LXR linux/Documentation/smart-config.txt

   1Smart CONFIG_* Dependencies
   21 August 1999
   3
   4Michael Chastain   <mec@shout.net>
   5Werner Almesberger <almesber@lrc.di.epfl.ch>
   6Martin von Loewis  <martin@mira.isdn.cs.tu-berlin.de>
   7
   8Here is the problem:
   9
  10    Suppose that drivers/net/foo.c has the following lines:
  11
  12        #include <linux/config.h>
  13
  14        ...
  15
  16        #ifdef CONFIG_FOO_AUTOFROB
  17            /* Code for auto-frobbing */
  18        #else
  19            /* Manual frobbing only */
  20        #endif
  21
  22        ...
  23
  24        #ifdef CONFIG_FOO_MODEL_TWO
  25            /* Code for model two */
  26        #endif
  27
  28    Now suppose the user (the person building kernels) reconfigures the
  29    kernel to change some unrelated setting.  This will regenerate the
  30    file include/linux/autoconf.h, which will cause include/linux/config.h
  31    to be out of date, which will cause drivers/net/foo.c to be recompiled.
  32
  33    Most kernel sources, perhaps 80% of them, have at least one CONFIG_*
  34    dependency somewhere.  So changing _any_ CONFIG_* setting requires
  35    almost _all_ of the kernel to be recompiled.
  36
  37Here is the solution:
  38
  39    We've made the dependency generator, mkdep.c, smarter.  Instead of
  40    generating this dependency:
  41
  42        drivers/net/foo.c: include/linux/config.h
  43
  44    It now generates these dependencies:
  45
  46        drivers/net/foo.c: \
  47            include/config/foo/autofrob.h \
  48            include/config/foo/model/two.h
  49
  50    So drivers/net/foo.c depends only on the CONFIG_* lines that
  51    it actually uses.
  52
  53    A new program, split-include.c, runs at the beginning of
  54    compilation (make bzImage or make zImage).  split-include reads
  55    include/linux/autoconf.h and updates the include/config/ tree,
  56    writing one file per option.  It updates only the files for options
  57    that have changed.
  58
  59Flag Dependencies
  60
  61    Martin Von Loewis contributed another feature to this patch:
  62    'flag dependencies'.  The idea is that a .o file depends on
  63    the compilation flags used to build it.  The file foo.o has
  64    its flags stored in .flags.foo.o.
  65
  66    Suppose the user changes the foo driver from resident to modular.
  67    'make' will notice that the current foo.o was not compiled with
  68    -DMODULE and will recompile foo.c.
  69
  70    All .o files made from C source have flag dependencies.  So do .o
  71    files made with ld, and .a files made with ar.  However, .o files
  72    made from assembly source do not have flag dependencies (nobody
  73    needs this yet, but it would be good to fix).
  74
  75Per-source-file Flags
  76
  77    Flag dependencies also work with per-source-file flags.
  78    You can specify compilation flags for individual source files
  79    like this:
  80
  81        CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
  82
  83    This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
  84    and several other Makefiles.
  85
  86Credit
  87
  88    Werner Almesberger had the original idea and wrote the first
  89    version of this patch.
  90    
  91    Michael Chastain picked it up and continued development.  He is
  92    now the principal author and maintainer.  Please report any bugs
  93    to him.
  94
  95    Martin von Loewis wrote flag dependencies, with some modifications
  96    by Michael Chastain.
  97
  98    Thanks to all of the beta testers.
  99