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31    to be out of date, which will cause drivers/net/foo.c to be recompiled.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
59    mkdep.c no longer generates warning messages for missing or unneeded
60    <linux/config.h> lines.  The new top-level target 'make checkconfig'
61    checks for these problems.
62
63Flag Dependencies
64
65    Martin Von Loewis contributed another feature to this patch:
66    'flag dependencies'.  The idea is that a .o file depends on
67    the compilation flags used to build it.  The file foo.o has
68    its flags stored in .flags.foo.o.
69
70    Suppose the user changes the foo driver from resident to modular.
71    'make' will notice that the current foo.o was not compiled with
72    -DMODULE and will recompile foo.c.
73
74    All .o files made from C source have flag dependencies.  So do .o
75    files made with ld, and .a files made with ar.  However, .o files
76    made from assembly source do not have flag dependencies (nobody
77    needs this yet, but it would be good to fix).
78
79Per-source-file Flags
80
81    Flag dependencies also work with per-source-file flags.
82    You can specify compilation flags for individual source files
83    like this:
84
85	CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
86
87    This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
88    and several other Makefiles.
89
90Credit
91
92    Werner Almesberger had the original idea and wrote the first
93    version of this patch.
94    
95    Michael Chastain picked it up and continued development.  He is
96    now the principal author and maintainer.  Please report any bugs
97    to him.
98
99    Martin von Loewis wrote flag dependencies, with some modifications
100    by Michael Chastain.
101
102    Thanks to all of the beta testers.
103