make参数传递
参数-C
# We process the rest of the Makefile if this is the final invocation of make
ifeq ($(skip-makefile),)
# Do not print "Entering directory ...",
# but we want to display it when entering to the output directory
# so that IDEs/editors are able to understand relative filenames.
MAKEFLAGS += --no-print-directory
# Call a source code checker (by default, "sparse") as part of the
# C compilation.
#
# Use 'make C=1' to enable checking of only re-compiled files.
# Use 'make C=2' to enable checking of *all* source files, regardless
# of whether they are re-compiled or not.
#
# See the file "Documentation/sparse.txt" for more details, including
# where to get the "sparse" utility.
ifeq ("$(origin C)", "command line")
KBUILD_CHECKSRC = $(C)
endif
ifndef KBUILD_CHECKSRC
KBUILD_CHECKSRC = 0
endif
首先检查了skip-makefile ，这个变量在配置选项O的时候设置为1
然后ifeq语句会检查传递给make的选项C
参数 -M
用来编译外部模块
# Use make M=dir to specify directory of external module to build
# Old syntax make ... SUBDIRS=$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 := $(M)
endif
# If building an external module we do not care about the all: rule
# but instead _all depend on modules
PHONY += all
ifeq ($(KBUILD_EXTMOD),)
_all: all
else
_all: modules
endif
设置objtree
OBJTREE：编译出的.o文件存放的目录的根目录；
在默认编译下，OBJTREE等于当前目录；
在O=xx编译下，OBJTREE就等于我们设置的那个输出目录。
SRCTREE: 源码目录，其实就是源代码的根目录，也就是当前目录。
ifeq ($(KBUILD_SRC),)
# building in the source tree
srctree := .
else
ifeq ($(KBUILD_SRC)/,$(dir $(CURDIR)))
# building in a subdirectory of the source tree
srctree := ..
else
srctree := $(KBUILD_SRC)
endif
endif
objtree := .
src := $(srctree)
obj := $(objtree)
VPATH := $(srctree)$(if $(KBUILD_EXTMOD),:$(KBUILD_EXTMOD))
export srctree objtree VPATH
系统检查了变量KBUILD_SRC ，如果KBUILD_SRC 未设置，系统会设置变量srctree为当前目录。
设置objtree和其他变量为这个目录，并且将这些变量导出
