1. Preliminary work of Linux kernel top-level Makefile
This article continues to analyze the preparation work done by the top-level Makefile of the Linux kernel source code .
This article is a continuation of the previous article at the following address:
2. Preliminary work of kernel top-level Makefile
7. Module compilation
Linux
allows you to compile a module separately, just use the command "
make M=dir
", the old syntax "
make SUBDIRS=dir
"
It is also supported.
The code in the top-level
Makefile is as follows:
182 ifdef SUBDIRS
183 KBUILD_EXTMOD ?= $(SUBDIRS)
184 endif
185
186 ifeq ("$(origin M)", "command line")
187 KBUILD_EXTMOD := $(M)
188 endif
189
190 # If building an external module we do not care about the all: rule
191 # but instead _all depend on modules
192 PHONY += all
193 ifeq ($(KBUILD_EXTMOD),)
194 _all: all
195 else
196 _all: modules
197 endif
198
199 ifeq ($(KBUILD_SRC),)
200 # building in the source tree
201 srctree := .
202 else
203 ifeq ($(KBUILD_SRC)/,$(dir $(CURDIR)))
204 # building in a subdirectory of the source tree
205 srctree := ..
206 else
207 srctree := $(KBUILD_SRC)
208 endif
209 endif
210 objtree := .
211 src := $(srctree)
212 obj := $(objtree)
213
214 VPATH := $(srctree)$(if $(KBUILD_EXTMOD),:$(KBUILD_EXTMOD))
215
216 export srctree objtree VPATH
The external module compilation process
is the same as
uboot , and
the values of the three variables srctree , objtree and VPATH are finally exported.
Among them,
srctree=.
, which is the current directory,
and objtree
is also "
.
".
8. Set target architecture and cross-compiler
Like
uboot
,
when compiling
Linux , you need to set the target board architecture
ARCH and cross-compiler CROSS_COMPILE .
The code in the top-level
Makefile
is as follows:
252 ARCH ?= $(SUBARCH)
253 CROSS_COMPILE ?= $(CONFIG_CROSS_COMPILE:"%"=%)
For convenience, generally directly modify
ARCH and CROSS_COMPILE in the top-level
Makefile
and set them directly.
for the corresponding architecture and compiler. For example, change this to:
252 ARCH ?= arm
253 CROSS_COMPILE ?= arm-linux-gnueabihf-9. Call scripts/Kbuild.include file
Like
uboot
,
the Linux
top-level
Makefile
will also call the file
scripts/Kbuild.include
. The top-level
Makefile
is also
The code should be as follows:
349 scripts/Kbuild.include: ;
350 include scripts/Kbuild.include10. Set cross-compilation tool variables
Other cross-compiler-related variables in the top-level
Makefile are set as follows:
353 AS = $(CROSS_COMPILE)as
354 LD = $(CROSS_COMPILE)ld
355 CC = $(CROSS_COMPILE)gcc
356 CPP = $(CC) -E
357 AR = $(CROSS_COMPILE)ar
358 NM = $(CROSS_COMPILE)nm
359 STRIP = $(CROSS_COMPILE)strip
360 OBJCOPY = $(CROSS_COMPILE)objcopy
361 OBJDUMP = $(CROSS_COMPILE)objdump
LA
,
LD
,
CC
, etc. are all tools used by cross-compilers.
11. Header file path variable
The top-level
Makefile
defines two variables to save the header file path:
USERINCLUDE
and
LINUXINCLUDE
, respectively.
The relevant code is as follows:
381 USERINCLUDE := \
382 -I$(srctree)/arch/$(hdr-arch)/include/uapi \
383 -Iarch/$(hdr-arch)/include/generated/uapi \
384 -I$(srctree)/include/uapi \
385 -Iinclude/generated/uapi \
386 -include $(srctree)/include/linux/kconfig.h
387
388 # Use LINUXINCLUDE when you must reference the include/ directory.
389 # Needed to be compatible with the O= option
390 LINUXINCLUDE := \
391 -I$(srctree)/arch/$(hdr-arch)/include \
392 -Iarch/$(hdr-arch)/include/generated/uapi \
393 -Iarch/$(hdr-arch)/include/generated \
394 $(if $(KBUILD_SRC), -I$(srctree)/include) \
395 -Iinclude \
396 $(USERINCLUDE)
Lines 381
~386
are
USERINCLUDE
, which is
the header file path related to
UAPI
, and lines 390~396 are LINUXINCLUDE, which is the header file path of the Linux kernel source code.
12. Export variables
The top-level
Makefile
will export many variables for use by sub-
Makefiles
. The exported variables are as follows:
417 export VERSION PATCHLEVEL SUBLEVEL KERNELRELEASE KERNELVERSION
418 export ARCH SRCARCH CONFIG_SHELL HOSTCC HOSTCFLAGS CROSS_COMPILE AS
LD CC
419 export CPP AR NM STRIP OBJCOPY OBJDUMP
420 export MAKE AWK GENKSYMS INSTALLKERNEL PERL PYTHON UTS_MACHINE
421 export HOSTCXX HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
422
423 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS
LDFLAGS
424 export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE CFLAGS_GCOV
CFLAGS_KASAN
425 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
426 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE
KBUILD_LDFLAGS_MODULE
427 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
428 export KBUILD_ARFLAGSThe next article continues to understand the preparation of the top-level Makefile of the Linux kernel source code.