1410 lines
46 KiB
Text
1410 lines
46 KiB
Text
Linux Kernel Makefiles
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This document describes the Linux kernel Makefiles.
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=== Table of Contents
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=== 1 Overview
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=== 2 Who does what
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=== 3 The kbuild files
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--- 3.1 Goal definitions
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--- 3.2 Built-in object goals - obj-y
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--- 3.3 Loadable module goals - obj-m
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--- 3.4 Objects which export symbols
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--- 3.5 Library file goals - lib-y
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--- 3.6 Descending down in directories
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--- 3.7 Compilation flags
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--- 3.8 Command line dependency
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--- 3.9 Dependency tracking
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--- 3.10 Special Rules
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--- 3.11 $(CC) support functions
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--- 3.12 $(LD) support functions
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=== 4 Host Program support
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--- 4.1 Simple Host Program
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--- 4.2 Composite Host Programs
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--- 4.3 Using C++ for host programs
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--- 4.4 Controlling compiler options for host programs
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--- 4.5 When host programs are actually built
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--- 4.6 Using hostprogs-$(CONFIG_FOO)
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=== 5 Kbuild clean infrastructure
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=== 6 Architecture Makefiles
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--- 6.1 Set variables to tweak the build to the architecture
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--- 6.2 Add prerequisites to archheaders:
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--- 6.3 Add prerequisites to archprepare:
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--- 6.4 List directories to visit when descending
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--- 6.5 Architecture-specific boot images
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--- 6.6 Building non-kbuild targets
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--- 6.7 Commands useful for building a boot image
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--- 6.8 Custom kbuild commands
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--- 6.9 Preprocessing linker scripts
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--- 6.10 Generic header files
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=== 7 Kbuild syntax for exported headers
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--- 7.1 header-y
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--- 7.2 genhdr-y
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--- 7.3 destination-y
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--- 7.4 generic-y
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=== 8 Kbuild Variables
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=== 9 Makefile language
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=== 10 Credits
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=== 11 TODO
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=== 1 Overview
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The Makefiles have five parts:
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Makefile the top Makefile.
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.config the kernel configuration file.
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arch/$(ARCH)/Makefile the arch Makefile.
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scripts/Makefile.* common rules etc. for all kbuild Makefiles.
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kbuild Makefiles there are about 500 of these.
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The top Makefile reads the .config file, which comes from the kernel
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configuration process.
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The top Makefile is responsible for building two major products: vmlinux
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(the resident kernel image) and modules (any module files).
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It builds these goals by recursively descending into the subdirectories of
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the kernel source tree.
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The list of subdirectories which are visited depends upon the kernel
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configuration. The top Makefile textually includes an arch Makefile
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with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
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architecture-specific information to the top Makefile.
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Each subdirectory has a kbuild Makefile which carries out the commands
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passed down from above. The kbuild Makefile uses information from the
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.config file to construct various file lists used by kbuild to build
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any built-in or modular targets.
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scripts/Makefile.* contains all the definitions/rules etc. that
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are used to build the kernel based on the kbuild makefiles.
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=== 2 Who does what
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People have four different relationships with the kernel Makefiles.
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*Users* are people who build kernels. These people type commands such as
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"make menuconfig" or "make". They usually do not read or edit
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any kernel Makefiles (or any other source files).
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*Normal developers* are people who work on features such as device
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drivers, file systems, and network protocols. These people need to
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maintain the kbuild Makefiles for the subsystem they are
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working on. In order to do this effectively, they need some overall
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knowledge about the kernel Makefiles, plus detailed knowledge about the
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public interface for kbuild.
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*Arch developers* are people who work on an entire architecture, such
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as sparc or ia64. Arch developers need to know about the arch Makefile
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as well as kbuild Makefiles.
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*Kbuild developers* are people who work on the kernel build system itself.
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These people need to know about all aspects of the kernel Makefiles.
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This document is aimed towards normal developers and arch developers.
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=== 3 The kbuild files
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Most Makefiles within the kernel are kbuild Makefiles that use the
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kbuild infrastructure. This chapter introduces the syntax used in the
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kbuild makefiles.
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The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
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be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
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file will be used.
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Section 3.1 "Goal definitions" is a quick intro, further chapters provide
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more details, with real examples.
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--- 3.1 Goal definitions
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Goal definitions are the main part (heart) of the kbuild Makefile.
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These lines define the files to be built, any special compilation
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options, and any subdirectories to be entered recursively.
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The most simple kbuild makefile contains one line:
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Example:
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obj-y += foo.o
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This tells kbuild that there is one object in that directory, named
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foo.o. foo.o will be built from foo.c or foo.S.
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If foo.o shall be built as a module, the variable obj-m is used.
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Therefore the following pattern is often used:
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Example:
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obj-$(CONFIG_FOO) += foo.o
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$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
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If CONFIG_FOO is neither y nor m, then the file will not be compiled
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nor linked.
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--- 3.2 Built-in object goals - obj-y
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The kbuild Makefile specifies object files for vmlinux
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in the $(obj-y) lists. These lists depend on the kernel
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configuration.
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Kbuild compiles all the $(obj-y) files. It then calls
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"$(LD) -r" to merge these files into one built-in.o file.
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built-in.o is later linked into vmlinux by the parent Makefile.
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The order of files in $(obj-y) is significant. Duplicates in
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the lists are allowed: the first instance will be linked into
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built-in.o and succeeding instances will be ignored.
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Link order is significant, because certain functions
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(module_init() / __initcall) will be called during boot in the
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order they appear. So keep in mind that changing the link
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order may e.g. change the order in which your SCSI
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controllers are detected, and thus your disks are renumbered.
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Example:
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#drivers/isdn/i4l/Makefile
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# Makefile for the kernel ISDN subsystem and device drivers.
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# Each configuration option enables a list of files.
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obj-$(CONFIG_ISDN_I4L) += isdn.o
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obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
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--- 3.3 Loadable module goals - obj-m
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$(obj-m) specifies object files which are built as loadable
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kernel modules.
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A module may be built from one source file or several source
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files. In the case of one source file, the kbuild makefile
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simply adds the file to $(obj-m).
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Example:
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#drivers/isdn/i4l/Makefile
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obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
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Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
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If a kernel module is built from several source files, you specify
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that you want to build a module in the same way as above; however,
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kbuild needs to know which object files you want to build your
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module from, so you have to tell it by setting a $(<module_name>-y)
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variable.
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Example:
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#drivers/isdn/i4l/Makefile
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obj-$(CONFIG_ISDN_I4L) += isdn.o
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isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
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In this example, the module name will be isdn.o. Kbuild will
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compile the objects listed in $(isdn-y) and then run
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"$(LD) -r" on the list of these files to generate isdn.o.
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Due to kbuild recognizing $(<module_name>-y) for composite objects,
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you can use the value of a CONFIG_ symbol to optionally include an
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object file as part of a composite object.
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Example:
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#fs/ext2/Makefile
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obj-$(CONFIG_EXT2_FS) += ext2.o
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ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
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namei.o super.o symlink.o
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ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
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xattr_trusted.o
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In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
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part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
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evaluates to 'y'.
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Note: Of course, when you are building objects into the kernel,
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the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
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kbuild will build an ext2.o file for you out of the individual
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parts and then link this into built-in.o, as you would expect.
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--- 3.4 Objects which export symbols
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No special notation is required in the makefiles for
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modules exporting symbols.
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--- 3.5 Library file goals - lib-y
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Objects listed with obj-* are used for modules, or
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combined in a built-in.o for that specific directory.
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There is also the possibility to list objects that will
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be included in a library, lib.a.
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All objects listed with lib-y are combined in a single
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library for that directory.
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Objects that are listed in obj-y and additionally listed in
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lib-y will not be included in the library, since they will
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be accessible anyway.
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For consistency, objects listed in lib-m will be included in lib.a.
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Note that the same kbuild makefile may list files to be built-in
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and to be part of a library. Therefore the same directory
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may contain both a built-in.o and a lib.a file.
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Example:
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#arch/x86/lib/Makefile
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lib-y := delay.o
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This will create a library lib.a based on delay.o. For kbuild to
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actually recognize that there is a lib.a being built, the directory
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shall be listed in libs-y.
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See also "6.4 List directories to visit when descending".
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Use of lib-y is normally restricted to lib/ and arch/*/lib.
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--- 3.6 Descending down in directories
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A Makefile is only responsible for building objects in its own
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directory. Files in subdirectories should be taken care of by
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Makefiles in these subdirs. The build system will automatically
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invoke make recursively in subdirectories, provided you let it know of
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them.
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To do so, obj-y and obj-m are used.
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ext2 lives in a separate directory, and the Makefile present in fs/
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tells kbuild to descend down using the following assignment.
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Example:
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#fs/Makefile
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obj-$(CONFIG_EXT2_FS) += ext2/
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If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
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the corresponding obj- variable will be set, and kbuild will descend
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down in the ext2 directory.
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Kbuild only uses this information to decide that it needs to visit
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the directory, it is the Makefile in the subdirectory that
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specifies what is modular and what is built-in.
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It is good practice to use a CONFIG_ variable when assigning directory
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names. This allows kbuild to totally skip the directory if the
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corresponding CONFIG_ option is neither 'y' nor 'm'.
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--- 3.7 Compilation flags
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ccflags-y, asflags-y and ldflags-y
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These three flags apply only to the kbuild makefile in which they
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are assigned. They are used for all the normal cc, as and ld
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invocations happening during a recursive build.
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Note: Flags with the same behaviour were previously named:
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EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
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They are still supported but their usage is deprecated.
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ccflags-y specifies options for compiling with $(CC).
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Example:
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# drivers/acpi/Makefile
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ccflags-y := -Os
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ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
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This variable is necessary because the top Makefile owns the
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variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
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entire tree.
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asflags-y specifies options for assembling with $(AS).
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Example:
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#arch/sparc/kernel/Makefile
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asflags-y := -ansi
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ldflags-y specifies options for linking with $(LD).
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Example:
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#arch/cris/boot/compressed/Makefile
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ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
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subdir-ccflags-y, subdir-asflags-y
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The two flags listed above are similar to ccflags-y and asflags-y.
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The difference is that the subdir- variants have effect for the kbuild
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file where they are present and all subdirectories.
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Options specified using subdir-* are added to the commandline before
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the options specified using the non-subdir variants.
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Example:
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subdir-ccflags-y := -Werror
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CFLAGS_$@, AFLAGS_$@
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CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
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kbuild makefile.
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$(CFLAGS_$@) specifies per-file options for $(CC). The $@
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part has a literal value which specifies the file that it is for.
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Example:
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# drivers/scsi/Makefile
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CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
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CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
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-DGDTH_STATISTICS
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These two lines specify compilation flags for aha152x.o and gdth.o.
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$(AFLAGS_$@) is a similar feature for source files in assembly
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languages.
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Example:
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# arch/arm/kernel/Makefile
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AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
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AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
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AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
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--- 3.9 Dependency tracking
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Kbuild tracks dependencies on the following:
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1) All prerequisite files (both *.c and *.h)
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2) CONFIG_ options used in all prerequisite files
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3) Command-line used to compile target
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Thus, if you change an option to $(CC) all affected files will
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be re-compiled.
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--- 3.10 Special Rules
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Special rules are used when the kbuild infrastructure does
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not provide the required support. A typical example is
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header files generated during the build process.
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Another example are the architecture-specific Makefiles which
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need special rules to prepare boot images etc.
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Special rules are written as normal Make rules.
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Kbuild is not executing in the directory where the Makefile is
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located, so all special rules shall provide a relative
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path to prerequisite files and target files.
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Two variables are used when defining special rules:
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$(src)
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$(src) is a relative path which points to the directory
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where the Makefile is located. Always use $(src) when
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referring to files located in the src tree.
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$(obj)
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$(obj) is a relative path which points to the directory
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where the target is saved. Always use $(obj) when
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referring to generated files.
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Example:
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#drivers/scsi/Makefile
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$(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
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$(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
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This is a special rule, following the normal syntax
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required by make.
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The target file depends on two prerequisite files. References
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to the target file are prefixed with $(obj), references
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to prerequisites are referenced with $(src) (because they are not
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generated files).
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$(kecho)
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echoing information to user in a rule is often a good practice
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but when execution "make -s" one does not expect to see any output
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except for warnings/errors.
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To support this kbuild defines $(kecho) which will echo out the
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text following $(kecho) to stdout except if "make -s" is used.
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Example:
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#arch/blackfin/boot/Makefile
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$(obj)/vmImage: $(obj)/vmlinux.gz
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$(call if_changed,uimage)
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@$(kecho) 'Kernel: $@ is ready'
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--- 3.11 $(CC) support functions
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The kernel may be built with several different versions of
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$(CC), each supporting a unique set of features and options.
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kbuild provides basic support to check for valid options for $(CC).
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$(CC) is usually the gcc compiler, but other alternatives are
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available.
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as-option
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as-option is used to check if $(CC) -- when used to compile
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assembler (*.S) files -- supports the given option. An optional
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second option may be specified if the first option is not supported.
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Example:
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#arch/sh/Makefile
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cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
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In the above example, cflags-y will be assigned the option
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-Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
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The second argument is optional, and if supplied will be used
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if first argument is not supported.
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cc-ldoption
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cc-ldoption is used to check if $(CC) when used to link object files
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supports the given option. An optional second option may be
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specified if first option are not supported.
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Example:
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#arch/x86/kernel/Makefile
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vsyscall-flags += $(call cc-ldoption, -Wl$(comma)--hash-style=sysv)
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In the above example, vsyscall-flags will be assigned the option
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-Wl$(comma)--hash-style=sysv if it is supported by $(CC).
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The second argument is optional, and if supplied will be used
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if first argument is not supported.
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as-instr
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as-instr checks if the assembler reports a specific instruction
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and then outputs either option1 or option2
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C escapes are supported in the test instruction
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Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
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cc-option
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cc-option is used to check if $(CC) supports a given option, and if
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not supported to use an optional second option.
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Example:
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#arch/x86/Makefile
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cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
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In the above example, cflags-y will be assigned the option
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-march=pentium-mmx if supported by $(CC), otherwise -march=i586.
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The second argument to cc-option is optional, and if omitted,
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cflags-y will be assigned no value if first option is not supported.
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Note: cc-option uses KBUILD_CFLAGS for $(CC) options
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cc-option-yn
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cc-option-yn is used to check if gcc supports a given option
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and return 'y' if supported, otherwise 'n'.
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Example:
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#arch/ppc/Makefile
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biarch := $(call cc-option-yn, -m32)
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aflags-$(biarch) += -a32
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cflags-$(biarch) += -m32
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In the above example, $(biarch) is set to y if $(CC) supports the -m32
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option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
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and $(cflags-y) will be assigned the values -a32 and -m32,
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respectively.
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Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
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cc-option-align
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gcc versions >= 3.0 changed the type of options used to specify
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alignment of functions, loops etc. $(cc-option-align), when used
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as prefix to the align options, will select the right prefix:
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gcc < 3.00
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cc-option-align = -malign
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gcc >= 3.00
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cc-option-align = -falign
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Example:
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KBUILD_CFLAGS += $(cc-option-align)-functions=4
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In the above example, the option -falign-functions=4 is used for
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gcc >= 3.00. For gcc < 3.00, -malign-functions=4 is used.
|
|
Note: cc-option-align uses KBUILD_CFLAGS for $(CC) options
|
|
|
|
cc-disable-warning
|
|
cc-disable-warning checks if gcc supports a given warning and returns
|
|
the commandline switch to disable it. This special function is needed,
|
|
because gcc 4.4 and later accept any unknown -Wno-* option and only
|
|
warn about it if there is another warning in the source file.
|
|
|
|
Example:
|
|
KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
|
|
|
|
In the above example, -Wno-unused-but-set-variable will be added to
|
|
KBUILD_CFLAGS only if gcc really accepts it.
|
|
|
|
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/x86/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 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 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 of 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
|
|
|
|
--- 4.4 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".
|
|
|
|
--- 4.5 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 := $(hostprogs-y)
|
|
|
|
This will tell kbuild to build lxdialog even if not referenced in
|
|
any rule.
|
|
|
|
--- 4.6 Using hostprogs-$(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 evaluates 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 the 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/x86/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 build the
|
|
final bootimage there is an optional target named archclean:
|
|
|
|
Example:
|
|
#arch/x86/Makefile
|
|
archclean:
|
|
$(Q)$(MAKE) $(clean)=arch/x86/boot
|
|
|
|
When "make clean" is executed, make will descend down in arch/x86/boot,
|
|
and clean as usual. The Makefile located in arch/x86/boot/ may use
|
|
the subdir- trick to descend further down.
|
|
|
|
Note 1: arch/$(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) Updating all other prerequisites to the target prepare:
|
|
- Additional prerequisites are specified in arch/$(ARCH)/Makefile
|
|
4) Recursively descend down in all directories listed in
|
|
init-* core* drivers-* net-* libs-* and build all targets.
|
|
- The values of the above variables are expanded in arch/$(ARCH)/Makefile.
|
|
5) 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.
|
|
6) 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/x86/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 $(CC) compiler flags
|
|
|
|
Default value - see top level Makefile
|
|
Append or modify as required per architecture.
|
|
|
|
Often, the KBUILD_CFLAGS variable depends on the configuration.
|
|
|
|
Example:
|
|
#arch/x86/boot/compressed/Makefile
|
|
cflags-$(CONFIG_X86_32) := -march=i386
|
|
cflags-$(CONFIG_X86_64) := -mcmodel=small
|
|
KBUILD_CFLAGS += $(cflags-y)
|
|
|
|
Many arch Makefiles dynamically run the target C compiler to
|
|
probe supported options:
|
|
|
|
#arch/x86/Makefile
|
|
|
|
...
|
|
cflags-$(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).
|
|
|
|
KBUILD_ARFLAGS Options for $(AR) when creating archives
|
|
|
|
$(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
|
|
mode) if this option is supported by $(AR).
|
|
|
|
--- 6.2 Add prerequisites to archheaders:
|
|
|
|
The archheaders: rule is used to generate header files that
|
|
may be installed into user space by "make header_install" or
|
|
"make headers_install_all". In order to support
|
|
"make headers_install_all", this target has to be able to run
|
|
on an unconfigured tree, or a tree configured for another
|
|
architecture.
|
|
|
|
It is run before "make archprepare" when run on the
|
|
architecture itself.
|
|
|
|
|
|
--- 6.3 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.
|
|
|
|
|
|
--- 6.4 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-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
|
|
|
|
|
|
--- 6.5 Architecture-specific boot images
|
|
|
|
An arch Makefile specifies goals that take the vmlinux file, compress
|
|
it, wrap it in bootstrapping code, and copy the resulting files
|
|
somewhere. This includes various kinds of installation commands.
|
|
The actual goals are not standardized across architectures.
|
|
|
|
It is common to locate any additional processing in a boot/
|
|
directory below arch/$(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/x86/Makefile
|
|
boot := arch/x86/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, $(archhelp) must be defined.
|
|
|
|
Example:
|
|
#arch/x86/Makefile
|
|
define archhelp
|
|
echo '* bzImage - Image (arch/$(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/x86/Makefile
|
|
all: bzImage
|
|
|
|
When "make" is executed without arguments, bzImage will be built.
|
|
|
|
--- 6.6 Building non-kbuild targets
|
|
|
|
extra-y
|
|
|
|
extra-y specifies 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/x86/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.
|
|
|
|
|
|
--- 6.7 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 6.8 "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/x86/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
|
|
frees us from listing the setup.o and bootsect.o files.
|
|
Note: It is a common mistake to forget the "targets :=" assignment,
|
|
resulting in the target file being recompiled for no
|
|
obvious reason.
|
|
|
|
dtc
|
|
Create flattened device tree blob object suitable for linking
|
|
into vmlinux. Device tree blobs linked into vmlinux are placed
|
|
in an init section in the image. Platform code *must* copy the
|
|
blob to non-init memory prior to calling unflatten_device_tree().
|
|
|
|
To use this command, simply add *.dtb into obj-y or targets, or make
|
|
some other target depend on %.dtb
|
|
|
|
A central rule exists to create $(obj)/%.dtb from $(src)/%.dts;
|
|
architecture Makefiles do no need to explicitly write out that rule.
|
|
|
|
Example:
|
|
targets += $(dtb-y)
|
|
clean-files += *.dtb
|
|
DTC_FLAGS ?= -p 1024
|
|
|
|
--- 6.8 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 $(obj)/bzImage target, the line
|
|
|
|
BUILD arch/x86/boot/bzImage
|
|
|
|
will be displayed with "make KBUILD_VERBOSE=0".
|
|
|
|
|
|
--- 6.9 Preprocessing linker scripts
|
|
|
|
When the vmlinux image is built, the linker script
|
|
arch/$(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/x86/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 files is used in several
|
|
architecture-specific files.
|
|
|
|
--- 6.10 Generic header files
|
|
|
|
The directory include/asm-generic contains the header files
|
|
that may be shared between individual architectures.
|
|
The recommended approach how to use a generic header file is
|
|
to list the file in the Kbuild file.
|
|
See "7.4 generic-y" for further info on syntax etc.
|
|
|
|
=== 7 Kbuild syntax for exported headers
|
|
|
|
The kernel includes a set of headers that is exported to userspace.
|
|
Many headers can be exported as-is but other headers require 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 are kernel internal (guarded by ifdef __KERNEL__)
|
|
|
|
Each relevant directory contains a file name "Kbuild" which specifies the
|
|
headers to be exported.
|
|
See subsequent chapter for the syntax of the Kbuild file.
|
|
|
|
--- 7.1 header-y
|
|
|
|
header-y specifies 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 specifies 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 genhdr-y
|
|
|
|
genhdr-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
|
|
genhdr-y += version.h
|
|
|
|
--- 7.3 destination-y
|
|
|
|
When an architecture has a set of exported headers that needs to be
|
|
exported to a different directory destination-y is used.
|
|
destination-y specifies 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 generic-y
|
|
|
|
If an architecture uses a verbatim copy of a header from
|
|
include/asm-generic then this is listed in the file
|
|
arch/$(ARCH)/include/asm/Kbuild like this:
|
|
|
|
Example:
|
|
#arch/x86/include/asm/Kbuild
|
|
generic-y += termios.h
|
|
generic-y += rtc.h
|
|
|
|
During the prepare phase of the build a wrapper include
|
|
file is generated in the directory:
|
|
|
|
arch/$(ARCH)/include/generated/asm
|
|
|
|
When a header is exported where the architecture uses
|
|
the generic header a similar wrapper is generated as part
|
|
of the set of exported headers in the directory:
|
|
|
|
usr/include/asm
|
|
|
|
The generated wrapper will in both cases look like the following:
|
|
|
|
Example: termios.h
|
|
#include <asm-generic/termios.h>
|
|
|
|
=== 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, it 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, the
|
|
INSTALL_MOD_STRIP value will be 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?
|
|
|
|
|
|
|