99 lines
3 KiB
Text
99 lines
3 KiB
Text
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Smart CONFIG_* Dependencies
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1 August 1999
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Michael Chastain <mec@shout.net>
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Werner Almesberger <almesber@lrc.di.epfl.ch>
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Martin von Loewis <martin@mira.isdn.cs.tu-berlin.de>
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Here is the problem:
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Suppose that drivers/net/foo.c has the following lines:
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#include <linux/config.h>
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...
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#ifdef CONFIG_FOO_AUTOFROB
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/* Code for auto-frobbing */
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#else
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/* Manual frobbing only */
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#endif
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...
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#ifdef CONFIG_FOO_MODEL_TWO
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/* Code for model two */
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#endif
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Now suppose the user (the person building kernels) reconfigures the
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kernel to change some unrelated setting. This will regenerate the
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file include/linux/autoconf.h, which will cause include/linux/config.h
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to be out of date, which will cause drivers/net/foo.c to be recompiled.
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Most kernel sources, perhaps 80% of them, have at least one CONFIG_*
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dependency somewhere. So changing _any_ CONFIG_* setting requires
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almost _all_ of the kernel to be recompiled.
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Here is the solution:
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We've made the dependency generator, mkdep.c, smarter. Instead of
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generating this dependency:
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drivers/net/foo.c: include/linux/config.h
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It now generates these dependencies:
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drivers/net/foo.c: \
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include/config/foo/autofrob.h \
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include/config/foo/model/two.h
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So drivers/net/foo.c depends only on the CONFIG_* lines that
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it actually uses.
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A new program, split-include.c, runs at the beginning of
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compilation (make bzImage or make zImage). split-include reads
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include/linux/autoconf.h and updates the include/config/ tree,
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writing one file per option. It updates only the files for options
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that have changed.
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Flag Dependencies
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Martin Von Loewis contributed another feature to this patch:
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'flag dependencies'. The idea is that a .o file depends on
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the compilation flags used to build it. The file foo.o has
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its flags stored in .flags.foo.o.
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Suppose the user changes the foo driver from resident to modular.
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'make' will notice that the current foo.o was not compiled with
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-DMODULE and will recompile foo.c.
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All .o files made from C source have flag dependencies. So do .o
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files made with ld, and .a files made with ar. However, .o files
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made from assembly source do not have flag dependencies (nobody
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needs this yet, but it would be good to fix).
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Per-source-file Flags
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Flag dependencies also work with per-source-file flags.
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You can specify compilation flags for individual source files
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like this:
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CFLAGS_foo.o = -DSPECIAL_FOO_DEFINE
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This helps clean up drivers/net/Makefile, drivers/scsi/Makefile,
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and several other Makefiles.
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Credit
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Werner Almesberger had the original idea and wrote the first
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version of this patch.
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Michael Chastain picked it up and continued development. He is
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now the principal author and maintainer. Please report any bugs
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to him.
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Martin von Loewis wrote flag dependencies, with some modifications
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by Michael Chastain.
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Thanks to all of the beta testers.
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