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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.
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:
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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"...]...];
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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 possiblity 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.
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