Merge "Add more javadocs to SyncFence" into tm-dev am: e60fa2d0

package android.hardware;

import android.annotation.NonNull;

import android.media.Image;

import android.media.ImageWriter;

import android.opengl.EGLDisplay;

import android.opengl.EGLSync;

import android.os.Parcel;

import java.time.Duration;

/**

 * A SyncFence represents a synchronization primitive which signals when hardware buffers have

 * completed work on a particular resource.

 * A SyncFence represents a synchronization primitive which signals when hardware units have

 * completed work on a particular resource. They initially start in an unsignaled state and make

 * a one-time transition to either a signaled or error state. SyncFences are created by various

 * device APIs in response to submitting tasks to the device. They cannot be created nor signaled

 * by userspace. As a result, this means that a SyncFence will make always make forward progress.

 *

 * <p>SyncFence's generally come in one of two varieties. "Presentation fences" refer to

 *  a SyncFence when the writing to a buffer has finished. "Release fences" then refer

 *  to when the reading from a buffer has finished.</p>

 *

 * <p>For example, a GPU rendering to a framebuffer may generate a synchronization fence,

 * e.g., a VkFence, which signals when rendering has completed.

 * e.g., an EGLSync or VkFence, which signals when rendering has completed. Once the fence signals,

 * then the backing storage for the framebuffer may be safely read from, such as for display or

 * for media encoding. This would be referred to as a "presentation fence."</p>

 *

 * Once the fence signals, then the backing storage for the framebuffer may be safely read from,

 * such as for display or for media encoding.</p>

 * <p>Similarly when using an {@link android.media.ImageWriter} it is possible that an

 * {@link android.media.Image} returned by {@link ImageWriter#dequeueInputImage()} may already

 * have a {@link Image#getFence() fence} set on it. This would be what is referred to as either

 * a "release fence" or an "acqurie fence" and indicates the fence that the writer must wait

 * on before writing to the underlying buffer. In the case of ImageWriter this is done

 * automatically when eg {@link Image#getPlanes()} is called, however when using

 * {@link Image#getHardwareBuffer()} it is the caller's responsibility to ensure the

 * release fence has signaled before writing to the buffer.</p>

 *

 * @see android.opengl.EGLExt#eglDupNativeFenceFDANDROID(EGLDisplay, EGLSync)

 * @see android.media.Image#getFence()