0-test6-pre5):

    balo		mips
    bootimage		alpha
    bootpfile		alpha, ia64
    bzImage		i386, m68k
    bzdisk		i386
    bzlilo		i386
    compressed		i386, m68k, mips, mips64, sh
    dasdfmt		s390
    Image		arm
    image		s390
    install		arm, i386
    lilo		m68k
    msb			alpha, ia64
    my-special-boot	alpha, ia64
    orionboot		mips
    rawboot		alpha
    silo		s390
    srmboot		alpha
    tftpboot.img	sparc, sparc64
    vmlinux.64		mips64
    vmlinux.aout	sparc64
    zImage		arm, i386, m68k, mips, mips64, ppc, sh
    zImage.initrd	ppc
    zdisk		i386, mips, mips64, sh
    zinstall		arm
    zlilo		i386
    znetboot.initrd	ppc



--- 5.4 Mandatory arch-specific goals

An arch Makefile must define the following arch-specific goals.
These goals provide arch-specific actions for the corresponding goals
in the top Makefile:

    archclean		clean
    archdep		dep
    archmrproper	mrproper



=== 6 The structure of a subdirectory Makefile

A subdirectory Makefile has five sections.



--- 6.1 Comments

The first section is a comment header.  Just write what you would
write if you were editing a C source file, but use "# ..." instead of
"/* ... */".  Historically, many anonymous people have edited kernel
Makefiles without leaving any change histories in the header; comments
from them would have been valuable.



--- 6.2 Goal definitions

The second section is a bunch of definitions that are the heart of the
subdirectory Makefile.  These lines define the files to be built, any
special compilation options, and any subdirectories to be recursively
entered.  The declarations in these lines depend heavily on the kernel
configuration variables (CONFIG_* symbols).

In some Makefiles ("old-style Makefiles"), the second section looks
like this:

	# drivers/parport/Makefile
	ifeq ($(CONFIG_PARPORT_PC),y)
	  LX_OBJS += parport_pc.o
	else
	  ifeq ($(CONFIG_PARPORT_PC),m)
	    MX_OBJS += parport_pc.o
	  endif
	endif

In most Makefiles ("new-style Makefiles"), the second section looks
like this:

	# drivers/block/Makefile
	obj-$(CONFIG_MAC_FLOPPY)	+= swim3.o
	obj-$(CONFIG_BLK_DEV_FD)	+= floppy.o
	obj-$(CONFIG_AMIGA_FLOPPY)	+= amiflop.o
	obj-$(CONFIG_ATARI_FLOPPY)	+= ataflop.o

The new-style Makefiles are more compact and easier to get correct
for certain features (such as CONFIG_* options that enable more than
one file).  If you have a choice, please write a new-style Makefile.



--- 6.3 Adapter section

The third section is an adapter section.  In old-style Makefiles, this
third section is not present.  In new-style Makefiles, the third section
contains boilerplate code which converts from new-style variables to
old-style variables.  This is because Rules.make processes only the
old-style variables.

See section 8.2 ("Converting to old-style") for examples.



--- 6.4 Rules.make section

The fourth section is the single line:

	include $(TOPDIR)/Rules.make



--- 6.5 Special rules

The fifth section contains any special Makefile rules needed that are
not available through the common rules in Rules.make.



=== 7 Rules.make variables

The public interface of Rules.make consists of the following variables:



--- 7.1 Subdirectories

    ALL_SUB_DIRS, SUB_DIRS, MOD_IN_SUB_DIRS, MOD_SUB_DIRS

	$(ALL_SUB_DIRS) is an unconditional list of *all* the
	subdirectories in a given directory.  This list should not depend
	on the kernel configuration.

	$(SUB_DIRS) is a list of subdirectories which may contribute code
	to vmlinux.  This list may depend on the kernel configuration.

	$(MOD_SUB_DIRS) and $(MOD_IN_SUB_DIRS) are lists of subdirectories
	which may build kernel modules.  Both names have exactly the
	same meaning.  (In version 2.2 and earlier kernels, these
	variables had different meanings -- hence the different names).

	For new code, $(MOD_SUB_DIRS) is recommended and $(MOD_IN_SUB_DIRS)
	is deprecated.

	Example:

		# fs/Makefile
		ALL_SUB_DIRS = coda minix ext2 fat msdos vfat proc isofs nfs \
			       umsdos ntfs hpfs sysv smbfs ncpfs ufs efs affs \
			       romfs autofs hfs lockd nfsd nls devpts devfs \
			       adfs partitions qnx4 udf bfs cramfs openpromfs \
			       autofs4 ramfs jffs
		SUB_DIRS :=

		...

		ifeq ($(CONFIG_EXT2_FS),y)
		SUB_DIRS += ext2
		else
		  ifeq ($(CONFIG_EXT2_FS),m)
		  MOD_SUB_DIRS += ext2
		  endif
		endif

		ifeq ($(CONFIG_CRAMFS),y)
		SUB_DIRS += cramfs
		else
		  ifeq ($(CONFIG_CRAMFS),m)
		  MOD_SUB_DIRS += cramfs
		  endif
		endif

	Example:

		# drivers/net/Makefile
		SUB_DIRS     := 
		MOD_SUB_DIRS :=
		MOD_IN_SUB_DIRS :=
		ALL_SUB_DIRS := $(SUB_DIRS) fc hamradio irda pcmcia tokenring \
				wan sk98lin arcnet skfp tulip appletalk

		...

		ifeq ($(CONFIG_IRDA),y)
		SUB_DIRS += irda
		MOD_IN_SUB_DIRS += irda
		else
		  ifeq ($(CONFIG_IRDA),m)
		  MOD_IN_SUB_DIRS += irda
		  endif
		endif

		ifeq ($(CONFIG_TR),y)
		SUB_DIRS += tokenring
		MOD_IN_SUB_DIRS += tokenring
		else
		  ifeq ($(CONFIG_TR),m)
		  MOD_IN_SUB_DIRS += tokenring
		  endif
		endif



--- 7.2 Object file goals

    O_TARGET, O_OBJS, OX_OBJS

	The subdirectory Makefile specifies object files for vmlinux in
	the lists $(O_OBJS) and $(OX_OBJS).  These lists depend on the
	kernel configuration.

	The "X" in "OX_OBJS" stands for "eXport".  Files in $(OX_OBJS)
	may use the EXPORT_SYMBOL macro to define public symbols which
	loadable kernel modules can see.  Files in $(O_OBJS) may not use
	EXPORT_SYMBOL (and you will get a funky error message if you try).

	[Yes, it's kludgy to do this by hand.  Yes, you can define all
	your objects as $(OX_OBJS) whether they define symbols or not;
	but then you will notice a lot of extra compiles when you edit
	any source file.  Blame CONFIG_MODVERSIONS for this.]

	Data that is passed to other objects via registration functions
	(e.g. pci_register_driver, pm_register) does not need to be marked
	as EXPORT_SYMBOL.  The objects that pass data via registration
	functions do not need to be marked as OX_OBJS, unless they also have
	exported symbols.

	Rules.make compiles all the $(O_OBJS) and $(OX_OBJS) files.
	It then calls "$(LD) -r" to merge these files into one .o file
	with the name $(O_TARGET).  This $(O_TARGET) name also appears
	in the top Makefile.

	The order of files in $(O_OBJS) and $(OX_OBJS) is significant.
	All $(OX_OBJS) files come first, in the order listed, followed by
	all $(O_OBJS) files, in the order listed.  Duplicates in the lists
	are allowed: the first instance will be linked into $(O_TARGET)
	and succeeding instances will be ignored.  (Note: Rules.make may
	emit warning messages for duplicates, but this is harmless).

	Example:

		# arch/alpha/kernel/Makefile
		O_TARGET := kernel.o
		O_OBJS   := entry.o traps.o process.o osf_sys.o irq.o \
			    irq_alpha.o signal.o setup.o ptrace.o time.o \
			    semaphore.o
		OX_OBJS  := alpha_ksyms.o

		ifdef CONFIG_SMP
		O_OBJS   += smp.o irq_smp.o
		endif

		ifdef CONFIG_PCI
		O_OBJS   += pci.o pci_iommu.o
		endif

	Even if a subdirectory Makefile has an $(O_TARGET), the .config
	options still control whether or not its $(O_TARGET) goes into
	vmlinux.  See the $(M_OBJS) example below.

	Sometimes the ordering of all $(OX_OBJS) files before all
	$(O_OBJS) files can be a problem, particularly if both
	$(O_OBJS) files and $(OX_OBJS) files contain __initcall
	declarations where order is important.   To avoid this imposed
	ordering, the use of $(OX_OBJS) can be dropped altogether and
	$(MIX_OBJS) used instead.

	If this approach is used, then:
	 - All objects to be linked into vmlinux should be listed in
	   $(O_OBJS) in the desired order.
	 - All objects to be created as modules should be listed in
	   $(M_OBJS)
	 - All objects that export symbols should also be listed in
	   $(MIX_OBJS).

	This has the same effect as maintaining the
	exported/non-exported split, except that there is more control
	over the ordering of object files in vmlinux.
	


--- 7.3 Library file goals

    L_TARGET, L_OBJS, LX_OBJS

	These names are similar to the O_* names.  Once again, $(L_OBJS)
	and $(LX_OBJS) specify object files for the resident kernel;
	once again, the lists depend on the current configuration; and
	once again, the files that call EXPORT_SYMBOL go on the "X" list.

	The difference is that "L" stands for "Library".  After making
	$(L_OBJS) and $(LX_OBJS), Rules.make uses the "$(AR) rcs" command
	to put these files into an archive file (a library) with the
	name $(L_TARGET).  This name also appears in the top Makefile.

	Example:

		# arch/i386/lib/Makefile
		L_TARGET = lib.a
		L_OBJS  = checksum.o old-checksum.o delay.o \
			usercopy.o getuser.o putuser.o iodebug.o

		ifdef CONFIG_X86_USE_3DNOW
		L_OBJS += mmx.o
		endif

		ifdef CONFIG_HAVE_DEC_LOCK
		L_OBJS += dec_and_lock.o
		endif

	The order of files in $(L_OBJS) and $(LX_OBJS) is not significant.
	Duplicates in the lists are allowed.  (Note: Rules.make may emit
	warning messages for duplicates, but this is harmless).

	A subdirectory Makefile can specify either an $(O_TARGET),
	an $(L_TARGET), or both.  Here is a discussion of the differences.

	All of the files in an $(O_TARGET) are guaranteed to appear in
	the resident vmlinux image.  In an $(L_TARGET), only the files
	that satisfy undefined symbol references from other files will
	appear in vmlinux.

	In a conventional link process, the linker processes some
	object files and creates a list of unresolved external symbols.
	The linker then looks in a set of libraries to resolve these
	symbols.  Indeed, the Linux kernel used to be linked this way,
	with the bulk of the code stored in libraries.

	But vmlinux contains two types of object files that cannot be
	fetched out of libraries this way:

	    (1) object files that are purely EXPORT_SYMBOL definitions
	    (2) object files that use module_init or __initcall initializers
		(instead of an initialization routine called externally)

	These files contain autonomous initializer sections which provide
	code and data without being explicitly called.	If these files
	were stored in $(L_TARGET) libraries, the linker would fail
	to include them in vmlinux.  Thus, most subdirectory Makefiles
	specify an $(O_TARGET) and do not use $(L_TARGET).

	Other considerations: $(O_TARGET) leads to faster re-link times
	during development activity, but $(L_TARGET) gives better error
	messages for unresolved symbols.



--- 7.4 Loadable module goals

    M_OBJS, MX_OBJS

	$(M_OBJS) and $(MX_OBJS) specify object files which are built
	as loadable kernel modules.  As usual, the "X" in $(MX_OBJS)
	stands for "eXport"; source files that use EXPORT_SYMBOL must
	appear on an $(MX_OBJS) list.

	A module may be built from one source file or several source
	files.	In the case of one source file, the subdirectory
	Makefile simply adds the file to either $(M_OBJS) or $(MX_OBJS),
	as appropriate.

	Example:

		# drivers/net/irda/Makefile
		ifeq ($(CONFIG_IRTTY_SIR),y)
		L_OBJS += irtty.o
		else
		  ifeq ($(CONFIG_IRTTY_SIR),m)
		  M_OBJS += irtty.o
		  endif
		endif

		ifeq ($(CONFIG_IRPORT_SIR),y)
		LX_OBJS += irport.o
		else
		  ifeq ($(CONFIG_IRPORT_SIR),m)
		  MX_OBJS += irport.o
		  endif
		endif

	If a kernel module is built from several source files, there
	are two ways to specify the set of source files.  One way is to
	build a single module for the entire subdirectory.  This way is
	popular in the file system and network protocol stacks.

	Example:

		# fs/ext2/Makefile
		O_TARGET := ext2.o
		O_OBJS   := acl.o balloc.o bitmap.o dir.o file.o fsync.o \
			    ialloc.o inode.o ioctl.o namei.o super.o symlink.o \
			    truncate.o
		M_OBJS   := $(O_TARGET)

	In this example, the module name will be ext2.o.  Because this
	file has the same name has $(O_TARGET), Rules.make will use
	the $(O_TARGET) rule to build ext2.o: it will run "$(LD) -r"
	on the list of $(O_OBJS) files.

	Note that this subdirectory Makefile defines both an $(O_TARGET)
	and an $(M_OBJS).  The control code, up in fs/Makefile, will
	select between these two.  If CONFIG_EXT2_FS=y, then fs/Makefile
	will build $(O_TARGET); and if CONFIG_EXT_FS=m, then fs/Makefile
	will build $(M_OBJS) instead.  (Yes, this is a little delicate
	and a little confusing).



--- 7.5 Multi-part modules

    MI_OBJS, MIX_OBJS

	Some kernel modules are composed of several object files
	linked together, but do not include every object file in their
	subdirectory.  $(MI_OBJS) and $(MIX_OBJS) are for this case.

	"M" stands for Module.
	"I" stands for Intermediate.
	"X", as usual, stands for "eXport symbol".

	Example:

		# drivers/sound/Makefile
		gus-objs  := gus_card.o gus_midi.o gus_vol.o gus_wave.o ics2101.o
		pas2-objs := pas2_card.o pas2_midi.o pas2_mixer.o pas2_pcm.o
		sb-objs	  := sb_card.o

		gus.o: $(gus-objs)
			$(LD) -r -o $@ $(gus-objs)

		pas2.o: $(pas2-objs)
			$(LD) -r -o $@ $(pas2-objs)

		sb.o: $(sb-objs)
			$(LD) -r -o $@ $(sb-objs)

	The kernel modules gus.o, pas2.o, and sb.o are the *composite
	files*.  The object files gus_card.o, gus_midi.o, gus_vol.o,
	gus_wave.o, ics2101.o, pas2_card.o, pas2_midi.o, pas2_mixer.o,
	pas2_pcm.o, and sb_card.o are *component files*.  The component
	files are also called *intermediate files*.

	In another part of drivers/sound/Makefile (not shown), all of
	the component files are split out.  For the resident drivers:
	the component object files go onto $(O_OBJS) and $(OX_OBJS)
	lists, depending on whether they export symbols or not; and the
	composite files are never built.  For the kernel modules: the
	component object files go onto $(MI_OBJS) and $(MIX_OBJS);
	the composite files go onto $(M_OBJS).

	The subdirectory Makefile must also specify the linking rule
	for a multi-object-file module:

		# drivers/sound/Makefile

		gus.o: $(gus-objs)
			$(LD) -r -o $@ $(gus-objs)

		pas2.o: $(pas2-objs)
			$(LD) -r -o $@ $(pas2-objs)

		sb.o: $(sb-objs)
			$(LD) -r -o $@ $(sb-objs)


	As is mentioned in section 7.2 ("Object file goals"),
	$(MIX_OBJS) can also be used simply to list all objects that
	export any symbols.  If this approach is taken, then
	$(O_OBJS), $(L_OBJS), $(M_OBJS) and $(MI_OBJS) should simply
	lists all of the vmlinux object files, library object files,
	module object files and intermediate module files
	respectively.  Duplication between $(MI_OBJS) and $(MIX_OBJS)
	is not a problem.

--- 7.6 Compilation flags

    EXTRA_CFLAGS, EXTRA_AFLAGS, EXTRA_LDFLAGS, EXTRA_ARFLAGS

	$(EXTRA_CFLAGS) specifies options for compiling C files with
	$(CC).	The options in this variable apply to all $(CC) commands
	for files in the current directory.

	Example:

		# drivers/sound/emu10k1/Makefile
		EXTRA_CFLAGS += -I.
		ifdef DEBUG
		    EXTRA_CFLAGS += -DEMU10K1_DEBUG
		endif

	$(EXTRA_CFLAGS) does not apply to subdirectories of the current
	directory.  Also, it does not apply to files compiled with
	$(HOSTCC).

	This variable is necessary because the top Makefile owns the
	variable $(CFLAGS) and uses it for compilation flags for the
	entire tree.

	$(EXTRA_AFLAGS) is a similar string for per-directory options
	when compiling assembly language source.

	Example: at the time of writing, there were no examples of
	$(EXTRA_AFLAGS) in the kernel corpus.

	$(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
	per-directory options to $(LD) and $(AR).

	Example: at the time of writing, there were no examples of
	$(EXTRA_LDFLAGS) or $(EXTRA_ARFLAGS) in the kernel corpus.

    CFLAGS_$@, AFLAGS_$@

	$(CFLAGS_$@) specifies per-file options for $(CC).  The $@
	part has a literal value which specifies the file that it's 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

	Rules.make has a feature where an object file depends on the
	value of $(CFLAGS_$@) that was used to compile it.  (It also
	depends on the values of $(CFLAGS) and $(EXTRA_CFLAGS)).  Thus,
	if you change the value of $(CFLAGS_$@) for a file, either by
	editing the Makefile or overriding the value some other way,
	Rules.make will do the right thing and re-compile your source
	file with the new options.

	Note: because of a deficiency in Rules.make, assembly language
	files do not have flag dependencies.  If you edit $(AFLAGS_$@)
	for such a file, you will have to remove the object file in order
	to re-build from source.

    LD_RFLAG

	This variable is used, but never defined.  It appears to be a
	vestige of some abandoned experiment.



--- 7.7 Miscellaneous variables

    IGNORE_FLAGS_OBJS

	$(IGNORE_FLAGS_OBJS) is a list of object files which will not have
	their flag dependencies automatically tracked.	This is a hackish
	feature, used to kludge around a problem in the implementation
	of flag dependencies.  (The problem is that flag dependencies
	assume that a %.o file is built from a matching %.S or %.c file.
	This is sometimes not true).

    USE_STANDARD_AS_RULE

	This is a transition variable.	If $(USE_STANDARD_AS_RULE)
	is defined, then Rules.make will provide standard rules for
	assembling %.S files into %.o files or %.s files (%.s files
	are useful only to developers).

	If $(USE_STANDARD_AS_RULE) is not defined, then Rules.make
	will not provide these standard rules.	In this case, the
	subdirectory Makefile must provide its own private rules for
	assembling %.S files.

	In the past, all Makefiles provided private %.S rules.	Newer
	Makefiles should define USE_STANDARD_AS_RULE and use the standard
	Rules.make rules.  As soon as all the Makefiles across all
	architectures have been converted to USE_STANDARD_AS_RULE, then
	Rules.make can drop the conditional test on USE_STANDARD_AS_RULE.
	After that, all the other Makefiles can drop the definition of
	USE_STANDARD_AS_RULE.



=== 8 New-style variables

The "new-style variables" are simpler and more powerful than the
"old-style variables".  As a result, many subdirectory Makefiles shrank
more than 60%.  This author hopes that, in time, all arch Makefiles and
subdirectory Makefiles will convert to the new style.

Rules.make does not understand new-style variables.  Thus, each new-style
Makefile has a section of boilerplate code that converts the new-style
variables into old-style variables.  There is also some mixing, where
people define most variables using "new style" but then fall back to
"old style" for a few lines.

--- 8.1 New variables

    obj-y obj-m obj-n obj-

	These variables replace $(O_OBJS), $(OX_OBJS), $(M_OBJS),
	and $(MX_OBJS).

	Example:

		# drivers/block/Makefile
		obj-$(CONFIG_MAC_FLOPPY)        += swim3.o
		obj-$(CONFIG_BLK_DEV_FD)        += floppy.o
		obj-$(CONFIG_AMIGA_FLOPPY)      += amiflop.o
		obj-$(CONFIG_ATARI_FLOPPY)      += ataflop.o

	Notice the use of $(CONFIG_...) substitutions on the left hand
	side of an assignment operator.  This gives Gnu Make the power
	of associative indexing!  Each of these assignments replaces
	eight lines of code in an old-style Makefile.

	After executing all of the assignments, the subdirectory
	Makefile has built up four lists: $(obj-y), $(obj-m), $(obj-n),
	and $(obj-).

	$(obj-y) is a list of files to include in vmlinux.
	$(obj-m) is a list of files to build as single-file modules.
	$(obj-n) and $(obj-) are ignored.

	Each list may contain duplicates items; duplicates are
	automatically removed later.  Also, if a file appears in both
	$(obj-y) and $(obj-m), it will automatically be removed from
	the $(obj-m) list.

	Example:

		# drivers/net/Makefile

		...
		obj-$(CONFIG_OAKNET) += oaknet.o 8390.o
		...
		obj-$(CONFIG_NE2K_PCI) += ne2k-pci.o 8390.o
		...
		obj-$(CONFIG_STNIC) += stnic.o 8390.o
		...
		obj-$(CONFIG_MAC8390) += daynaport.o 8390.o
		...

	In this example, four different drivers require the code in
	8390.o.  If one or more of these four drivers are built into
	vmlinux, then 8390.o will also be built into vmlinux, and will
	*not* be built as a module -- even if another driver which needs
	8390.o is built as a module.  (The modular driver is able to
	use services of the 8390.o code in the resident vmlinux image).

    export-objs

	$(export-objs) is a list of all the files in the subdirectory
	which potentially export symbols.  The canonical way to construct
	this list is:

	    grep -l EXPORT_SYMBOL *.c

	(but watch out for sneaky files that call EXPORT_SYMBOL from an
	included header file!)

	This is a potential list, independent of the kernel configuration.
	All files that export symbols go into $(export-objs).  The
	boilerplate code then uses the $(export-objs) list to separate
	the real file lists into $(*_OBJS) and $(*X_OBJS).

	Experience has shown that maintaining the proper X's in an
	old-style Makefile is difficult and error-prone.  Maintaining the
	$(export-objs) list in a new-style Makefile is simpler and easier
	to audit.

    list-multi
    $(foo)-objs

	Some kernel modules are composed of multiple object files linked
	together.  $(list-multi) is a list of such kernel modules.
	This is a static list; it does not depend on the configuration.

	For each kernel module in $(list-multi) there is another list
	of all the object files which make up that module.  For a kernel
	module named foo.o, its object file list is foo-objs.

	Example:

		# drivers/scsi/Makefile
		list-multi	:= scsi_mod.o sr_mod.o initio.o a100u2w.o

		...

		scsi_mod-objs	:= hosts.o scsi.o scsi_ioctl.o constants.o \
				   scsicam.o scsi_proc.o scsi_error.o \
				   scsi_obsolete.o scsi_queue.o scsi_lib.o \
				   scsi_merge.o scsi_dma.o scsi_scan.o \
				   scsi_syms.o
		sr_mod-objs	:= sr.o sr_ioctl.o sr_vendor.o
		initio-objs	:= ini9100u.o i91uscsi.o
		a100u2w-objs	:= inia100.o i60uscsi.o

	The subdirectory Makefile puts the modules onto obj-* lists in
	the usual configuration-dependent way:

		obj-$(CONFIG_SCSI)		+= scsi_mod.o
		obj-$(CONFIG_BLK_DEV_SR)	+= sr_mod.o
		obj-$(CONFIG_SCSI_INITIO)	+= initio.o
		obj-$(CONFIG_SCSI_INIA100)	+= a100u2w.o

	Suppose that CONFIG_SCSI=y.  Then vmlinux needs to link in all
	14 components of scsi_mod.o, so these components will go onto
	$(O_OBJS) and $(OX_OBJS).  The composite file scsi_mod.o will
	never be created.  The boilerplate conversion code produces this
	result with a few lines of list processing commands.

	Suppose that CONFIG_BLK_DEV_SR=m.  Then the 3 components
	of sr_mod.o will linked together with "$(LD) -r" to make the
	kernel module sr_mod.o, so these 3 components need to go onto
	the $(MI_OBJS) and $(MIX_OBJS) lists; the composite file sr_mod.o
	goes onto $(M_OBJS).  The boilerplate conversion code takes care
	of this, too.

	And suppose CONFIG_SCSI_INITIO=n.  Then initio.o goes onto the
	$(obj-n) list and that's the end of it.  Its component files
	are not compiled, and the composite file is not created.

	Finally, the subdirectory Makefile needs to define rules to
	build each multi-object kernel module from its component list.
	Example:

		# drivers/scsi/Makefile

		scsi_mod.o: $(scsi_mod-objs)
			$(LD) -r -o $@ $(scsi_mod-objs)

		sr_mod.o: $(sr_mod-objs)
			$(LD) -r -o $@ $(sr_mod-objs)

		initio.o: $(initio-objs)
			$(LD) -r -o $@ $(initio-objs)

		a100u2w.o: $(a100u2w-objs)
			$(LD) -r -o $@ $(a100u2w-objs)

	These rules are very regular; it would be nice for the boilerplate
	code or Rules.make to synthesize these rules automatically.
	But until that happens, the subdirectory Makefile needs to define
	these rules explicitly.

    subdir-y subdir-m subdir-n subdir-

	These variables replace $(ALL_SUB_DIRS), $(SUB_DIRS) and
	$(MOD_SUB_DIRS).

	Example:

		# drivers/Makefile
		subdir-$(CONFIG_PCI)		+= pci
		subdir-$(CONFIG_PCMCIA)		+= pcmcia
		subdir-$(CONFIG_MTD)		+= mtd
		subdir-$(CONFIG_SBUS)		+= sbus

	These variables work similar to obj-*, but are used for
	subdirectories instead of object files.

	After executing all of the assignments, the subdirectory
	Makefile has built up four lists: $(subdir-y), $(subdir-m),
	$(subdir-n), and $(subdir-).

	$(subdir-y) is a list of directories that should be entered
		for making vmlinux.
	$(subdir-m) is a list of directories that should be entered
		for making modules.
	$(subdir-n) and $(subdir-) are only used for collecting a list
		of all subdirectories of this directory.

	Each list besides subdir-y may contain duplicates items; duplicates
	are automatically removed later.

    mod-subdirs

	$(mod-subdirs) is a list of all the subdirectories that should
	be added to $(subdir-m), too if they appear in $(subdir-y)

	Example:

		# fs/Makefile
		mod-subdirs :=	nls

	This means nls should be added to (subdir-y) and $(subdir-m) if
	CONFIG_NFS = y.

--- 8.2 Converting to old-style

	The following example is taken from drivers/usb/Makefile.
	Note that this uses MIX_OBJS to avoid the need for OX_OBJS and
	MX_OBJS and thus to maintain the ordering of objects in $(obj-y)

		# Translate to Rules.make lists.
		multi-used	:= $(filter $(list-multi), $(obj-y) $(obj-m))
		multi-objs	:= $(foreach m, $(multi-used), $($(basename $(m))-objs))
		active-objs	:= $(sort $(multi-objs) $(obj-y) $(obj-m))

		O_OBJS		:= $(obj-y)
		M_OBJS		:= $(obj-m)
		MIX_OBJS	:= $(filter $(export-objs), $(active-objs))

	An example for libraries from drivers/acorn/scsi/Makefile:

		# Translate to Rules.make lists.

		L_OBJS		:= $(filter-out $(export-objs), $(obj-y))
		LX_OBJS		:= $(filter     $(export-objs), $(obj-y))
		M_OBJS		:= $(sort $(filter-out $(export-objs), $(obj-m)))
		MX_OBJS		:= $(sort $(filter     $(export-objs), $(obj-m)))

	As ordering is not so important in libraries, this still uses
	LX_OBJS and MX_OBJS, though (presumably) it could be changed to
	use MIX_OBJS as follows:

		active-objs	:= $(sort $(obj-y) $(obj-m))
		L_OBJS		:= $(obj-y)
		M_OBJS		:= $(obj-m)
		MIX_OBJS	:= $(filter $(export-objs), $(active-objs))
		

	which is clearly shorted and arguably clearer.

=== 9 Compatibility with Linux Kernel 2.2

Most of the information in this document also applies to 2.2, although
there is no indication of which things have changed when.  Here are some
hints for writing subdirectory Makefiles that are compatible with Linux
kernel 2.2.

You can write either an old-style Makefile or a new-style Makefile
with a boilerplate adapter section.  See the 2.2 version of
drivers/sound/Makefile for a copy of the boilerplate code.

In 2.2, Rules.make makes a distinction between $(MOD_SUB_DIRS)
and $(MOD_IN_SUB_DIRS).  If you have a single directory with no
subdirectories, this will not matter to you.  If you have a whole
tree, then you need to know the difference between $(MOD_SUB_DIRS)
and $(MOD_IN_SUB_DIRS).  For example code: $(MOD_SUB_DIRS) is used
extensively in fs/Makefile; $(MOD_IN_SUB_DIRS) is used extensively in
drivers/net/Makefile.

If you are already using MOD_LIST_NAME, go ahead and keep using it.
If you don't already have a MOD_LIST_NAME, go ahead and keep not using
one; your module will be a 'misc' module in 2.2.

Assembly language rules were a mess in 2.2.  If you have assembly language
files, this author recommends that you write your own explicit rules
for each file by name.



=== 10 Credits

Thanks to the members of the linux-kbuild mailing list for reviewing
drafts of this document, with particular thanks to Peter Samuelson
and Thomas Molina.