Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
1 Smart CONFIG_* Dependencies 2 1 August 1999 3 4 Michael Chastain <mec[AT]shout[DOT]net> 5 Werner Almesberger <almesber[AT]lrc.di.epfl[DOT]ch> 6 Martin von Loewis <martin[AT]mira.isdn.cs.tu-berlin[DOT]de> 7 8 Here 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 This will regenerate the 30 file include/linux/autoconf.h, which will cause include/linux/config.h 31 to be out of date, which will cause drivers/net/foo.c to be recompiled. 32 33 Most kernel sources, perhaps 80% of them, have at least one CONFIG_* 34 dependency somewhere.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 37 Here 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 Flag 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 75 Per-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 86 Credit 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.
