前言

之前对uboot的构建进行了分析，现在再对linux kernel的构建进行分析。几年前的确也分析过，但是只是停留在笔记层面，没有转为文章，这次下定决定来完善它。

环境

同样，采用的还是zynq平台的linux，从Makefile可以看到版本：

VERSION = 3

PATCHLEVEL = 15

SUBLEVEL = 0

EXTRAVERSION =

NAME = Shuffling Zombie Juror

linux Makefile支持的选项（最常用到的）

选项V，用于开启或者关闭执行make时编译信息的打印

    @echo  '  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build'

    @echo  '  make V=2   [targets] 2 => give reason for rebuild of target'

ifeq ("$(origin V)", "command line")

  KBUILD_VERBOSE = $(V)

endif

ifndef KBUILD_VERBOSE

  KBUILD_VERBOSE = 0

endif

选项C，用于开启或者关闭静态代码检查

    @echo  '  make C=1   [targets] Check all c source with $$CHECK (sparse by default)'

    @echo  '  make C=2   [targets] Force check of all c source with $$CHECK'

ifeq ("$(origin C)", "command line")

  KBUILD_CHECKSRC = $(C)

endif

ifndef KBUILD_CHECKSRC

  KBUILD_CHECKSRC = 0

endif

选项M/SUBDIRS，源码外模块编译时会用到

# Use make M=dir to specify directory of external module to build

# Old syntax make ... SUBDIRS=$PWD is still supported

# Setting the environment variable KBUILD_EXTMOD take precedence

ifdef SUBDIRS

  KBUILD_EXTMOD ?= $(SUBDIRS)

endif

ifeq ("$(origin M)", "command line")

  KBUILD_EXTMOD := $(M)

endif

选项O/KBUILD_OUTPUT，指定out-of-build时的输出目录

    @echo  '  make O=dir [targets] Locate all output files in "dir", including .config'

ifeq ("$(origin O)", "command line")

  KBUILD_OUTPUT := $(O)

endif

选项W，也是代码检查用的，很少用到

    @echo  '  make W=n   [targets] Enable extra gcc checks, n=1,2,3 where'

    @echo  '        1: warnings which may be relevant and do not occur too often'

    @echo  '        2: warnings which occur quite often but may still be relevant'

    @echo  '        3: more obscure warnings, can most likely be ignored'

    @echo  '        Multiple levels can be combined with W=12 or W=123' 

ifeq ("$(origin W)", "command line")

  export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)

endif

构建分析

内核的构建一般包含两步（如果算上distclean的话，就是三步，如果再算上dtbs构建的话，就是四步，这里忽略那两个步骤的分析），下面会对每一个步骤进行跟踪分析

第一步，配置，对应的命令make ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zynq_zturn_defconfig

对应的规则是(顶层Makefile里)：

%config: scripts_basic outputmakefile FORCE

    $(Q)mkdir -p include/linux include/config

    $(Q)$(MAKE) $(build)=scripts/kconfig $@

这里先解释两个地方

第一个，关于Q，它只是在KBUILD_VERBOSE为1的时候为空，不为1的时候，为@。在make中，一条命令前加@表示执行该命令的时候，不打印出执行的命令。而KBUILD_VERBOSE的设置，请参考关于"支持的选项"那节里的V选项的描述。

# A simple variant is to prefix commands with $(Q) - that's useful

# for commands that shall be hidden in non-verbose mode.

#

#   $(Q)ln $@ :<

ifeq ($(KBUILD_VERBOSE),1)

  quiet =

  Q =

else

  quiet=quiet_

  Q = @

endif

也顺便说说quiet

# Normally, we echo the whole command before executing it. By making

# that echo $($(quiet)$(cmd)), we now have the possibility to set

# $(quiet) to choose other forms of output instead, e.g.

#

#         quiet_cmd_cc_o_c = Compiling $(RELDIR)/$@

#         cmd_cc_o_c       = $(CC) $(c_flags) -c -o $@ $<

#

# If $(quiet) is empty, the whole command will be printed.

# If it is set to "quiet_", only the short version will be printed.

# If it is set to "silent_", nothing will be printed at all, since

# the variable $(silent_cmd_cc_o_c) doesn't exist.

# If the user is running make -s (silent mode), suppress echoing of

# commands                                                                      

ifneq ($(filter 4.%,$(MAKE_VERSION)),)  # make-4

ifneq ($(filter %s ,$(firstword x$(MAKEFLAGS))),)

  quiet=silent_

endif

else                    # make-3.8x

ifneq ($(filter s% -s%,$(MAKEFLAGS)),)

  quiet=silent_

endif

endif

第二个，关于build(在scripts/Kbuild.include定义，它是由顶层Makefile所包含进来的):

# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=

# Usage:

# $(Q)$(MAKE) $(build)=dir

build := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build obj

现在重点关注这条规则的依赖，scripts_basic和outputmakefile

依赖1 scripts_basic（顶层Makefile中）

PHONY += scripts_basic

scripts_basic:

    $(Q)$(MAKE) $(build)=scripts/basic

    $(Q)rm -f .tmp_quiet_recordmcount

也就是说，会先执行

$(Q)$(MAKE) $(build)=scripts/basic

以及

$(Q)rm -f .tmp_quiet_recordmcount(这个就不用说了，删除临时文件）

经过上面对Q和build的说明，我们可以知道

$(Q)$(MAKE) $(build)=scripts/basic

等价于

$(Q)$(MAKE) -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build obj=scripts/basic

这里假设KBUILD_SRC为空（一般都是为空），那么上面的命令最终变成了

$(Q)$(MAKE) -f scripts/Makefile.build obj=scripts/basic

也就是说会执行scripts/Makefile.build这个Makefile，且设置输入变量obj为 scripts/basic

经过分析scripts/Makefile.build，最终执行的规则为：

__build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) \

     $(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) \

     $(subdir-ym) $(always)

    @:

其中KBUILD_BUILTIN := 1在顶层Makefile有定义，如果执行”make modules”，会在214行开始对其进行一些处理

ifeq ($(MAKECMDGOALS),modules)

  KBUILD_BUILTIN := $(if $(CONFIG_MODVERSIONS),1)

endif

所以我们这里 KBUILD_BUILTIN :=1

如果执行”make all”、”make _all”、”make modules”、”make”中任一个命令，则会对这个变量进行处理

ifneq ($(filter all _all modules,$(MAKECMDGOALS)),)

  KBUILD_MODULES := 1

endif

ifeq ($(MAKECMDGOALS),)

  KBUILD_MODULES := 1

endif

因此，我们这里KBUILD_MODULES :=

分析了这两个变量后，上面的规则可重新写为

__build: $(builtin-target) $(lib-target) $(extra-y)) $(subdir-ym) $(always)

    @:

规则的命令是一个冒号命令”:”，冒号(:)命令是bash的内建命令，通常把它看作true命令。

下面来一个个分析在执行make ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zynq_zturn_defconfig的情况下，$(builtin-target) $(lib-target) $(extra-y)) $(subdir-ym) $(always)这些变量的值。

ifneq ($(strip $(obj-y) $(obj-m) $(obj-n) $(obj-) $(subdir-m) $(lib-target)),)

builtin-target := $(obj)/built-in.o

endif 

ifneq ($(strip $(lib-y) $(lib-m) $(lib-n) $(lib-)),)

lib-target := $(obj)/lib.a

endif

这里的变量除了always，其他都为空，always来自于scripts/Makefile.build中，有

kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))

kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)

include $(kbuild-file)

由于我们输入的obj为scripts/basic，而该Makefile开头就有src := $(obj)，于是我们可以知道，它最终包含了头文件scripts/basic/Makefile，该Makefile内容如下：

hostprogs-y := fixdep

always      := $(hostprogs-y)                                                   

# fixdep is needed to compile other host programs

$(addprefix $(obj)/,$(filter-out fixdep,$(always))): $(obj)/fixdep

由此可知，always的内容为fixdep。也就是说scripts_basic在执行make ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zynq_zturn_defconfig的结果就是构建fixdep

依赖2,outputmakefile

outputmakefile:

ifneq ($(KBUILD_SRC),)

    $(Q)ln -fsn $(srctree) source

    $(Q)$(CONFIG_SHELL) $(srctree)/scripts/mkmakefile \

        $(srctree) $(objtree) $(VERSION) $(PATCHLEVEL)

endif

这个规则的命令运行一个shell脚本scripts/makefile，并传递四个参数。这个脚本主要是在$(objtree)参数指定的目录中生成一个Makefile文件。由于这里KBUILD_SRC为空，所以这个脚本并不会被执行

分析完两个依赖后，再来看

%config: scripts_basic outputmakefile FORCE

    $(Q)mkdir -p include/linux include/config

    $(Q)$(MAKE) $(build)=scripts/kconfig $@

在他的依赖被处理完后，开始执行规则的命令。第一个命令创建了两个目录，第二个命令扩展后为

$(Q)$(MAKE) -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build  obj=scripts/kconfig zynq_zturn_defconfig

这个命令依然是执行scripts/Makefile.build这个makefile文件。并执行它里面zynq_zturn_defconfig的规则。根据上面的分析，在Makefile.build会包含scripts/kconfig/Makefile文件。然后执行以zynq_zturn_defconfig为目标的规则，在scripts/kconfig/Makefile中有规则：

%_defconfig: $(obj)/conf

    $(Q)$< --defconfig=arch/$(SRCARCH)/configs/$@ $(Kconfig)

其中Kconfig来自：

# Read arch specific Makefile to set KBUILD_DEFCONFIG as needed.

# KBUILD_DEFCONFIG may point out an alternative default configuration

# used for 'make defconfig'

include $(srctree)/arch/$(SRCARCH)/Makefile

export KBUILD_DEFCONFIG KBUILD_KCONFIG  

ifdef KBUILD_KCONFIG

Kconfig := $(KBUILD_KCONFIG)

else

Kconfig := Kconfig

endif

arch/arm/Makefile中没有定义KBUILD_KCONFIG,因此Kconfig := Kconfig

等价于

scripts/kconfig/conf --defconfig=arch/$(SRCARCH)/configs/zynq_zturn_defconfig Kconfig

继续分析%_defconfig: $(obj)/conf，它会先构建conf（请参考scripts/Makefile.host），然后就是调用conf处理--defconfig=arch/$(SRCARCH)/configs/zynq_zturn_defconfig Kconfig。这就是我们实际配置的过程。

这里可以总结下，执行make ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zynq_zturn_defconfig，make第一步会先编译fixdep这个程序，它在内核的编译过程会用到，然后会构建conf，它用来解析Kconfig，最后会根据命令行情况生成.config

我们可以通过执行命令来验证下上面的分析（记得先distclean下）：

$ make V=1 ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zynq_zturn_defconfig

make -f scripts/Makefile.build obj=scripts/basic

  gcc -Wp,-MD,scripts/basic/.fixdep.d -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer     -o scripts/basic/fixdep scripts/basic/fixdep.c

rm -f .tmp_quiet_recordmcount

mkdir -p include/linux include/config

make -f scripts/Makefile.build obj=scripts/kconfig zynq_zturn_defconfig

  gcc -Wp,-MD,scripts/kconfig/.conf.o.d -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer   -DCURSES_LOC="<ncurses.h>" -DLOCALE   -c -o scripts/kconfig/conf.o scripts/kconfig/conf.c

  cat scripts/kconfig/zconf.tab.c_shipped > scripts/kconfig/zconf.tab.c

  cat scripts/kconfig/zconf.lex.c_shipped > scripts/kconfig/zconf.lex.c

  cat scripts/kconfig/zconf.hash.c_shipped > scripts/kconfig/zconf.hash.c

  gcc -Wp,-MD,scripts/kconfig/.zconf.tab.o.d -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer   -DCURSES_LOC="<ncurses.h>" -DLOCALE  -Iscripts/kconfig -c -o scripts/kconfig/zconf.tab.o scripts/kconfig/zconf.tab.c

In file included from scripts/kconfig/zconf.tab.c:2537:0:

scripts/kconfig/menu.c: In function ‘get_symbol_str’:

scripts/kconfig/menu.c:590:18: warning: ‘jump’ may be used uninitialized in this function [-Wmaybe-uninitialized]

     jump->offset = strlen(r->s);

                  ^

scripts/kconfig/menu.c:551:19: note: ‘jump’ was declared here

  struct jump_key *jump;

                   ^

  gcc  -o scripts/kconfig/conf scripts/kconfig/conf.o scripts/kconfig/zconf.tab.o

scripts/kconfig/conf --defconfig=arch/arm/configs/zynq_zturn_defconfig Kconfig

#

# configuration written to .config

#

第二步，构建，对应的命令make ARCH=arm CROSS_COMPILE=arm-xilinx-linux-gnueabi- zImage

BOOT_TARGETS    = zImage Image xipImage bootpImage uImage

$(BOOT_TARGETS): vmlinux

    $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@

由此可以看出依赖vmlinux,下面先看vmlinux

# Include targets which we want to

# execute if the rest of the kernel build went well.

vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE

ifdef CONFIG_HEADERS_CHECK

    $(Q)$(MAKE) -f $(srctree)/Makefile headers_check

endif

ifdef CONFIG_SAMPLES

    $(Q)$(MAKE) $(build)=samples

endif

ifdef CONFIG_BUILD_DOCSRC

    $(Q)$(MAKE) $(build)=Documentation

endif

    +$(call if_changed,link-vmlinux)

关于命令前加号说明

makefile中以+开头的命令的执行不受到 make的-n,-t,-q三个参数的影响。比方说你的 makefile 是这样写的

all:

echo hello

+echo world

正常你 make,两个echo命令都会执行

hello

world

然后你 make -n,就只有第二个echo命令会被执行了

接着看依赖$(vmlinux-deps)：

export KBUILD_VMLINUX_INIT := $(head-y) $(init-y)

export KBUILD_VMLINUX_MAIN := $(core-y) $(libs-y) $(drivers-y) $(net-y)

export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds  

vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)

由此可以看出主要是依赖$(head-y) $(init-y) $(core-y) $(libs-y) $(drivers-y) $(net-y)

其中

head-y      := arch/arm/kernel/head$(MMUEXT).o  (arch/arm/Makefile)

其他的都是在顶层的Makefile里定义

init-y      := $(patsubst %/, %/built-in.o, $(init-y))

core-y      := $(patsubst %/, %/built-in.o, $(core-y))

drivers-y   := $(patsubst %/, %/built-in.o, $(drivers-y))

net-y       := $(patsubst %/, %/built-in.o, $(net-y))

libs-y1     := $(patsubst %/, %/lib.a, $(libs-y))

libs-y2     := $(patsubst %/, %/built-in.o, $(libs-y))

libs-y      := $(libs-y1) $(libs-y2)

写到这的时候，我发现网上有一个blog已经详细分析了整个构建过程，而且写的非常好（之前没看到它！！！），因此我就打算停止继续写这篇了，不重复造*，请大家直接参考 这里

完！

2016年5月