kernel makefile

--- 3.2 Built-in object goals - obj-y

     The kbuild Makefile specifies object files for vmlinux          vmlinux中的目标文件包含在$(obj-y) lists 中，
     in the $(obj-y) lists.  These lists depend on the kernel         $(obj-？) lists 中的目标是否编译进内核取决于.config。

     configuration.

     Kbuild compiles all the $(obj-y) files.  It then calls               Kbuild会编译所有的 $(obj-y) files ，并用"$(LD) -r" 将各级子目录
     "$(LD) -r" to merge these files into one built-in.o file.          下的built-in.o 连接成顶层目录下的一个built-in.o ，
     built-in.o is later linked into vmlinux by the parent Makefile. 上一级目录的Makefile将负责打包子目录的各个built-in.o，依此类推 

     The order of files in $(obj-y) is significant.  Duplicates in     $(obj-y) 中的文件是有一定次序的。列表中允许重复的目标，
     the lists are allowed: the first instance will be linked into          最先遇到的目标将被链接进built-in.o，之后的重复目标将被忽略。
     built-in.o and succeeding instances will be ignored.

     Link order is significant, because certain functions               链接也是有序的。在boot过程中，有些函数将根据其出现的次序被
     (module_init() / __initcall) will be called during boot in the     依次调用。 (如module_init() / __initcall) 
     order they appear. So keep in mind that changing the link     改变链接次序可能导致SCSI控制器的次序，将会使磁盘
     order may e.g. change the order in which your SCSI               被重新编号。
     controllers are detected, and thus your disks are renumbered.

     Example:
          #drivers/isdn/i4l/Makefile
          # Makefile for the kernel ISDN subsystem and device drivers.
          # Each configuration option enables a list of files.
          obj-$(CONFIG_ISDN_I4L)         += isdn.o
          obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o

 

--- 3.3 Loadable module goals - obj-m

     $(obj-m) specify object files which are built as loadable                $(obj-m) 指定的目标文件将被编译成可加载的内核模块
     kernel modules.

     A module may be built from one source file or several source          仅有一个源文件的内核模块的简单Makefile的写法
     files. In the case of one source file, the kbuild makefile
     simply adds the file to $(obj-m).

     Example:
          #drivers/isdn/i4l/Makefile
          obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o

     Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to ‘m‘

     If a kernel module is built from several source files, you specify               当一个内核模块是由多个目标文件构成时，需要通过
     that you want to build a module in the same way as above; however,      $(<module_name>-y)变量设置需要编译的目标文件。 
     kbuild needs to know which object files you want to build your
     module from, so you have to tell it by setting a $(<module_name>-y)
     variable.

     Example:
          #drivers/isdn/i4l/Makefile
          obj-$(CONFIG_ISDN_I4L) += isdn.o
          isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o

     In this example, the module name will be isdn.o. Kbuild will                    在这个例子中，模块名是isdn.o，Kbuild将编译
     compile the objects listed in $(isdn-y) and then run                         $(isdn-y)中的对象，然后执行"$(LD) -r"将这些目标文件 
     "$(LD) -r" on the list of these files to generate isdn.o.                         链接成isdn.o。

     Due to kbuild recognizing $(<module_name>-y) for composite objects,
     you can use the value of a CONFIG_ symbol to optionally include an
     object file as part of a composite object.

     Example:
          #fs/ext2/Makefile
             obj-$(CONFIG_EXT2_FS) += ext2.o
          ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
                 namei.o super.o symlink.o
             ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
                              xattr_trusted.o

     In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
     part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
     evaluates to ‘y‘.

     Note: Of course, when you are building objects into the kernel,
     the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
     kbuild will build an ext2.o file for you out of the individual
     parts and then link this into built-in.o, as you would expect.

 

--- 3.4 Objects which export symbols

     No special notation is required in the makefiles for
     modules exporting symbols.

 

--- 3.5 Library file goals - lib-y

     Objects listed with obj-* are used for modules, or
     combined in a built-in.o for that specific directory.
     There is also the possibility to list objects that will
     be included in a library, lib.a.
     All objects listed with lib-y are combined in a single          lib-y中的目标将被组合成一个lib.a库，放在当前目录下
     library for that directory.
     Objects that are listed in obj-y and additionally listed in     既在obj-y 列表中又在lib-y 列表中的目标将不会被包含在库中
     lib-y will not be included in the library, since they will          因为obj-y中的目标总能访问到。
     be accessible anyway.
     For consistency, objects listed in lib-m will be included in lib.a.     lib-m 中的目标会包含在lib.a中。

     Note that the same kbuild makefile may list files to be built-in
     and to be part of a library. Therefore the same directory
     may contain both a built-in.o and a lib.a file.

     Example:
          #arch/x86/lib/Makefile
          lib-y    := delay.o

     This will create a library lib.a based on delay.o. For kbuild to
     actually recognize that there is a lib.a being built, the directory
     shall be listed in libs-y.
     See also "6.4 List directories to visit when descending".

     Use of lib-y is normally restricted to lib/ and arch/*/lib.

 

--- 3.6 Descending down in directories

     A Makefile is only responsible for building objects in its own
     directory. Files in subdirectories should be taken care of by
     Makefiles in these subdirs. The build system will automatically
     invoke make recursively in subdirectories, provided you let it know of
     them.

     To do so, obj-y and obj-m are used.
     ext2 lives in a separate directory, and the Makefile present in fs/
     tells kbuild to descend down using the following assignment.                            fs

                                                                                                                        |-- Makefile
     Example:                                                                                                      `-- ext2
          #fs/Makefile                                                                                                 |-- Makefile
          obj-$(CONFIG_EXT2_FS) += ext2/                                                                 |-- *

     If CONFIG_EXT2_FS is set to either ‘y‘ (built-in) or ‘m‘ (modular)
     the corresponding obj- variable will be set, and kbuild will descend
     down in the ext2 directory.
     Kbuild only uses this information to decide that it needs to visit                   Kbuild只是根据该信息判断要访问子目录，
     the directory, it is the Makefile in the subdirectory that                                至于在子目录下做什么，有子目录的Makefile决定。
     specifies what is modules and what is built-in.

     It is good practice to use a CONFIG_ variable when assigning directory
     names. This allows kbuild to totally skip the directory if the
     corresponding CONFIG_ option is neither ‘y‘ nor ‘m‘.