Advanced Configuration and Power Interface Specification

ACPI-Defined Devices and Device-Specific Objects
The OS derives the user’s display brightness preference (this reference value) either from the
Brightness Control Levels (_BCL) object or another OS-specific mechanism. See Section 9.2.8,
“Relationship to Backlight Control Methods”, for more information.
9.2.6 _ALP (Ambient Light Polling)
This optional object evaluates to a recommended polling frequency (in tenths of seconds) for this
ambient light sensor. A value of zero – or the absence of this object when other ALS objects are
defined – indicates that OSPM does not need to poll the sensor in order to detect meaningful changes
in ambient light (the hardware is capable of generating asynchronous notifications).
The use of polling is allowed but strongly discouraged by this specification. OEMs should design
systems that asynchronously notify OSPM whenever a meaningful change in the ambient light
occurs—relieving the OS of the overhead associated with polling.
This value is specified as tenths of seconds. For example, a value of 10 would be used to indicate a 1
second polling frequency. As this is a recommended value, OSPM will consider other factors when
determining the actual polling frequency to use.
Arguments:
None
Return Value:
An Integer containing the recommended polling frequency in tenths of seconds
0 – Polling by the host OS is not required
Other –
9.2.7 Ambient Light Sensor Events
The recommended polling frequency in tenths of seconds
To communicate meaningful changes in ALS illuminance to OSPM, AML code should issue a
Notify(als_device, 0x80) whenever the lux reading changes more than 10% (from the last
reading that resulted in a notification). OSPM receives this notification and evaluates the _ALI
control method to determine the current ambient light status. The OS then adjusts the display
brightness based upon its ALS policy (derived from _ALR).
The definition of what constitutes a meaningful change is left to the system integrator, but should be
at a level of granularity that provides an appropriate response without overly taxing the system with
unnecessary interrupts. For example, an ALS configuration may be tuned to generate events for all
changes in ambient light illuminance that result in a minimum ±5% display brightness response (as
defined by _ALR).
To communicate meaningful changes in ALS color temperature to OSPM, AML code should issue a
Notify(als_device, 0x81) whenever the lux reading changes more than 10% (from the last
reading that resulted in a notification). OSPM receives this notification and evaluates the _ALT and
_ALC control method to determine the current ambient light color temperature.
To communicate meaningful changes in ALS response to OSPM, AML code should issue a
Notify(als_device, 0x82) whenever the set of points used to convey ambient light
response has changed. OSPM receives this notification and evaluates the _ALR object to determine
the current response points.
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