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>Changing Power Modes</TITLE
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TITLE="eCos Power Management Support"
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TITLE="Power Management Information"
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TITLE="Support for Policy Modules"
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>eCos Reference Manual</TH
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><A
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><HR
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><H1
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><A
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NAME="POWER-CHANGE">Changing Power Modes</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN15752"
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></A
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><H2
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>Name</H2
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>Changing Power Modes&nbsp;--&nbsp;reducing or increasing power consumption as needed</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN15755"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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NAME="AEN15756"><P
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></P
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><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
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CLASS="FUNCSYNOPSISINFO"
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>#include &lt;cyg/power/power.h&gt;</PRE
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></TD
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></TR
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>    void power_set_mode
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  </CODE
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>(    PowerMode new_mode
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  );</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>    void power_set_controller_mode
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  </CODE
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>(    PowerController* controller
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  ,     PowerMode new_mode
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  );</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>    void power_set_controller_mode_now
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  </CODE
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>(    PowerController* controller
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  ,     PowerMode new_mode
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  );</CODE
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></P
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><P
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></P
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></DIV
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="POWER-CHANGE-GLOBAL"
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></A
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><H2
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>Changing the Global Power Mode</H2
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><P
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>The primary functionality supported by the power management package is
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to change the system's global power mode. This is achieved by calling
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the function <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> with a single
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argument, which should be one of <TT
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CLASS="LITERAL"
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>PowerMode_Active</TT
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>,
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<TT
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CLASS="LITERAL"
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>PowerMode_Idle</TT
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>, <TT
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CLASS="LITERAL"
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>PowerMode_Sleep</TT
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>
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or <TT
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CLASS="LITERAL"
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>PowerMode_Off</TT
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>. Typically this function will only
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be invoked in certain scenarios:</P
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><P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>A typical system will contain a policy module which is primarily
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responsible for initiating power mode changes, and a thread inside the
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power management package. The policy module will call
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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>, which has the effect of
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manipulating some internal state in the power management package and
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waking up its thread. When this thread gets scheduled to run (its
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priority is controlled by a configuration option), it will iterate
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over the power controllers and invoke each controller to change its
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power mode. There is support for a <A
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HREF="power-policy.html#POWER-POLICY-CALLBACK"
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>callback function</A
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>, and for
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<A
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HREF="power-attached.html"
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>detached</A
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> power controllers.</P
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></LI
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><LI
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><P
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>After a call to <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> but before the
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power management thread has had a chance to iterate over all the
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controllers, or even before the thread has been rescheduled at all,
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the policy module may decide that a different power mode would be more
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appropriate for the current situation and calls
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> again. This has the effect of
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aborting the previous mode change, followed by the power management
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thread iterating over the power controllers again for the new mode.</P
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></LI
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><LI
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><P
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>If there is no single policy module responsible for power mode
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changes, any code can call <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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>. If
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there are multiple calls in quick succession, earlier calls will
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be aborted and the system should end up in the power mode
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corresponding to the last call</P
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></LI
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><LI
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><P
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>As a special case, it is possible for a power controller to call
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> when invoked by the power
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management thread. For example a power controller could decide that it
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is inappropriate for the system to go to sleep because the device it
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is associated with is still busy. The effect is as if the policy
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module had called <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> again before
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the mode change had completed.</P
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></LI
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></OL
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><P
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>If the power management package has been configured not to use a
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separate thread then obviously the behaviour is somewhat different.
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The call to <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> will now iterate over
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the various power controllers immediately, rather than leaving this to
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a separate thread, and the whole mode change completes before
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> returns. If some other thread or a
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DSR calls <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> concurrently the
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behaviour of the system is undefined. However, it is still legal for a
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power controller to call <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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>:
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effectively this is a recursive call; it is detected by the system,
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and internal state is updated; the recursive
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> call now returns, and when the
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power controller returns back to the original
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> call it detects what has happened,
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aborts the previous mode change, and starts a new mode change as
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requested by the controller.</P
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><P
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><TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> is normally invoked from thread
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context. If a separate power management thread is used it can be
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invoked safely from DSR context. If the system is configured not to
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use such a thread, it may or may not be safe to invoke this function
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from DSR context: essentially the function just iterates through
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the various power controllers, and the documentation or source code of
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each controller present in the current system will have to be examined
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to determine whether or not this can happen safely in DSR context.
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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> should never be invoked from
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ISR context.</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="POWER-CHANGE-CONTROLLER"
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></A
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><H2
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>Manipulating an Individual Power Controller</H2
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><P
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>In some cases it is desirable to set the power mode of an individual
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controller separately from the mode for the system as a whole. For
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example if a device is not currently being used then the associated
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power controller could be set to <TT
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CLASS="LITERAL"
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>PowerMode_Off</TT
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>,
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even while the system as a whole is still active. This can be achieved
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by calling the function
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<TT
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CLASS="FUNCTION"
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>power_set_controller_mode</TT
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>. It takes two
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arguments: the first identifies a particular controller; the second
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specifies the desired new power mode for that controller. The function
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operates in much the same way as <TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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>,
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for example if a separate power management thread is being used then
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<TT
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CLASS="FUNCTION"
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>power_set_controller_mode</TT
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> operates by
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manipulating some internal state and waking up that thread. The
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limitations are also much the same as for
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<TT
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CLASS="FUNCTION"
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>power_set_mode</TT
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>, so for example
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<TT
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CLASS="FUNCTION"
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>power_set_controller_mode</TT
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> should not be invoked
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from inside ISRs.</P
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><P
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>Manipulating individual controllers is often used in conjunction with
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the function <A
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HREF="power-attached.html"
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><TT
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CLASS="FUNCTION"
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>power_set_controller_attached</TT
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></A
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>,
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allowing the policy module to specify which controllers are affected
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by global mode changes.</P
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="POWER-CHANGE-CONTROLLER-NOW"
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></A
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><H2
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>Direct Manipulation of a Power Controller</H2
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><P
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>In exceptional circumstances it may be necessary to invoke a power
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controller directly, bypassing the power management thread and
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higher-level functionality such as <A
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HREF="power-policy.html#POWER-POLICY-CALLBACK"
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>callback functions</A
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>. The
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function <TT
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CLASS="FUNCTION"
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>power_set_controller_mode_now</TT
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> allows
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this. It takes two arguments, a controller and a mode, just like
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<TT
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CLASS="FUNCTION"
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>power_set_controller_mode</TT
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>.</P
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><P
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>Use of <TT
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CLASS="FUNCTION"
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>power_set_controller_mode_now</TT
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> is
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dangerous. For example no attempt is made to synchronise with any
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other power mode changes that might be happening concurrently. A
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possible use is when the system gets woken up out of
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<SPAN
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CLASS="TYPE"
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>sleep</SPAN
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> mode: depending on the hardware, on which power
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controllers are present, and on the application code it may be
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necessary to wake up some power controllers immediately before the
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system as a whole is ready to run again.</P
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