Symbols From Another External Module

转载说明：来自Linux内核的：Documentation/kbuild/modules.txt

说明怎么正确在模块A中使用模块B的符号。（模块A使用模块B提供的符号)

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.

注：使用最后一种方式比较方便，如果有多个模块的信赖，在不同的Module.symvers之前使用空格来区别。