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 ?

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

The top-level Makefile does
```
include $(srctree)/scripts/Kbuild.include
```
, where $(srctree) is the top-level kernel directory.
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.
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
```

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.