System Decorations

In Android 9 (and lower), SurfaceFlinger and DisplayManagerService assumed the existence of at most two physical displays with hard-coded IDs 0 and 1. As described in Static Display Identifiers, SurfaceFlinger now leverages a Hardware Composer (HWC) API to generate stable display IDs, which enables it to manage an arbitrary number of physical displays.

The framework can look up the IBinder token for a physical display via SurfaceControl#getPhysicalDisplayToken after obtaining the 64-bit display ID from SurfaceControl#getPhysicalDisplayIds or from a DisplayEventReceiver hotplug event.

In Android 10, primary internal display is TYPE_BUILT_IN, and all secondary displays are flagged as TYPE_HDMI regardless of connection type. Therefore, additional internal displays are currently treated as external. As a workaround, device-specific code can make assumptions about DisplayAddress.Physical#getPort if the HWC is known and the port allocation logic is predictable.


Previously, displays were identified by 32-bit IDs, where 0 is the internal display, 1 is the external display, [2, INT32_MAX] are HWC virtual displays, and -1 represents an invalid display or a non-HWC virtual display. For SurfaceFlinger and DisplayManagerService to track more than two displays and recognize previously seen displays, displays should be given stable and persistent IDs.

If the HWC supports IComposerClient.getDisplayIdentificationData and provides display identification data, SurfaceFlinger parses the EDID structure and allocates stable 64-bit display IDs for physical and HWC virtual displays. The IDs are expressed using an option type, where the null value represents an invalid display or non-HWC virtual display. Without HWC support, SurfaceFlinger falls back to legacy behavior with at most two physical displays.