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Supporting brightness controls on integrated display panels

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This article describes the mechanisms and requirements for supporting brightness controls on integrated display panels. It outlines the collaboration between the system-supplied monitor driver (Monitor.sys), kernel-mode display miniport drivers (KMD), and the BIOS through ACPI methods. Whether through KMD or ACPI, the goal is to ensure that users can easily adjust their display's brightness, enhancing their computing experience while optimizing power usage.

Brightness control support in Monitor.sys

Brightness controls are implemented in Monitor.sys. This system-supplied monitor driver provides the following support:

  • Implements a Windows Management Instrumentation (WMI) interface to allow applications (such as the operating system's brightness slider) to interact with the brightness level.

  • Registers with the Device Power Policy Engine (DPPE) so that brightness levels respond to changes in power policy.

  • Registers with the Advanced Configuration and Power Interface (ACPI) to process ACPI-based brightness shortcut keys. For compatibility with the legacy Windows 2000 Display Driver Model, the monitor driver implements the IOCTL-based brightness controls.

Either the kernel-mode display miniport driver (KMD) or the ACPI methods exposed by the BIOS can support changing the brightness of an integrated display panel. For the first video target that is marked as having output technology that connects internally in a computer (D3DKMDT_VOT_INTERNAL), the monitor driver calls the KMD's DxgkDdiQueryInterface function to query for both of the following interfaces:

If the KMD doesn't support at least the Brightness Control Interface, the monitor driver uses ACPI to query for the _BCL, _BCM, and _BQC methods on the child device. For more information about these methods, see the ACPI specification on the ACPI website.

Note

In the Windows Display Driver Model (WDDM), an ACPI identifier is not used to identify an integrated display panel. This is different from the Windows 2000 Display Driver Model, which supports only display panels with an identifier of 0x0110.

If either the KMD or BIOS-exposed ACPI methods support brightness controls, the monitor driver registers for ACPI notifications of brightness shortcut keys. No alternative mechanism exists to signal the monitor driver about shortcut key notifications. If the monitor driver can't use either brightness-control mechanism or if the KMD supplies the brightness control interface but fails a call to the DxgkDdiGetPossibleBrightness function, the monitor driver doesn't support brightness controls.

Brightness levels

Brightness levels are represented as single-byte values in the range from zero to 100 where zero is off and 100 is the maximum brightness that a laptop computer supports. Every laptop computer must report a maximum brightness level of 100; however, a laptop computer isn't required to support a level of zero.

The only requirement for values from zero to 100 is that larger values must represent higher brightness levels. The increment between levels isn't required to be uniform, and a laptop computer can support any number of distinct values up to the maximum of 101 levels. You must decide how to map hardware levels to the range of brightness level values. However, a call to the KMD's DxgkDdiGetPossibleBrightness function shouldn't report more brightness level values than the hardware supports.

Disabling automatic brightness changes by the BIOS

The KMD should set bit 2 of the argument to the _DOS method. Setting this bit avoids problems that might occur if the system BIOS and monitor driver both control display panel brightness. For more information about the _DOS method and its arguments, see the ACPI specification. By setting bit 2, the system BIOS is informed that it shouldn't perform any automatic brightness changes.

BIOS requirements to support brightness controls

For the KMD to support controlling integrated panel brightness in an optimum way, the system BIOS must provide the following items through the ACPI.

Brightness control methods

An integrated panel device should support the ACPI brightness control methods (_BCL, _BCM, and _BQC). _BCL and _BCM are unchanged since version 1.0b of the ACPI specification; you can find their definitions in the ACPI 3.0 specification in sections B.6.2 and B.6.3. _BQC is optional and is defined in the ACPI 3.0 specification in section B.6.4. For definitions of brightness levels, see Brightness Levels.

The following are the aliases for the ACPI brightness control methods defined in Dispmprt.h:

  • ACPI_METHOD_OUTPUT_BCL - Allows Windows to query a list of brightness levels supported by the display output devices. This method is required if an integrated LCD is present and supports brightness levels.

  • ACPI_METHOD_OUTPUT_BCM - Allows Windows to set the brightness level of the display output device. Windows only sets levels reported by the ACPI_METHOD_OUTPUT_BCL method. The ACPI_METHOD_OUTPUT_BCM method is required if the ACPI_METHOD_OUTPUT_BCL method is implemented.

Disabling the automatic system BIOS brightness control

The system BIOS should support setting bit 2 of the argument to the _DOS method on the graphics adapter to allow automatic system BIOS brightness changes to be disabled. This bit is an addition to the previously defined values for the bits in this method. For details about this bit, see section B.4.1 in the ACPI 3.0 specification.

If this bit isn't supported, the monitor driver and the system BIOS can both change the brightness level, resulting in a flicker of brightness. In addition, the brightness might potentially be set to a value that isn't what the user requested.

ACPI_METHOD_DISPLAY_DOS, defined in Dispmprt.h, is the alias for the ACPI automatic brightness control method. This alias indicates that the system BIOS is capable of automatically switching the active display output or controlling the brightness of the LCD. Its parameter can be one of the following values.

Value Meaning
ACPI_ARG_ENABLE_AUTO_LCD_BRIGHTNESS The system BIOS should automatically control the brightness level of the LCD when the power changes from AC to DC.
ACPI_ARG_DISABLE_AUTO_LCD_BRIGHTNESS The system BIOS shouldn't automatically control the brightness level of the LCD when the power changes from AC to DC.

Notifications of brightness shortcut keys

Brightness shortcut key notifications should be targeted to the integrated display panel device, not to the graphics adapter.

The following notifications are supported as defined in Dispmprt.h:

Notification Meaning
ACPI_NOTIFY_CYCLE_BRIGHTNESS_HOTKEY The user pressed the hotkey for cycling display brightness.
ACPI_NOTIFY_INC_BRIGHTNESS_HOTKEY The user pressed the hotkey for increasing display brightness.
ACPI_NOTIFY_DEC_BRIGHTNESS_HOTKEY The user pressed the hotkey for decreasing display brightness.
ACPI_NOTIFY_ZERO_BRIGHTNESS_HOTKEY The user pressed the hotkey for reducing display brightness to zero.

These shortcut key notifications were new to the ACPI 3.0 specification and are described in section B.7. Typically, a laptop computer wouldn't support all of these shortcut key notifications.

For increasing and decreasing brightness levels, the monitor driver's default behavior is:

  • For ACPI_NOTIFY_INC_BRIGHTNESS_HOTKEY, increment brightness by at least five percent more than the previous brightness level until the next available five percent step level is reached (5, 10, 15, ..., 95, 100).

  • For ACPI_NOTIFY_DEC_BRIGHTNESS_HOTKEY, decrement brightness by at least five percent less than the previous brightness level until the next available five percent step level is reached (100, 95, ..., 15, 10, 5).

Incrementing or decrementing with shortcut keys can create asymmetrical patterns in brightness levels, as the following examples show.

  • Available _BCL brightness control levels specified as 0, 1, 5, 10, ..., 95, 100

    • Results using the ACPI_NOTIFY_INC_BRIGHTNESS_HOTKEY notification:
      0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100

    • Results using the ACPI_NOTIFY_DEC_BRIGHTNESS_HOTKEY notification:
      100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, 0

  • Available _BCL brightness control levels specified as 1, 5, 10, ..., 95, 100

    • Results using the ACPI_NOTIFY_INC_BRIGHTNESS_HOTKEY notification:
      1, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100

    • Results using the ACPI_NOTIFY_DEC_BRIGHTNESS_HOTKEY notification:
      100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10, 5, 1

    In the latter example, 1 is the last available value, so the driver sets the brightness level to 1 even though it's less than five percentage units different from the previous value of 5.

An IHV or OEM can change the default monitor driver behavior on Windows Vista and Windows 7 by setting a DWORD value for MinimumStepPercentage in the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\monitor\Parameters\

Supporting Display Output and ACPI Events