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5 <chapt><heading>Adding New Kernel Configuration Options<label id="kernelopts"></heading>
7 <p><em>Contributed by &a.joerg;</em>
9 <em/Note:/ You should be familiar with the section about <ref
10 id="kernelconfig" name="kernel configuration"> before reading here.
12 <sect><heading>What's a <em>kernel option</em>, anyway?</heading>
14 <p>The use of kernel options is basically described in the <ref
15 id="kernelconfig:options" name="kernel configuration"> section.
16 There's also an explanation about ``historic'' and ``new-style''
17 options. The ultimate goal is to eventually turn all the supported
18 options in the kernel into new-style ones, so for people who
19 correctly did a <tt/make depend/ in their kernel compile directory
20 after running <tt/config(8)/, the build process will automatically
21 pick up modified options, and only recompile those files where it is
22 necessary. Wiping out the old compile directory on each run of
23 <tt/config(8)/ as it is still done now can then be eliminated again.
25 <p>Basically, a kernel option is nothing else than the definition of
26 a C preprocessor macro for the kernel compilation process. To make
27 the build truly optional, the corresponding part of the kernel
28 source (or kernel <tt/.h/ file) must be written with the option
29 concept in mind, i. e. the default must have been made overridable
30 by the config option. This is usually done with something like:
34 #define THIS_OPTION (some_default_value)
35 #endif /* THIS_OPTION */
37 <p>This way, an administrator mentioning another value for the
38 option in his config file will take the default out of effect, and
39 replace it with his new value. Apparently, the new value will be
40 substituted into the source code during the preprocessor run, so it
41 must be a valid C expression in whatever context the default value
44 <p>It is also possible to create value-less options that simply
45 enable or disable a particular piece of code by embracing it in
54 <p>Simply mentioning <tt/THAT_OPTION/ in the config file (with or
55 without any value) will then turn on the corresponding piece of
58 <p>People familiar with the C language will immediately recognize
59 that everything could be counted as a ``config option'' where
60 there is at least a single <tt/#ifdef/ referencing it... Now only
61 few people probably would try to say
66 <p>in their config file however, and watch the kernel compilation
69 <p>Apparently, using arbitrary names for the options makes it very
70 hard to track their usage throughout the kernel source tree. That is
71 the rationale behind the <em/new-style/ option scheme, where each
72 option goes into a separate <tt/.h/ file in the kernel compile
73 directory, which is by convention named <tt>opt_<em>foo</em>.h</tt>.
74 This way, the usual Makefile dependencies could be applied, and
75 <tt/make/ can determine what needs to be recompiled once an option
78 <p>The old-style option mechanism still has one advantage for local
79 options or maybe experimental options that have a short anticipated
80 lifetime: since it is easy to add a new <tt/#ifdef/ to the kernel
81 source, this has already made it a kernel config option.
82 In this case, the administrator using such an
83 option is responsible himself for knowing about its implications
84 (and maybe manually forcing the recompilation of parts of his
85 kernel). Once the transition of all supported options has been
86 done, <tt/config(8)/ will warn whenever an unsupported option
87 appears in the config file, but it will nevertheless include it into
91 <sect><heading>Now what do I have to do for it?</heading>
93 <p>First, edit <tt>sys/conf/options</tt> (or
94 <tt>sys/i386/conf/options.<em><arch></em></tt>, e. g.
95 <tt>sys/i386/conf/options.i386</tt>), and select an
96 <tt>opt_<em>foo</em>.h</tt> file where your new option would best go
99 <p>If there is already something that comes close to the purpose of
100 the new option, pick this. For example, options modifying the
101 overall behaviour of the SCSI subsystem can go into <tt/opt_scsi.h/.
102 By default, simply mentioning an option in the appropriate option
103 file, say <tt/FOO/, implies its value will go into the
104 corresponding file <tt/opt_foo.h/. This can be overridden on the
105 right-hand side of a rule by specifying another filename.
107 <p>If there is no <tt>opt_<em>foo</em>.h</tt> already available for
108 the intended new option, invent a new name. Make it meaningful, and
109 comment the new section in the
110 <tt>options[<em>.<arch></em>]</tt> file. <tt/config(8)/ will
111 automagically pick up the change, and create that file next time it
112 is run. Most options should go in a header file by themselves..
114 <p>Packing too many options into a single
115 <tt>opt_<em>foo</em>.h</tt> will cause too many kernel files to be
116 rebuilt when one of the options has been changed in the config file.
118 <p>Finally, find out which kernel files depend on the new option.
119 Unless you have just invented your option, and it does not exist
123 find /usr/src/sys -name type f | xargs fgrep NEW_OPTION
125 <p>is your friend in finding them. Go and edit all those files, and
131 <p><em>on top</em>, before all the <tt/#include <xxx.h>/
132 stuff. The sequence is most important in case the options will
133 override some defaults from the regular include files, where the
134 defaults are protected by
138 #define NEW_OPTION (something)
141 <p>in the regular header.
143 <p>Adding an option that overrides something in a system header file
144 (i. e., a file sitting in <tt>/usr/include/sys/</tt>) is almost
145 always a mistake. <tt>opt_<em>foo</em>.h</tt> cannot be included
146 into those files since it would break the headers more seriously,
147 but if it is not included, then places that include it may get an
148 inconsistent value for the option. Yes, there are precedents for
149 this right now, but that does not make them more correct.