在Linux内核里，编译内核文件时，先要配置.config文件，然后Makefile在编译时通过读取.config文件的配置来选择要编译的文件，选择驱动的加载方式。
 
defconfig 一般在arch/arm64/configs/目录下，是一个没有展开的内核配置，需要配合Kconfig展开成.config
从defconfig到.config不是简单的复制操作，而是make ARCH=arm64 defconfig
.confg也不是直接拷贝成defconfig，而是使用make ARCH=arm64 savedefconfig
 
 
正确使用和保存deconfig的流程：
1. 要修改在arch/arm/configs下的文件xxx_defconfig
2. make ARCH=arm64 xxx_defconfig 会生成.config文件
3. make ARCH=arm64 menuconfig 修改配置后保存
4. make ARCH=arm64 savedefconfig 生成defconfg文件
5. cp defconfig arch/arm/configs/xxx_defconfig 保存
这样保存的defconfig文件，配置最小化，且日后能恢复成.config。
 
 
.config
All config symbol values are saved in a special file called  .config . Every time you want to change a kernel compile configuration, you execute a make target, such as  menuconfig  or  xconfig . These read the  Kconfig  files to create the menus and update the config symbols' values using the values defined in the  .config  file. Additionally, these tools update the  .config  file with the new options you chose and also can generate one if it didn't exist before.
Because the  .config  file is plain text, you also can change it without needing any specialized tool. It is very convenient for saving and restoring previous kernel compilation configurations as well.
 
 
deconfig
The  .config  file is not simply copied from your  defconfig  file. The motivation for storing  defconfig  in such a format is next: in  defconfig  we can only specify options with non-default values (i.e. options we changed for our board). This way we can keep it small and clear. Every new kernel version brings a bunch of new options, and this way we don't need to update our  defconfig  file each time the kernel releases. Also, it should be mentioned that kernel build system keeps very specific order of options in  defconfig  file, so it's better to avoid modifying it by hand. Instead you should use  make savedefconfig  rule.
When  .config  file is being generated, kernel build system goes through all  Kconfig  files (from all subdirs), checking all options in those  Kconfig  files:
if option is mentioned in  defconfig , build system puts that option into  .config  with value chosen in  defconfig
if option isn't mentioned in  defconfig , build system puts that option into  .config  using its default value, specified in corresponding  Kconfig
根据上述描述，xxx_deconfig中只保存那些没有默认值的option（但被用户修改过的option除外，如config_xxx默认值为y，但是被用户修改为n，那么config_xxx将被保存进deconfig），因为有默认值的option保存在Kconfig中，没必要重复保存。