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><H1
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><A
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NAME="POWER-CONTROLLER">Implementing a Power Controller</H1
83 
><DIV
84 
CLASS="REFNAMEDIV"
85 
><A
86 
NAME="AEN15936"
87 
></A
88 
><H2
89 
>Name</H2
90 
>Implementing a Power Controller&nbsp;--&nbsp;adding power management support to device drivers and
91 
other packages</DIV
92 
><DIV
93 
CLASS="REFSECT1"
94 
><A
95 
NAME="AEN15939"
96 
></A
97 
><H2
98 
>Implementing a Power Controller</H2
99 
><P
100 
>A system will have some number of power controllers. Usually there
101 
will be one power controller for the cpu,
102 
<TT
103 
CLASS="VARNAME"
104 
>power_controller_cpu</TT
105 
>, typically provided by one of
106 
the HAL packages and responsible for managing the processor itself and
107 
associated critical components such as memory. Some or all of the
108 
device drivers will provide power controllers, allowing the power
109 
consumption of the associated devices to be controlled. There may be
110 
some arbitrary number of other controllers present in the system. The
111 
power management package does not impose any restrictions on the
112 
number or nature of the power controllers in the system, other than
113 
insisting that at most one <TT
114 
CLASS="VARNAME"
115 
>power_controller_cpu</TT
116 
> be
117 
provided.</P
118 
><P
119 
>Each power controller involves a single data structure of type
120 
<SPAN
121 
CLASS="STRUCTNAME"
122 
>PowerController</SPAN
123 
>, defined in the header file
124 
<TT
125 
CLASS="FILENAME"
126 
>cyg/power/power.h</TT
127 
>. These data
128 
structures should all be placed in the table
129 
<TT
130 
CLASS="LITERAL"
131 
>__POWER__</TT
132 
>, so that the power management package and
133 
other code can easily locate all the controllers in the system. This
134 
table is constructed at link-time, avoiding code-size or run-time
135 
overheads. To facilitate this the package provides two macros which
136 
should be used to define a power controller,
137 
<TT
138 
CLASS="LITERAL"
139 
>POWER_CONTROLLER()</TT
140 
> and
141 
<TT
142 
CLASS="LITERAL"
143 
>POWER_CONTROLLER_CPU()</TT
144 
>.</P
145 
><P
146 
>The macro <TT
147 
CLASS="LITERAL"
148 
>POWER_CONTROLLER</TT
149 
> takes four arguments:</P
150 
><P
151 
></P
152 
><OL
153 
TYPE="1"
154 
><LI
155 
><P
156 
>A variable name. This can be used to access the power controller
157 
directly, as well as via the table.</P
158 
></LI
159 
><LI
160 
><P
161 
>A priority. The table of power controllers is sorted, such that power
162 
controllers with a numerically lower priority come earlier in the
163 
table. The special controller <TT
164 
CLASS="VARNAME"
165 
>power_controller_cpu</TT
166 
>
167 
always comes at the end of the table. When moving from a high-power
168 
mode to a lower-powered mode, the power management package iterates
169 
through the table from front to back. When moving to a higher-powered
170 
mode the reverse direction is used. The intention is that the power
171 
controller for a software-only package such as a TCP/IP stack should
172 
appear near the start of the table, whereas the controllers for the
173 
ethernet and similar devices would be near the end of the table. Hence
174 
when the policy module initiates a mode change to a lower-powered mode
175 
the TCP/IP stack gets a chance to cancel this mode change, before the
176 
devices it depends on are powered down. Similarly when moving to a
177 
higher-powered mode the devices will be re-activated before any
178 
software that depends on those devices.</P
179 
><P
180 
>The header file <TT
181 
CLASS="FILENAME"
182 
>cyg/power/power.h</TT
183 
> defines three
184 
priorities <TT
185 
CLASS="LITERAL"
186 
>PowerPri_Early</TT
187 
>,
188 
<TT
189 
CLASS="LITERAL"
190 
>PowerPri_Typical</TT
191 
> and
192 
<TT
193 
CLASS="LITERAL"
194 
>PowerPri_Late</TT
195 
>. For most controllers one of these
196 
priorities, possibly with a small number added or subtracted, will
197 
give sufficient control. If an application developer is uncertain
198 
about the relative priorities of the various controllers, a simple
199 
<A
200 
HREF="power-info.html#POWER-INFO-IDS"
201 
>test program</A
202 
> that iterates over
203 
the table will quickly eliminate any confusion.</P
204 
></LI
205 
><LI
206 
><P
207 
>A constant string identifier. If the system has been configured
208 
without support for such identifiers
209 
(<TT
210 
CLASS="VARNAME"
211 
>CYGIMP_POWER_PROVIDE_STRINGS</TT
212 
>) then this identifer
213 
will be discarded at compile-time. Otherwise it will be made available
214 
to higher-level code using the function
215 
<TT
216 
CLASS="FUNCTION"
217 
>power_get_controller_id</TT
218 
>. </P
219 
></LI
220 
><LI
221 
><P
222 
>A function pointer. This will be invoked to perform actual mode
223 
changes, as described below.</P
224 
></LI
225 
></OL
226 
><P
227 
>A typical example of the use of the
228 
<TT
229 
CLASS="LITERAL"
230 
>POWER_CONTROLLER</TT
231 
> macro would be as follows:</P
232 
><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
239 
CLASS="PROGRAMLISTING"
240 
>#include &lt;pkgconf/system.h&gt;
241 
 
242 
#ifdef CYGPKG_POWER
243 
# include &lt;cyg/power/power.h&gt;
244 
 
245 
static void
246 
xyzzy_device_power_mode_change(
247 
    PowerController* controller,
248 
    PowerMode        desired_mode,
249 
    PowerModeChange  change)
250 
{
251 
   // Do the work
252 
}
253 
 
254 
static POWER_CONTROLLER(xyzzy_power_controller, \
255 
                        PowerPri_Late,          \
256 
                        "xyzzy device",         \
257 
                        &amp;xyzzy_device_power_mode_change);
258 
#endif</PRE
259 
></TD
260 
></TR
261 
></TABLE
262 
><P
263 
>This creates a variable <TT
264 
CLASS="VARNAME"
265 
>xyzzy_power_controller</TT
266 
>,
267 
which is a power controller data structure that will end up near the
268 
end of the table of power controllers. Higher-level code can
269 
iterate through this table and report the string <TT
270 
CLASS="LITERAL"
271 
>"xyzzy
272 
device"</TT
273 
> to the user. Whenever there is a mode change
274 
operation that affects this controller, the function
275 
<TT
276 
CLASS="FUNCTION"
277 
>xyzzy_device_power_mode_change</TT
278 
> will be invoked.
279 
The variable is declared static so this controller cannot be
280 
manipulated by name in any other code. Alternatively, if the variable
281 
had not been declared static other code could manipulate this
282 
controller by name as well as through the table, especially if the
283 
package for the xyzzy device driver explicitly declared this
284 
variable in an exported header file. Obviously exporting the variable
285 
involves a slight risk of a name clash at link time.</P
286 
><P
287 
>The above code explicitly checks for the presence of the power
288 
management package before including that package's header file or
289 
providing any related functionality. Since power management
290 
functionality is optional, such checks are recommended.</P
291 
><P
292 
>The macro <TT
293 
CLASS="LITERAL"
294 
>POWER_CONTROLLER_CPU</TT
295 
> only takes two
296 
arguments, a string identifier and a mode change function pointer.
297 
This macro always instantiates a variable
298 
<TT
299 
CLASS="VARNAME"
300 
>power_controller_cpu</TT
301 
> so there is no need to provide
302 
a variable name. The resulting power controller structure always
303 
appears at the end of the table, so there is no need to specify a
304 
priority. Typical usage of the <TT
305 
CLASS="LITERAL"
306 
>POWER_CONTROLLER_CPU</TT
307 
>
308 
macro would be:</P
309 
><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
314 
><TD
315 
><PRE
316 
CLASS="PROGRAMLISTING"
317 
>static void
318 
wumpus_processor_power_mode_change(
319 
    PowerController* controller,
320 
    PowerMode        desired_mode,
321 
    PowerModeChange  change)
322 
{
323 
   // Do the work
324 
}
325 
 
326 
POWER_CONTROLLER_CPU("wumpus processor", \
327 
                     &amp;wumpus_processor_power_mode_change);</PRE
328 
></TD
329 
></TR
330 
></TABLE
331 
><P
332 
>This defines a power controller structure
333 
<TT
334 
CLASS="VARNAME"
335 
>power_controller_cpu</TT
336 
>. It should not be declared
337 
static since higher-level code may well want to manipulate the cpu's
338 
power mode directly, and the variable is declared by the power
339 
management package's header file.</P
340 
><P
341 
>Some care has to be taken to ensure that the power controllers
342 
actually end up in the final executable. If a power controller
343 
variable ends up in an ordinary library and is never referenced
344 
directly then typically the linker will believe that the variable is
345 
not needed and it will not end up in the executable. For eCos packages
346 
this can be achieved in the CDL, by specifying that the containing
347 
source file should end up in <TT
348 
CLASS="FILENAME"
349 
>libextras.a</TT
350 
> rather
351 
than the default <TT
352 
CLASS="FILENAME"
353 
>libtarget.a</TT
354 
>:</P
355 
><TABLE
356 
BORDER="5"
357 
BGCOLOR="#E0E0F0"
358 
WIDTH="70%"
359 
><TR
360 
><TD
361 
><PRE
362 
CLASS="PROGRAMLISTING"
363 
>cdl_package CYGPKG_HAL_WUMPUS_ARCH {
364 
    &#8230;
365 
    compile -library=libextras.a data.c
366 
}</PRE
367 
></TD
368 
></TR
369 
></TABLE
370 
><P
371 
>If the file <TT
372 
CLASS="FILENAME"
373 
>data.c</TT
374 
> instantiates a power
375 
controller this is now guaranteed to end up in the final executable,
376 
as intended. Typically HAL and device driver packages will already
377 
have some data that must not be eliminated by the linker, so they will
378 
already contain a file that gets built into
379 
<TT
380 
CLASS="FILENAME"
381 
>libextras.a</TT
382 
>. For power controllers defined inside
383 
application code it is important that the power controllers end up in
384 
<TT
385 
CLASS="FILENAME"
386 
>.o</TT
387 
> object files rather than in
388 
<TT
389 
CLASS="FILENAME"
390 
>.a</TT
391 
> library archive files.</P
392 
><P
393 
>All the real work of a power controller is done by the mode change
394 
function. If the power management package has been configured to use a
395 
separate thread then this mode change function will be invoked by that
396 
thread (except for the special case of <A
397 
HREF="power-change.html#POWER-CHANGE-CONTROLLER-NOW"
398 
><TT
399 
CLASS="FUNCTION"
400 
>power_set_controller_mode_now</TT
401 
></A
402 
>).
403 
If no separate thread is used then the mode change function will be
404 
invoked directly by <TT
405 
CLASS="FUNCTION"
406 
>power_set_mode</TT
407 
> or
408 
<TT
409 
CLASS="FUNCTION"
410 
>power_set_controller_mode</TT
411 
>.</P
412 
><P
413 
>The mode change function will be invoked with three arguments. The
414 
first argument identifies the power controller. Usually this argument
415 
is not actually required since a given mode change function will only
416 
ever be invoked for a single power controller. For example,
417 
<TT
418 
CLASS="FUNCTION"
419 
>xyzzy_device_power_mode_change</TT
420 
> will only ever be
421 
used in conjunction with <TT
422 
CLASS="VARNAME"
423 
>xyzzy_power_controller</TT
424 
>.
425 
However there may be some packages which contain multiple controllers,
426 
all of which can share a single mode change function, and in that case
427 
it is essential to identify the specific controller. The second
428 
argument specifies the mode the controller should switch to, if
429 
possible: it will be one of <TT
430 
CLASS="LITERAL"
431 
>PowerMode_Active</TT
432 
>,
433 
<TT
434 
CLASS="LITERAL"
435 
>PowerMode_Idle</TT
436 
>, <TT
437 
CLASS="LITERAL"
438 
>PowerMode_Sleep</TT
439 
>
440 
or <TT
441 
CLASS="LITERAL"
442 
>PowerMode_Off</TT
443 
>. The final argument will be one of
444 
<TT
445 
CLASS="LITERAL"
446 
>PowerModeChange_Controller</TT
447 
>,
448 
PowerModeChange_ControllerNow, or
449 
<TT
450 
CLASS="LITERAL"
451 
>PowerModeChange_Global</TT
452 
>, and identifies the call
453 
that caused this invocation. For example, if the mode change function
454 
was invoked because of a call to <TT
455 
CLASS="FUNCTION"
456 
>power_set_mode</TT
457 
>
458 
then this argument will be <TT
459 
CLASS="LITERAL"
460 
>PowerModeChange_Global</TT
461 
>.
462 
It is up to each controller to decide how to interpret this final
463 
argument. A typical controller might reject a global request to switch
464 
to <SPAN
465 
CLASS="TYPE"
466 
>off</SPAN
467 
> mode if the associated device is still busy, but
468 
if the request was aimed specifically at this controller then it could
469 
instead abort any current I/O operations and switch off the device.</P
470 
><P
471 
>The <SPAN
472 
CLASS="STRUCTNAME"
473 
>PowerController</SPAN
474 
> data structure contains
475 
one field, <TT
476 
CLASS="STRUCTFIELD"
477 
><I
478 
>mode</I
479 
></TT
480 
>, that needs to be updated
481 
by the power mode change function. At all times it should indicate the
482 
current mode for this controller. When a mode change is requested the
483 
desired mode is passed as the second argument. The exact operation of
484 
the power mode change function depends very much on what is being
485 
controlled and the current circumstances, but some guidelines are
486 
possible:</P
487 
><P
488 
></P
489 
><OL
490 
TYPE="1"
491 
><LI
492 
><P
493 
>If the request can be satisfied without obvious detriment, do so and
494 
update the <TT
495 
CLASS="STRUCTFIELD"
496 
><I
497 
>mode</I
498 
></TT
499 
> field. Reducing the power
500 
consumption of a device that is not currently being used is generally
501 
harmless.</P
502 
></LI
503 
><LI
504 
><P
505 
>If a request is a no-op, for example if the system is switching
506 
from <SPAN
507 
CLASS="TYPE"
508 
>idle</SPAN
509 
> to <SPAN
510 
CLASS="TYPE"
511 
>sleep</SPAN
512 
> mode and the controller
513 
does not distinguish between these modes, simply act as if the request
514 
was satisfied.</P
515 
></LI
516 
><LI
517 
><P
518 
>If a request is felt to be unsafe, for example shutting down a
519 
device that is still in use, then the controller may decide
520 
to reject this request. This is especially true if the request was a
521 
global mode change as opposed to one intended specifically for this
522 
controller: in the latter case the policy module should be given due
523 
deference. There are a number of ways in which a request can be
524 
rejected:</P
525 
><P
526 
></P
527 
><OL
528 
TYPE="a"
529 
><LI
530 
><P
531 
>If the request cannot be satisfied immediately but may be feasible in
532 
a short while, leave the <TT
533 
CLASS="STRUCTFIELD"
534 
><I
535 
>mode</I
536 
></TT
537 
> field
538 
unchanged. Higher-level code in the policy module can interpret this
539 
as a hint to retry the operation a little bit later. This approach is
540 
also useful if the mode change can be started but will take some time
541 
to complete, for example shutting down a socket connection, and
542 
additional processing will be needed later on.</P
543 
></LI
544 
><LI
545 
><P
546 
>If the request is felt to be inappropriate, for example switching off
547 
a device that is still in use, the mode change function can
548 
call <TT
549 
CLASS="FUNCTION"
550 
>power_set_controller_mode</TT
551 
> to reset the
552 
desired mode for this controller back to the current mode.
553 
Higher-level code can then interpret this as a hint that there is more
554 
activity in the system than had been apparent.</P
555 
></LI
556 
><LI
557 
><P
558 
>For a global mode change, if the new mode is felt to be inappropriate
559 
then the power controller can call <TT
560 
CLASS="FUNCTION"
561 
>power_set_mode</TT
562 
>
563 
to indicate this. An example of this would be the policy module
564 
deciding to switch off the whole unit while there is still I/O
565 
activity.</P
566 
></LI
567 
></OL
568 
></LI
569 
></OL
570 
><P
571 
>Mode change functions should not directly manipulate any other fields
572 
in the <SPAN
573 
CLASS="STRUCTNAME"
574 
>PowerController</SPAN
575 
> data structure. If it
576 
is necessary to keep track of additional data then static variables
577 
can be used.</P
578 
><P
579 
>It should be noted that the above are only guidelines. Their
580 
application in any given situation may be unclear. In addition the
581 
detailed requirements of specific systems will vary, so even if the
582 
power controller for a given device driver follows the above
583 
guidelines exactly it may turn out that slightly different behaviour
584 
would be more appropriate for the actual system that is being
585 
developed. Fortunately the open source nature of
586 
<SPAN
587 
CLASS="PRODUCTNAME"
588 
>eCos</SPAN
589 
> allows system developers to fine-tune
590 
power controllers to meet their exact requirements.</P
591 
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