Analysis of preliminary work of Linux kernel top-level Makefile 1

1. Linux kernel top-level Makefile

The top-level Makefile of Linux is very similar to the top-level Makefile of uboot, because uboot refers to Linux.

2. Linux kernel-top-level Makefile preliminary work

Let's take a look at the preliminary work done by the top-level Makefile of the Linux kernel.

1. Version number

The top-level Makefile starts with the version number of the Linux kernel, as shown below:

1 VERSION = 4
2 PATCHLEVEL = 1
3 SUBLEVEL = 15
4 EXTRAVERSION =

It can be seen that the Linux kernel version number is 4.1.15 .

2. MAKEFLAGS variable _

The MAKEFLAGS variable is set as follows:

16 MAKEFLAGS += -rR --include-dir=$(CURDIR)

3. Command output

When compiling Linux, you can also use " V=1 " to output the complete command. This is the same as uboot . The relevant code is as follows:

69 ifeq ("$(origin V)", "command line")
70 KBUILD_VERBOSE = $(V)
71 endif
72 ifndef KBUILD_VERBOSE
73 KBUILD_VERBOSE = 0
74 endif
75
76 ifeq ($(KBUILD_VERBOSE),1)
77 quiet =
78 Q =
79 else
80 quiet=quiet_
81 Q = @
82 endif

4. Silent output

When compiling Linux, use " make -s " to achieve silent compilation. No information will be printed during compilation. Just like uboot , the relevant code is as follows:

87 ifneq ($(filter 4.%,$(MAKE_VERSION)),) # make-4
88 ifneq ($(filter %s ,$(firstword x$(MAKEFLAGS))),)
89 quiet=silent_
90 endif
91 else # make-3.8x
92 ifneq ($(filter s% -s%,$(MAKEFLAGS)),)
93 quiet=silent_
94 endif
95 endif
96
97 export quiet Q KBUILD_VERBOSE

5. Set the compilation result output directory

When compiling Linux, use " O=xxx " to output the process file generated by compilation to the specified directory. The relevant code is as follows:

116 ifeq ($(KBUILD_SRC),)
117
118 # OK, Make called in directory where kernel src resides
119 # Do we want to locate output files in a separate directory?
120 ifeq ("$(origin O)", "command line")
121 KBUILD_OUTPUT := $(O)
122 endif

6. Code inspection

Linux also supports code checking. Use the command " make C=1 " to enable code checking and check those files that need to be recompiled. " make C=2 " is used to check all source code files. The code in the top-level Makefile is as follows:

172 ifeq ("$(origin C)", "command line")
173 KBUILD_CHECKSRC = $(C)
174 endif
175 ifndef KBUILD_CHECKSRC
176 KBUILD_CHECKSRC = 0
177 endif

The next article continues to analyze the preparation work done by the top-level Makefile of the Linux kernel source code.