事件和帧通知机制

在先前版本的外部视图系统 (EVS) 中，IEvsCameraStream 接口定义了一个回调方法，用于仅传递捕获的视频帧。虽然这简化了 EVS 服务客户端实现，但也使客户端难以识别任何流式传输事件，从而难以正确处理这些事件。为了增强 EVS 开发体验，AOSP 现在包含一个额外的回调来传递流式传输事件。

package android.hardware.automotive.evs@1.1;

import @1.0::IEvsCameraStream;

/**
 * Implemented on client side to receive asynchronous video frame deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
    /**
     * Receives calls from the HAL each time a video frame is ready for inspection.
     * Buffer handles received by this method must be returned via calls to
     * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
     * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
     * some time as the pipeline drains. Each frame must still be returned.
     * When the last frame in the stream has been delivered, STREAM_STOPPED
     * event must be delivered. No further frame deliveries may happen
     * thereafter.
     *
     * @param buffer a buffer descriptor of a delivered image frame.
     */
    oneway deliverFrame_1_1(BufferDesc buffer);

    /**
     * Receives calls from the HAL each time an event happens.
     *
     * @param  event EVS event with possible event information.
     */
    oneway notify(EvsEvent event);
};

此方法传递 EvsEventDesc，其中包含三个字段

  • 事件类型。
  • 用于标识事件来源的字符串。
  • 4x 32 位字数据，用于包含可能的事件信息。
/**
 * Structure that describes informative events occurred during EVS is streaming
 */
struct EvsEvent {
    /**
     * Type of an informative event
     */
    EvsEventType aType;
    /**
     * Device identifier
     */
    string deviceId;
    /**
     * Possible additional information
     */
    uint32_t[4] payload;
};

并且，为了避免 EVS 和其他 Android 图形组件之间图形缓冲区描述出现任何差异，BufferDesc 已被重新定义为使用从 android.hardware.graphics.common@1.2 接口导入的 HardwareBuffer。HardwareBuffer 包含 HardwareBufferDescription，它是 Android NDK 的 AHardwareBuffer_Desc 的 HIDL 对等项，带有一个缓冲区句柄。

/**
 * HIDL counterpart of AHardwareBuffer_Desc.
 *
 * An AHardwareBuffer_Desc object can be converted to and from a
 * HardwareBufferDescription object by memcpy().
 *
 * @sa +ndk libnativewindow#AHardwareBuffer_Desc.
 */
typedef uint32_t[10] HardwareBufferDescription;

/**
 * HIDL counterpart of AHardwareBuffer.
 *
 * AHardwareBuffer_createFromHandle() can be used to convert a HardwareBuffer
 * object to an AHardwareBuffer object.
 *
 * Conversely, AHardwareBuffer_getNativeHandle() can be used to extract a native
 * handle from an AHardwareBuffer object. Paired with AHardwareBuffer_Desc,
 * AHardwareBuffer_getNativeHandle() can be used to convert between
 * HardwareBuffer and AHardwareBuffer.
 *
 * @sa +ndk libnativewindow#AHardwareBuffer".
 */
struct HardwareBuffer {
    HardwareBufferDescription description;
    handle nativeHandle;
}

/**
 * Structure representing an image buffer through our APIs
 *
 * In addition to the handle to the graphics memory, need to retain
 * the properties of the buffer for easy reference and reconstruction of
 * an ANativeWindowBuffer object on the remote side of API calls.
 * Not least because OpenGL expect an ANativeWindowBuffer* for us as a
 * texture via eglCreateImageKHR().
 */
struct BufferDesc {
    /**
     * HIDL counterpart of AHardwareBuffer_Desc. Please see
     * hardware/interfaces/graphics/common/1.2/types.hal for more details.
     */
    HardwareBuffer buffer;
    /**
     * The size of a pixel in the units of bytes
     */
    uint32_t pixelSize;
    /**
     * Opaque value from driver
     */
    uint32_t bufferId;
    /**
     * Unique identifier of the physical camera device that produces this buffer.
     */
    string deviceId;
    /**
     * Time that this buffer is being filled
     */
    int64_t timestamp;
    /**
     * Frame metadata. This is opaque to EVS manager
     */
    vec<uint8_t> metadata
};

注意：HardwareBufferDescription 定义为十个 32 位字的数组。您可能需要将其强制转换为 AHardwareBuffer_Desc 类型并填写内容。

EvsEventDesc 是 enum EvsEventType 的结构，其中列出了多个流式传输事件和一个 32 位字负载，开发人员可以在其中放置可能的其他信息。例如，开发人员可以为流式传输错误事件放置一个错误代码。

/**
 * Types of informative streaming events
 */
enum EvsEventType : uint32_t {
    /**
     * Video stream is started
     */
    STREAM_STARTED = 0,
    /**
     * Video stream is stopped
     */
    STREAM_STOPPED,
    /**
     * Video frame is dropped
     */
    FRAME_DROPPED,
    /**
     * Timeout happens
     */
    TIMEOUT,
    /**
     * Camera parameter is changed; payload contains a changed parameter ID and
     * its value
     */
    PARAMETER_CHANGED,
    /**
     * Master role has become available
     */
    MASTER_RELEASED,
};

帧传递

使用新的 BufferDesc，IEvsCameraStream 还引入了新的回调方法，用于接收来自服务实现的帧和流式传输事件。

/**
 * Implemented on client side to receive asynchronous streaming event deliveries.
 */
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
   /**
    * Receives calls from the HAL each time video frames are ready for inspection.
    * Buffer handles received by this method must be returned via calls to
    * IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
    * to IEvsCamera::stopVideoStream(), this callback may continue to happen for
    * some time as the pipeline drains. Each frame must still be returned.
    * When the last frame in the stream has been delivered, STREAM_STOPPED
    * event must be delivered. No further frame deliveries may happen
    * thereafter.
    *
    * A camera device delivers the same number of frames as number of
    * backing physical camera devices; it means, a physical camera device
    * sends always a single frame and a logical camera device sends multiple
    * frames as many as the number of backing physical camera devices.
    *
    * @param buffer Buffer descriptors of delivered image frames.
    */
   oneway deliverFrame_1_1(vec<BufferDesc> buffer);

   /**
    * Receives calls from the HAL each time an event happens.
    *
    * @param  event EVS event with possible event information.
    */
   oneway notify(EvsEventDesc event);
};

较新版本的帧回调方法旨在传递多个缓冲区描述符。因此，如果 EVS 相机实现管理多个源，则可以通过单次调用转发多个帧。

此外，先前用于通知流结束的协议（即发送空帧）已被弃用，并替换为 STREAM_STOPPED 事件。

Event notification sequence diagram

图 1. 事件通知时序图

使用事件和帧通知机制

识别客户端实现的 IEvsCameraStream 版本

服务可以通过尝试向下转换来识别客户端实现的传入 IEvsCameraStream 接口的版本

using IEvsCameraStream_1_0 =
    ::android::hardware::automotive::evs::V1_0::IEvsCameraStream;
using IEvsCameraStream_1_1 =
    ::android::hardware::automotive::evs::V1_1::IEvsCameraStream;

Return<EvsResult> EvsV4lCamera::startVideoStream(
    const sp<IEvsCameraStream_1_0>& stream)  {

    IEvsCameraStream_1_0 aStream = stream;
    // Try to downcast. This succeeds if the client implements
    // IEvsCameraStream v1.1.
    IEvsCameraStream_1_1 aStream_1_1 =
        IEvsCameraStream_1_1::castFrom(aStream).withDefault(nullptr);
    if (aStream_1_1 == nullptr) {
        ALOGI("Start a stream for v1.0 client.");
    } else {
        ALOGI("Start a stream for v1.1 client.");
    }

    // Start a video stream
    ...
}

notify() 回调

EvsEvent 通过 notify() 回调传递，然后客户端可以根据鉴别器识别其类型，如下所示

Return<void> StreamHandler::notify(const EvsEvent& event) {
    ALOGD("Received an event id: %u", event.aType);
    // Handle each received event.
    switch(event.aType) {
        case EvsEventType::ERROR:
            // Do something to handle an error
            ...
            break;
        [More cases]
    }
    return Void();
}

使用 BufferDesc

AHardwareBuffer_Desc 是 Android NDK 的数据类型，用于表示可以绑定到 EGL/OpenGL 和 Vulkan 图元的原生硬件缓冲区。它包含先前 EVS BufferDesc 中的大多数缓冲区元数据，因此在新 BufferDesc 定义中取代了它。但是，由于这在 HIDL 接口中定义为数组，因此无法直接索引成员变量。相反，您可以将数组强制转换为 AHardwareBuffer_Desc 类型，如下所示

BufferDesc bufDesc = {};
AHardwareBuffer_Desc* pDesc =
    reinterpret_cast<AHardwareBuffer_Desc *>(&bufDesc.buffer.description);
pDesc->width  = mVideo.getWidth();
pDesc->height = mVideo.getHeight();
pDesc->layers = 1;
pDesc->format = mFormat;
pDesc->usage  = mUsage;
pDesc->stride = mStride;
bufDesc_1_1.buffer.nativeHandle = mBuffers[idx].handle;
bufDesc_1_1.bufferId = idx;