| 1 |
28 |
unneback |
<!-- Copyright (C) 2003 Red Hat, Inc. -->
|
| 2 |
|
|
<!-- This material may be distributed only subject to the terms -->
|
| 3 |
|
|
<!-- and conditions set forth in the Open Publication License, v1.0 -->
|
| 4 |
|
|
<!-- or later (the latest version is presently available at -->
|
| 5 |
|
|
<!-- http://www.opencontent.org/openpub/). -->
|
| 6 |
|
|
<!-- Distribution of the work or derivative of the work in any -->
|
| 7 |
|
|
<!-- standard (paper) book form is prohibited unless prior -->
|
| 8 |
|
|
<!-- permission is obtained from the copyright holder. -->
|
| 9 |
|
|
<HTML
|
| 10 |
|
|
><HEAD
|
| 11 |
|
|
><TITLE
|
| 12 |
|
|
>Interfaces</TITLE
|
| 13 |
|
|
><meta name="MSSmartTagsPreventParsing" content="TRUE">
|
| 14 |
|
|
<META
|
| 15 |
|
|
NAME="GENERATOR"
|
| 16 |
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
|
| 17 |
|
|
"><LINK
|
| 18 |
|
|
REL="HOME"
|
| 19 |
|
|
TITLE="The eCos Component Writer's Guide"
|
| 20 |
|
|
HREF="cdl-guide.html"><LINK
|
| 21 |
|
|
REL="UP"
|
| 22 |
|
|
TITLE="The CDL Language"
|
| 23 |
|
|
HREF="language.html"><LINK
|
| 24 |
|
|
REL="PREVIOUS"
|
| 25 |
|
|
TITLE="Values and Expressions"
|
| 26 |
|
|
HREF="language.values.html"><LINK
|
| 27 |
|
|
REL="NEXT"
|
| 28 |
|
|
TITLE="Updating the ecos.db database"
|
| 29 |
|
|
HREF="language.database.html"></HEAD
|
| 30 |
|
|
><BODY
|
| 31 |
|
|
CLASS="SECT1"
|
| 32 |
|
|
BGCOLOR="#FFFFFF"
|
| 33 |
|
|
TEXT="#000000"
|
| 34 |
|
|
LINK="#0000FF"
|
| 35 |
|
|
VLINK="#840084"
|
| 36 |
|
|
ALINK="#0000FF"
|
| 37 |
|
|
><DIV
|
| 38 |
|
|
CLASS="NAVHEADER"
|
| 39 |
|
|
><TABLE
|
| 40 |
|
|
SUMMARY="Header navigation table"
|
| 41 |
|
|
WIDTH="100%"
|
| 42 |
|
|
BORDER="0"
|
| 43 |
|
|
CELLPADDING="0"
|
| 44 |
|
|
CELLSPACING="0"
|
| 45 |
|
|
><TR
|
| 46 |
|
|
><TH
|
| 47 |
|
|
COLSPAN="3"
|
| 48 |
|
|
ALIGN="center"
|
| 49 |
|
|
>The <SPAN
|
| 50 |
|
|
CLASS="APPLICATION"
|
| 51 |
|
|
>eCos</SPAN
|
| 52 |
|
|
> Component Writer's Guide</TH
|
| 53 |
|
|
></TR
|
| 54 |
|
|
><TR
|
| 55 |
|
|
><TD
|
| 56 |
|
|
WIDTH="10%"
|
| 57 |
|
|
ALIGN="left"
|
| 58 |
|
|
VALIGN="bottom"
|
| 59 |
|
|
><A
|
| 60 |
|
|
HREF="language.values.html"
|
| 61 |
|
|
ACCESSKEY="P"
|
| 62 |
|
|
>Prev</A
|
| 63 |
|
|
></TD
|
| 64 |
|
|
><TD
|
| 65 |
|
|
WIDTH="80%"
|
| 66 |
|
|
ALIGN="center"
|
| 67 |
|
|
VALIGN="bottom"
|
| 68 |
|
|
>Chapter 3. The CDL Language</TD
|
| 69 |
|
|
><TD
|
| 70 |
|
|
WIDTH="10%"
|
| 71 |
|
|
ALIGN="right"
|
| 72 |
|
|
VALIGN="bottom"
|
| 73 |
|
|
><A
|
| 74 |
|
|
HREF="language.database.html"
|
| 75 |
|
|
ACCESSKEY="N"
|
| 76 |
|
|
>Next</A
|
| 77 |
|
|
></TD
|
| 78 |
|
|
></TR
|
| 79 |
|
|
></TABLE
|
| 80 |
|
|
><HR
|
| 81 |
|
|
ALIGN="LEFT"
|
| 82 |
|
|
WIDTH="100%"></DIV
|
| 83 |
|
|
><DIV
|
| 84 |
|
|
CLASS="SECT1"
|
| 85 |
|
|
><H1
|
| 86 |
|
|
CLASS="SECT1"
|
| 87 |
|
|
><A
|
| 88 |
|
|
NAME="LANGUAGE.INTERFACE">Interfaces</H1
|
| 89 |
|
|
><P
|
| 90 |
|
|
>For many configurability requirements, options provide sufficient
|
| 91 |
|
|
expressive power. However there are times when a higher level of
|
| 92 |
|
|
abstraction is appropriate. As an example, suppose that some package
|
| 93 |
|
|
relies on the presence of code that implements the standard kernel
|
| 94 |
|
|
scheduling interface. However the requirement is no more stringent
|
| 95 |
|
|
than this, so the constraint can be satisfied by the mlqueue
|
| 96 |
|
|
scheduler, the bitmap scheduler, or any additional schedulers that may
|
| 97 |
|
|
get implemented in future. A first attempt at expressing the
|
| 98 |
|
|
dependency might be:</P
|
| 99 |
|
|
><TABLE
|
| 100 |
|
|
BORDER="5"
|
| 101 |
|
|
BGCOLOR="#E0E0F0"
|
| 102 |
|
|
WIDTH="70%"
|
| 103 |
|
|
><TR
|
| 104 |
|
|
><TD
|
| 105 |
|
|
><PRE
|
| 106 |
|
|
CLASS="PROGRAMLISTING"
|
| 107 |
|
|
> requires CYGSEM_KERNEL_SCHED_MLQUEUE || CYGSEM_KERNEL_SCHED_BITMAP</PRE
|
| 108 |
|
|
></TD
|
| 109 |
|
|
></TR
|
| 110 |
|
|
></TABLE
|
| 111 |
|
|
><P
|
| 112 |
|
|
>This constraint will work with the current release, but it is limited.
|
| 113 |
|
|
Suppose there is a new release of the kernel which adds another
|
| 114 |
|
|
scheduler such as a deadline scheduler, or suppose that there is a new
|
| 115 |
|
|
third party package which adds such a scheduler. The package
|
| 116 |
|
|
containing the limited constraint would now have to be updated and
|
| 117 |
|
|
another release made, with possible knock-on effects.</P
|
| 118 |
|
|
><P
|
| 119 |
|
|
><SPAN
|
| 120 |
|
|
CLASS="APPLICATION"
|
| 121 |
|
|
>CDL</SPAN
|
| 122 |
|
|
> interfaces provide an abstraction mechanism: constraints can be
|
| 123 |
|
|
expressed in terms of an abstract concept, for example
|
| 124 |
|
|
“scheduler”, rather than specific implementations such as
|
| 125 |
|
|
<TT
|
| 126 |
|
|
CLASS="VARNAME"
|
| 127 |
|
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
| 128 |
|
|
> and
|
| 129 |
|
|
<TT
|
| 130 |
|
|
CLASS="VARNAME"
|
| 131 |
|
|
>CYGSEM_KERNEL_SCHED_BITMAP</TT
|
| 132 |
|
|
>. Basically an interface
|
| 133 |
|
|
is a calculated configuration option:</P
|
| 134 |
|
|
><TABLE
|
| 135 |
|
|
BORDER="5"
|
| 136 |
|
|
BGCOLOR="#E0E0F0"
|
| 137 |
|
|
WIDTH="70%"
|
| 138 |
|
|
><TR
|
| 139 |
|
|
><TD
|
| 140 |
|
|
><PRE
|
| 141 |
|
|
CLASS="PROGRAMLISTING"
|
| 142 |
|
|
>cdl_interface CYGINT_KERNEL_SCHEDULER {
|
| 143 |
|
|
display "Number of schedulers in this configuration"
|
| 144 |
|
|
…
|
| 145 |
|
|
}</PRE
|
| 146 |
|
|
></TD
|
| 147 |
|
|
></TR
|
| 148 |
|
|
></TABLE
|
| 149 |
|
|
><P
|
| 150 |
|
|
>The individual schedulers can then implement this interface:</P
|
| 151 |
|
|
><TABLE
|
| 152 |
|
|
BORDER="5"
|
| 153 |
|
|
BGCOLOR="#E0E0F0"
|
| 154 |
|
|
WIDTH="70%"
|
| 155 |
|
|
><TR
|
| 156 |
|
|
><TD
|
| 157 |
|
|
><PRE
|
| 158 |
|
|
CLASS="PROGRAMLISTING"
|
| 159 |
|
|
>cdl_option CYGSEM_KERNEL_SCHED_MLQUEUE {
|
| 160 |
|
|
display "Multi-level queue scheduler"
|
| 161 |
|
|
default_value 1
|
| 162 |
|
|
implements CYGINT_KERNEL_SCHEDULER
|
| 163 |
|
|
…
|
| 164 |
|
|
}
|
| 165 |
|
|
|
| 166 |
|
|
cdl_option CYGSEM_KERNEL_SCHED_BITMAP {
|
| 167 |
|
|
display "Bitmap scheduler"
|
| 168 |
|
|
default_value 0
|
| 169 |
|
|
implements CYGINT_KERNEL_SCHEDULER
|
| 170 |
|
|
…
|
| 171 |
|
|
}</PRE
|
| 172 |
|
|
></TD
|
| 173 |
|
|
></TR
|
| 174 |
|
|
></TABLE
|
| 175 |
|
|
><P
|
| 176 |
|
|
>Future schedulers can also implement this interface. The value of an
|
| 177 |
|
|
interface, for the purposes of expression evaluation, is the number of
|
| 178 |
|
|
active and enabled options which implement this interface. Packages
|
| 179 |
|
|
which rely on the presence of a scheduler can impose constraints such
|
| 180 |
|
|
as:</P
|
| 181 |
|
|
><TABLE
|
| 182 |
|
|
BORDER="5"
|
| 183 |
|
|
BGCOLOR="#E0E0F0"
|
| 184 |
|
|
WIDTH="70%"
|
| 185 |
|
|
><TR
|
| 186 |
|
|
><TD
|
| 187 |
|
|
><PRE
|
| 188 |
|
|
CLASS="PROGRAMLISTING"
|
| 189 |
|
|
> requires CYGINT_KERNEL_SCHEDULER</PRE
|
| 190 |
|
|
></TD
|
| 191 |
|
|
></TR
|
| 192 |
|
|
></TABLE
|
| 193 |
|
|
><P
|
| 194 |
|
|
>If none of the schedulers are enabled, or if the kernel package is not
|
| 195 |
|
|
loaded, then <TT
|
| 196 |
|
|
CLASS="VARNAME"
|
| 197 |
|
|
>CYGINT_KERNEL_SCHEDULER</TT
|
| 198 |
|
|
> will evaluate
|
| 199 |
|
|
to <TT
|
| 200 |
|
|
CLASS="LITERAL"
|
| 201 |
|
|
>0</TT
|
| 202 |
|
|
>. If at least one scheduler is active and
|
| 203 |
|
|
enabled then the constraint will be satisfied.</P
|
| 204 |
|
|
><P
|
| 205 |
|
|
>Because interfaces have a calculated value determined by the
|
| 206 |
|
|
implementors, the <SPAN
|
| 207 |
|
|
CLASS="PROPERTY"
|
| 208 |
|
|
>default_value</SPAN
|
| 209 |
|
|
> and <SPAN
|
| 210 |
|
|
CLASS="PROPERTY"
|
| 211 |
|
|
>calculated</SPAN
|
| 212 |
|
|
> properties are not
|
| 213 |
|
|
applicable and should not appear in the body of a <TT
|
| 214 |
|
|
CLASS="LITERAL"
|
| 215 |
|
|
>cdl_interface</TT
|
| 216 |
|
|
>
|
| 217 |
|
|
command. Interfaces have the <TT
|
| 218 |
|
|
CLASS="LITERAL"
|
| 219 |
|
|
>data</TT
|
| 220 |
|
|
> flavor by
|
| 221 |
|
|
default, but the <TT
|
| 222 |
|
|
CLASS="LITERAL"
|
| 223 |
|
|
>bool</TT
|
| 224 |
|
|
> and
|
| 225 |
|
|
<TT
|
| 226 |
|
|
CLASS="LITERAL"
|
| 227 |
|
|
>booldata</TT
|
| 228 |
|
|
> flavors may be specified instead. A
|
| 229 |
|
|
<TT
|
| 230 |
|
|
CLASS="LITERAL"
|
| 231 |
|
|
>bool</TT
|
| 232 |
|
|
> interface is disabled if there are no active
|
| 233 |
|
|
and enabled implementors, otherwise it is enabled. A
|
| 234 |
|
|
<TT
|
| 235 |
|
|
CLASS="LITERAL"
|
| 236 |
|
|
>booldata</TT
|
| 237 |
|
|
> interface is disabled if there are no
|
| 238 |
|
|
active and enabled implementors, otherwise it is enabled and has a
|
| 239 |
|
|
value corresponding to the number of these implementors. Other
|
| 240 |
|
|
properties such as <SPAN
|
| 241 |
|
|
CLASS="PROPERTY"
|
| 242 |
|
|
>requires</SPAN
|
| 243 |
|
|
> and <SPAN
|
| 244 |
|
|
CLASS="PROPERTY"
|
| 245 |
|
|
>compile</SPAN
|
| 246 |
|
|
> can be used as normal.</P
|
| 247 |
|
|
><P
|
| 248 |
|
|
>Some component writers will not want to use interfaces in this way.
|
| 249 |
|
|
The reasoning is that their code will only have been tested with the
|
| 250 |
|
|
existing schedulers, so the <SPAN
|
| 251 |
|
|
CLASS="PROPERTY"
|
| 252 |
|
|
>requires</SPAN
|
| 253 |
|
|
> constraint needs to be
|
| 254 |
|
|
expressed in terms of those schedulers; it is possible that the
|
| 255 |
|
|
component will still work with a new scheduler, but there are no
|
| 256 |
|
|
guarantees. Other component writers may take a more optimistic view
|
| 257 |
|
|
and assume that their code will work with any scheduler until proven
|
| 258 |
|
|
otherwise. It is up to individual component writers to decide which
|
| 259 |
|
|
approach is most appropriate in any given case.</P
|
| 260 |
|
|
><P
|
| 261 |
|
|
>One common use for interfaces is to describe the hardware
|
| 262 |
|
|
functionality provided by a given target. For example the <SPAN
|
| 263 |
|
|
CLASS="APPLICATION"
|
| 264 |
|
|
>CDL</SPAN
|
| 265 |
|
|
>
|
| 266 |
|
|
scripts for a TCP/IP package might want to know whether or not the
|
| 267 |
|
|
target hardware has an ethernet interface. Generally it is not
|
| 268 |
|
|
necessary for the TCP/IP stack to know exactly which ethernet hardware
|
| 269 |
|
|
is present, since there should be a device driver which implements the
|
| 270 |
|
|
appropriate functionality. In <SPAN
|
| 271 |
|
|
CLASS="APPLICATION"
|
| 272 |
|
|
>CDL</SPAN
|
| 273 |
|
|
> terms the device drivers should
|
| 274 |
|
|
implement an interface <TT
|
| 275 |
|
|
CLASS="VARNAME"
|
| 276 |
|
|
>CYGHWR_NET_DRIVERS</TT
|
| 277 |
|
|
>, and the
|
| 278 |
|
|
<SPAN
|
| 279 |
|
|
CLASS="APPLICATION"
|
| 280 |
|
|
>CDL</SPAN
|
| 281 |
|
|
> scripts for the TCP/IP stack can use this in appropriate
|
| 282 |
|
|
expressions. </P
|
| 283 |
|
|
><DIV
|
| 284 |
|
|
CLASS="NOTE"
|
| 285 |
|
|
><BLOCKQUOTE
|
| 286 |
|
|
CLASS="NOTE"
|
| 287 |
|
|
><P
|
| 288 |
|
|
><B
|
| 289 |
|
|
>Note: </B
|
| 290 |
|
|
>Using the term <SPAN
|
| 291 |
|
|
CLASS="emphasis"
|
| 292 |
|
|
><I
|
| 293 |
|
|
CLASS="EMPHASIS"
|
| 294 |
|
|
>interface</I
|
| 295 |
|
|
></SPAN
|
| 296 |
|
|
> for this concept is
|
| 297 |
|
|
sometimes confusing, since the term has various other meanings as
|
| 298 |
|
|
well. In practice, it is often correct. If there is a configuration
|
| 299 |
|
|
option that implements a given <SPAN
|
| 300 |
|
|
CLASS="APPLICATION"
|
| 301 |
|
|
>CDL</SPAN
|
| 302 |
|
|
> interface, then usually this
|
| 303 |
|
|
option will enable some code that provides a particular interface at
|
| 304 |
|
|
the C or C++ level. For example an ethernet device driver implements
|
| 305 |
|
|
the <SPAN
|
| 306 |
|
|
CLASS="APPLICATION"
|
| 307 |
|
|
>CDL</SPAN
|
| 308 |
|
|
> interface <TT
|
| 309 |
|
|
CLASS="VARNAME"
|
| 310 |
|
|
>CYGHWR_NET_DRIVERS</TT
|
| 311 |
|
|
>, and also
|
| 312 |
|
|
implements a set of C functions that can be used by the TCP/IP stack.
|
| 313 |
|
|
Similarly <TT
|
| 314 |
|
|
CLASS="VARNAME"
|
| 315 |
|
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
| 316 |
|
|
> implements
|
| 317 |
|
|
the <SPAN
|
| 318 |
|
|
CLASS="APPLICATION"
|
| 319 |
|
|
>CDL</SPAN
|
| 320 |
|
|
> interface <TT
|
| 321 |
|
|
CLASS="VARNAME"
|
| 322 |
|
|
>CYGINT_KERNEL_SCHEDULER</TT
|
| 323 |
|
|
> and
|
| 324 |
|
|
also provides the appropriate scheduling functions.</P
|
| 325 |
|
|
></BLOCKQUOTE
|
| 326 |
|
|
></DIV
|
| 327 |
|
|
></DIV
|
| 328 |
|
|
><DIV
|
| 329 |
|
|
CLASS="NAVFOOTER"
|
| 330 |
|
|
><HR
|
| 331 |
|
|
ALIGN="LEFT"
|
| 332 |
|
|
WIDTH="100%"><TABLE
|
| 333 |
|
|
SUMMARY="Footer navigation table"
|
| 334 |
|
|
WIDTH="100%"
|
| 335 |
|
|
BORDER="0"
|
| 336 |
|
|
CELLPADDING="0"
|
| 337 |
|
|
CELLSPACING="0"
|
| 338 |
|
|
><TR
|
| 339 |
|
|
><TD
|
| 340 |
|
|
WIDTH="33%"
|
| 341 |
|
|
ALIGN="left"
|
| 342 |
|
|
VALIGN="top"
|
| 343 |
|
|
><A
|
| 344 |
|
|
HREF="language.values.html"
|
| 345 |
|
|
ACCESSKEY="P"
|
| 346 |
|
|
>Prev</A
|
| 347 |
|
|
></TD
|
| 348 |
|
|
><TD
|
| 349 |
|
|
WIDTH="34%"
|
| 350 |
|
|
ALIGN="center"
|
| 351 |
|
|
VALIGN="top"
|
| 352 |
|
|
><A
|
| 353 |
|
|
HREF="cdl-guide.html"
|
| 354 |
|
|
ACCESSKEY="H"
|
| 355 |
|
|
>Home</A
|
| 356 |
|
|
></TD
|
| 357 |
|
|
><TD
|
| 358 |
|
|
WIDTH="33%"
|
| 359 |
|
|
ALIGN="right"
|
| 360 |
|
|
VALIGN="top"
|
| 361 |
|
|
><A
|
| 362 |
|
|
HREF="language.database.html"
|
| 363 |
|
|
ACCESSKEY="N"
|
| 364 |
|
|
>Next</A
|
| 365 |
|
|
></TD
|
| 366 |
|
|
></TR
|
| 367 |
|
|
><TR
|
| 368 |
|
|
><TD
|
| 369 |
|
|
WIDTH="33%"
|
| 370 |
|
|
ALIGN="left"
|
| 371 |
|
|
VALIGN="top"
|
| 372 |
|
|
>Values and Expressions</TD
|
| 373 |
|
|
><TD
|
| 374 |
|
|
WIDTH="34%"
|
| 375 |
|
|
ALIGN="center"
|
| 376 |
|
|
VALIGN="top"
|
| 377 |
|
|
><A
|
| 378 |
|
|
HREF="language.html"
|
| 379 |
|
|
ACCESSKEY="U"
|
| 380 |
|
|
>Up</A
|
| 381 |
|
|
></TD
|
| 382 |
|
|
><TD
|
| 383 |
|
|
WIDTH="33%"
|
| 384 |
|
|
ALIGN="right"
|
| 385 |
|
|
VALIGN="top"
|
| 386 |
|
|
>Updating the <SPAN
|
| 387 |
|
|
CLASS="DATABASE"
|
| 388 |
|
|
>ecos.db</SPAN
|
| 389 |
|
|
> database</TD
|
| 390 |
|
|
></TR
|
| 391 |
|
|
></TABLE
|
| 392 |
|
|
></DIV
|
| 393 |
|
|
></BODY
|
| 394 |
|
|
></HTML
|
| 395 |
|
|
>
|
