src/devices/graphics/arch-sf-hwc.jd

page.title=SurfaceFlinger and Hardware Composer
@jd:body

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    <h2>In this document</h2>
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<p>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.</p>


<h2 id=surfaceflinger>SurfaceFlinger</h2>

<p>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.
<code>/dev/graphics/fb0</code>), but those days are long gone.</p>

<p>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.</p>

<p class="note"><strong>Note:</strong> While this section uses SurfaceFlinger
terminology, WindowManager uses the term <em>window</em> instead of
<em>layer</em>&hellip;and uses layer to mean something else. (It can be argued
that SurfaceFlinger should really be called LayerFlinger.)</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>After SurfaceFlinger has collected all buffers for visible layers, it asks
the Hardware Composer how composition should be performed.</p>

<h2 id=hwc>Hardware Composer</h2>

<p>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.</p>

<p>The value of this approach is easy to recognize when you consider <em>overlay
planes</em>, 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:</p>

<ul>
<li>Rendering the app content into a scratch buffer, then rendering the status
bar over it, the navigation bar on top of that, and finally passing the scratch
buffer to the display hardware.</li>
<li>Passing all three buffers to the display hardware and tell it to read data
from different buffers for different parts of the screen.</li>
</ul>

<p>The latter approach can be significantly more efficient.</p>

<p>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:</p>

<ol>
<li>SurfaceFlinger provides HWC with a full list of layers and asks, "How do
you want to handle this?"</li>
<li>HWC responds by marking each layer as overlay or GLES composition.</li>
<li>SurfaceFlinger takes care of any GLES composition, passing the output buffer
to HWC, and lets HWC handle the rest.</li>
</ol>

<p>Since hardware vendors can custom tailor decision-making code, it's possible
to get the best performance out of every device.</p>

<p>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.</p>

<p>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.</p>

<h2 id=virtual-displays>Virtual displays</h2>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>As you might expect, frames generated for a virtual display are written to a
BufferQueue.</p>

<h2 id=screenrecord>Case Study: screenrecord</h2>

<p>The <a href="https://android.googlesource.com/platform/frameworks/av/+/marshmallow-release/cmds/screenrecord/">screenrecord
command</a> 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.</p>

<p>The MediaCodec class allows an app to provide data as raw bytes in buffers,
or through a <a href="{@docRoot}devices/graphics/arch-sh.html">Surface</a>. 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.</p>

<p>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.</p>

<p>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.</p>

<h2 id=simulate-secondary>Case Study: Simulate secondary displays</h2>

<p>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?</p>

<p>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
<a href="https://en.wikipedia.org/wiki/Turtles_all_the_way_down">turtles all the
way down)</a>. To see this in action, enable
<a href="http://developer.android.com/tools/index.html">Developer options</a> in
settings, select <strong>Simulate secondary displays</strong>, and enable a
window. For bonus points, use screenrecord to capture the act of enabling the
display then play it back frame-by-frame.</p>