Top level makefile analysis of u-boot (1)

top floor Makefile almost 3200, Next, analyze this file in detail .

u-boot Version and environment variables

VERSION = 1
PATCHLEVEL = 3
SUBLEVEL = 4
EXTRAVERSION =
U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
VERSION_FILE = $(obj)include/version_autogenerated.h

HOSTARCH := $(shell uname -m | \ sed -e s/i.86/i386/ \ -e s/sun4u/sparc64/ \ -e s/arm.*/arm/ \ -e s/sa110/arm/ \ -e s/powerpc/ppc/ \ -e s/ppc64/ppc/ \ -e s/macppc/ppc/)

HOSTOS := $(shell uname -s | tr '[:upper:]' '[:lower:]' | \ sed -e 's/\(cygwin\).*/cygwin/')
#$(error HOSTARCH = $(HOSTARCH) HOSTOS = $(HOSTOS) "WAITING for value!")
export	HOSTARCH HOSTOS

# Deal with colliding definitions from tcsh etc.
VENDOR=

• The first three variables are combined into one U_BOOT_VERSION Variable , To form the u-boot Version number of , This version number is finally stored in include/version_autogenerated.h In file . After opening, the content is #define U_BOOT_VERSION "U-Boot 1.3.4"
• sed Of sed s/i.86/i386/ Is a kind of substitution grammar , It means that i.86 Replace with i386, for example echo "33i.86" | sed s/i.86/i386/ The output of is 33i386,sed in -e It means that multiple substitutions can be carried out at the same time .
• uname -m Is to show Linux The architecture of , For example, the command result of the current virtual machine is x86_64, It can be seen from the value that there is no i.86,sun4u,…,macppc, therefore HOSTARCH The value of is equal to uname -m Output result of .
• uname -s Is to display the kernel name , So here it shows "Linux",tr '[:upper:]' '[:lower:]' The result of the command is to set all letters to lowercase . At last HOSTOS The value of is linux.
• To verify whether the judgment is correct , Can open export Comments on the previous line . intend Makefile Stop after this sentence （ take error Switch to info After that, it will not stop ）, And output the values of these two variables .
• Cancel the above comments and run make, The result is Makefile:43: *** HOSTARCH = x86_64 HOSTOS = linux "WAITING for value!". stop it ., Therefore, our analysis is correct .
• And then HOSTARCH and HOSTOS export (export) For environment variables , In order to make the son shell Use .
• Definition VENDOR The variable is empty

Silent compilation

# Allow for silent builds
ifeq (,$(findstring s,$(MAKEFLAGS)))
XECHO = echo
else
XECHO = :
endif

When we use make -s Command compilation u-boot No information will be output , As Silent compilation .

Set a separate compile output folder

#########################################################################
#
# U-boot build supports producing a object files to the separate external
# directory. Two use cases are supported:
#
# 1) Add O= to the make command line
# 'make O=/tmp/build all'
#
# 2) Set environement variable BUILD_DIR to point to the desired location
# 'export BUILD_DIR=/tmp/build'
# 'make'
#
# The second approach can also be used with a MAKEALL script
# 'export BUILD_DIR=/tmp/build'
# './MAKEALL'
#
# Command line 'O=' setting overrides BUILD_DIR environent variable.
#
# When none of the above methods is used the local build is performed and
# the object files are placed in the source directory.
#

ifdef O
ifeq ("$(origin O)", "command line")
BUILD_DIR := $(O)
endif
endif

ifneq ($(BUILD_DIR),)
saved-output := $(BUILD_DIR)

# Attempt to create a output directory.
$(shell [ -d ${
     BUILD_DIR} ] || mkdir -p ${
     BUILD_DIR})

# Verify if it was successful.
BUILD_DIR := $(shell cd $(BUILD_DIR) && /bin/pwd)
$(if $(BUILD_DIR),,$(error output directory "$(saved-output)" does not exist))
endif # ifneq ($(BUILD_DIR),)

It can be seen from the initial comments ,u-boot It supports the storage of source code directories and compiled directories separately , But the manufacturer seems to give up this function when porting , No matter how I tried, I failed , So I won't talk about it here , Just for completeness .

Several important variables

OBJTREE		:= $(if $(BUILD_DIR),$(BUILD_DIR),$(CURDIR))
SRCTREE		:= $(CURDIR)
TOPDIR		:= $(SRCTREE)
LNDIR		:= $(OBJTREE)
export	TOPDIR SRCTREE OBJTREE

MKCONFIG	:= $(SRCTREE)/mkconfig
export MKCONFIG
#$(info OBJTREE = $(OBJTREE))
#$(info SRCTREE = $(SRCTREE))
#$(error )

ifneq ($(OBJTREE),$(SRCTREE))
REMOTE_BUILD	:= 1
export REMOTE_BUILD
endif
# $(obj) and (src) are defined in config.mk but here in main Makefile
# we also need them before config.mk is included which is the case for
# some targets like unconfig, clean, clobber, distclean, etc.
ifneq ($(OBJTREE),$(SRCTREE))
obj := $(OBJTREE)/
src := $(SRCTREE)/
else
obj :=
src :=
endif
export obj src

• The above code will generate the compiled file storage root directory (OBJTREE) Source code root directory in the future (SRCTREE). Because we didn't set up different directories , So the values of both are the same , After uncommenting, you will see the running result .
• Set up MKCONFIG Variable , Define mkconfig file
• because OBJTREE Value and SRCTREE Same value , be obj,src And REMOTE_BUILD All three variables are empty .

Makefile The analysis is stuck

ifeq ($(obj)include/config.mk,$(wildcard $(obj)include/config.mk))

• stay u-boot The top-level directory uses the command make distclean Delete all generated files .
• And then use make Will find the newspaper System not configured - see README error . This is because there is no such config.mk The document created .
• In our uboot Environment construction and compilation In the article ,make Only before make x210_sd_config command , therefore config.mk The file is generated after this command .
• Next, we analyze the generation process of the file .