KBUILD_CFLAGS += $(call cc-option,-Wno-format-nonliteral)
# Prohibit date/time macros, which would make the build non-deterministic
KBUILD_CFLAGS += $(call cc-option,-Werror=date-time)
# Report stack usage if supported
ifeq ($(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-stack-usage.sh $(CC)),y)
KBUILD_CFLAGS += -fstack-usage
endif
#(1) 顶层config.mk
KBUILD_CFLAGS += $(KCFLAGS)
展开为：
KBUILD_CFLAGS=-Wall -Wstrict-prototypes -Wno-format-security -fno-builtin -ffreestanding -fshort-wchar -Os -fno-stack-protector -fno-delete-null-pointer-checks -g -fstack-usage -Wno-format-nonliteral -Werror=date-time
参数讲解见本文末尾参考小节。
7. 预处理需要的编译选项$(cpp_flags)
#(1) 顶层config.mk
# Use UBOOTINCLUDE when you must reference the include/ directory.
# Needed to be compatible with the O= option
UBOOTINCLUDE := \
-Iinclude \
$(if $(KBUILD_SRC), -I$(srctree)/include) \
$(if $(CONFIG_$(SPL_)SYS_THUMB_BUILD), \
$(if $(CONFIG_HAS_THUMB2),, \
-I$(srctree)/arch/$(ARCH)/thumb1/include),) \
-I$(srctree)/arch/$(ARCH)/include \ //-I./arch/arm/include
-include $(srctree)/include/linux/kconfig.h //-include ./include/linux/kconfig.h
......
NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
// $(CC) -print-file-name=include : 打印GCC默认搜索include头文件的路径
......
# FIX ME
cpp_flags := $(KBUILD_CPPFLAGS) $(PLATFORM_CPPFLAGS) $(UBOOTINCLUDE) $(NOSTDINC_FLAGS)
预处理编译选项$(cpp_flags）定义在顶层Makefile中，主要由：编译系统kbuild需要的预处理选项 $(KBUILD_CPPFLAGS)、平台代码预处理需要的编译选项 $(PLATFORM_CPPFLAGS)、UBOOT要使用的头文件路径、CC默认搜索include头文件的路径组成。
展开为：
cpp_flags=-D__KERNEL__ -D__UBOOT__ -D__ARM__ -fno-pic -mstrict-align -ffunction-sections -fdata-sections -fno-common -ffixed-r9 -fno-common -ffixed-x18 -pipe -march=armv8-a -mgeneral-regs-only -D__LINUX_ARM_ARCH__=8 -Iinclude -I./arch/arm/include -include ./include/linux/kconfig.h -nostdinc -isystem /home/h/my-work/03_toolchain/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu/bin/../lib/gcc/aarch64-linux-gnu/7.3.1/include
8. 编译需要的编译选项$(c_flags)
#(1) 顶层config.mk
c_flags := $(KBUILD_CFLAGS) $(cpp_flags)
编译选项$(c_flags）定义在顶层Makefile中，主要由 $(KBUILD_CFLAGS) （编译系统kbuild需要的编译选项）和 $(cpp_flags)（uboot整体代码预处理需要的编译选项）组成。
展开为：
c_flags=-Wall -Wstrict-prototypes -Wno-format-security -fno-builtin -ffreestanding -fshort-wchar -Os -fno-stack-protector -fno-delete-null-pointer-checks -g -fstack-usage -Wno-format-nonliteral -Werror=date-time -D__KERNEL__ -D__UBOOT__ -D__ARM__ -fno-pic -mstrict-align -ffunction-sections -fdata-sections -fno-common -ffixed-r9 -fno-common -ffixed-x18 -pipe -march=armv8-a -mgeneral-regs-only -D__LINUX_ARM_ARCH__=8 -Iinclude -I./arch/arm/include -include ./include/linux/kconfig.h -nostdinc -isystem /home/h/my-work/03_toolchain/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu/bin/../lib/gcc/aarch64-linux-gnu/7.3.1/include
参数讲解见本文末尾参考小节。
9. 链接需要的编译选项$(LDFLAGS_FINAL)
#(1) 顶层config.mk
LDFLAGS_FINAL :=
......
LDFLAGS_FINAL += -Bstatic
export LDFLAGS_FINAL
#(2) arch/arm/config.mk
LDFLAGS_FINAL += --gc-sections
展开为：
LDFLAGS_FINAL= --gc-sections -Bstatic //指定从-L指定的目录列表中查找libfoo.a
参数讲解见本文末尾参考小节。
10. 参考
[1] -Wall：打开全部编译告警；
[2] -Wstrict-prototypes：如果在未指定参数类型的情况下声明或定义函数，则发出警告。 （如果前面有一个指定参数类型的声明，则允许在没有警告的情