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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31 to be out of date, which will cause drivers/net/foo.c to be recompiled.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.