Smart CONFIG_* Dependencies
Fri 2 Dec 1997

Michael Chastain   <mec@shout.net>
Werner Almesberger <almesber@lrc.di.epfl.ch>
Martin von Loewis  <martin@mira.isdn.cs.tu-berlin.de>

Here is the problem:

    Suppose that drivers/net/foo.c has the following lines:

	#include <linux/config.h>

	...

	#ifdef CONFIG_FOO_AUTOFROB
	    /* Code for auto-frobbing */
	#else
	    /* Manual frobbing only */
	#endif

	...

	#ifdef CONFIG_FOO_MODEL_TWO
	    /* Code for model two */
	#endif

    Now suppose the user (the person building kernels) reconfigures the
    kernel to change some unrelated setting.  This will regenerate the
    file include/linux/autoconf.h, which will cause include/linux/config.h
    to be out of date, which will cause drivers/net/foo.c to be recompiled.

    Most kernel sources, perhaps 80% of them, have at least one CONFIG_*
    dependency somewhere.  So changing _any_ CONFIG_* setting requires
    almost _all_ of the kernel to be recompiled.

Here is the solution:

    We've made the dependency generator, mkdep.c, smarter.  Instead of
    generating this dependency:

	drivers/net/foo.c: include/linux/config.h

    It now generates these dependencies:

	drivers/net/foo.c: \
	    include/config/foo_autofrob.h \
	    include/config/foo_model_two.h

    So drivers/net/foo.c depends only on the CONFIG_* lines that
    it actually uses.

    A new program, split-include.c, runs at the end of make config (also
    make oldconfig, make menuconfig, and make xconfig).  split-include
    reads include/linux/autoconf.h and updates the include/linux/*.h
    directory, writing one file per option.  It updates only the files
    that changed.

    mkdep.c also generates much better warning messages for missing
    or unneeded <linux/config.h> lines.  In fact, you can get these
    messages without generating dependencies with the new top-level
    target 'make checkconfig'.

Flag Dependencies

    Martin Von Loewis contributed another feature to this patch:
    'flag dependencies'.  The idea is that a .o file depends on
    the compilation flags used to build it.  The file foo.o has
    its flags stored in .flags.foo.o.

    Suppose the user changes the foo driver from resident to
    modular, 'make' will notice that the foo.o was not compiled
    with -DMODULE and will recompile foo.c.

    Flag dependencies also work with per-source-file flags such
    as those in drivers/net/CONFIG.

    All .a and .o files made from C source or with 'ld' or 'ar'
    have flag dependencies.  .S files do not have flag dependencies.

Per-source-file Flags

    You can specify compilation flags for individual source files
    like this:

	CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE

    This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
    and several other Makefiles.

Credit

    Werner Almesberger had the original idea and wrote the first
    version of this patch.
    
    Michael Chastain picked it up and continued development.  He is
    now the principal author and maintainer.  Report bugs to him,
    or to all three people together.

    Martin von Loewis wrote flag dependencies, with some modifications
    by Michael Chastain.

    Thanks to all of the beta testers.