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+Smart CONFIG_* Dependencies
+1 August 1999
+Michael Chastain <email@example.com>
+Werner Almesberger <firstname.lastname@example.org>
+Martin von Loewis <email@example.com>
+Here is the problem:
+ Suppose that drivers/net/foo.c has the following lines:
+ #include <linux/config.h>
+ #ifdef CONFIG_FOO_AUTOFROB
+ /* Code for auto-frobbing */
+ /* Manual frobbing only */
+ #ifdef CONFIG_FOO_MODEL_TWO
+ /* Code for model two */
+ Now suppose the user (the person building kernels) reconfigures the
+ kernel to change some unrelated setting. This will regenerate the
+ file include/linux/autoconf.h, which will cause include/linux/config.h
+ to be out of date, which will cause drivers/net/foo.c to be recompiled.
+ Most kernel sources, perhaps 80% of them, have at least one CONFIG_*
+ dependency somewhere. So changing _any_ CONFIG_* setting requires
+ almost _all_ of the kernel to be recompiled.
+Here is the solution:
+ We've made the dependency generator, mkdep.c, smarter. Instead of
+ generating this dependency:
+ drivers/net/foo.c: include/linux/config.h
+ It now generates these dependencies:
+ drivers/net/foo.c: \
+ include/config/foo/autofrob.h \
+ So drivers/net/foo.c depends only on the CONFIG_* lines that
+ it actually uses.
+ A new program, split-include.c, runs at the beginning of
+ compilation (make bzImage or make zImage). split-include reads
+ include/linux/autoconf.h and updates the include/config/ tree,
+ writing one file per option. It updates only the files for options
+ that have changed.
+ mkdep.c no longer generates warning messages for missing or unneeded
+ <linux/config.h> lines. The new top-level target 'make checkconfig'
+ checks for these problems.
+ Martin Von Loewis contributed another feature to this patch:
+ 'flag dependencies'. The idea is that a .o file depends on
+ the compilation flags used to build it. The file foo.o has
+ its flags stored in .flags.foo.o.
+ Suppose the user changes the foo driver from resident to modular.
+ 'make' will notice that the current foo.o was not compiled with
+ -DMODULE and will recompile foo.c.
+ All .o files made from C source have flag dependencies. So do .o
+ files made with ld, and .a files made with ar. However, .o files
+ made from assembly source do not have flag dependencies (nobody
+ needs this yet, but it would be good to fix).
+ Flag dependencies also work with per-source-file flags.
+ You can specify compilation flags for individual source files
+ like this:
+ CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
+ This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
+ and several other Makefiles.
+ Werner Almesberger had the original idea and wrote the first
+ version of this patch.
+ Michael Chastain picked it up and continued development. He is
+ now the principal author and maintainer. Please report any bugs
+ to him.
+ Martin von Loewis wrote flag dependencies, with some modifications
+ by Michael Chastain.
+ Thanks to all of the beta testers.