A device is power manageable if the power consumption of the device can be reduced when it is idle. In general, a power manageable device consists of a number of power manageable hardware units called components. Each component is separately controllable and has its own set of power parameters.

An example of a one-component power manageable device is a disk whose spindle motor can be stopped to save power when the disk is idle. An example of a two-component power manageable device is a frame buffer card with a connected monitor. The frame buffer electronics (with power that can be reduced when not in use) comprises the first component. The second component is the monitor, which can enter in a lower power mode when not in use. The combination of frame buffer electronics and monitor is considered as one device by the system.

In the Power Management framework, all components are considered equal and completely independent of each other. If this is not true for a particular device, the device driver must ensure that undesirable state combinations do not occur. Each component is created in the idle state.

The pm-components property describes the Power Management model of a device driver to the Power Management framework. It lists each power manageable component by name and lists the power level supported by each component by numerical value and name. Its syntax and interpretation is described below.

This property is only interpreted by the system immediately after the device has successfully attached, or upon the first call into Power Management framework, whichever comes first. Changes in the property made by the driver after the property has been interpreted will not be recognized.

pm-components is a string array property. The existence of the pm-components property indicates that a device implements power manageable components and describes the Power Management model implemented by the device driver. The existence of pm-components also indicates to the framework that device is ready for Power Management if automatic device Power Management is enabled. See power.conf(4).

The pm-component property syntax is:

pm-components="NAME=component name","numeric power level=power level name", "numeric power level=power level name" [, "numeric power level=power level name" ...] [, "NAME=component name", "numeric power level=power level name", "numeric power level=power level name" [, "numeric power level=power level name"...]...]; 

The start of each new component is represented by a string consisting of NAME= followed by the name of the component. This should be a short name that a user would recognize, such as "Monitor" or "Spindle Motor." The succeeding elements in the string array must be strings consisting of the numeric value (can be decimal or 0x <hexadecimal number>) of a power level the component supports, followed by an equal sign followed by a short descriptive name for that power level. Again, the names should be descriptive, such as "On," "Off," "Suspend", "Standby," etc. The next component continues the array in the same manner, with a string that starts out NAME=, specifying the beginning of a new component (and its name), followed by specifications of the power levels the component supports.

The components must be listed in increasing order according to the component number as interpreted by the driver's power(9E) routine. (Components are numbered sequentially from 0). The power levels must be listed in increasing order of power consumption. Each component must support at least two power levels, or there is no possibility of power level transitions. If a power level value of 0 is used, it must be the first one listed for that component. A power level value of 0 has a special meaning (off) to the Power Management framework.

An example of a pm-components entry from the .conf file of a driver which implements a single power managed component consisting of a disk spindle motor is shown below. This is component 0 and it supports 2 power level, which represent spindle stopped or full speed.

pm-components="NAME=Spindle Motor", "0=Stopped", "1=Full Speed"; 
...

Below is an example of how the above entry would be implemented in the attach(9E) function of the driver.

static char *pmcomps[] = {
 "NAME=Spindle Motor",
 "0=Stopped",
 "1=Full Speed"
};

...

xxattach(dev_info_t *dip, ddi_attach_cmd_t cmd)
{
...
 if (ddi_prop_update_string_array(DDI_DEV_T_NONE, dip, "pm-components",
 &pmcomp[0], sizeof (pmcomps) / sizeof (char *)) !=DDI_PROP_SUCCESS)
 goto failed;
}

Below is an example for a frame buffer which implements two components. Component 0 is the frame buffer electronics which supports four different power levels. Component 1 represents the state of Power Management of the attached monitor.

pm-components="NAME=Frame Buffer", "0=Off" 
 "1=Suspend", "2=Standby", "3=On",
 "NAME=Monitor", "0=Off", "1=Suspend", "2=Standby," 
 "3=On;

See attributes(5) for descriptions of the following attributes:

power.conf(4), pm(7D), attach(9E), detach(9E), ddi_prop_update_string_array(9F) pm_busy_component(9F), pm_idle_component(9F)

Writing Device Drivers