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<!--     Generic power management documentation.                     -->
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<!-- Author(s):   bartv                                              -->
<!-- Contact(s):  bartv                                              -->
<!-- Date:        2001/06/26                                         -->
<!-- Version:     0.01                                               -->
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<!-- }}} -->
<part id="services-power">
<!-- reference id="services-power" -->
  <title>eCos Power Management Support</title>

<!-- {{{ Intro                          -->

<refentry id="power-intro">
<refmeta>
<refentrytitle>Introduction</refentrytitle>
</refmeta>
<refnamediv>
<refname>Introduction</refname>
<refpurpose>eCos support for Power Management</refpurpose>
</refnamediv>

<!-- {{{ Intro                          -->

<refsect1 id="power-intro-intro">
<title>Introduction</title>
<para>
The eCos Power Management package provides a framework for
incorporating power management facilities in an embedded application.
However its functionality is deliberately limited.
</para>
<orderedlist>
<listitem>
<para>
The package does not contain any support for controlling the current
power mode of any given processor, device or board. Instead it is the
responsibility of the appropriate HAL or device driver package to
implement such support, by implementing <firstterm>power
controllers</firstterm>. The power management package groups these
power controllers together and provides an interface for manipulating
them.
</para>
</listitem>
<listitem>
<para>
The package does not contain any power management policy support.
Specifically, including this package in an application does not by
itself ever cause the system to go into low-power mode. Instead it is
the responsibility of a separate policy module, provided by
higher-level application code or by some other package, to decide when
it would be appropriate to switch from one power mode to another. The
power management package then provides the mechanisms for making it
happen.
</para>
</listitem>
</orderedlist>
</refsect1>

<!-- }}} -->
<!-- {{{ including                      -->

<refsect1 id="power-intro-include">
<title>Including Power Management</title>
<para>
The power management package is never included automatically in an
eCos configuration: it is not part of any target specification or of
any template. Instead it must be added explicitly to a configuration
if the intended application requires power management functionality.
When using the command-line <command>ecosconfig</command> tool this
can be achieved using a command such as:
</para>
<screen>
$ ecosconfig add power
</screen>
<para>
The generic eCos user documentation should be consulted for more
information on how to use the various tools. The functionality
provided by the power management package is defined in the header file
<filename class="headerfile">cyg/power/power.h</filename>. This header
file can be used by both C and C++ code.
</para>

</refsect1>

<!-- }}} -->
<!-- {{{ modes                          -->

<refsect1 id="power-intro-modes">
<title>Power Modes</title>
<para>
There are four defined modes of operation:
</para>
<variablelist>

<varlistentry><term>active</term>
<listitem>
<para>
The system is fully operational, and power consumption is expected to
be high.
</para>
</listitem>
</varlistentry>

<varlistentry><term>idle</term>
<listitem>
<para>
There has been little or no activity for a short period of time. It is
up to the policy module to determine what constitutes a short period
of time, but typically it will be some tenths of a second or some
small number of seconds. A possible action when entering idle mode is
to reduce the system's clock speed, thus reducing the power drawn by
the cpu.
</para>
<para>
Note that typically this power mode is not entered automatically
whenever the idle thread starts running. Instead it is entered when
the policy module discovers that for a certain period of time the
system has been spending most of its time in the idle thread.
Theoretically it is possible to implement a policy module that would
cause a switch to idle mode as soon as the idle thread starts running,
but that could result in a great many power mode changes for no
immediate benefit.
</para>
</listitem>
</varlistentry>

<varlistentry><term>sleep</term>
<listitem>
<para>
The system has been idle for a significant period of time, perhaps
some tens of seconds. It is desirable to shut down any hardware that
is drawing a significant amount of power, for example a screen
backlight.
</para>
</listitem>
</varlistentry>

<varlistentry><term>off</term>
<listitem>
<para>
The system is powered down. Power consumption should be minimized.
Some special action may be needed before the system comes back up, for
example the user may need to press a specific button.
</para>
</listitem>
</varlistentry>

</variablelist>

<para>
The exact transitions that will happen are decided by the policy
module. One policy module might include transitions from active to
idle, from idle to sleep, from sleep to off, and from any of idle,
sleep or off directly back to active. Another policy module might
only use the active and off states, bypassing the intermediate ones.
</para>

</refsect1>

<!-- }}} -->
<!-- {{{ Power Controllers              -->

<refsect1 id="power-intro-controllers">
<title>Power Controllers</title>
<para>
The power management package operates primarily on power controllers.
The main functionality provided by a power controller is to switch the
power mode for some part of the system, for example the lcd display or
the cpu. A power controller consists primarily of a function which
will be invoked to switch the power mode for the part of the overall
system being controlled, plus some auxiliary data. A typical system
will include a number of different power controllers:
</para>
<orderedlist>
<listitem>
<para>
Usually there will be one power controller
<varname>power_controller_cpu</varname> associated with the processor
or with the target platform, and provided by the corresponding HAL
package. It is this controller which is responsible for switching off
the system when entering the <type>off</type> mode, which makes it
somewhat special: attempting to switch off the cpu before other
devices like the lcd display does not make sense because the cpu would
no longer be executing any instructions for the latter operation.
Therefore this power controller has to be invoked last when switching
to a lower-power mode, and similarly when switching back to a
higher-power mode it will be invoked first.
</para>
<para>
It should be noted that providing power management support is not a
hard requirement when porting eCos to a new processor or platform, and
many eCos ports predate the availability of power management support.
Therefore for any given platform it is distinctly possible that
<varname>power_controller_cpu</varname> is not yet provided, and if
full power management functionality is desired then the appropriate
HAL package would have to be extended first. System developers should
examine the relevant HAL documentation and sources to determine what
is actually available.
</para>
</listitem>
<listitem>
<para>
Some or all of the device drivers will supply their own power
controllers, as part of the device driver package. It is not required
that all device drivers provide power controllers. In some cases,
especially for devices that are integrated with the processor,
<varname>power_controller_cpu</varname> will take care of the
integrated devices as a side effect. In other cases the hardware may
not provide any functionality that allows power consumption to be
controlled. For any given device driver it is also possible that no
power controller exists either because it was not required when the
driver was written, or because the driver predates the availability of
power management. Again the relevant documentation and sources should
be consulted for further information.
</para>
</listitem>
<listitem>
<para>
There may be power controllers which are not associated directly with
any specific hardware. For example a TCP/IP stack could provide a
power controller so that it gets informed when the system has been
reactivated: by looking at the system clock it can determine for how
long the system has been switched off; using this information it can
then recover from expired dhcp leases, or even to shut down any stream
connections that may have become invalid (although arguably the stack
should have refused to go to <type>off</type> mode while there were
open connections).
</para>
</listitem>
</orderedlist>
</refsect1>

<!-- }}} -->
<!-- {{{ Basic Operation                -->

<refsect1 id="power-intro-operation">
<title>Basic Operation</title>
<para>
By default the Power Management package creates a thread during
initialization. It is also possible for the package to be used without
such a thread, for example in configurations which do not include a
full kernel, and this alternative is described below. When a separate
thread is used the stacksize and priority for this thread can be
controlled by configuration options
<varname>CYGNUM_POWER_THREAD_STACKSIZE</varname> and
<varname>CYGNUM_POWER_THREAD_PRIORITY</varname>. Typically the thread
will just wait on a semaphore internal to the package, and will do
nothing until some other part of the system requests a change to the
power mode.
</para>
<para>
At some point the policy module will decide that the system should
move into a lower-power mode, for example from active to idle. This is
achieved by calling the function <function>power_set_mode</function>,
provided by the power management package and declared in <filename
class="headerfile">cyg/power/power.h</filename>, with a single
argument, <literal>PowerMode_Idle</literal>. This function manipulates
some internal state and posts the semaphore, thus waking up the power
management thread. Note that the function returns before the mode
change has completed, and in fact depending on thread priorities this
return may happen before any power controller has been invoked.
</para>
<para>
When the power management thread wakes up it examines the internal
state to figure out what it should be doing. In this case it is
supposed to change the global power mode, so it will iterate over all
the power controllers requesting each one to switch to the
<type>idle</type> mode. It is up to each power controller to handle
this request appropriately. Optionally the thread will invoke a
callback function after processing each power controller, so that
higher-level code such as the policy module can more easily keep
track of the actual state of each controller. Once the thread has
iterated through all the power controllers it will again wait on the
internal semaphore for the next request to arrive.
</para>
<note>
<para>
At present the power management thread always runs at a single
priority, which defaults to a low priority. A possible future
enhancement would be to support two separate priorities. When
switching to a lower-powered mode the thread would run at a low
priority as before, thus allowing other threads to run and get a
chance to cancel this mode change. When switching to a higher-powered
mode the thread would run at a high priority. This could be especially
important when moving out of the <type>off</type> state: for example
it would ensure that all device drivers get a chance to wake up before
ordinary application threads get to run again and possibly attempt I/O
operations.
</para>
</note>
<para>
Although usually calls to <function>power_set_mode</function> will
come from just one place in the policy module, this is not a hard
requirement. It is possible for multiple threads to call this
function, with no need for any synchronization. If the power
management thread is in the middle of performing a mode change and a
new request comes in, the thread will detect this, abort the current
operation, and start iterating through the power controllers again
with the new mode. This check happens between every power controller
invocation. Usefully this makes it possible for power controllers
themselves to manipulate power modes: a power controller is invoked to
change mode; for some reason it determines that the new mode is
inappropriate; it calls <function>power_set_mode</function> to move
the system back to another mode; when the power controller returns
this event will be detected; the power management thread will abort
the current mode change, and start the new one.
</para>
<para>
In addition to changing the power mode for the system as a whole,
individual controllers can be manipulated using the function
<function>power_set_controller_mode</function>. For example, while the
system as a whole might be in <type>active</type> mode certain devices
might be kept in <type>sleep</type> mode until they are explicitly
activated. It is possible to mix concurrent calls to
<function>power_set_mode</function> and
<function>power_set_controller_mode</function>, and when a power
controller is invoked it may use
<function>power_set_controller_mode</function> to request further
changes to its own or to another controller's mode as required.
</para>
<para>
There are some scenarios where the power management package should not
use its own thread. One scenario is if the configuration is
specifically for a single-threaded application such as RedBoot.
Another scenario is if the policy module already involves a separate
thread: it may make more sense if the various power management
operations are synchronous with respect to the calling thread. The use
of a separate thread inside the power management package is controlled
by the configuration option <varname>CYGPKG_POWER_THREAD</varname>,
which is active only if the kernel package is present and enabled by
default.
</para>
<para>
If no separate power management thread is used then obviously the
implementations of <function>power_set_mode</function> and
<function>power_set_controller_mode</function> will be somewhat
different: instead of waking up a separate thread to do the work,
these functions will now manipulate the power controllers directly. If
the system does still involve multiple threads then only one thread
may call <function>power_set_mode</function> or
<function>power_set_controller_mode</function> at a time: the power
management package will not provide any synchronization, that must
happen at a higher level. However when a power controller is invoked
it can still call these functions as required.
</para>
</refsect1>

<!-- }}} -->

</refentry>

<!-- }}} -->
<!-- {{{ Power Management Info          -->

<refentry id="power-info">
<refmeta>
<refentrytitle>Power Management Information</refentrytitle>
</refmeta>
<refnamediv>
<refname>Obtaining Power Management Information</refname>
<refpurpose>finding out about the various power controllers in the system</refpurpose>
</refnamediv>

<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>
#include &lt;cyg/power/power.h&gt;

extern PowerController __POWER__[], __POWER_END__;
extern PowerController power_controller_cpu;
extern cyg_handle_t    power_thread_handle;
</funcsynopsisinfo>

<funcprototype>
  <funcdef>
    PowerMode <function>power_get_mode</function>
  </funcdef>
  <void>
</funcprototype>
<funcprototype>
  <funcdef>
    PowerMode <function>power_get_desired_mode</function>
  </funcdef>
  <void>
</funcprototype>
<funcprototype>
  <funcdef>
    PowerMode <function>power_get_controller_mode</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    PowerMode <function>power_get_controller_desired_mode</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    const char* <function>power_get_controller_id</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>

<refsect1 id="power-info-access">
<title>Accessing Power Controllers</title>
<para>
All the power controllers in a system are held in a table, filled in
at link-time. The symbols <varname>__POWER__</varname> and
<varname>__POWER_END</varname> can be used to iterate through this
table, for example:
</para>
<programlisting>
PowerController* controller;
for (controller  = &amp;(__POWER__[0]);
     controller != &amp;(__POWER_END__);
     controller++) {

     &hellip;
}
</programlisting>
<para>
Each controller has an associated priority, controlling the order in
which they appear in the table. Typically a software-only component
such as a TCP/IP stack would use a small number for the priority, so
that it appears near the start of the table, whereas a device driver
would be nearer the back of the table. When switching to a
lower-powered mode the power management package will iterate through
this table from front to back, thus ensuring that for example the
TCP/IP stack gets a chance to shut down before the underlying ethernet
or other hardware that the stack depends on. Similarly when switching
to a higher-powered mode the power management package will iterate
through this table from back to front.
</para>
<para>
In most systems there will be one special controller,
<varname>power_controller_cpu</varname>, which should be provided by
one of the architectural, variant or platform HAL packages. This
controller will always be the last entry in the table. It is
responsible for the final power down operation when switching to
<type>off</type> mode. Other packages such as device drivers may or
may not declare variable identifiers for their power controllers,
allowing those controllers to be accessed by name as well as by their
entries in the global table.
</para>
</refsect1>

<refsect1 id="power-info-global">
<title>Global Power Modes</title>
<para>
The function <function>power_get_mode</function> can be called at any
time to determine the current power mode for the system as a whole.
The return value will be one of <literal>PowerMode_Active</literal>,
<literal>PowerMode_Idle</literal>, <literal>PowerMode_Sleep</literal>
or <literal>PowerMode_Off</literal>. In normal circumstances it is
unlikely that <literal>PowerMode_Off</literal> would be returned since
that mode generally means that the cpu is no longer running.
</para>
<para>
The function <function>power_get_desired_mode</function> returns the
power mode that the system should be running at. Most of the time this
will be the same value as returned by
<function>power_get_mode</function>. However a different value may be
returned when in the middle of changing power modes. For example, if
the current thread runs at a higher priority than the power management
thread then the latter may have been pre-empted in the middle of a
mode change: <function>power_get_mode</function> will return the mode
the system was running at before the mode change started, and
<function>power_get_desired_mode</function> will return the mode the
system should end up in when the mode change completes, barring
further calls to <function>power_set_mode</function>.
</para>
</refsect1>

<refsect1 id="power-info-individual">
<title>Individual Controller Power Modes</title>
<para>
The power management package keeps track of the current and desired
modes for each power controller, as well as the modes for the system as
a whole. The function <function>power_get_controller_mode</function>
takes a single argument, a pointer to a power controller, and returns
the power mode that controller is currently running at. Similarly
<function>power_get_controller_desired_mode</function> returns the
power mode that controller should be running at. Most of the time the
current and desired modes for a given controller will be the same, and
will also be the same as the global power mode. However if the power
management thread is preeempted in the middle of a mode change then
some of the controllers will have been updated to the desired global
mode, whereas others will still be at the old mode. The power
management package also provides functionality for manipulating
<link linkend="power-change-controller">
individual controllers</link>, and for <link
linkend="power-attached">detaching</link> controllers from
global mode changes.
</para>
</refsect1>

<refsect1 id="power-info-ids">
<title>Power Controller Identification</title>
<para>
In some scenarios the power management package will run completely
automated, and there is no need to identify individual power
controllers. Any form of identification such as a string
description would serve no purpose, but would still consume memory in
the final system. In other scenarios it may be very desirable to
provide some means of identification. For example, while still
debugging it may be useful to see a simple string when printing the
contents of a power controller structure. Alternatively, if the
application is expected to provide some sort of user interface that
gives control over which parts of the system are enabled or disabled,
a string identifier for each controller would be useful. To cope with
these scenarios the power management package provides a configuration
option <varname>CYGIMP_POWER_PROVIDE_STRINGS</varname>. When enabled,
each power controller will contain a pointer to a constant string
which can be accessed via a function
<function>power_get_controller_id</function>. When disabled the system
will not contain these strings, and the function will not be provided.
The following code illustrates how to use this function.
</para>
<programlisting>
#include &lt;stdio.h&gt;
#include &lt;pkgconf/system.h&gt;
#ifndef CYGPKG_POWER
# error The power management package is not present.
#endif
#include &lt;pkgconf/power.h&gt;
#ifndef CYGIMP_POWER_PROVIDE_STRINGS
# error Power controller identifiers are not available.
#endif
#include &lt;cyg/power/power.h&gt;

static const char*
mode_to_string(PowerMode mode)
{
    const char* result;
    switch(mode) {
      case PowerMode_Active : result = "active"; break;
      case PowerMode_Idle   : result = "idle"; break;
      case PowerMode_Sleep  : result = "sleep"; break;
      case PowerMode_Off    : result = "off"; break;
      default               : result = "&lt;unknown&gt;"; break;
    }
    return result;
}

int
main(int argc, char** argv)
{
    PowerController* controller;

    for (controller = &amp;(__POWER__[0]);
         controller != &amp;(__POWER_END__);
         controller++) {
        printf("Controller @ %p: %s, %s\n", controller,
               power_get_controller_id(controller),
               mode_to_string(power_get_controller_mode(controller)));
    }
    return 0;
}
</programlisting>
</refsect1>
<refsect1 id="power-info-thread">
<title>The Power Management Thread</title>
<para>
If the power management package is configured to use a separate thread
then a handle for that thread is made available to higher-level code
via the variable <varname>power_thread_handle</varname>. This handle
can be used for a variety of purposes, including manipulating that
thread's priority.
</para>
</refsect1>

</refentry>

<!-- }}} -->
<!-- {{{ Changing Power Modes           -->

<refentry id="power-change">
<refmeta>
<refentrytitle>Changing Power Modes</refentrytitle>
</refmeta>
<refnamediv>
<refname>Changing Power Modes</refname>
<refpurpose>reducing or increasing power consumption as needed</refpurpose>
</refnamediv>

<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>
#include &lt;cyg/power/power.h&gt;
</funcsynopsisinfo>

<funcprototype>
  <funcdef>
    void <function>power_set_mode</function>
  </funcdef>
  <paramdef>
    PowerMode <parameter>new_mode</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    void <function>power_set_controller_mode</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
  <paramdef>
    PowerMode <parameter>new_mode</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    void <function>power_set_controller_mode_now</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
  <paramdef>
    PowerMode <parameter>new_mode</parameter>
  </paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>

<refsect1 id="power-change-global">
<title>Changing the Global Power Mode</title>
<para>
The primary functionality supported by the power management package is
to change the system's global power mode. This is achieved by calling
the function <function>power_set_mode</function> with a single
argument, which should be one of <literal>PowerMode_Active</literal>,
<literal>PowerMode_Idle</literal>, <literal>PowerMode_Sleep</literal>
or <literal>PowerMode_Off</literal>. Typically this function will only
be invoked in certain scenarios:
</para>
<orderedlist>
<listitem>
<para>
A typical system will contain a policy module which is primarily
responsible for initiating power mode changes, and a thread inside the
power management package. The policy module will call
<function>power_set_mode</function>, which has the effect of
manipulating some internal state in the power management package and
waking up its thread. When this thread gets scheduled to run (its
priority is controlled by a configuration option), it will iterate
over the power controllers and invoke each controller to change its
power mode. There is support for a <link
linkend="power-policy-callback">callback function</link>, and for
<link linkend="power-attached">detached</link> power controllers.
</para>
</listitem>
<listitem>
<para>
After a call to <function>power_set_mode</function> but before the
power management thread has had a chance to iterate over all the
controllers, or even before the thread has been rescheduled at all,
the policy module may decide that a different power mode would be more
appropriate for the current situation and calls
<function>power_set_mode</function> again. This has the effect of
aborting the previous mode change, followed by the power management
thread iterating over the power controllers again for the new mode.
</para>
</listitem>
<listitem>
<para>
If there is no single policy module responsible for power mode
changes, any code can call <function>power_set_mode</function>. If
there are multiple calls in quick succession, earlier calls will
be aborted and the system should end up in the power mode
corresponding to the last call
</para>
</listitem>
<listitem>
<para>
As a special case, it is possible for a power controller to call
<function>power_set_mode</function> when invoked by the power
management thread. For example a power controller could decide that it
is inappropriate for the system to go to sleep because the device it
is associated with is still busy. The effect is as if the policy
module had called <function>power_set_mode</function> again before 
the mode change had completed.
</para>
</listitem>
</orderedlist>
<para>
If the power management package has been configured not to use a
separate thread then obviously the behaviour is somewhat different.
The call to <function>power_set_mode</function> will now iterate over
the various power controllers immediately, rather than leaving this to
a separate thread, and the whole mode change completes before
<function>power_set_mode</function> returns. If some other thread or a
DSR calls <function>power_set_mode</function> concurrently the
behaviour of the system is undefined. However, it is still legal for a
power controller to call <function>power_set_mode</function>:
effectively this is a recursive call; it is detected by the system,
and internal state is updated; the recursive
<function>power_set_mode</function> call now returns, and when the
power controller returns back to the original
<function>power_set_mode</function> call it detects what has happened,
aborts the previous mode change, and starts a new mode change as
requested by the controller.
</para>
<para>
<function>power_set_mode</function> is normally invoked from thread
context. If a separate power management thread is used it can be
invoked safely from DSR context. If the system is configured not to
use such a thread, it may or may not be safe to invoke this function
from DSR context: essentially the function just iterates through
the various power controllers, and the documentation or source code of
each controller present in the current system will have to be examined
to determine whether or not this can happen safely in DSR context.
<function>power_set_mode</function> should never be invoked from
ISR context.
</para>
</refsect1>

<refsect1 id="power-change-controller">
<title>Manipulating an Individual Power Controller</title>
<para>
In some cases it is desirable to set the power mode of an individual
controller separately from the mode for the system as a whole. For
example if a device is not currently being used then the associated
power controller could be set to <literal>PowerMode_Off</literal>,
even while the system as a whole is still active. This can be achieved
by calling the function
<function>power_set_controller_mode</function>. It takes two
arguments: the first identifies a particular controller; the second
specifies the desired new power mode for that controller. The function
operates in much the same way as <function>power_set_mode</function>,
for example if a separate power management thread is being used then
<function>power_set_controller_mode</function> operates by
manipulating some internal state and waking up that thread. The
limitations are also much the same as for
<function>power_set_mode</function>, so for example
<function>power_set_controller_mode</function> should not be invoked
from inside ISRs.
</para>
<para>
Manipulating individual controllers is often used in conjunction with
the function <link
linkend="power-attached"><function>power_set_controller_attached</function></link>,
allowing the policy module to specify which controllers are affected
by global mode changes.
</para>
</refsect1>

<refsect1 id="power-change-controller-now">
<title>Direct Manipulation of a Power Controller</title>
<para>
In exceptional circumstances it may be necessary to invoke a power
controller directly, bypassing the power management thread and
higher-level functionality such as <link
linkend="power-policy-callback">callback functions</link>. The
function <function>power_set_controller_mode_now</function> allows
this. It takes two arguments, a controller and a mode, just like
<function>power_set_controller_mode</function>.
</para>
<para>
Use of <function>power_set_controller_mode_now</function> is
dangerous. For example no attempt is made to synchronise with any
other power mode changes that might be happening concurrently. A
possible use is when the system gets woken up out of
<type>sleep</type> mode: depending on the hardware, on which power
controllers are present, and on the application code it may be
necessary to wake up some power controllers immediately before the
system as a whole is ready to run again.
</para>
</refsect1>

</refentry>

<!-- }}} -->
<!-- {{{ Policy support                 -->

<refentry id="power-policy">
<refmeta>
<refentrytitle>Support for Policy Modules</refentrytitle>
</refmeta>
<refnamediv>
<refname>Support for Policy Modules</refname>
<refpurpose>closer integration with higher-level code</refpurpose>
</refnamediv>

<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>
#include &lt;cyg/power/power.h&gt;
</funcsynopsisinfo>

<funcprototype>
  <funcdef>
    void <function>power_set_policy_callback</function>
  </funcdef>
  <paramdef>
    void (*)(PowerController*, PowerMode, PowerMode, PowerMode, PowerMode) <parameter>callback</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    void (*)(PowerController*, PowerMode, PowerMode, PowerMode, PowerMode) <function>power_get_policy_callback</function>
  </funcdef>
  <void>
</funcprototype>
<funcprototype>
  <funcdef>
    CYG_ADDRWORD <function>power_get_controller_policy_data</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    void <function>power_set_controller_policy_data</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
  <paramdef>
    CYG_ADDRWORD <parameter>data</parameter>
  </paramdef>
</funcprototype>

</funcsynopsis>
</refsynopsisdiv>

<refsect1 id="power-policy-callback">
<title>Policy Callbacks</title>
<para>
The use of a separate thread to perform power mode changes in typical
configurations can cause problems for some policy modules.
Specifically, the policy module can request a mode change for the
system as a whole or for an individual controller, but it does not
know when the power management thread actually gets scheduled to run
again and carry out the request. Although it would be possible for the
policy module to perform some sort of polling, in general that is
undesirable.
</para>
<para>
To avoid such problems the policy module can install a callback
function using <function>power_set_policy_callback</function>. The
current callback function can be retrieved using
<function>power_get_policy_callback</function>. If a callback function
has been installed then it will be called by the power management
package whenever a power controller has been invoked to perform a mode
change. The callback will be called in the context of the power
management thread, so usually it will have to make use of thread
synchronisation primitives to interact with the main policy module. It
is passed five arguments:
</para>
<orderedlist>
<listitem>
<para>
The power controller that has just been invoked to perform a mode
change. 
</para>
</listitem>
<listitem>
<para>
The mode this controller was running at before the invocation.
</para>
</listitem>
<listitem>
<para>
The current mode this controller is now running at.
</para>
</listitem>
<listitem>
<para>
The desired mode before the power controller was invoked. Usually this
will be the same as the current mode, unless the controller has
decided for some reason that this was inappropriate.
</para>
</listitem>
<listitem>
<para>
The current desired mode. This will differ from the previous argument
only if there has was another call to
<function>power_set_mode</function> or
<function>power_set_controller_mode</function> while the power
controller was being invoked, probably by the power controller itself.
</para>
</listitem>
</orderedlist>
<para>
A simple example of a policy callback function would be:
</para>
<programlisting>
static void
power_callback(
    PowerController* controller,
    PowerMode old_mode,
    PowerMode new_mode,
    PowerMode old_desired_mode,
    powerMode new_desired_mode)
{
    printf("Power mode change: %s, %s -> %d\n",
        power_get_controller_id(controller),
        mode_to_string(old_mode),
        mode_to_string(new_mode));

    CYG_UNUSED_PARAM(PowerMode, old_desired_mode);
    CYG_UNUSED_PARAM(PowerMode, new_desired_mode);
}

int
main(int argc, char** argv)
{
    &hellip;
    power_set_policy_callback(&amp;power_callback);
    &hellip;
}
</programlisting>
<para>
If <function>power_set_controller_mode_now</function> is used to
manipulate an individual controller the policy callback will not be
invoked. This function may get called from any context including DSRs,
and even if there is already a call to the policy callback happening
in some other context, so invoking the callback would usually be
unsafe.
</para>
<para>
If the power management package has not been configured to use a
separate thread then <function>power_set_mode</function> and
<function>power_set_controller_mode</function> will manipulate the
power controllers immediately and invoke the policy callback
afterwards. Therefore the policy callback will typically run in the
same context as the main policy module.
</para>
</refsect1>

<refsect1 id="power-policy-data">
<title>Policy-specific Controller Data</title>
<para>
Some policy modules may want to associate some additional data with
each power controller. This could be achieved by for example
maintaining a hash table or similar data structure, but for
convenience the power management package allows higher-level code,
typically the policy module, to store and retrieve one word of data in
each power controller. The function
<function>power_set_controller_policy_data</function> takes two
arguments, a pointer to a power controller and a
<type>CYG_ADDRWORD</type> of data: by appropriate use of casts this
word could be an integer or a pointer to some data structure. The
matching function
<function>power_get_controller_policy_data</function> retrieves the
word previously installed, and can be cast back to an integer or
pointer. The default value for the policy data is 0.
</para>
<para>
For example the following code fragment stores a simple index value in
each power controller. This could then be retrieved by the policy
callback.
</para>
<programlisting>
    unsigned int     i = 0;
    PowerController* controller;

    for (controller = &amp;(__POWER__[0]);
         controller != &amp;(__POWER_END__);
         controller++) {
        power_set_controller_policy_data(controller, (CYG_ADDRWORD) i++);
    }
</programlisting>
<para>
Not all policy modules will require per-controller data. The
configuration option
<varname>CYGIMP_POWER_PROVIDE_POLICY_DATA</varname> can be used to
control this functionality, thus avoiding wasting a small amount of
memory inside each power controller structure.
</para>

</refsect1>
</refentry>

<!-- }}} -->
<!-- {{{ Attached/detached controllers  -->

<refentry id="power-attached">
<refmeta>
<refentrytitle>Attached and Detached Controllers</refentrytitle>
</refmeta>
<refnamediv>
<refname>Attached and Detached Controllers</refname>
<refpurpose>control which power controllers are affected by global changes</refpurpose>
</refnamediv>

<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>
#include &lt;cyg/power/power.h&gt;
</funcsynopsisinfo>
<funcprototype>
  <funcdef>
    cyg_bool <function>power_get_controller_attached</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
</funcprototype>
<funcprototype>
  <funcdef>
    void <function>power_set_controller_attached</function>
  </funcdef>
  <paramdef>
    PowerController* <parameter>controller</parameter>
  </paramdef>
  <paramdef>
    cyg_bool <parameter>new_state</parameter>
  </paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>

<refsect1>
<title>Detaching Power Controllers</title>
<para>
By default the global operation <function>power_set_mode</function>
affects all power controllers. There may be circumstances when this is
not desirable. For example if a particular device is not currently
being used then it can be left switched off: the rest of the system
could be moving between <type>active</type>, <type>idle</type> and
<type>sleep</type> modes, but there is no point in invoking the power
controller for the unused device. To support this the power management
package supports the concept of attached and detached controllers. By
default all controllers are attached, and hence will be affected by
global mode changes. A specific controller can be detached using the
function <function>power_set_controller_attached</function>. This
function takes two arguments, one to specify a particular controller
and another to specify the desired new state.
<function>power_get_controller_attached</function> can be used to
determine whether or not a specific controller is currently attached.
</para>
<para>
The attached or detached state of a controller only affects what
happens during a global mode change, in other words following a call
to <function>power_set_mode</function>. It is still possible to
manipulate a detached controller using
<function>power_set_controller_mode</function> or
<function>power_set_controller_mode_now</function>.
</para>

</refsect1>
</refentry>

<!-- }}} -->
<!-- {{{ Implementing                   -->

<refentry id="power-controller">
<refmeta>
<refentrytitle>Implementing a Power Controller</refentrytitle>
</refmeta>
<refnamediv>
<refname>Implementing a Power Controller</refname>
<refpurpose>adding power management support to device drivers and
other packages</refpurpose>
</refnamediv>

<refsect1>
<title>Implementing a Power Controller</title>
<para>
A system will have some number of power controllers. Usually there
will be one power controller for the cpu,
<varname>power_controller_cpu</varname>, typically provided by one of
the HAL packages and responsible for managing the processor itself and
associated critical components such as memory. Some or all of the
device drivers will provide power controllers, allowing the power
consumption of the associated devices to be controlled. There may be
some arbitrary number of other controllers present in the system. The
power management package does not impose any restrictions on the
number or nature of the power controllers in the system, other than
insisting that at most one <varname>power_controller_cpu</varname> be
provided.
</para>
<para>
Each power controller involves a single data structure of type
<structname>PowerController</structname>, defined in the header file
<filename class="headerfile">cyg/power/power.h</filename>. These data
structures should all be placed in the table
<literal>__POWER__</literal>, so that the power management package and
other code can easily locate all the controllers in the system. This
table is constructed at link-time, avoiding code-size or run-time
overheads. To facilitate this the package provides two macros which
should be used to define a power controller,
<literal>POWER_CONTROLLER()</literal> and
<literal>POWER_CONTROLLER_CPU()</literal>.
</para>
<para>
The macro <literal>POWER_CONTROLLER</literal> takes four arguments:
</para>
<orderedlist>
<listitem>
<para>
A variable name. This can be used to access the power controller
directly, as well as via the table.
</para>
</listitem>
<listitem>
<para>
A priority. The table of power controllers is sorted, such that power
controllers with a numerically lower priority come earlier in the
table. The special controller <varname>power_controller_cpu</varname>
always comes at the end of the table. When moving from a high-power
mode to a lower-powered mode, the power management package iterates
through the table from front to back. When moving to a higher-powered
mode the reverse direction is used. The intention is that the power
controller for a software-only package such as a TCP/IP stack should
appear near the start of the table, whereas the controllers for the
ethernet and similar devices would be near the end of the table. Hence
when the policy module initiates a mode change to a lower-powered mode
the TCP/IP stack gets a chance to cancel this mode change, before the
devices it depends on are powered down. Similarly when moving to a
higher-powered mode the devices will be re-activated before any
software that depends on those devices.
</para>
<para>
The header file <filename
class="headerfile">cyg/power/power.h</filename> defines three
priorities <literal>PowerPri_Early</literal>,
<literal>PowerPri_Typical</literal> and
<literal>PowerPri_Late</literal>. For most controllers one of these
priorities, possibly with a small number added or subtracted, will
give sufficient control. If an application developer is uncertain
about the relative priorities of the various controllers, a simple
<link linkend="power-info-ids">test program</link> that iterates over
the table will quickly eliminate any confusion.
</para>
</listitem>
<listitem>
<para>
A constant string identifier. If the system has been configured
without support for such identifiers
(<varname>CYGIMP_POWER_PROVIDE_STRINGS</varname>) then this identifer
will be discarded at compile-time. Otherwise it will be made available
to higher-level code using the function
<function>power_get_controller_id</function>. 
</para>
</listitem>
<listitem>
<para>
A function pointer. This will be invoked to perform actual mode
changes, as described below.
</para>
</listitem>
</orderedlist>
<para>
A typical example of the use of the
<literal>POWER_CONTROLLER</literal> macro would be as follows:
</para>
<programlisting>
#include &lt;pkgconf/system.h&gt;

#ifdef CYGPKG_POWER
# include &lt;cyg/power/power.h&gt;

static void
xyzzy_device_power_mode_change(
    PowerController* controller,
    PowerMode        desired_mode,
    PowerModeChange  change)
{
   // Do the work
}

static POWER_CONTROLLER(xyzzy_power_controller, \
                        PowerPri_Late,          \
                        "xyzzy device",         \
                        &amp;xyzzy_device_power_mode_change);
#endif
</programlisting>
<para>
This creates a variable <varname>xyzzy_power_controller</varname>,
which is a power controller data structure that will end up near the
end of the table of power controllers. Higher-level code can
iterate through this table and report the string <literal>"xyzzy
device"</literal> to the user. Whenever there is a mode change
operation that affects this controller, the function
<function>xyzzy_device_power_mode_change</function> will be invoked.
The variable is declared static so this controller cannot be
manipulated by name in any other code. Alternatively, if the variable
had not been declared static other code could manipulate this
controller by name as well as through the table, especially if the
package for the xyzzy device driver explicitly declared this
variable in an exported header file. Obviously exporting the variable
involves a slight risk of a name clash at link time.
</para>
<para>
The above code explicitly checks for the presence of the power
management package before including that package's header file or
providing any related functionality. Since power management
functionality is optional, such checks are recommended.
</para>
<para>
The macro <literal>POWER_CONTROLLER_CPU</literal> only takes two
arguments, a string identifier and a mode change function pointer.
This macro always instantiates a variable
<varname>power_controller_cpu</varname> so there is no need to provide
a variable name. The resulting power controller structure always
appears at the end of the table, so there is no need to specify a
priority. Typical usage of the <literal>POWER_CONTROLLER_CPU</literal>
macro would be:
</para>
<programlisting>
static void
wumpus_processor_power_mode_change(
    PowerController* controller,
    PowerMode        desired_mode,
    PowerModeChange  change)
{
   // Do the work
}

POWER_CONTROLLER_CPU("wumpus processor", \
                     &amp;wumpus_processor_power_mode_change);
</programlisting>
<para>
This defines a power controller structure
<varname>power_controller_cpu</varname>. It should not be declared
static since higher-level code may well want to manipulate the cpu's
power mode directly, and the variable is declared by the power
management package's header file.
</para>
<para>
Some care has to be taken to ensure that the power controllers
actually end up in the final executable. If a power controller
variable ends up in an ordinary library and is never referenced
directly then typically the linker will believe that the variable is
not needed and it will not end up in the executable. For eCos packages
this can be achieved in the CDL, by specifying that the containing
source file should end up in <filename>libextras.a</filename> rather
than the default <filename>libtarget.a</filename>:
</para>
<programlisting>
cdl_package CYGPKG_HAL_WUMPUS_ARCH {
    &hellip;
    compile -library=libextras.a data.c
}
</programlisting>
<para>
If the file <filename>data.c</filename> instantiates a power
controller this is now guaranteed to end up in the final executable,
as intended. Typically HAL and device driver packages will already
have some data that must not be eliminated by the linker, so they will
already contain a file that gets built into
<filename>libextras.a</filename>. For power controllers defined inside
application code it is important that the power controllers end up in
<filename>.o</filename> object files rather than in
<filename>.a</filename> library archive files.
</para>
<para>
All the real work of a power controller is done by the mode change
function. If the power management package has been configured to use a
separate thread then this mode change function will be invoked by that
thread (except for the special case of <link
linkend="power-change-controller-now"><function>power_set_controller_mode_now</function></link>). 
If no separate thread is used then the mode change function will be
invoked directly by <function>power_set_mode</function> or
<function>power_set_controller_mode</function>.
</para>
<para>
The mode change function will be invoked with three arguments. The
first argument identifies the power controller. Usually this argument
is not actually required since a given mode change function will only
ever be invoked for a single power controller. For example,
<function>xyzzy_device_power_mode_change</function> will only ever be
used in conjunction with <varname>xyzzy_power_controller</varname>.
However there may be some packages which contain multiple controllers,
all of which can share a single mode change function, and in that case
it is essential to identify the specific controller. The second
argument specifies the mode the controller should switch to, if
possible: it will be one of <literal>PowerMode_Active</literal>,
<literal>PowerMode_Idle</literal>, <literal>PowerMode_Sleep</literal>
or <literal>PowerMode_Off</literal>. The final argument will be one of
<literal>PowerModeChange_Controller</literal>,
PowerModeChange_ControllerNow, or
<literal>PowerModeChange_Global</literal>, and identifies the call
that caused this invocation. For example, if the mode change function
was invoked because of a call to <function>power_set_mode</function>
then this argument will be <literal>PowerModeChange_Global</literal>.
It is up to each controller to decide how to interpret this final
argument. A typical controller might reject a global request to switch
to <type>off</type> mode if the associated device is still busy, but
if the request was aimed specifically at this controller then it could
instead abort any current I/O operations and switch off the device.
</para>
<para>
The <structname>PowerController</structname> data structure contains
one field, <structfield>mode</structfield>, that needs to be updated
by the power mode change function. At all times it should indicate the
current mode for this controller. When a mode change is requested the
desired mode is passed as the second argument. The exact operation of
the power mode change function depends very much on what is being
controlled and the current circumstances, but some guidelines are
possible:
</para>
<orderedlist>
<listitem>
<para>
If the request can be satisfied without obvious detriment, do so and
update the <structfield>mode</structfield> field. Reducing the power
consumption of a device that is not currently being used is generally
harmless.
</para>
</listitem>
<listitem>
<para>
If a request is a no-op, for example if the system is switching
from <type>idle</type> to <type>sleep</type> mode and the controller
does not distinguish between these modes, simply act as if the request
was satisfied.
</para>
</listitem>
<listitem>
<para>
If a request is felt to be unsafe, for example shutting down a
device that is still in use, then the controller may decide
to reject this request. This is especially true if the request was a
global mode change as opposed to one intended specifically for this
controller: in the latter case the policy module should be given due
deference. There are a number of ways in which a request can be
rejected:
</para>
<orderedlist>
<listitem>
<para>
If the request cannot be satisfied immediately but may be feasible in
a short while, leave the <structfield>mode</structfield> field
unchanged. Higher-level code in the policy module can interpret this
as a hint to retry the operation a little bit later. This approach is
also useful if the mode change can be started but will take some time
to complete, for example shutting down a socket connection, and
additional processing will be needed later on.
</para>
</listitem>
<listitem>
<para>
If the request is felt to be inappropriate, for example switching off
a device that is still in use, the mode change function can
call <function>power_set_controller_mode</function> to reset the
desired mode for this controller back to the current mode.
Higher-level code can then interpret this as a hint that there is more
activity in the system than had been apparent.
</para>
</listitem>
<listitem>
<para>
For a global mode change, if the new mode is felt to be inappropriate
then the power controller can call <function>power_set_mode</function>
to indicate this. An example of this would be the policy module
deciding to switch off the whole unit while there is still I/O
activity.
</para>
</listitem>
</orderedlist>
</listitem>
</orderedlist>
<para>
Mode change functions should not directly manipulate any other fields
in the <structname>PowerController</structname> data structure. If it
is necessary to keep track of additional data then static variables
can be used.
</para>
<para>
It should be noted that the above are only guidelines. Their
application in any given situation may be unclear. In addition the
detailed requirements of specific systems will vary, so even if the
power controller for a given device driver follows the above
guidelines exactly it may turn out that slightly different behaviour
would be more appropriate for the actual system that is being
developed. Fortunately the open source nature of
<productname>eCos</productname> allows system developers to fine-tune
power controllers to meet their exact requirements.
</para>

</refsect1>
</refentry>

<!-- }}} -->

<!-- /reference -->
</part>
