关于kbuild中KERNELRELEASE的疑问

[雁子依然]

有个小问题，关于KERNELRELEASE，我的理解是不是也可以不用去判断这个变量的定义呢？
LDD3里面hello的例子makefile如下

# To build modules outside of the kernel tree, we run "make"
# in the kernel source tree; the Makefile these then includes this
# Makefile once again.
# This conditional selects whether we are being included from the
# kernel Makefile or not.
ifeq ($(KERNELRELEASE),)

    # Assume the source tree is where the running kernel was built
    # You should set KERNELDIR in the environment if it's elsewhere
    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    # The current directory is passed to sub-makes as argument
    PWD := $(shell pwd)

modules:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

modules_install:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install

clean:
	rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions

.PHONY: modules modules_install clean

else
    # called from kernel build system: just declare what our modules are
    obj-m := hello.o hellop.o seq.o jit.o jiq.o sleepy.o complete.o \
             silly.o faulty.o kdatasize.o kdataalign.o
endif

我试着把ifeq else endif这三行注释，也可以编译成功，我理解是不是这样：
1. 在模块所在目录执行make，根据makefile规则，会先去找第一个目标modules，执行后面的命令$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

2. 进入kernel目录，kbuild开始工作
3. 根据M=参数，生成模块之前回到当前目录再次读取makefile里面的obj-m

所以是不是说KERNELRELEASE定义与否，效果都是一样的呢

当然有一个例外就是，如果在当前目录下make的时候直接就make KERNELRELEASE=y，那就会编译出错，但是应该不会有人这么做吧，或者说不会出现这样的情况吧？

可以帮住解答一下么？

[雁子依然]

求指点啊，要沉下去了