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<!-- $Id: kernelopts.sgml,v 1.4 1997/01/02 17:00:26 max Exp $ -->
<!-- The FreeBSD Documentation Project -->
<!-- <!DOCTYPE linuxdoc PUBLIC '-//FreeBSD//DTD linuxdoc//EN'> -->
<chapt><heading>Adding New Kernel Configuration Options<label id="kernelopts"></heading>
<p><em>Contributed by &a.joerg;</em>
<em/Note:/ You should be familiar with the section about <ref
id="kernelconfig" name="kernel configuration"> before reading here.
<sect><heading>What's a <em>kernel option</em>, anyway?</heading>
<p>The use of kernel options is basically described in the <ref
id="kernelconfig:options" name="kernel configuration"> section.
There's also an explanation about ``historic'' and ``new-style''
options. The ultimate goal is to eventually turn all the supported
options in the kernel into new-style ones, so for people who
correctly did a <tt/make depend/ in their kernel compile directory
after running <tt/config(8)/, the build process will automatically
pick up modified options, and only recompile those files where it is
necessary. Wiping out the old compile directory on each run of
<tt/config(8)/ as it is still done now can then be eliminated again.
<p>Basically, a kernel option is nothing else than the definition of
a C preprocessor macro for the kernel compilation process. To make
the build truly optional, the corresponding part of the kernel
source (or kernel <tt/.h/ file) must be written with the option
concept in mind, i. e. the default must have been made overridable
by the config option. This is usually done with something like:
<verb>
#ifndef THIS_OPTION
#define THIS_OPTION (some_default_value)
#endif /* THIS_OPTION */
</verb>
<p>This way, an administrator mentioning another value for the
option in his config file will take the default out of effect, and
replace it with his new value. Apparently, the new value will be
substituted into the source code during the preprocessor run, so it
must be a valid C expression in whatever context the default value
would have been used.
<p>It is also possible to create value-less options that simply
enable or disable a particular piece of code by embracing it in
<verb>
#ifdef THAT_OPTION
[your code here]
#endif
</verb>
<p>Simply mentioning <tt/THAT_OPTION/ in the config file (with or
without any value) will then turn on the corresponding piece of
code.
<p>People familiar with the C language will immediately recognize
that everything could be counted as a ``config option'' where
there is at least a single <tt/#ifdef/ referencing it... Now only
few people probably would try to say
<verb>
options notyet,notdef
</verb>
<p>in their config file however, and watch the kernel compilation
fall over. :-)
<p>Apparently, using arbitrary names for the options makes it very
hard to track their usage throughout the kernel source tree. That is
the rationale behind the <em/new-style/ option scheme, where each
option goes into a separate <tt/.h/ file in the kernel compile
directory, which is by convention named <tt>opt_<em>foo</em>.h</tt>.
This way, the usual Makefile dependencies could be applied, and
<tt/make/ can determine what needs to be recompiled once an option
has been changed.
<p>The old-style option mechanism still has one advantage for local
options or maybe experimental options that have a short anticipated
lifetime: since it is easy to add a new <tt/#ifdef/ to the kernel
source, this has already made it a kernel config option.
In this case, the administrator using such an
option is responsible himself for knowing about its implications
(and maybe manually forcing the recompilation of parts of his
kernel). Once the transition of all supported options has been
done, <tt/config(8)/ will warn whenever an unsupported option
appears in the config file, but it will nevertheless include it into
the kernel Makefile.
<sect><heading>Now what do I have to do for it?</heading>
<p>First, edit <tt>sys/conf/options</tt> (or
<tt>sys/i386/conf/options.<em><arch></em></tt>, e. g.
<tt>sys/i386/conf/options.i386</tt>), and select an
<tt>opt_<em>foo</em>.h</tt> file where your new option would best go
into.
<p>If there is already something that comes close to the purpose of
the new option, pick this. For example, options modifying the
overall behaviour of the SCSI subsystem can go into <tt/opt_scsi.h/.
By default, simply mentioning an option in the appropriate option
file, say <tt/FOO/, implies its value will go into the
corresponding file <tt/opt_foo.h/. This can be overridden on the
right-hand side of a rule by specifying another filename.
<p>If there is no <tt>opt_<em>foo</em>.h</tt> already available for
the intended new option, invent a new name. Make it meaningful, and
comment the new section in the
<tt>options[<em>.<arch></em>]</tt> file. <tt/config(8)/ will
automagically pick up the change, and create that file next time it
is run. Most options should go in a header file by themselves..
<p>Packing too many options into a single
<tt>opt_<em>foo</em>.h</tt> will cause too many kernel files to be
rebuilt when one of the options has been changed in the config file.
<p>Finally, find out which kernel files depend on the new option.
Unless you have just invented your option, and it does not exist
anywhere yet,
<verb>
find /usr/src/sys -name type f | xargs fgrep NEW_OPTION
</verb>
<p>is your friend in finding them. Go and edit all those files, and
add
<verb>
#include "opt_foo.h"
</verb>
<p><em>on top</em>, before all the <tt/#include <xxx.h>/
stuff. The sequence is most important in case the options will
override some defaults from the regular include files, where the
defaults are protected by
<verb>
#ifndef NEW_OPTION
#define NEW_OPTION (something)
#endif
</verb>
<p>in the regular header.
<p>Adding an option that overrides something in a system header file
(i. e., a file sitting in <tt>/usr/include/sys/</tt>) is almost
always a mistake. <tt>opt_<em>foo</em>.h</tt> cannot be included
into those files since it would break the headers more seriously,
but if it is not included, then places that include it may get an
inconsistent value for the option. Yes, there are precedents for
this right now, but that does not make them more correct.
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