PM-COMPONENTS(9P) Kernel Properties for Drivers PM-COMPONENTS(9P)


pm-components - Power Management device property


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

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(5).

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

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",
"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,"


See attributes(7) for descriptions of the following attributes:

|Interface stability | Committed |


pm(4D), power.conf(5), attach(9E), detach(9E),
ddi_prop_update_string_array(9F), pm_busy_component(9F),

Writing Device Drivers

May 23, 2021 PM-COMPONENTS(9P)