make参数传递

设置输出目录

参数-O

# kbuild supports saving output files in a separate directory.

# To locate output files in a separate directory two syntaxes are supported.
# In both cases the working directory must be the root of the kernel src.
# 1) O=
# Use "make O=dir/to/store/output/files/"
#
# 2) Set KBUILD_OUTPUT
# Set the environment variable KBUILD_OUTPUT to point to the directory
# where the output files shall be placed.
# export KBUILD_OUTPUT=dir/to/store/output/files/
# make
#
# The O= assignment takes precedence over the KBUILD_OUTPUT environment
# variable.

Kbuild支持将输出文件保存在单独的目录中，由两种方式可以实现：
1、在命令行中设置变量O，例如：make O=dir/to/store/output/files/
2、设置变量KBUILD_OUTPUT的值，例如export KBUILD_OUTPUT=dir/to/store/output/files/ 变量O的优先级高于变量KBUILD_OUTPUT。

# KBUILD_SRC is set on invocation of make in OBJ directory
# KBUILD_SRC is not intended to be used by the regular user (for now)
ifeq ($(KBUILD_SRC),)

# OK, Make called in directory where kernel src resides
# Do we want to locate output files in a separate directory?
ifeq ("$(origin O)", "command line")
  KBUILD_OUTPUT := $(O)
endif

# That's our default target when none is given on the command line
PHONY := _all
_all:

# Cancel implicit rules on top Makefile
$(CURDIR)/Makefile Makefile: ;

ifneq ($(KBUILD_OUTPUT),)
# Invoke a second make in the output directory, passing relevant variables
# check that the output directory actually exists
saved-output := $(KBUILD_OUTPUT)
KBUILD_OUTPUT := $(shell mkdir -p $(KBUILD_OUTPUT) && cd $(KBUILD_OUTPUT) \
                                && /bin/pwd)
$(if $(KBUILD_OUTPUT),, \
     $(error failed to create output directory "$(saved-output)"))

PHONY += $(MAKECMDGOALS) sub-make

$(filter-out _all sub-make $(CURDIR)/Makefile, $(MAKECMDGOALS)) _all: sub-make
    @:

sub-make: FORCE
    $(Q)$(MAKE) -C $(KBUILD_OUTPUT) KBUILD_SRC=$(CURDIR) \
    -f $(CURDIR)/Makefile $(filter-out _all sub-make,$(MAKECMDGOALS))

# Leave processing to above invocation of make
skip-makefile := 1
endif # ifneq ($(KBUILD_OUTPUT),)
endif # ifeq ($(KBUILD_SRC),)

首先系统会检查变量KBUILD_SRC是否为空，它代表代码的顶层目录。如果它是空的，将会设置变量KBUILD_OUTPUT为传递给选项O的值。
然后检查变量KBUILD_OUTPUT，如果已经设置好，那么接下来会做以下几件事：
将变量KBUILD_OUTPUT的值保存到临时变量saved-output；
尝试创建给定的输出目录；
检查创建的输出目录，如果失败了就打印错误；
如果成功创建了输出目录，那么就在新目录重新执行make命令。