Comtrol eCos - Manual (Page 666)

Changing Power Modes
However, it is still legal for a power controller to call power_set_mode : effectively this is a recursive call; it is
detected by the system, and internal state is updated; the recursive power_set_mode call now returns, and when
the power controller returns back to the original power_set_mode call it detects what has happened, aborts the
previous mode change, and starts a new mode change as requested by the controller.
power_set_mode 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.
power_set_mode should never be
invoked from ISR context.
Manipulating an Individual Power Controller
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 con-
troller could be set to
PowerMode_Off , even while the system as a whole is still active. This can be achieved
by calling the function power_set_controller_mode . 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
power_set_mode , for example if a separate power management thread is being used then
power_set_controller_mode operates by manipulating some internal state and waking up that thread. The lim-
itations are also much the same as for power_set_mode , so for example power_set_controller_mode should
not be invoked from inside ISRs.
Manipulating individual controllers is often used in conjunction with the function
power_set_controller_attached , allowing the policy module to specify which controllers are affected by
global mode changes.
Direct Manipulation of a Power Controller
In exceptional circumstances it may be necessary to invoke a power controller directly, bypassing the
power management thread and higher-level functionality such as
callback functions . The function
power_set_controller_mode_now allows this. It takes two arguments, a controller and a mode, just like
power_set_controller_mode .
Use of power_set_controller_mode_now 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 sleep 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.
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