1 Smart CONFIG_* Dependencies
4 Michael Chastain <mec@shout.net>
5 Werner Almesberger <almesber@lrc.di.epfl.ch>
6 Martin von Loewis <martin@mira.isdn.cs.tu-berlin.de>
10 Suppose that drivers/net/foo.c has the following lines:
12 #include <linux/config.h>
16 #ifdef CONFIG_FOO_AUTOFROB
17 /* Code for auto-frobbing */
19 /* Manual frobbing only */
24 #ifdef CONFIG_FOO_MODEL_TWO
25 /* Code for model two */
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31 to be out of date, which will cause drivers/net/foo.c to be recompiled.
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.
39 We've made the dependency generator, mkdep.c, smarter. Instead of
40 generating this dependency:
42 drivers/net/foo.c: include/linux/config.h
44 It now generates these dependencies:
47 include/config/foo_autofrob.h \
48 include/config/foo_model_two.h
50 So drivers/net/foo.c depends only on the CONFIG_* lines that
53 A new program, split-include.c, runs at the end of make config (also
54 make oldconfig, make menuconfig, and make xconfig). split-include
55 reads include/linux/autoconf.h and updates the include/linux/*.h
56 directory, writing one file per option. It updates only the files
59 mkdep.c also generates much better warning messages for missing
60 or unneeded <linux/config.h> lines. In fact, you can get these
61 messages without generating dependencies with the new top-level
62 target 'make checkconfig'.
66 Martin Von Loewis contributed another feature to this patch:
67 'flag dependencies'. The idea is that a .o file depends on
68 the compilation flags used to build it. The file foo.o has
69 its flags stored in .flags.foo.o.
71 Suppose the user changes the foo driver from resident to
72 modular, 'make' will notice that the foo.o was not compiled
73 with -DMODULE and will recompile foo.c.
75 All .a and .o files made from C source or with 'ld' or 'ar'
76 have flag dependencies. .S files do not have flag dependencies.
80 Flag dependencies also work with per-source-file flags.
81 You can specify compilation flags for individual source files
84 CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
86 This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
87 and several other Makefiles.
91 Werner Almesberger had the original idea and wrote the first
92 version of this patch.
94 Michael Chastain picked it up and continued development. He is
95 now the principal author and maintainer. Report bugs to him,
96 or to all three people together.
98 Martin von Loewis wrote flag dependencies, with some modifications
101 Thanks to all of the beta testers.