Having buffers of graphical data is wonderful, but life is even better when you get to see them on your device's screen. That's where SurfaceFlinger and the Hardware Composer HAL come in.
SurfaceFlinger's role is to accept buffers of data from multiple sources,
composite them, and send them to the display. Once upon a time this was done
with software blitting to a hardware framebuffer (e.g.
/dev/graphics/fb0), but those days are long gone.
When an app comes to the foreground, the WindowManager service asks SurfaceFlinger for a drawing surface. SurfaceFlinger creates a layer (the primary component of which is a BufferQueue) for which SurfaceFlinger acts as the consumer. A Binder object for the producer side is passed through the WindowManager to the app, which can then start sending frames directly to SurfaceFlinger.
Note: While this section uses SurfaceFlinger terminology, WindowManager uses the term window instead of layer…and uses layer to mean something else. (It can be argued that SurfaceFlinger should really be called LayerFlinger.)
Most applications have three layers on screen at any time: the status bar at the top of the screen, the navigation bar at the bottom or side, and the application UI. Some apps have more, some less (e.g. the default home app has a separate layer for the wallpaper, while a full-screen game might hide the status bar. Each layer can be updated independently. The status and navigation bars are rendered by a system process, while the app layers are rendered by the app, with no coordination between the two.
Device displays refresh at a certain rate, typically 60 frames per second on phones and tablets. If the display contents are updated mid-refresh, tearing will be visible; so it's important to update the contents only between cycles. The system receives a signal from the display when it's safe to update the contents. For historical reasons we'll call this the VSYNC signal.
The refresh rate may vary over time, e.g. some mobile devices will range from 58 to 62fps depending on current conditions. For an HDMI-attached television, this could theoretically dip to 24 or 48Hz to match a video. Because we can update the screen only once per refresh cycle, submitting buffers for display at 200fps would be a waste of effort as most of the frames would never be seen. Instead of taking action whenever an app submits a buffer, SurfaceFlinger wakes up when the display is ready for something new.
When the VSYNC signal arrives, SurfaceFlinger walks through its list of layers looking for new buffers. If it finds a new one, it acquires it; if not, it continues to use the previously-acquired buffer. SurfaceFlinger always wants to have something to display, so it will hang 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 how composition should be performed.
The Hardware Composer HAL (HWC) was introduced in Android 3.0 and has evolved steadily over the years. Its primary purpose is to determine the most efficient way to composite buffers with the available hardware. As a HAL, its implementation is device-specific and usually done by the display hardware OEM.
The value of this approach is easy to recognize when you consider overlay planes, the purpose of which is to composite multiple buffers together in the display hardware rather than the GPU. For example, consider a typical Android phone in portrait orientation, with the status bar on top, navigation bar at the bottom, and app content everywhere else. The contents for each layer are in separate buffers. You could handle composition using either of the following methods:
The latter approach can be significantly more efficient.
Display processor capabilities vary significantly. The number of overlays, whether layers can be rotated or blended, and restrictions on positioning and overlap can be difficult to express through an API. The HWC attempts to accommodate such diversity through a series of decisions:
Since hardware vendors can custom tailor decision-making code, it's possible to get the best performance out of every device.
Overlay planes may be less efficient than GL composition when nothing on the screen is changing. This is particularly true when overlay contents have transparent pixels and overlapping layers are blended together. In such cases, the HWC can choose to request GLES composition for some or all layers and retain the composited buffer. If SurfaceFlinger comes back asking to composite the same set of buffers, the HWC can continue to show the previously-composited scratch buffer. This can improve the battery life of an idle device.
Devices running Android 4.4 and later typically support four overlay planes. Attempting to composite more layers than overlays causes the system to use GLES composition for some of them, meaning the number of layers used by an app can have a measurable impact on power consumption and performance.
SurfaceFlinger supports a primary display (i.e. what's built into your phone or tablet), an external display (such as a television connected through HDMI), and one or more virtual displays that make composited output available within the system. Virtual displays can be used to record the screen or send it over a network.
Virtual displays may share the same set of layers as the main display (the layer stack) or have its own set. There is no VSYNC for a virtual display, so the VSYNC for the primary display is used to trigger composition for all displays.
In older versions of Android, virtual displays were always composited with GLES and the Hardware Composer managed composition for the primary display only. In Android 4.4, the Hardware Composer gained the ability to participate in virtual display composition.
As you might expect, frames generated for a virtual display are written to a BufferQueue.
The screenrecord command allows you to record everything that appears on the screen as an .mp4 file on disk. To implement, we have to receive composited frames from SurfaceFlinger, write them to the video encoder, and then write the encoded video data to a file. The video codecs are managed by a separate process (mediaserver) so we have to move large graphics buffers around the system. To make it more challenging, we're trying to record 60fps video at full resolution. The key to making this work efficiently is BufferQueue.
The MediaCodec class allows an app to provide data as raw bytes in buffers, or through a Surface. When screenrecord requests access to a video encoder, mediaserver creates a BufferQueue, connects itself to the consumer side, then passes the producer side back to screenrecord as a Surface.
The screenrecord command then asks SurfaceFlinger to create a virtual display that mirrors the main display (i.e. it has all of the same layers), and directs it to send output to the Surface that came from mediaserver. In this case, SurfaceFlinger is the producer of buffers rather than the consumer.
After the configuration is complete, screenrecord waits for encoded data to appear. As apps draw, their buffers travel to SurfaceFlinger, which composites them into a single buffer that gets sent directly to the video encoder in mediaserver. The full frames are never even seen by the screenrecord process. Internally, mediaserver has its own way of moving buffers around that also passes data by handle, minimizing overhead.
The WindowManager can ask SurfaceFlinger to create a visible layer for which SurfaceFlinger acts as the BufferQueue consumer. It's also possible to ask SurfaceFlinger to create a virtual display, for which SurfaceFlinger acts as the BufferQueue producer. What happens if you connect them, configuring a virtual display that renders to a visible layer?
You create a closed loop, where the composited screen appears in a window. That window is now part of the composited output, so on the next refresh the composited image inside the window will show the window contents as well (and then it's turtles all the way down). To see this in action, enable Developer options in settings, select Simulate secondary displays, and enable a window. For bonus points, use screenrecord to capture the act of enabling the display then play it back frame-by-frame.