Docs: HWC2 updates, new section+subsections
Removing Framebuffer bullet
Making OpenGL 3.x drivers optional
Fixing horrendous typo in nav
Adding Clay's feedback
Bug: 28419158
Change-Id: I19ce49d22f7bdb54229363580d9075f71037ef9c
diff --git a/src/devices/graphics/implement.jd b/src/devices/graphics/implement.jd
index 3f3654a..178f4b8 100644
--- a/src/devices/graphics/implement.jd
+++ b/src/devices/graphics/implement.jd
@@ -26,580 +26,140 @@
</div>
-<p>Follow the instructions here to implement the Android graphics HAL.</p>
+<p>To implement the Android graphics HAL, review the following requirements,
+implementation details, and testing advice.</p>
<h2 id=requirements>Requirements</h2>
-<p>The following list and sections describe what you need to provide to support
-graphics in your product:</p>
+<p>Android graphics support requires the following components:</p>
-<ul> <li> OpenGL ES 1.x Driver <li> OpenGL ES 2.0 Driver <li> OpenGL ES 3.0
-Driver (optional) <li> EGL Driver <li> Gralloc HAL implementation <li> Hardware
-Composer HAL implementation <li> Framebuffer HAL implementation </ul>
+<ul>
+ <li>EGL driver</li>
+ <li>OpenGL ES 1.x driver</li>
+ <li>OpenGL ES 2.0 driver</li>
+ <li>OpenGL ES 3.x driver (optional)</li>
+ <li>Gralloc HAL implementation</li>
+ <li>Hardware Composer HAL implementation</li>
+</ul>
<h2 id=implementation>Implementation</h2>
<h3 id=opengl_and_egl_drivers>OpenGL and EGL drivers</h3>
-<p>You must provide drivers for OpenGL ES 1.x, OpenGL ES 2.0, and EGL. Here are
-some key considerations:</p>
+<p>You must provide drivers for EGL, OpenGL ES 1.x, and OpenGL ES 2.0 (support
+for OpenGL 3.x is optional). Key considerations include:</p>
-<ul> <li> The GL driver needs to be robust and conformant to OpenGL ES
-standards. <li> Do not limit the number of GL contexts. Because Android allows
-apps in the background and tries to keep GL contexts alive, you should not
-limit the number of contexts in your driver. <li> It is not uncommon to have
-20-30 active GL contexts at once, so you should also be careful with the amount
-of memory allocated for each context. <li> Support the YV12 image format and
-any other YUV image formats that come from other components in the system such
-as media codecs or the camera. <li> Support the mandatory extensions:
-<code>GL_OES_texture_external</code>,
-<code>EGL_ANDROID_image_native_buffer</code>, and
-<code>EGL_ANDROID_recordable</code>. The
-<code>EGL_ANDROID_framebuffer_target</code> extension is required for Hardware
-Composer 1.1 and higher, as well. <li> We highly recommend also supporting
-<code>EGL_ANDROID_blob_cache</code>, <code>EGL_KHR_fence_sync</code>,
-<code>EGL_KHR_wait_sync</code>, and <code>EGL_ANDROID_native_fence_sync</code>.
-</ul>
+<ul>
+ <li>GL driver must be robust and conformant to OpenGL ES standards.</li>
+ <li>Do not limit the number of GL contexts. Because Android allows apps in
+ the background and tries to keep GL contexts alive, you should not limit the
+ number of contexts in your driver.</li>
+ <li> It is common to have 20-30 active GL contexts at once, so be
+ mindful of the amount of memory allocated for each context.</li>
+ <li>Support the YV12 image format and other YUV image formats that come from
+ other components in the system, such as media codecs or the camera.</li>
+ <li>Support the mandatory extensions: <code>GL_OES_texture_external</code>,
+ <code>EGL_ANDROID_image_native_buffer</code>, and
+ <code>EGL_ANDROID_recordable</code>. In addition, the
+ <code>EGL_ANDROID_framebuffer_target</code> extension is required for
+ Hardware Composer v1.1 and higher.</li>
+ </ul>
+<p>We highly recommend also supporting <code>EGL_ANDROID_blob_cache</code>,
+<code>EGL_KHR_fence_sync</code>, <code>EGL_KHR_wait_sync</code>, and <code>EGL_ANDROID_native_fence_sync</code>.</p>
-<p>Note the OpenGL API exposed to app developers is different from the OpenGL
-interface that you are implementing. Apps do not have access to the GL driver
-layer and must go through the interface provided by the APIs.</p>
+<p class="note"><strong>Note</strong>: The OpenGL API exposed to app developers
+differs from the OpenGL implemented on the device. Apps cannot directly access
+the GL driver layer and must go through the interface provided by the APIs.</p>
<h3 id=pre-rotation>Pre-rotation</h3>
-<p>Many hardware overlays do not support rotation, and even if they do it costs
-processing power. So the solution is to pre-transform the buffer before it
-reaches SurfaceFlinger. A query hint in <code>ANativeWindow</code> was added
-(<code>NATIVE_WINDOW_TRANSFORM_HINT</code>) that represents the most likely
-transform to be applied to the buffer by SurfaceFlinger. Your GL driver can use
-this hint to pre-transform the buffer before it reaches SurfaceFlinger so when
-the buffer arrives, it is correctly transformed.</p>
+<p>Many hardware overlays do not support rotation (and even if they do it costs
+processing power); the solution is to pre-transform the buffer before it reaches
+SurfaceFlinger. Android supports a query hint
+(<code>NATIVE_WINDOW_TRANSFORM_HINT</code>) in <code>ANativeWindow</code> 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 when the buffer arrives, it is correctly
+transformed.</p>
-<p>For example, you may receive a hint to rotate 90 degrees. You must generate
-a matrix and apply it to the buffer to prevent it from running off the end of
-the page. To save power, this should be done in pre-rotation. See the
-<code>ANativeWindow</code> interface defined in
-<code>system/core/include/system/window.h</code> for more details.</p>
+<p>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 <code>ANativeWindow</code>
+interface defined in <code>system/core/include/system/window.h</code>.</p>
<h3 id=gralloc_hal>Gralloc HAL</h3>
-<p>The graphics memory allocator is needed to allocate memory that is requested
-by image producers. You can find the interface definition of the HAL at:
-<code>hardware/libhardware/modules/gralloc.h</code></p>
+<p>The graphics memory allocator allocates memory requested by image producers.
+You can find the interface definition of the HAL at
+<code>hardware/libhardware/modules/gralloc.h</code>.</p>
<h3 id=protected_buffers>Protected buffers</h3>
<p>The gralloc usage flag <code>GRALLOC_USAGE_PROTECTED</code> allows the
graphics buffer to be displayed only through a hardware-protected path. These
-overlay planes are the only way to display DRM content. DRM-protected buffers
-cannot be accessed by SurfaceFlinger or the OpenGL ES driver.</p>
+overlay planes are the only way to display DRM content (DRM-protected buffers
+cannot be accessed by SurfaceFlinger or the OpenGL ES driver).</p>
<p>DRM-protected video can be presented only on an overlay plane. Video players
that support protected content must be implemented with SurfaceView. Software
-running on unprotected hardware cannot read or write the buffer.
-Hardware-protected paths must appear on the Hardware Composer overlay. For
-instance, protected videos will disappear from the display if Hardware Composer
-switches to OpenGL ES composition.</p>
+running on unprotected hardware cannot read or write the buffer;
+hardware-protected paths must appear on the Hardware Composer overlay (i.e.,
+protected videos will disappear from the display if Hardware Composer switches
+to OpenGL ES composition).</p>
-<p>See the <a href="{@docRoot}devices/drm.html">DRM</a> page for a description
-of protected content.</p>
+<p>For details on protected content, see
+<a href="{@docRoot}devices/drm.html">DRM</a>.</p>
<h3 id=hardware_composer_hal>Hardware Composer HAL</h3>
-<p>The Hardware Composer HAL is used by SurfaceFlinger to composite surfaces to
-the screen. The Hardware Composer abstracts objects like overlays and 2D
-blitters and helps offload some work that would normally be done with
-OpenGL.</p>
-
-<p>We recommend you start using version 1.3 of the Hardware Composer HAL as it
-will provide support for the newest features (explicit synchronization,
-external displays, and more). Because the physical display hardware behind the
-Hardware Composer abstraction layer can vary from device to device, it is
-difficult to define recommended features. But here is some guidance:</p>
-
-<ul> <li> The Hardware Composer should support at least four overlays (status
-bar, system bar, application, and wallpaper/background). <li> Layers can be
-bigger than the screen, so the Hardware Composer should be able to handle
-layers that are larger than the display (for example, a wallpaper). <li>
-Pre-multiplied per-pixel alpha blending and per-plane alpha blending should be
-supported at the same time. <li> The Hardware Composer should be able to
-consume the same buffers that the GPU, camera, video decoder, and Skia buffers
-are producing, so supporting some of the following properties is helpful: <ul>
-<li> RGBA packing order <li> YUV formats <li> Tiling, swizzling, and stride
-properties </ul> <li> A hardware path for protected video playback must be
-present if you want to support protected content. </ul>
-
-<p>The general recommendation when implementing your Hardware Composer is to
-implement a non-operational Hardware Composer first. Once you have the
-structure done, implement a simple algorithm to delegate composition to the
-Hardware Composer. For example, just delegate the first three or four surfaces
-to the overlay hardware of the Hardware Composer.</p>
-
-<p>Focus on optimization, such as intelligently selecting the surfaces to send
-to the overlay hardware that maximizes the load taken off of the GPU. Another
-optimization is to detect whether the screen is updating. If not, delegate
-composition to OpenGL instead of the Hardware Composer to save power. When the
-screen updates again, continue to offload composition to the Hardware
-Composer.</p>
-
-<p>Devices must report the display mode (or resolution). Android uses the first
-mode reported by the device. To support televisions, have the TV device report
-the mode selected for it by the manufacturer to Hardware Composer. See
-hwcomposer.h for more details.</p>
-
-<p>Prepare for common use cases, such as:</p>
-
-<ul> <li> Full-screen games in portrait and landscape mode <li> Full-screen
-video with closed captioning and playback control <li> The home screen
-(compositing the status bar, system bar, application window, and live
-wallpapers) <li> Protected video playback <li> Multiple display support </ul>
-
-<p>These use cases should address regular, predictable uses rather than edge
-cases that are rarely encountered. Otherwise, any optimization will have little
-benefit. Implementations must balance two competing goals: animation smoothness
-and interaction latency.</p>
-
-<p>Further, to make best use of Android graphics, you must develop a robust
-clocking strategy. Performance matters little if clocks have been turned down
-to make every operation slow. You need a clocking strategy that puts the clocks
-at high speed when needed, such as to make animations seamless, and then slows
-the clocks whenever the increased speed is no longer needed.</p>
-
-<p>Use the <code>adb shell dumpsys SurfaceFlinger</code> command to see
-precisely what SurfaceFlinger is doing. See the <a
-href="{@docRoot}devices/graphics/architecture.html#hwcomposer">Hardware
-Composer</a> section of the Architecture page for example output and a
-description of relevant fields.</p>
-
-<p>You can find the HAL for the Hardware Composer and additional documentation
-in: <code>hardware/libhardware/include/hardware/hwcomposer.h
-hardware/libhardware/include/hardware/hwcomposer_defs.h</code></p>
-
-<p>A stub implementation is available in the
-<code>hardware/libhardware/modules/hwcomposer</code> directory.</p>
+<p>The Hardware Composer HAL (HWC) is used by SurfaceFlinger to composite
+surfaces to the screen. It abstracts objects such as overlays and 2D blitters
+and helps offload some work that would normally be done with OpenGL. For details
+on the HWC, see <a href="{@docRoot}devices/graphics/implement-hwc.html">Hardware
+Composer HAL</a>.</p>
<h3 id=vsync>VSYNC</h3>
<p>VSYNC synchronizes certain events to the refresh cycle of the display.
-Applications always start drawing on a VSYNC boundary, and SurfaceFlinger
-always composites on a VSYNC boundary. This eliminates stutters and improves
-visual performance of graphics. The Hardware Composer has a function
-pointer:</p>
-
-<pre class=prettyprint> int (waitForVsync*) (int64_t *timestamp) </pre>
-
-
-<p>This points to a function you must implement for VSYNC. This function blocks
-until a VSYNC occurs and returns the timestamp of the actual VSYNC. A message
-must be sent every time VSYNC occurs. A client can receive a VSYNC timestamp
-once, at specified intervals, or continuously (interval of 1). You must
-implement VSYNC to have no more than a 1ms lag at the maximum (0.5ms or less is
-recommended), and the timestamps returned must be extremely accurate.</p>
-
-<h4 id=explicit_synchronization>Explicit synchronization</h4>
-
-<p>Explicit synchronization is required and provides a mechanism for Gralloc
-buffers to be acquired and released in a synchronized way. Explicit
-synchronization allows producers and consumers of graphics buffers to signal
-when they are done with a buffer. This allows the Android system to
-asynchronously queue buffers to be read or written with the certainty that
-another consumer or producer does not currently need them. See the
-<a href="{@docRoot}devices/graphics/index.html#synchronization_framework">Synchronization
-framework</a> section for an overview of this mechanism.</p>
-
-<p>The benefits of explicit synchronization include less behavior variation
-between devices, better debugging support, and improved testing metrics. For
-instance, the sync framework output readily identifies problem areas and root
-causes. And centralized SurfaceFlinger presentation timestamps show when events
-occur in the normal flow of the system.</p>
-
-<p>This communication is facilitated by the use of synchronization fences,
-which are now required when requesting a buffer for consuming or producing. The
-synchronization framework consists of three main building blocks:
-sync_timeline, sync_pt, and sync_fence.</p>
-
-<h5 id=sync_timeline>sync_timeline</h5>
-
-<p>A sync_timeline is a monotonically increasing timeline that should be
-implemented for each driver instance, such as a GL context, display controller,
-or 2D blitter. This is essentially a counter of jobs submitted to the kernel
-for a particular piece of hardware. It provides guarantees about the order of
-operations and allows hardware-specific implementations.</p>
-
-<p>Please note, the sync_timeline is offered as a CPU-only reference
-implementation called sw_sync (which stands for software sync). If possible,
-use sw_sync instead of a sync_timeline to save resources and avoid complexity.
-If you’re not employing a hardware resource, sw_sync should be sufficient.</p>
-
-<p>If you must implement a sync_timeline, use the sw_sync driver as a starting
-point. Follow these guidelines:</p>
-
-<ul> <li> Provide useful names for all drivers, timelines, and fences. This
-simplifies debugging. <li> Implement timeline_value str and pt_value_str
-operators in your timelines as they make debugging output much more readable.
-<li> If you want your userspace libraries (such as the GL library) to have
-access to the private data of your timelines, implement the fill driver_data
-operator. This lets you get information about the immutable sync_fence and
-sync_pts so you might build command lines based upon them. </ul>
-
-<p>When implementing a sync_timeline, <strong>don’t</strong>:</p>
-
-<ul> <li> Base it on any real view of time, such as when a wall clock or other
-piece of work might finish. It is better to create an abstract timeline that
-you can control. <li> Allow userspace to explicitly create or signal a fence.
-This can result in one piece of the user pipeline creating a denial-of-service
-attack that halts all functionality. This is because the userspace cannot make
-promises on behalf of the kernel. <li> Access sync_timeline, sync_pt, or
-sync_fence elements explicitly, as the API should provide all required
-functions. </ul>
-
-<h5 id=sync_pt>sync_pt</h5>
-
-<p>A sync_pt is a single value or point on a sync_timeline. A point has three
-states: active, signaled, and error. Points start in the active state and
-transition to the signaled or error states. For instance, when a buffer is no
-longer needed by an image consumer, this sync_point is signaled so that image
-producers know it is okay to write into the buffer again.</p>
-
-<h5 id=sync_fence>sync_fence</h5>
-
-<p>A sync_fence is a collection of sync_pts that often have different
-sync_timeline parents (such as for the display controller and GPU). These are
-the main primitives over which drivers and userspace communicate their
-dependencies. A fence is a promise from the kernel that it gives upon accepting
-work that has been queued and assures completion in a finite amount of
-time.</p>
-
-<p>This allows multiple consumers or producers to signal they are using a
-buffer and to allow this information to be communicated with one function
-parameter. Fences are backed by a file descriptor and can be passed from
-kernel-space to user-space. For instance, a fence can contain two sync_points
-that signify when two separate image consumers are done reading a buffer. When
-the fence is signaled, the image producers know both consumers are done
-consuming.
-
-Fences, like sync_pts, start active and then change state based upon the state
-of their points. If all sync_pts become signaled, the sync_fence becomes
-signaled. If one sync_pt falls into an error state, the entire sync_fence has
-an error state.
-
-Membership in the sync_fence is immutable once the fence is created. And since
-a sync_pt can be in only one fence, it is included as a copy. Even if two
-points have the same value, there will be two copies of the sync_pt in the
-fence.
-
-To get more than one point in a fence, a merge operation is conducted. In the
-merge, the points from two distinct fences are added to a third fence. If one
-of those points was signaled in the originating fence, and the other was not,
-the third fence will also not be in a signaled state.</p>
-
-<p>To implement explicit synchronization, you need to provide the
-following:</p>
-
-<ul> <li> A kernel-space driver that implements a synchronization timeline for
-a particular piece of hardware. Drivers that need to be fence-aware are
-generally anything that accesses or communicates with the Hardware Composer.
-Here are the key files (found in the android-3.4 kernel branch): <ul> <li> Core
-implementation: <ul> <li> <code>kernel/common/include/linux/sync.h</code> <li>
-<code>kernel/common/drivers/base/sync.c</code> </ul> <li> sw_sync: <ul> <li>
-<code>kernel/common/include/linux/sw_sync.h</code> <li>
-<code>kernel/common/drivers/base/sw_sync.c</code> </ul> <li> Documentation:
-<li> <code>kernel/common//Documentation/sync.txt</code> Finally, the
-<code>platform/system/core/libsync</code> directory includes a library to
-communicate with the kernel-space. </ul> <li> A Hardware Composer HAL module
-(version 1.3 or later) that supports the new synchronization functionality. You
-will need to provide the appropriate synchronization fences as parameters to
-the set() and prepare() functions in the HAL. <li> Two GL-specific extensions
-related to fences, <code>EGL_ANDROID_native_fence_sync</code> and
-<code>EGL_ANDROID_wait_sync</code>, along with incorporating fence support into
-your graphics drivers. </ul>
-
-<p>For example, to use the API supporting the synchronization function, you
-might develop a display driver that has a display buffer function. Before the
-synchronization framework existed, this function would receive dma-bufs, put
-those buffers on the display, and block while the buffer is visible, like
-so:</p>
-
-<pre class=prettyprint>
-/*
- * assumes buf is ready to be displayed. returns when buffer is no longer on
- * screen.
- */
-void display_buffer(struct dma_buf *buf); </pre>
-
-
-<p>With the synchronization framework, the API call is slightly more complex.
-While putting a buffer on display, you associate it with a fence that says when
-the buffer will be ready. So you queue up the work, which you will initiate
-once the fence clears.</p>
-
-<p>In this manner, you are not blocking anything. You immediately return your
-own fence, which is a guarantee of when the buffer will be off of the display.
-As you queue up buffers, the kernel will list dependencies. With the
-synchronization framework:</p>
-
-<pre class=prettyprint>
-/*
- * will display buf when fence is signaled. returns immediately with a fence
- * that will signal when buf is no longer displayed.
- */
-struct sync_fence* display_buffer(struct dma_buf *buf, struct sync_fence
-*fence); </pre>
-
-
-<h4 id=sync_integration>Sync integration</h4>
-
-<h5 id=integration_conventions>Integration conventions</h5>
-
-<p>This section explains how to integrate the low-level sync framework with
-different parts of the Android framework and the drivers that need to
-communicate with one another.</p>
-
-<p>The Android HAL interfaces for graphics follow consistent conventions so
-when file descriptors are passed across a HAL interface, ownership of the file
-descriptor is always transferred. This means:</p>
-
-<ul> <li> if you receive a fence file descriptor from the sync framework, you
-must close it. <li> if you return a fence file descriptor to the sync
-framework, the framework will close it. <li> if you want to continue using the
-fence file descriptor, you must duplicate the descriptor. </ul>
-
-<p>Every time a fence is passed through BufferQueue - such as for a window that
-passes a fence to BufferQueue saying when its new contents will be ready - the
-fence object is renamed. Since kernel fence support allows fences to have
-strings for names, the sync framework uses the window name and buffer index
-that is being queued to name the fence, for example:
-<code>SurfaceView:0</code></p>
-
-<p>This is helpful in debugging to identify the source of a deadlock. Those
-names appear in the output of <code>/d/sync</code> and bug reports when
-taken.</p>
-
-<h5 id=anativewindow_integration>ANativeWindow integration</h5>
-
-<p>ANativeWindow is fence aware. <code>dequeueBuffer</code>,
-<code>queueBuffer</code>, and <code>cancelBuffer</code> have fence
-parameters.</p>
-
-<h5 id=opengl_es_integration>OpenGL ES integration</h5>
-
-<p>OpenGL ES sync integration relies upon these two EGL extensions:</p>
-
-<ul> <li> <code>EGL_ANDROID_native_fence_sync</code> - provides a way to either
-wrap or create native Android fence file descriptors in EGLSyncKHR objects.
-<li> <code>EGL_ANDROID_wait_sync</code> - allows GPU-side stalls rather than in
-CPU, making the GPU wait for an EGLSyncKHR. This is essentially the same as the
-<code>EGL_KHR_wait_sync</code> extension. See the
-<code>EGL_KHR_wait_sync</code> specification for details. </ul>
-
-<p>These extensions can be used independently and are controlled by a compile
-flag in libgui. To use them, first implement the
-<code>EGL_ANDROID_native_fence_sync</code> extension along with the associated
-kernel support. Next add a ANativeWindow support for fences to your driver and
-then turn on support in libgui to make use of the
-<code>EGL_ANDROID_native_fence_sync</code> extension.</p>
-
-<p>Then, as a second pass, enable the <code>EGL_ANDROID_wait_sync</code>
-extension in your driver and turn it on separately. The
-<code>EGL_ANDROID_native_fence_sync</code> extension consists of a distinct
-native fence EGLSync object type so extensions that apply to existing EGLSync
-object types don’t necessarily apply to <code>EGL_ANDROID_native_fence</code>
-objects to avoid unwanted interactions.</p>
-
-<p>The EGL_ANDROID_native_fence_sync extension employs a corresponding native
-fence file descriptor attribute that can be set only at creation time and
-cannot be directly queried onward from an existing sync object. This attribute
-can be set to one of two modes:</p>
-
-<ul> <li> A valid fence file descriptor - wraps an existing native Android
-fence file descriptor in an EGLSyncKHR object. <li> -1 - creates a native
-Android fence file descriptor from an EGLSyncKHR object. </ul>
-
-<p>The DupNativeFenceFD function call is used to extract the EGLSyncKHR object
-from the native Android fence file descriptor. This has the same result as
-querying the attribute that was set but adheres to the convention that the
-recipient closes the fence (hence the duplicate operation). Finally, destroying
-the EGLSync object should close the internal fence attribute.</p>
-
-<h5 id=hardware_composer_integration>Hardware Composer integration</h5>
-
-<p>Hardware Composer handles three types of sync fences:</p>
-
-<ul> <li> <em>Acquire fence</em> - one per layer, this is set before calling
-HWC::set. It signals when Hardware Composer may read the buffer. <li>
-<em>Release fence</em> - one per layer, this is filled in by the driver in
-HWC::set. It signals when Hardware Composer is done reading the buffer so the
-framework can start using that buffer again for that particular layer. <li>
-<em>Retire fence</em> - one per the entire frame, this is filled in by the
-driver each time HWC::set is called. This covers all of the layers for the set
-operation. It signals to the framework when all of the effects of this set
-operation has completed. The retire fence signals when the next set operation
-takes place on the screen. </ul>
-
-<p>The retire fence can be used to determine how long each frame appears on the
-screen. This is useful in identifying the location and source of delays, such
-as a stuttering animation. </p>
-
-<h4 id=vsync_offset>VSYNC Offset</h4>
-
-<p>Application and SurfaceFlinger render loops should be synchronized to the
-hardware VSYNC. On a VSYNC event, the display begins showing frame N while
-SurfaceFlinger begins compositing windows for frame N+1. The app handles
-pending input and generates frame N+2.</p>
-
-<p>Synchronizing with VSYNC delivers consistent latency. It reduces errors in
-apps and SurfaceFlinger and the drifting of displays in and out of phase with
-each other. This, however, does assume application and SurfaceFlinger per-frame
-times don’t vary widely. Nevertheless, the latency is at least two frames.</p>
-
-<p>To remedy this, you may employ VSYNC offsets to reduce the input-to-display
-latency by making application and composition signal relative to hardware
-VSYNC. This is possible because application plus composition usually takes less
-than 33 ms.</p>
-
-<p>The result of VSYNC offset is three signals with same period, offset
-phase:</p>
-
-<ul> <li> <em>HW_VSYNC_0</em> - Display begins showing next frame <li>
-<em>VSYNC</em> - App reads input and generates next frame <li> <em>SF
-VSYNC</em> - SurfaceFlinger begins compositing for next frame </ul>
-
-<p>With VSYNC offset, SurfaceFlinger receives the buffer and composites the
-frame, while the application processes the input and renders the frame, all
-within a single frame of time.</p>
-
-<p>Please note, VSYNC offsets reduce the time available for app and composition
-and therefore provide a greater chance for error.</p>
-
-<h5 id=dispsync>DispSync</h5>
-
-<p>DispSync maintains a model of the periodic hardware-based VSYNC events of a
-display and uses that model to execute periodic callbacks at specific phase
-offsets from the hardware VSYNC events.</p>
-
-<p>DispSync is essentially a software phase lock loop (PLL) that generates the
-VSYNC and SF VSYNC signals used by Choreographer and SurfaceFlinger, even if
-not offset from hardware VSYNC.</p>
-
-<img src="images/dispsync.png" alt="DispSync flow">
-
-<p class="img-caption"><strong>Figure 4.</strong> DispSync flow</p>
-
-<p>DispSync has these qualities:</p>
-
-<ul> <li> <em>Reference</em> - HW_VSYNC_0 <li> <em>Output</em> - VSYNC and SF
-VSYNC <li> <em>Feedback</em> - Retire fence signal timestamps from Hardware
-Composer </ul>
-
-<h5 id=vsync_retire_offset>VSYNC/Retire Offset</h5>
-
-<p>The signal timestamp of retire fences must match HW VSYNC even on devices
-that don’t use the offset phase. Otherwise, errors appear to have greater
-severity than reality.</p>
-
-<p>“Smart” panels often have a delta. Retire fence is the end of direct memory
-access (DMA) to display memory. The actual display switch and HW VSYNC is some
-time later.</p>
-
-<p><code>PRESENT_TIME_OFFSET_FROM_VSYNC_NS</code> is set in the device’s
-BoardConfig.mk make file. It is based upon the display controller and panel
-characteristics. Time from retire fence timestamp to HW Vsync signal is
-measured in nanoseconds.</p>
-
-<h5 id=vsync_and_sf_vsync_offsets>VSYNC and SF_VSYNC Offsets</h5>
-
-<p>The <code>VSYNC_EVENT_PHASE_OFFSET_NS</code> and
-<code>SF_VSYNC_EVENT_PHASE_OFFSET_NS</code> are set conservatively based on
-high-load use cases, such as partial GPU composition during window transition
-or Chrome scrolling through a webpage containing animations. These offsets
-allow for long application render time and long GPU composition time.</p>
-
-<p>More than a millisecond or two of latency is noticeable. We recommend
-integrating thorough automated error testing to minimize latency without
-significantly increasing error counts.</p>
-
-<p>Note these offsets are also set in the device’s BoardConfig.mk make file.
-The default if not set is zero offset. Both settings are offset in nanoseconds
-after HW_VSYNC_0. Either can be negative.</p>
+Applications always start drawing on a VSYNC boundary, and SurfaceFlinger always
+composites on a VSYNC boundary. This eliminates stutters and improves visual
+performance of graphics. For details on VSYNC, see
+<a href="{@docRoot}devices/graphics/implement-vsync.html">Implementing
+VSYNC</a>.</p>
<h3 id=virtual_displays>Virtual displays</h3>
-<p>Android added support for virtual displays to Hardware Composer in version
-1.3. This support was implemented in the Android platform and can be used by
-Miracast.</p>
-
-<p>The virtual display composition is similar to the physical display: Input
+<p>Android added platform support for virtual displays in Hardware Composer v1.3.
+The virtual display composition is similar to the physical display: Input
layers are described in prepare(), SurfaceFlinger conducts GPU composition, and
-layers and GPU framebuffer are provided to Hardware Composer in set().</p>
-
-<p>Instead of the output going to the screen, it is sent to a gralloc buffer.
-Hardware Composer writes output to a buffer and provides the completion fence.
-The buffer is sent to an arbitrary consumer: video encoder, GPU, CPU, etc.
-Virtual displays can use 2D/blitter or overlays if the display pipeline can
-write to memory.</p>
-
-<h4 id=modes>Modes</h4>
-
-<p>Each frame is in one of three modes after prepare():</p>
-
-<ul> <li> <em>GLES</em> - All layers composited by GPU. GPU writes directly to
-the output buffer while Hardware Composer does nothing. This is equivalent to
-virtual display composition with Hardware Composer <1.3. <li> <em>MIXED</em> -
-GPU composites some layers to framebuffer, and Hardware Composer composites
-framebuffer and remaining layers. GPU writes to scratch buffer (framebuffer).
-Hardware Composer reads scratch buffer and writes to the output buffer. Buffers
-may have different formats, e.g. RGBA and YCbCr. <li> <em>HWC</em> - All
-layers composited by Hardware Composer. Hardware Composer writes directly to
-the output buffer. </ul>
-
-<h4 id=output_format>Output format</h4>
-
-<p><em>MIXED and HWC modes</em>: If the consumer needs CPU access, the consumer
-chooses the format. Otherwise, the format is IMPLEMENTATION_DEFINED. Gralloc
-can choose best format based on usage flags. For example, choose a YCbCr format
-if the consumer is video encoder, and Hardware Composer can write the format
-efficiently.</p>
-
-<p><em>GLES mode</em>: EGL driver chooses output buffer format in
-dequeueBuffer(), typically RGBA8888. The consumer must be able to accept this
-format.</p>
-
-<h4 id=egl_requirement>EGL requirement</h4>
-
-<p>Hardware Composer 1.3 virtual displays require that eglSwapBuffers() does
-not dequeue the next buffer immediately. Instead, it should defer dequeueing
-the buffer until rendering begins. Otherwise, EGL always owns the “next” output
-buffer. SurfaceFlinger can’t get the output buffer for Hardware Composer in
-MIXED/HWC mode. </p>
-
-<p>If Hardware Composer always sends all virtual display layers to GPU, all
-frames will be in GLES mode. Although it is not recommended, you may use this
-method if you need to support Hardware Composer 1.3 for some other reason but
-can’t conduct virtual display composition.</p>
+layers and GPU framebuffer are provided to Hardware Composer in set(). For
+details on virtual displays, see
+<a href="{@docRoot}devices/graphics/implement-vdisplays.html">Implementing
+Virtual Displays</a>.</p>
<h2 id=testing>Testing</h2>
-<p>For benchmarking, we suggest following this flow by phase:</p>
+<p>For benchmarking, use the following flow by phase:</p>
-<ul> <li> <em>Specification</em> - When initially specifying the device, such
-as when using immature drivers, you should use predefined (fixed) clocks and
-workloads to measure the frames per second rendered. This gives a clear view of
-what the hardware is capable of doing. <li> <em>Development</em> - In the
-development phase as drivers mature, you should use a fixed set of user actions
-to measure the number of visible stutters (janks) in animations. <li>
-<em>Production</em> - Once the device is ready for production and you want to
-compare against competitors, you should increase the workload until stutters
-increase. Determine if the current clock settings can keep up with the load.
-This can help you identify where you might be able to slow the clocks and
-reduce power use. </ul>
+<ul>
+ <li><em>Specification</em>. When initially specifying the device (such as when
+ using immature drivers), use predefined (fixed) clocks and workloads to
+ measure frames per second (fps) rendered. This gives a clear view of hardware
+ capabilities.</li>
+ <li><em>Development</em>. As drivers mature, use a fixed set of user actions
+ to measure the number of visible stutters (janks) in animations.</li>
+ <li><em>Production</em>. When a device is ready for comparison against
+ competitors, increase the workload until stutters increase. Determine if the
+ current clock settings can keep up with the load. This can help you identify
+ where to slow the clocks and reduce power use.</li>
+</ul>
-<p>For the specification phase, Android offers the Flatland tool to help derive
-device capabilities. It can be found at:
-<code>platform/frameworks/native/cmds/flatland/</code></p>
+<p>For help deriving device capabilities during the specification phase, use the
+Flatland tool at <code>platform/frameworks/native/cmds/flatland/</code>.
+Flatland relies upon fixed clocks and shows the throughput achievable with
+composition-based workloads. It uses gralloc buffers to simulate multiple window
+scenarios, filling in the window with GL then measuring the compositing.</p>
-<p>Flatland relies upon fixed clocks and shows the throughput that can be
-achieved with composition-based workloads. It uses gralloc buffers to simulate
-multiple window scenarios, filling in the window with GL and then measuring the
-compositing. Please note, Flatland uses the synchronization framework to
-measure time. So you must support the synchronization framework to readily use
-Flatland.</p>
+<p class="note"><strong>Note:</strong> Flatland uses the synchronization
+framework to measure time, so your implementation must support the
+synchronization framework.</p>