一个模块使用另一个模块导出的符号,该模块怎样编译?很经典!

6.3 Symbols From Another External Module

    Sometimes, an external module uses exported symbols from
    another external module. kbuild needs to have full knowledge of
    all symbols to avoid spitting out warnings about undefined
    symbols. Three solutions exist for this situation.

    NOTE: The method with a top-level kbuild file is recommended
    but may be impractical in certain situations.

    Use a top-level kbuild file
        If you have two modules, foo.ko and bar.ko, where
        foo.ko needs symbols from bar.ko, you can use a
        common top-level kbuild file so both modules are
        compiled in the same build. Consider the following
        directory layout:

        ./foo/ <= contains foo.ko
        ./bar/ <= contains bar.ko

        The top-level kbuild file would then look like:

        #./Kbuild (or ./Makefile):
            obj-y := foo/ bar/

        And executing

            $ make -C $KDIR M=$PWD

        will then do the expected and compile both modules with
        full knowledge of symbols from either module.

    Use an extra Module.symvers file
        When an external module is built, a Module.symvers file
        is generated containing all exported symbols which are
        not defined in the kernel. To get access to symbols
        from bar.ko, copy the Module.symvers file from the
        compilation of bar.ko to the directory where foo.ko is
        built. During the module build, kbuild will read the
        Module.symvers file in the directory of the external
        module, and when the build is finished, a new
        Module.symvers file is created containing the sum of
        all symbols defined and not part of the kernel.

    Use "make" variable KBUILD_EXTRA_SYMBOLS
        If it is impractical to copy Module.symvers from
        another module, you can assign a space separated list
        of files to KBUILD_EXTRA_SYMBOLS in your build file.
        These files will be loaded by modpost during the
        initialization of its symbol tables.


=== 7. Tips & Tricks

--- 7.1 Testing for CONFIG_FOO_BAR

    Modules often need to check for certain CONFIG_ options to
    decide if a specific feature is included in the module. In
    kbuild this is done by referencing the CONFIG_ variable
    directly.

        #fs/ext2/Makefile
        obj-$(CONFIG_EXT2_FS) += ext2.o

        ext2-y := balloc.o bitmap.o dir.o
        ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o

    External modules have traditionally used "grep" to check for
    specific CONFIG_ settings directly in .config. This usage is
    broken. As introduced before, external modules should use
    kbuild for building and can therefore use the same methods as
    in-tree modules when testing for CONFIG_ definitions.

你可能感兴趣的:(驱动开发)