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|
Smart CONFIG_* Dependencies
Fri 2 Dec 1997
Michael Chastain <mec@shout.net>
Werner Almesberger <almesber@lrc.di.epfl.ch>
Martin von Loewis <martin@mira.isdn.cs.tu-berlin.de>
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 */
#else
/* Manual frobbing only */
#endif
...
#ifdef CONFIG_FOO_MODEL_TWO
/* Code for model two */
#endif
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to be out of date, which will cause drivers/net/foo.c to be recompiled.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 \
include/config/foo_model_two.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 end of make config (also
make oldconfig, make menuconfig, and make xconfig). split-include
reads include/linux/autoconf.h and updates the include/linux/*.h
directory, writing one file per option. It updates only the files
that changed.
mkdep.c also generates much better warning messages for missing
or unneeded <linux/config.h> lines. In fact, you can get these
messages without generating dependencies with the new top-level
target 'make checkconfig'.
Flag Dependencies
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 foo.o was not compiled
with -DMODULE and will recompile foo.c.
Flag dependencies also work with per-source-file flags such
as those in drivers/net/CONFIG.
All .a and .o files made from C source or with 'ld' or 'ar'
have flag dependencies. .S files do not have flag dependencies.
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.
Credit
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. Report bugs to him,
or to all three people together.
Martin von Loewis wrote flag dependencies, with some modifications
by Michael Chastain.
Thanks to all of the beta testers.
|
