执行make menuconfig，开启了devtmpfs，然后重新编译内核，发现基本上整个内核重编了。以为是自己的Make写的有问题，开启编译verbose分析如下：
make V=1
显示完整的编译命令，比如：
/home/yuan/toolchain/crosstool-ipq4018/bin/arm-linux-gnueabihf-gcc -Wp,-MD,kernel/power/.qos.o.d -nostdinc -isystem /home/yuan/toolchain/crosstool-ipq4018/lib/gcc/arm-linux-gnueabihf/8.3.0/include -I/work/git/Xrouter/staging/source/linux-4.14.120/arch/arm/include -I./arch/arm/include/generated -I/work/git/Xrouter/staging/source/linux-4.14.120/include -I./include -I/work/git/Xrouter/staging/source/linux-4.14.120/arch/arm/include/uapi -I./arch/arm/include/generated/uapi -I/work/git/Xrouter/staging/source/linux-4.14.120/include/uapi -I./include/generated/uapi -include /work/git/Xrouter/staging/source/linux-4.14.120/include/linux/kconfig.h -I/work/git/Xrouter/staging/source/linux-4.14.120/kernel/power -Ikernel/power -D__KERNEL__ -mlittle-endian -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -Werror-implicit-function-declaration -Wno-format-security -std=gnu89 -fno-PIE -fno-dwarf2-cfi-asm -fno-ipa-sra -mabi=aapcs-linux -mfpu=vfp -funwind-tables -marm -D__LINUX_ARM_ARCH__=7 -march=armv7-a -msoft-float -Uarm -fno-delete-null-pointer-checks -Wno-frame-address -Wno-format-truncation -Wno-format-overflow -Wno-int-in-bool-context -Wno-attribute-alias -O2 -fno-reorder-blocks -fno-tree-ch --param=allow-store-data-races=0 -DCC_HAVE_ASM_GOTO -Wframe-larger-than=1024 -fstack-protector -Wno-unused-but-set-variable -Wno-unused-const-variable -fomit-frame-pointer -fno-var-tracking-assignments -g -femit-struct-debug-baseonly -fno-var-tracking -Wdeclaration-after-statement -Wno-pointer-sign -Wno-stringop-truncation -fno-strict-overflow -fno-merge-all-constants -fmerge-constants -fno-stack-check -fconserve-stack -Werror=implicit-int -Werror=strict-prototypes -Werror=date-time -Werror=incompatible-pointer-types -Werror=designated-init -Wno-packed-not-aligned -DKBUILD_BASENAME='"qos"' -DKBUILD_MODNAME='"qos"' -c -o kernel/power/qos.o /work/git/Xrouter/staging/source/linux-4.14.120/kernel/power/qos.c
make V=2
显示是哪个文件更新导致重编，比如
CC kernel/cpu.o - due to: include/config/devtmpfs.h
还可以单独编译某个目录或文件，直接跟上路径即可，比如
make drivers/mtd/ubi/vtbl.o
参考：
https://www.kernel.org/doc/makehelp.txt