1. 前言
UBOOT版本：uboot2018.03，开发板myimx8mmek240。
2. 概述
在编译uboot的过程中，有两个特别的依赖version_h 和 timestamp_h，它们定义在顶层Makefile中（这里只讲解编译直接在源目录的情况，即srctree 为空），如下：
# 顶层Makefile
version_h := include/generated/version_autogenerated.h
timestamp_h := include/generated/timestamp_autogenerated.h
......
$(version_h): include/config/uboot.release FORCE
$(call filechk,version.h)
$(timestamp_h): $(srctree)/Makefile FORCE
$(call filechk,timestamp.h)
3. 目标$(version_h)
# 顶层Makefile
$(version_h): include/config/uboot.release FORCE
$(call filechk,version.h)
3.1 $(version_h)依赖分析
3.1.1 依赖include/config/uboot.release
使用scripts/setlocalversion工具来生成include/config/uboot.release，只是一个版本号，不需要特别关注，参见LINUX 内核编译 LOCALVERSION 配置(分析内核版本号自动添加的"+"号)。
# 顶层Makefile
# Store (new) UBOOTRELEASE string in include/config/uboot.release
include/config/uboot.release: include/config/auto.conf FORCE
$(call filechk,uboot.release)
include/config/uboot.release定义在顶层Makefile中，该目标又依赖于include/config/auto.conf（参见UBOOT编译--- include/config/auto.conf、 include/config/auto.conf.cmd、 include/generated/autoconf.h (二)） 和 FORCE。
接下来我们看规则$(call filechk,uboot.release)：
#note：scripts/Kbuild.include
###
# filechk is used to check if the content of a generated file is updated.
# Sample usage:
# define filechk_sample
# echo $KERNELRELEASE
# endef
# version.h : Makefile
# $(call filechk,sample)
# The rule defined shall write to stdout the content of the new file.
# The existing file will be compared with the new one.
# - If no file exist it is created
# - If the content differ the new file is used
# - If they are equal no change, and no timestamp update
# - stdin is piped in from the first prerequisite ($<) so one has
# to specify a valid file as first prerequisite (often the kbuild file)
define filechk
$(Q)set -e; \
$(kecho) ' CHK $@'; \
mkdir -p $(dir $@); \
$(filechk_$(1)) < $< > $@.tmp; \
if [ -r $@ ] && cmp -s $@ $@.tmp; then \
rm -f $@.tmp; \
else \
$(kecho) ' UPD $@'; \
mv -f $@.tmp $@; \
fi
endef
上述代码中：
- ‘$@’ ：目标include/config/uboot.release ；
- ‘$(1)’ ：第一个参数uboot.release；
- ‘$ <’ ：第一个依赖include/config/auto.conf ；
- ‘filechk_$(1)’ ：filechk_uboot.release；
filechk_uboot.release定义在顶层Makefile中：
#note：顶层Makefile
# Read UBOOTRELEASE from include/config/uboot.release (if it exists)
UBOOTRELEASE = $(shell cat include/config/uboot.release 2> /dev/null)
UBOOTVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
......
define filechk_uboot.release
echo "$(UBOOTVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
endef
全部展开，具体编译命令如下：
set -e; : ' CHK include/config/uboot.release'; mkdir -p include/config/; echo "2018.03$(/bin/sh ./scripts/setlocalversion .)" < include/config/auto.conf > include/config/uboot.release.tmp; if [ -r include/config/uboot.release ] && cmp -s include/config/uboot.release include/config/uboot.release.tmp; then rm -f include/config/uboot.release.tmp; else : ' UPD include/config/uboot.release'; mv -f include/config/uboot.release.tmp include/config/uboot.release; fi
最终产生的include/config/uboot.release内容如下：
3.1.2 依赖FORCE
参见UBOOT编译--- make xxx_deconfig过程详解(一) 4.3小节 - 依赖 FORCE。