In this design, the primary plane can be overlayed and/or alpha-blended with up to eight additional planes inside the video mixer. The z-order (foreground or background position) of the planes is fixed. The global alpha mode can be configured per plane through the driver by means of custom KMS properties: an alpha value of 0% (or 0) means the layer is fully transparent (invisible); an alpha value of 100% (or 255) means that the layer is fully opaque.
Each overlay plane’s width, height, X- and Y-offset is run-time programmable relative to the primary plane or CRTC which determines the display resolution. The pixel formats of the primary plane as well as the eight overlay planes are fixed: one BGR plane (primary) plus four YUY2 planes (overlay) plus four BGR planes (overlay) from bottom to top.
The Xilinx DRM driver supports the universal plane feature, therefore the primary plane and overlay planes can be configured through the same API. A page-flip is the operation that configures a plane with the new buffer index to be selected for the next scan-out. The new buffer is prepared while the current buffer is being scanned out and the flip typically happens during vertical blanking to avoid image tearing.