How does kbuild actually work?

When i’m developing a linux driver, i’ve read about how to write linux kbuild makefile through this document

I know kbuild system use makefile variables such as obj-y obj-m to determine what to build and how to build.

But what i’m confused about is where does kbuild system really execute build process.In a word, if i have obj-m = a.o, then where does kbuild system parse obj-m and execute gcc a.c ?

Answer

Kbuild’s Makefiles aren’t the easiest to read, but here’s a high-level untangling (using the 4.0-rc3 kernel):

1. The top-level Makefile does

   include $(srctree)/scripts/Kbuild.include
   

   , where $(srctree) is the top-level kernel directory.

2. Kbuild.include defines various common stuff and helpers. Among these is build:

   ###
   # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=
   # Usage:
   # $(Q)$(MAKE) $(build)=dir
   build := -f $(srctree)/scripts/Makefile.build obj
   

   build is used with a command like $(MAKE) $(build)=dir to perform the build for the directory dir. It makes use of scripts/Makefile.build.

3. Returning to the top-level Makefile, there’s the following:

   $(vmlinux-dirs): prepare scripts
           $(Q)$(MAKE) $(build)=$@
   

   vmlinux-dirs contains a list of subdirectories to build (init, usr, kernel, etc.). $(Q)$(MAKE) $(build)=<subdirectory> will be run for each subdirectory.

   The rule above compiles object files for both the kernel image and modules. Further down in the top-level Makefile, there’s some additional module-specific stuff:

   ifdef CONFIG_MODULES
   ...
   modules: $(vmlinux-dirs) $(if $(KBUILD_BUILTIN),vmlinux) modules.builtin
           # Do additional module-specific stuff using
           # scripts/Makefile.modpost among other things
           # (my comment).
           ...
   ...
   endif # CONFIG_MODULES
   

4. Looking into scripts/Makefile.build (the Makefile used by $(build)) now, it begins by initializing the obj-* lists and various other lists:

   # Init all relevant variables used in kbuild files so
   # 1) they have correct type
   # 2) they do not inherit any value from the environment
   obj-y :=
   obj-m :=
   lib-y :=
   lib-m :=
   

   A bit further down, it loads in the Kbuild file where obj-y, obj-m, etc., are set:

   include $(kbuild-file)
   

   Further down is the default rule, which has the $(obj-y) and $(obj-m) lists as prerequisites:

   __build: $(if $(KBUILD_BUILTIN),$(builtin-target) $(lib-target) $(extra-y)) 
            $(if $(KBUILD_MODULES),$(obj-m) $(modorder-target)) 
            $(subdir-ym) $(always)
           @:
   

   The $(obj-y) prerequisites come from $(builtin-target), which is defined as follows:

   builtin-target := $(obj)/built-in.o
   ...
   $(builtin-target): $(obj-y) FORCE
           $(call if_changed,link_o_target)
   

   The actual building seems to be performed by the following rule:

   # Built-in and composite module parts
   $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
           $(call cmd,force_checksrc)
           $(call if_changed_rule,cc_o_c)
   

   if_changed_rule is from Kbuild.include. The rule ends up running the following commands in Makefile.build:

   define rule_cc_o_c
           $(call echo-cmd,checksrc) $(cmd_checksrc)                         
           $(call echo-cmd,cc_o_c) $(cmd_cc_o_c);                            
           ...
   endef
   

   $(cmd_cc_o_c) seems to be the actual compilation command. The usual definition (there are two possibilities in Makefile.build, AFAICS) seems to be the following:

   cmd_cc_o_c = $(CC) $(c_flags) -c -o $@ $<
   

   Unless set explicitly using e.g. make CC=clang, CC defaults to gcc, as can be seen here in the top-level Makefile:

   ifneq ($(CC),)
   ifeq ($(shell $(CC) -v 2>&1 | grep -c "clang version"), 1)
   COMPILER := clang
   else
   COMPILER := gcc
   endif
   export COMPILER
   endif
   

The way I untangled this was by doing a CTRL-C during a kernel build and seeing where make reported the error. Another handy make debugging technique is to use $(warning $(variable)) to print the value of variable.