[PATCH] doc: Bring in Makefile documentation
Simon Glass
sjg at chromium.org
Wed Jul 21 22:56:58 CEST 2021
U-Boot uses the Linux Kbuild build system. Add the associated
documentation so that people can understand the Makefiles better.
This is taken from Linux v5.12
Signed-off-by: Simon Glass <sjg at chromium.org>
---
doc/develop/index.rst | 1 +
doc/develop/makefiles.rst | 1674 +++++++++++++++++++++++++++++++++++++
2 files changed, 1675 insertions(+)
create mode 100644 doc/develop/makefiles.rst
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 3edffbc6373..c1bbca617c9 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -16,6 +16,7 @@ Implementation
menus
uefi/index
version
+ makefiles
Debugging
---------
diff --git a/doc/develop/makefiles.rst b/doc/develop/makefiles.rst
new file mode 100644
index 00000000000..c3b5b66992b
--- /dev/null
+++ b/doc/develop/makefiles.rst
@@ -0,0 +1,1674 @@
+======================
+Linux Kernel Makefiles
+======================
+
+Note: This document mostly applies to U-Boot so is included here.
+
+This document describes the Linux kernel Makefiles.
+
+.. Table of Contents
+
+ === 1 Overview
+ === 2 Who does what
+ === 3 The kbuild files
+ --- 3.1 Goal definitions
+ --- 3.2 Built-in object goals - obj-y
+ --- 3.3 Loadable module goals - obj-m
+ --- 3.4 <deleted>
+ --- 3.5 Library file goals - lib-y
+ --- 3.6 Descending down in directories
+ --- 3.7 Non-builtin vmlinux targets - extra-y
+ --- 3.8 Always built goals - always-y
+ --- 3.9 Compilation flags
+ --- 3.10 Dependency tracking
+ --- 3.11 Custom Rules
+ --- 3.12 Command change detection
+ --- 3.13 $(CC) support functions
+ --- 3.14 $(LD) support functions
+ --- 3.15 Script Invocation
+
+ === 4 Host Program support
+ --- 4.1 Simple Host Program
+ --- 4.2 Composite Host Programs
+ --- 4.3 Using C++ for host programs
+ --- 4.4 Controlling compiler options for host programs
+ --- 4.5 When host programs are actually built
+
+ === 5 Userspace Program support
+ --- 5.1 Simple Userspace Program
+ --- 5.2 Composite Userspace Programs
+ --- 5.3 Controlling compiler options for userspace programs
+ --- 5.4 When userspace programs are actually built
+
+ === 6 Kbuild clean infrastructure
+
+ === 7 Architecture Makefiles
+ --- 7.1 Set variables to tweak the build to the architecture
+ --- 7.2 Add prerequisites to archheaders
+ --- 7.3 Add prerequisites to archprepare
+ --- 7.4 List directories to visit when descending
+ --- 7.5 Architecture-specific boot images
+ --- 7.6 Building non-kbuild targets
+ --- 7.7 Commands useful for building a boot image
+ --- 7.8 <deleted>
+ --- 7.9 Preprocessing linker scripts
+ --- 7.10 Generic header files
+ --- 7.11 Post-link pass
+
+ === 8 Kbuild syntax for exported headers
+ --- 8.1 no-export-headers
+ --- 8.2 generic-y
+ --- 8.3 generated-y
+ --- 8.4 mandatory-y
+
+ === 9 Kbuild Variables
+ === 10 Makefile language
+ === 11 Credits
+ === 12 TODO
+
+1 Overview
+==========
+
+The Makefiles have five parts::
+
+ Makefile the top Makefile.
+ .config the kernel configuration file.
+ arch/$(SRCARCH)/Makefile the arch Makefile.
+ scripts/Makefile.* common rules etc. for all kbuild Makefiles.
+ kbuild Makefiles exist in every subdirectory
+
+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/$(SRCARCH)/Makefile. The arch Makefile supplies
+architecture-specific information to the top Makefile.
+
+Each subdirectory has a kbuild Makefile which carries out the commands
+passed down from above. The kbuild Makefile uses information from the
+.config file to construct various file lists used by kbuild to build
+any built-in or modular targets.
+
+scripts/Makefile.* contains all the definitions/rules etc. that
+are used to build the kernel based on the kbuild makefiles.
+
+
+2 Who does what
+===============
+
+People have four different relationships with the kernel Makefiles.
+
+*Users* are people who build kernels. These people type commands such as
+"make menuconfig" or "make". They usually do not read or edit
+any kernel Makefiles (or any other source files).
+
+*Normal developers* are people who work on features such as device
+drivers, file systems, and network protocols. These people need to
+maintain the kbuild Makefiles for the subsystem they are
+working on. In order to do this effectively, they need some overall
+knowledge about the kernel Makefiles, plus detailed knowledge about the
+public interface for kbuild.
+
+*Arch developers* are people who work on an entire architecture, such
+as sparc or ia64. Arch developers need to know about the arch Makefile
+as well as kbuild Makefiles.
+
+*Kbuild developers* are people who work on the kernel build system itself.
+These people need to know about all aspects of the kernel Makefiles.
+
+This document is aimed towards normal developers and arch developers.
+
+
+3 The kbuild files
+==================
+
+Most Makefiles within the kernel are kbuild Makefiles that use the
+kbuild infrastructure. This chapter introduces the syntax used in the
+kbuild makefiles.
+The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
+be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
+file will be used.
+
+Section 3.1 "Goal definitions" is a quick intro; further chapters provide
+more details, with real examples.
+
+3.1 Goal definitions
+--------------------
+
+ Goal definitions are the main part (heart) of the kbuild Makefile.
+ These lines define the files to be built, any special compilation
+ options, and any subdirectories to be entered recursively.
+
+ The most simple kbuild makefile contains one line:
+
+ Example::
+
+ obj-y += foo.o
+
+ This tells kbuild that there is one object in that directory, named
+ foo.o. foo.o will be built from foo.c or foo.S.
+
+ If foo.o shall be built as a module, the variable obj-m is used.
+ Therefore the following pattern is often used:
+
+ Example::
+
+ obj-$(CONFIG_FOO) += foo.o
+
+ $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
+ If CONFIG_FOO is neither y nor m, then the file will not be compiled
+ nor linked.
+
+3.2 Built-in object goals - obj-y
+---------------------------------
+
+ The kbuild Makefile specifies object files for vmlinux
+ in the $(obj-y) lists. These lists depend on the kernel
+ configuration.
+
+ Kbuild compiles all the $(obj-y) files. It then calls
+ "$(AR) rcSTP" to merge these files into one built-in.a file.
+ This is a thin archive without a symbol table. It will be later
+ linked into vmlinux by scripts/link-vmlinux.sh
+
+ The order of files in $(obj-y) is significant. Duplicates in
+ the lists are allowed: the first instance will be linked into
+ built-in.a and succeeding instances will be ignored.
+
+ Link order is significant, because certain functions
+ (module_init() / __initcall) will be called during boot in the
+ order they appear. So keep in mind that changing the link
+ order may e.g. change the order in which your SCSI
+ controllers are detected, and thus your disks are renumbered.
+
+ Example::
+
+ #drivers/isdn/i4l/Makefile
+ # Makefile for the kernel ISDN subsystem and device drivers.
+ # Each configuration option enables a list of files.
+ obj-$(CONFIG_ISDN_I4L) += isdn.o
+ obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+3.3 Loadable module goals - obj-m
+---------------------------------
+
+ $(obj-m) specifies object files which are built as loadable
+ kernel modules.
+
+ A module may be built from one source file or several source
+ files. In the case of one source file, the kbuild makefile
+ simply adds the file to $(obj-m).
+
+ Example::
+
+ #drivers/isdn/i4l/Makefile
+ obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
+
+ Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
+
+ If a kernel module is built from several source files, you specify
+ that you want to build a module in the same way as above; however,
+ kbuild needs to know which object files you want to build your
+ module from, so you have to tell it by setting a $(<module_name>-y)
+ variable.
+
+ Example::
+
+ #drivers/isdn/i4l/Makefile
+ obj-$(CONFIG_ISDN_I4L) += isdn.o
+ isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
+
+ In this example, the module name will be isdn.o. Kbuild will
+ compile the objects listed in $(isdn-y) and then run
+ "$(LD) -r" on the list of these files to generate isdn.o.
+
+ Due to kbuild recognizing $(<module_name>-y) for composite objects,
+ you can use the value of a `CONFIG_` symbol to optionally include an
+ object file as part of a composite object.
+
+ Example::
+
+ #fs/ext2/Makefile
+ obj-$(CONFIG_EXT2_FS) += ext2.o
+ ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
+ namei.o super.o symlink.o
+ ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
+ xattr_trusted.o
+
+ In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
+ part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
+ evaluates to 'y'.
+
+ Note: Of course, when you are building objects into the kernel,
+ the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
+ kbuild will build an ext2.o file for you out of the individual
+ parts and then link this into built-in.a, as you would expect.
+
+3.5 Library file goals - lib-y
+------------------------------
+
+ Objects listed with obj-* are used for modules, or
+ combined in a built-in.a for that specific directory.
+ There is also the possibility to list objects that will
+ be included in a library, lib.a.
+ All objects listed with lib-y are combined in a single
+ library for that directory.
+ Objects that are listed in obj-y and additionally listed in
+ lib-y will not be included in the library, since they will
+ be accessible anyway.
+ For consistency, objects listed in lib-m will be included in lib.a.
+
+ Note that the same kbuild makefile may list files to be built-in
+ and to be part of a library. Therefore the same directory
+ may contain both a built-in.a and a lib.a file.
+
+ Example::
+
+ #arch/x86/lib/Makefile
+ lib-y := delay.o
+
+ This will create a library lib.a based on delay.o. For kbuild to
+ actually recognize that there is a lib.a being built, the directory
+ shall be listed in libs-y.
+
+ See also "7.4 List directories to visit when descending".
+
+ Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
+
+3.6 Descending down in directories
+----------------------------------
+
+ A Makefile is only responsible for building objects in its own
+ directory. Files in subdirectories should be taken care of by
+ Makefiles in these subdirs. The build system will automatically
+ invoke make recursively in subdirectories, provided you let it know of
+ them.
+
+ To do so, obj-y and obj-m are used.
+ ext2 lives in a separate directory, and the Makefile present in fs/
+ tells kbuild to descend down using the following assignment.
+
+ Example::
+
+ #fs/Makefile
+ obj-$(CONFIG_EXT2_FS) += ext2/
+
+ If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
+ the corresponding obj- variable will be set, and kbuild will descend
+ down in the ext2 directory.
+
+ Kbuild uses this information not only to decide that it needs to visit
+ the directory, but also to decide whether or not to link objects from
+ the directory into vmlinux.
+
+ When Kbuild descends into the directory with 'y', all built-in objects
+ from that directory are combined into the built-in.a, which will be
+ eventually linked into vmlinux.
+
+ When Kbuild descends into the directory with 'm', in contrast, nothing
+ from that directory will be linked into vmlinux. If the Makefile in
+ that directory specifies obj-y, those objects will be left orphan.
+ It is very likely a bug of the Makefile or of dependencies in Kconfig.
+
+ Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
+ descending into subdirectories. It is a good fit when you know they
+ do not contain kernel-space objects at all. A typical usage is to let
+ Kbuild descend into subdirectories to build tools.
+
+ Examples::
+
+ # scripts/Makefile
+ subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
+ subdir-$(CONFIG_MODVERSIONS) += genksyms
+ subdir-$(CONFIG_SECURITY_SELINUX) += selinux
+
+ Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
+ syntax is always used for directories.
+
+ It is good practice to use a `CONFIG_` variable when assigning directory
+ names. This allows kbuild to totally skip the directory if the
+ corresponding `CONFIG_` option is neither 'y' nor 'm'.
+
+3.7 Non-builtin vmlinux targets - extra-y
+-----------------------------------------
+
+ extra-y specifies targets which are needed for building vmlinux,
+ but not combined into built-in.a.
+
+ Examples are:
+
+ 1) head objects
+
+ Some objects must be placed at the head of vmlinux. They are
+ directly linked to vmlinux without going through built-in.a
+ A typical use-case is an object that contains the entry point.
+
+ arch/$(SRCARCH)/Makefile should specify such objects as head-y.
+
+ Discussion:
+ Given that we can control the section order in the linker script,
+ why do we need head-y?
+
+ 2) vmlinux linker script
+
+ The linker script for vmlinux is located at
+ arch/$(SRCARCH)/kernel/vmlinux.lds
+
+ Example::
+
+ # arch/x86/kernel/Makefile
+ extra-y := head_$(BITS).o
+ extra-y += head$(BITS).o
+ extra-y += ebda.o
+ extra-y += platform-quirks.o
+ extra-y += vmlinux.lds
+
+ $(extra-y) should only contain targets needed for vmlinux.
+
+ Kbuild skips extra-y when vmlinux is apparently not a final goal.
+ (e.g. 'make modules', or building external modules)
+
+ If you intend to build targets unconditionally, always-y (explained
+ in the next section) is the correct syntax to use.
+
+3.8 Always built goals - always-y
+---------------------------------
+
+ always-y specifies targets which are literally always built when
+ Kbuild visits the Makefile.
+
+ Example::
+ # ./Kbuild
+ offsets-file := include/generated/asm-offsets.h
+ always-y += $(offsets-file)
+
+3.9 Compilation flags
+---------------------
+
+ ccflags-y, asflags-y and ldflags-y
+ These three flags apply only to the kbuild makefile in which they
+ are assigned. They are used for all the normal cc, as and ld
+ invocations happening during a recursive build.
+ Note: Flags with the same behaviour were previously named:
+ EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
+ They are still supported but their usage is deprecated.
+
+ ccflags-y specifies options for compiling with $(CC).
+
+ Example::
+
+ # drivers/acpi/acpica/Makefile
+ ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA
+ ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
+
+ This variable is necessary because the top Makefile owns the
+ variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
+ entire tree.
+
+ asflags-y specifies assembler options.
+
+ Example::
+
+ #arch/sparc/kernel/Makefile
+ asflags-y := -ansi
+
+ ldflags-y specifies options for linking with $(LD).
+
+ Example::
+
+ #arch/cris/boot/compressed/Makefile
+ ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
+
+ subdir-ccflags-y, subdir-asflags-y
+ The two flags listed above are similar to ccflags-y and asflags-y.
+ The difference is that the subdir- variants have effect for the kbuild
+ file where they are present and all subdirectories.
+ Options specified using subdir-* are added to the commandline before
+ the options specified using the non-subdir variants.
+
+ Example::
+
+ subdir-ccflags-y := -Werror
+
+ ccflags-remove-y, asflags-remove-y
+ These flags are used to remove particular flags for the compiler,
+ assembler invocations.
+
+ Example::
+
+ ccflags-remove-$(CONFIG_MCOUNT) += -pg
+
+ CFLAGS_$@, AFLAGS_$@
+ CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
+ kbuild makefile.
+
+ $(CFLAGS_$@) specifies per-file options for $(CC). The $@
+ part has a literal value which specifies the file that it is for.
+
+ CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
+ can re-add compiler flags that were removed by ccflags-remove-y.
+
+ Example::
+
+ # drivers/scsi/Makefile
+ CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
+
+ This line specify compilation flags for aha152x.o.
+
+ $(AFLAGS_$@) is a similar feature for source files in assembly
+ languages.
+
+ AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
+ can re-add assembler flags that were removed by asflags-remove-y.
+
+ Example::
+
+ # arch/arm/kernel/Makefile
+ AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
+ AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
+ AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
+
+
+3.10 Dependency tracking
+------------------------
+
+ Kbuild tracks dependencies on the following:
+
+ 1) All prerequisite files (both `*.c` and `*.h`)
+ 2) `CONFIG_` options used in all prerequisite files
+ 3) Command-line used to compile target
+
+ Thus, if you change an option to $(CC) all affected files will
+ be re-compiled.
+
+3.11 Custom Rules
+-----------------
+
+ Custom rules are used when the kbuild infrastructure does
+ not provide the required support. A typical example is
+ header files generated during the build process.
+ Another example are the architecture-specific Makefiles which
+ need custom rules to prepare boot images etc.
+
+ Custom rules are written as normal Make rules.
+ Kbuild is not executing in the directory where the Makefile is
+ located, so all custom rules shall use a relative
+ path to prerequisite files and target files.
+
+ Two variables are used when defining custom rules:
+
+ $(src)
+ $(src) is a relative path which points to the directory
+ where the Makefile is located. Always use $(src) when
+ referring to files located in the src tree.
+
+ $(obj)
+ $(obj) is a relative path which points to the directory
+ where the target is saved. Always use $(obj) when
+ referring to generated files.
+
+ Example::
+
+ #drivers/scsi/Makefile
+ $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
+ $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
+
+ This is a custom rule, following the normal syntax
+ required by make.
+
+ The target file depends on two prerequisite files. References
+ to the target file are prefixed with $(obj), references
+ to prerequisites are referenced with $(src) (because they are not
+ generated files).
+
+ $(kecho)
+ echoing information to user in a rule is often a good practice
+ but when execution "make -s" one does not expect to see any output
+ except for warnings/errors.
+ To support this kbuild defines $(kecho) which will echo out the
+ text following $(kecho) to stdout except if "make -s" is used.
+
+ Example::
+
+ # arch/arm/Makefile
+ $(BOOT_TARGETS): vmlinux
+ $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
+ @$(kecho) ' Kernel: $(boot)/$@ is ready'
+
+ 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::
+
+ # lib/Makefile
+ quiet_cmd_crc32 = GEN $@
+ cmd_crc32 = $< > $@
+
+ $(obj)/crc32table.h: $(obj)/gen_crc32table
+ $(call cmd,crc32)
+
+ When updating the $(obj)/crc32table.h target, the line:
+
+ GEN lib/crc32table.h
+
+ will be displayed with "make KBUILD_VERBOSE=0".
+
+3.12 Command change detection
+-----------------------------
+
+ When the rule is evaluated, timestamps are compared between the target
+ and its prerequisite files. GNU Make updates the target when any of the
+ prerequisites is newer than that.
+
+ The target should be rebuilt also when the command line has changed
+ since the last invocation. This is not supported by Make itself, so
+ Kbuild achieves this by a kind of meta-programming.
+
+ if_changed is the macro used for this purpose, in the following form::
+
+ quiet_cmd_<command> = ...
+ cmd_<command> = ...
+
+ <target>: <source(s)> FORCE
+ $(call if_changed,<command>)
+
+ Any target that utilizes if_changed must be listed in $(targets),
+ otherwise the command line check will fail, and the target will
+ always be built.
+
+ If the target is already listed in the recognized syntax such as
+ obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
+ automatically adds it to $(targets). Otherwise, the target must be
+ explicitly added to $(targets).
+
+ Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
+ used in conjunction with custom rules as defined in "3.11 Custom Rules".
+
+ 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, objcopy)
+
+ Note:
+ if_changed should not be used more than once per target.
+ It stores the executed command in a corresponding .cmd
+ file and multiple calls would result in overwrites and
+ unwanted results when the target is up to date and only the
+ tests on changed commands trigger execution of commands.
+
+3.13 $(CC) support functions
+----------------------------
+
+ The kernel may be built with several different versions of
+ $(CC), each supporting a unique set of features and options.
+ kbuild provides basic support to check for valid options for $(CC).
+ $(CC) is usually the gcc compiler, but other alternatives are
+ available.
+
+ as-option
+ as-option is used to check if $(CC) -- when used to compile
+ assembler (`*.S`) files -- supports the given option. An optional
+ second option may be specified if the first option is not supported.
+
+ Example::
+
+ #arch/sh/Makefile
+ cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
+
+ In the above example, cflags-y will be assigned the option
+ -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
+ The second argument is optional, and if supplied will be used
+ if first argument is not supported.
+
+ as-instr
+ as-instr checks if the assembler reports a specific instruction
+ and then outputs either option1 or option2
+ C escapes are supported in the test instruction
+ Note: as-instr-option uses KBUILD_AFLAGS for assembler options
+
+ cc-option
+ cc-option is used to check if $(CC) supports a given option, and if
+ not supported to use an optional second option.
+
+ Example::
+
+ #arch/x86/Makefile
+ cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
+
+ In the above example, cflags-y will be assigned the option
+ -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
+ The second argument to cc-option is optional, and if omitted,
+ cflags-y will be assigned no value if first option is not supported.
+ Note: cc-option uses KBUILD_CFLAGS for $(CC) options
+
+ cc-option-yn
+ cc-option-yn is used to check if gcc supports a given option
+ and return 'y' if supported, otherwise 'n'.
+
+ Example::
+
+ #arch/ppc/Makefile
+ biarch := $(call cc-option-yn, -m32)
+ aflags-$(biarch) += -a32
+ cflags-$(biarch) += -m32
+
+ In the above example, $(biarch) is set to y if $(CC) supports the -m32
+ option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
+ and $(cflags-y) will be assigned the values -a32 and -m32,
+ respectively.
+ Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
+
+ cc-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-ifversion
+ cc-ifversion tests the version of $(CC) and equals the fourth parameter
+ if version expression is true, or the fifth (if given) if the version
+ expression is false.
+
+ 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-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
+
+3.14 $(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)
+
+3.15 Script invocation
+----------------------
+
+ Make rules may invoke scripts to build the kernel. The rules shall
+ always provide the appropriate interpreter to execute the script. They
+ shall not rely on the execute bits being set, and shall not invoke the
+ script directly. For the convenience of manual script invocation, such
+ as invoking ./scripts/checkpatch.pl, it is recommended to set execute
+ bits on the scripts nonetheless.
+
+ Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
+ and $(PYTHON3) to refer to interpreters for the respective
+ scripts.
+
+ Example::
+
+ #Makefile
+ cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
+ $(KERNELRELEASE)
+
+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".
+
+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-y".
+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 := 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 := 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 := 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 := 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 $(KBUILD_HOSTCFLAGS).
+ To set flags that will take effect for all host programs created
+ in that Makefile, use the variable HOST_EXTRACFLAGS.
+
+ Example::
+
+ #scripts/lxdialog/Makefile
+ HOST_EXTRACFLAGS += -I/usr/include/ncurses
+
+ To set specific flags for a single file the following construction
+ is used:
+
+ Example::
+
+ #arch/ppc64/boot/Makefile
+ HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
+
+ It is also possible to specify additional options to the linker.
+
+ Example::
+
+ #scripts/kconfig/Makefile
+ HOSTLDLIBS_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 custom rule.
+
+ Example::
+
+ #drivers/pci/Makefile
+ hostprogs := 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 custom rules must be prefixed with $(obj).
+
+ (2) Use always-y
+
+ When there is no suitable custom rule, and the host program
+ shall be built when a makefile is entered, the always-y
+ variable shall be used.
+
+ Example::
+
+ #scripts/lxdialog/Makefile
+ hostprogs := lxdialog
+ always-y := $(hostprogs)
+
+ Kbuild provides the following shorthand for this:
+
+ hostprogs-always-y := lxdialog
+
+ This will tell kbuild to build lxdialog even if not referenced in
+ any rule.
+
+5 Userspace Program support
+===========================
+
+Just like host programs, Kbuild also supports building userspace executables
+for the target architecture (i.e. the same architecture as you are building
+the kernel for).
+
+The syntax is quite similar. The difference is to use "userprogs" instead of
+"hostprogs".
+
+5.1 Simple Userspace Program
+----------------------------
+
+ The following line tells kbuild that the program bpf-direct shall be
+ built for the target architecture.
+
+ Example::
+
+ userprogs := bpf-direct
+
+ Kbuild assumes in the above example that bpf-direct is made from a
+ single C source file named bpf-direct.c located in the same directory
+ as the Makefile.
+
+5.2 Composite Userspace Programs
+--------------------------------
+
+ Userspace programs can be made up based on composite objects.
+ The syntax used to define composite objects for userspace programs is
+ similar to the syntax used for kernel objects.
+ $(<executable>-objs) lists all objects used to link the final
+ executable.
+
+ Example::
+
+ #samples/seccomp/Makefile
+ userprogs := bpf-fancy
+ bpf-fancy-objs := bpf-fancy.o bpf-helper.o
+
+ Objects with extension .o are compiled from the corresponding .c
+ files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
+ and bpf-helper.c is compiled to bpf-helper.o.
+
+ Finally, the two .o files are linked to the executable, bpf-fancy.
+ Note: The syntax <executable>-y is not permitted for userspace programs.
+
+5.3 Controlling compiler options for userspace programs
+-------------------------------------------------------
+
+ When compiling userspace programs, it is possible to set specific flags.
+ The programs will always be compiled utilising $(CC) passed
+ the options specified in $(KBUILD_USERCFLAGS).
+ To set flags that will take effect for all userspace programs created
+ in that Makefile, use the variable userccflags.
+
+ Example::
+
+ # samples/seccomp/Makefile
+ userccflags += -I usr/include
+
+ To set specific flags for a single file the following construction
+ is used:
+
+ Example::
+
+ bpf-helper-userccflags += -I user/include
+
+ It is also possible to specify additional options to the linker.
+
+ Example::
+
+ # net/bpfilter/Makefile
+ bpfilter_umh-userldflags += -static
+
+ When linking bpfilter_umh, it will be passed the extra option -static.
+
+5.4 When userspace programs are actually built
+----------------------------------------------
+
+ Kbuild builds userspace programs only when told to do so.
+ There are two ways to do this.
+
+ (1) Add it as the prerequisite of another file
+
+ Example::
+
+ #net/bpfilter/Makefile
+ userprogs := bpfilter_umh
+ $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
+
+ $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
+
+ (2) Use always-y
+
+ Example::
+
+ userprogs := binderfs_example
+ always-y := $(userprogs)
+
+ Kbuild provides the following shorthand for this:
+
+ userprogs-always-y := binderfs_example
+
+ This will tell Kbuild to build binderfs_example when it visits this
+ Makefile.
+
+6 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), $(always-y), $(always-m),
+$(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
+during "make clean". Files matching the patterns "*.[oas]", "*.ko", plus
+some additional files generated by kbuild are deleted all over the kernel
+source tree when "make clean" is executed.
+
+Additional files or directories can be specified in kbuild makefiles by use of
+$(clean-files).
+
+ Example::
+
+ #lib/Makefile
+ clean-files := crc32table.h
+
+When executing "make clean", the file "crc32table.h" will be deleted.
+Kbuild will assume files to be in the same relative directory as the
+Makefile, except if prefixed with $(objtree).
+
+To exclude certain files or directories from make clean, use the
+$(no-clean-files) variable.
+
+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) $(clean)=arch/x86/boot
+
+When "make clean" is executed, make will descend down in arch/x86/boot,
+and clean as usual. The Makefile located in arch/x86/boot/ may use
+the subdir- trick to descend further down.
+
+Note 1: arch/$(SRCARCH)/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".
+
+7 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/$(SRCARCH)/Makefile contains what is required to set up kbuild
+for said architecture.
+To do so, arch/$(SRCARCH)/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/$(SRCARCH)/Makefile
+4) Recursively descend down in all directories listed in
+ init-* core* drivers-* net-* libs-* and build all targets.
+ - The values of the above variables are expanded in arch/$(SRCARCH)/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/$(SRCARCH)/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
+
+
+7.1 Set variables to tweak the build to the architecture
+--------------------------------------------------------
+
+ KBUILD_LDFLAGS
+ Generic $(LD) options
+
+ Flags used for all invocations of the linker.
+ Often specifying the emulation is sufficient.
+
+ Example::
+
+ #arch/s390/Makefile
+ KBUILD_LDFLAGS := -m elf_s390
+
+ Note: ldflags-y can be used to further customise
+ the flags used. See section 3.7.
+
+ 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
+ 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
+ Assembler options specific for built-in
+
+ $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
+ resident kernel code.
+
+ KBUILD_AFLAGS_MODULE
+ Assembler options specific for modules
+
+ $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
+ are used for assembler.
+
+ From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
+
+ 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.rst).
+
+ 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.rst).
+
+ KBUILD_LDS
+
+ The linker script with full path. Assigned by the top-level Makefile.
+
+ KBUILD_LDS_MODULE
+
+ The module linker script with full path. Assigned by the top-level
+ Makefile and additionally by the arch Makefile.
+
+ KBUILD_VMLINUX_OBJS
+
+ All object files for vmlinux. They are linked to vmlinux in the same
+ order as listed in KBUILD_VMLINUX_OBJS.
+
+ KBUILD_VMLINUX_LIBS
+
+ All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
+ KBUILD_VMLINUX_LIBS together specify all the object files used to
+ link vmlinux.
+
+7.2 Add prerequisites to archheaders
+------------------------------------
+
+ The archheaders: rule is used to generate header files that
+ may be installed into user space by "make header_install".
+
+ It is run before "make archprepare" when run on the
+ architecture itself.
+
+
+7.3 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 describes how kbuild supports
+ generating offset header files.
+
+
+7.4 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, core-y, libs-y, drivers-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.a object file can be
+ located.
+
+ Then the rest follows in this order:
+
+ $(core-y), $(libs-y), $(drivers-y)
+
+ The top level Makefile defines values for all generic directories,
+ and arch/$(SRCARCH)/Makefile only adds architecture-specific
+ directories.
+
+ Example::
+
+ # arch/sparc/Makefile
+ core-y += arch/sparc/
+
+ libs-y += arch/sparc/prom/
+ libs-y += arch/sparc/lib/
+
+ drivers-$(CONFIG_PM) += arch/sparc/power/
+
+7.5 Architecture-specific boot images
+-------------------------------------
+
+ An arch Makefile specifies goals that take the vmlinux file, compress
+ it, wrap it in bootstrapping code, and copy the resulting files
+ somewhere. This includes various kinds of installation commands.
+ The actual goals are not standardized across architectures.
+
+ It is common to locate any additional processing in a boot/
+ directory below arch/$(SRCARCH)/.
+
+ Kbuild does not provide any smart way to support building a
+ target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
+ call make manually to build a target in boot/.
+
+ The recommended approach is to include shortcuts in
+ arch/$(SRCARCH)/Makefile, and use the full path when calling down
+ into the arch/$(SRCARCH)/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, $(archhelp) must be defined.
+
+ Example::
+
+ #arch/x86/Makefile
+ define archhelp
+ echo '* bzImage - Compressed kernel image (arch/x86/boot/bzImage)'
+ endif
+
+ When make is executed without arguments, the first goal encountered
+ will be built. In the top level Makefile the first goal present
+ is all:.
+ An architecture shall always, per default, build a bootable image.
+ In "make help", the default goal is highlighted with a '*'.
+ Add a new prerequisite to all: to select a default goal different
+ from vmlinux.
+
+ Example::
+
+ #arch/x86/Makefile
+ all: bzImage
+
+ When "make" is executed without arguments, bzImage will be built.
+
+7.7 Commands useful for building a boot image
+---------------------------------------------
+
+ Kbuild provides a few macros that are useful when building a
+ boot image.
+
+ ld
+ Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
+
+ 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.
+ $(targets) are assigned all potential targets, by which kbuild knows
+ the targets and will:
+
+ 1) check for commandline changes
+ 2) delete target during make clean
+
+ The ": %: %.o" part of the prerequisite is a shorthand that
+ frees us from listing the setup.o and bootsect.o files.
+
+ Note:
+ It is a common mistake to forget the "targets :=" assignment,
+ resulting in the target file being recompiled for no
+ obvious reason.
+
+ objcopy
+ Copy binary. Uses OBJCOPYFLAGS usually specified in
+ arch/$(SRCARCH)/Makefile.
+ OBJCOPYFLAGS_$@ may be used to set additional options.
+
+ gzip
+ Compress target. Use maximum compression to compress target.
+
+ Example::
+
+ #arch/x86/boot/compressed/Makefile
+ $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
+ $(call if_changed,gzip)
+
+ dtc
+ Create flattened device tree blob object suitable for linking
+ into vmlinux. Device tree blobs linked into vmlinux are placed
+ in an init section in the image. Platform code *must* copy the
+ blob to non-init memory prior to calling unflatten_device_tree().
+
+ To use this command, simply add `*.dtb` into obj-y or targets, or make
+ some other target depend on `%.dtb`
+
+ A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
+ architecture Makefiles do no need to explicitly write out that rule.
+
+ Example::
+
+ targets += $(dtb-y)
+ DTC_FLAGS ?= -p 1024
+
+7.9 Preprocessing linker scripts
+--------------------------------
+
+ When the vmlinux image is built, the linker script
+ arch/$(SRCARCH)/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
+ extra-y := vmlinux.lds
+
+ The assignment to extra-y 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.
+
+7.10 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 "8.2 generic-y" for further info on syntax etc.
+
+7.11 Post-link pass
+-------------------
+
+ If the file arch/xxx/Makefile.postlink exists, this makefile
+ will be invoked for post-link objects (vmlinux and modules.ko)
+ for architectures to run post-link passes on. Must also handle
+ the clean target.
+
+ This pass runs after kallsyms generation. If the architecture
+ needs to modify symbol locations, rather than manipulate the
+ kallsyms, it may be easier to add another postlink target for
+ .tmp_vmlinux? targets to be called from link-vmlinux.sh.
+
+ For example, powerpc uses this to check relocation sanity of
+ the linked vmlinux file.
+
+8 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__`)
+
+All headers under include/uapi/, include/generated/uapi/,
+arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
+are exported.
+
+A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
+arch/<arch>/include/asm/ to list asm files coming from asm-generic.
+See subsequent chapter for the syntax of the Kbuild file.
+
+8.1 no-export-headers
+---------------------
+
+ no-export-headers is essentially used by include/uapi/linux/Kbuild to
+ avoid exporting specific headers (e.g. kvm.h) on architectures that do
+ not support it. It should be avoided as much as possible.
+
+8.2 generic-y
+-------------
+
+ If an architecture uses a verbatim copy of a header from
+ include/asm-generic then this is listed in the file
+ arch/$(SRCARCH)/include/asm/Kbuild like this:
+
+ Example::
+
+ #arch/x86/include/asm/Kbuild
+ generic-y += termios.h
+ generic-y += rtc.h
+
+ During the prepare phase of the build a wrapper include
+ file is generated in the directory::
+
+ arch/$(SRCARCH)/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>
+
+8.3 generated-y
+---------------
+
+ If an architecture generates other header files alongside generic-y
+ wrappers, generated-y specifies them.
+
+ This prevents them being treated as stale asm-generic wrappers and
+ removed.
+
+ Example::
+
+ #arch/x86/include/asm/Kbuild
+ generated-y += syscalls_32.h
+
+8.4 mandatory-y
+---------------
+
+ mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
+ to define the minimum set of ASM headers that all architectures must have.
+
+ This works like optional generic-y. If a mandatory header is missing
+ in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
+ generate a wrapper of the asm-generic one.
+
+9 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 ...
+
+ SRCARCH
+ This variable specifies the directory in arch/ to build.
+
+ ARCH and SRCARCH may not necessarily match. A couple of arch
+ directories are biarch, that is, a single `arch/*/` directory supports
+ both 32-bit and 64-bit.
+
+ For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
+ For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
+ x86_64.
+
+ 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.
+
+
+10 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.
+
+11 Credits
+==========
+
+- Original version made by Michael Elizabeth Chastain, <mailto:mec at shout.net>
+- Updates by Kai Germaschewski <kai at tp1.ruhr-uni-bochum.de>
+- Updates by Sam Ravnborg <sam at ravnborg.org>
+- Language QA by Jan Engelhardt <jengelh at gmx.de>
+
+12 TODO
+=======
+
+- Describe how kbuild supports shipped files with _shipped.
+- Generating offset header files.
+- Add more variables to chapters 7 or 9?
--
2.32.0.402.g57bb445576-goog
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