Display and Graphics Power Saving

Feature descriptions

Framebuffer Compression

Intel mobile devices support run length encoding (RLE) of the framebuffer in certain configurations. The compressed buffer is stored in a separate memory region. The buffer’s coherency is maintained by a separate tag buffer, which is managed by hardware. When enabled, the graphics device will refresh the display from compressed lines where possible, and uncompressed lines where necessary, reducing bus traffic, which saves power.

Backlight Control

The backlight is a significant power consumer on mobile platforms. So users and applications need a way to control backlight brightness for power and usability reasons.

Minimized Vertical Blank Interrupts

Interrupts unconditionally wake the CPU from any sleep state, so they’re especially important to avoid when the system would otherwise be idle.

Vertical blank counts are required to support certain OpenGL and other graphics APIs. The counts are usually kept by vertical blank interrupts.

Unfortunately, these interrupts happen at refresh rate frequency (that is, usually higher than 60 Hz).

Intel hardware tracks vertical blank counts regardless of whether interrupts are enabled. So we need only enable them when an application is waiting for a specific counter value.

Display Refresh Rate Switching

Using lower refresh rates saves power. For example, it may be desirable to dynamically lowering refresh rate when switching to battery power.

This concept could be expanded to be driven by user activity, such as idleness or the requirements of the content on your display. Another power saving option, in addition to lowering refresh rates, could be using interlaced modes.

Dynamic Clock Control

The render clock drives all graphics operations. Dynamic Clock Control allows you to reduce the render clock’s frequency. This slows the graphics chip, reducing its power consumption. The render clock can be slowed, as long as graphics processing requirements are minimal.