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SurfaceFlinger and WindowManager

SurfaceFlinger accepts buffers, composes buffers, and sends buffers to the display. WindowManager provides SurfaceFlinger with buffers and window metadata, which SurfaceFlinger uses to composite surfaces to the display.

SurfaceFlinger

SurfaceFlinger can accept buffers in two ways: through BufferQueue and SurfaceControl, or through ASurfaceControl.

One way SurfaceFlinger accepts buffers is through BufferQueue and SurfaceControl. When an app comes to the foreground, it requests buffers from WindowManager. WindowManager then requests a layer from SurfaceFlinger. A layer is a combination of a surface, which contains the BufferQueue, and a SurfaceControl, which contains the layer metadata like the display frame. SurfaceFlinger creates the layer and sends it to WindowManager. WindowManager then sends the surface to the app, but keeps the SurfaceControl to manipulate the appearance of the app on the screen.

Android 10 adds ASurfaceControl, which is another way that SurfaceFlinger can accept buffers. ASurfaceControl combines a surface and a SurfaceControl into one transaction package that is sent to SurfaceFlinger. An ASurfaceControl is associated with a layer, which apps update through ASurfaceTransactions. Apps then get information about ASurfaceTransactions through callbacks that pass ASurfaceTransactionStats containing information, such as latch time, acquire times, and so on.

The following table includes more details about ASurfaceControl and its associated components.

Component Description
ASurfaceControl Wraps SurfaceControl and enables an app to create SurfaceControls that correspond to layers on the display.

Can be created as a child of ANativeWindow or as a child of another ASurfaceControl.
ASurfaceTransaction Wraps Transaction to enable the client to edit a layer's descriptive properties, such as geometry, and sends the updated buffers to SurfaceFlinger.
ASurfaceTransactionStats Sends information about transactions that have been presented, such as latch time, acquire times, and previous release fence, to an app through a preregistered callback.

Though apps can submit buffers at any time, SurfaceFlinger only wakes up to accept buffers between display refreshes, which can differ depending on the device. This minimizes memory usage and avoids visible tearing on the screen, which can occur when updating the display mid-refresh.

When the display is between refreshes, the display sends the VSYNC signal to SurfaceFlinger. The VSYNC signal indicates that the display can be refreshed without tearing. When SurfaceFlinger receives the VSYNC signal, SurfaceFlinger walks through its list of layers looking for new buffers. If SurfaceFlinger finds a new buffer, SurfaceFlinger acquires the buffer; if not, SurfaceFlinger continues to use the previously acquired buffer. SurfaceFlinger must always display something, so it hangs on to one buffer. If no buffers have ever been submitted on a layer, the layer is ignored.

After SurfaceFlinger has collected all buffers for visible layers, it asks the Hardware Composer (HWC) how composition should be performed. If the HWC marks layer composition type as client composition, SurfaceFlinger composites those layers. Then, SurfaceFlinger passes the output buffer to the HWC.

WindowManager

WindowManager controls window objects, which are containers for view objects. Window objects are always backed by surface objects. WindowManager oversees lifecycles, input and focus events, screen orientation, transitions, animations, position, transforms, z-order, and many other aspects of a window. WindowManager sends all of the window metadata to SurfaceFlinger so SurfaceFlinger can use that data to composite surfaces on the display.

Pre-rotation

Many hardware overlays don't support rotation (and even if they do, it costs processing power); the solution is to transform the buffer before it reaches SurfaceFlinger. Android supports a query hint (NATIVE_WINDOW_TRANSFORM_HINT) in ANativeWindow to represent the most likely transform to be applied to the buffer by SurfaceFlinger. GL drivers can use this hint to pre-transform the buffer before it reaches SurfaceFlinger so that when the buffer arrives, it's correctly transformed.

For example, when receiving a hint to rotate 90 degrees, generate and apply a matrix to the buffer to prevent it from running off the end of the page. To save power, do this pre-rotation. For details, see the ANativeWindow interface defined in system/core/include/system/window.h.