103 lines
3.1 KiB
Text
103 lines
3.1 KiB
Text
|
Smart CONFIG_* Dependencies
|
||
|
|
1 August 1999
|
||
|
|
|
||
|
|
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to be out of date, which will cause drivers/net/foo.c to be recompiled.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 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.
|
||
|
|
|
||
|
|
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 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).
|
||
|
|
|
||
|
|
Per-source-file Flags
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
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. 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.
|