KBuild question

[Tom Bart <tbartcz@gmail.com>]

Hello,
I have spend some time analysing KBuild source files. However there is
one thing that is not clear for me. In the main makefile I have found
the following statement:

Most importantly: sub-Makefiles should only ever modify files in
their own directory. If in some directory we have a dependency on
a file in another dir (which doesn't happen often, but it's often
unavoidable when linking the built-in.o targets which finally
turn into vmlinux), we will call a sub make in that other dir, and
after that we are sure that everything which is in that other dir
is now up to date.

Kernel is build in a recursive manner. I understand that according to
the citation firstly all dependencies between different directories
are identified and then the sub-directory walk through process is
started. Unfortunately I cannot fine where this functionality is
implemented? Can somebody point me the right place or maybe my line of
reasoning is simply wrong?
Regards
Tomasz Bartczak