FreeBSD/Linux Kernel Cross Reference
sys/Documentation/smart-config.txt

     Smart CONFIG_* Dependencies
     1 August 1999
     
     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        to be out of date, which will cause drivers/net/foo.c to be recompiled.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 beginning of
        compilation (make bzImage or make zImage).  split-include reads
        include/linux/autoconf.h and updates the include/config/ tree,
        writing one file per option.  It updates only the files for options
        that have changed.
    
    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 current foo.o was not compiled with
        -DMODULE and will recompile foo.c.
    
        All .o files made from C source have flag dependencies.  So do .o
        files made with ld, and .a files made with ar.  However, .o files
        made from assembly source do not have flag dependencies (nobody
        needs this yet, but it would be good to fix).
    
    Per-source-file Flags
    
        Flag dependencies also work with 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.  Please report any bugs
        to him.
    
        Martin von Loewis wrote flag dependencies, with some modifications
        by Michael Chastain.
    
        Thanks to all of the beta testers.

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