汽車顯示 Proxy 服務

這個簡單的架構服務可讓供應商程序在 HAL 實作中使用 SurfaceFlinger/EGL，而無須連結 libgui。AOSP 提供這項服務的預設實作方式，可提供完整功能。不過，供應商也必須實作 API，才能在平台上提供這項服務。

package android.frameworks.automotive.display@1.0;

import android.hardware.graphics.bufferqueue@2.0::IGraphicBufferProducer;

interface IAutomotiveDisplayProxyService {
    /**
     * Gets an IGraphicBufferProducer instance from the service.
     *
     * @param  id   Target's stable display identifier
     *
     * @return igbp Returns an IGraphicBufferProducer object, that can be
     *              converted to an ANativeWindow object.
     */
    getIGraphicBufferProducer(uint64_t id) generates (IGraphicBufferProducer igbp);

    /**
     * Sets the ANativeWindow, which is associated with the
     * IGraphicBufferProducer, to be visible and to take over the display.
     *
     * @param  id      Target display ID
     *
     * @return success Returns true on success.
     */
    showWindow(uint64_t id) generates (bool success);

    /**
     * Sets the ANativeWindow, which is associated with the
     * IGraphicBufferProducer, to be invisible and to release the control
     * over display.
     *
     * @param  id      Target display ID
     *
     * @return success Returns true on success.
     */
    hideWindow(uint64_t id) generates (bool success);

    /**
     * Returns the stable identifiers of all available displays.
     *
     * @return ids A list of stable display identifiers.
     */
    getDisplayIdList() generates (vec<uint64_t> ids);

    /**
     * Returns the descriptor of the target display.
     *
     * @param  id    Stable ID of a target display.
     * @return cfg   DisplayConfig of the active display.
     * @return state Current state of the active display.
     */
    getDisplayInfo(uint64_t id) generates (HwDisplayConfig cfg, HwDisplayState state);
}

如要使用這項服務，請按照下列步驟操作：

1. 取得 IAutomotiveDisplayProxyService。

   android::sp<IAutomotiveDisplayProxyService> windowProxyService =
       IAutomotiveDisplayProxyService::getService("default");
   if (windowProxyService == nullptr) {
       LOG(ERROR) << "Cannot use AutomotiveDisplayProxyService. Exiting.";
       return 1;
   }

2. 從服務中擷取有效的顯示資訊，以便判斷解析度。

   // We use the first display in the list as the primary.
   pWindowProxy->getDisplayInfo(displayId, [this](auto dpyConfig, auto dpyState) {
       DisplayConfig *pConfig = (DisplayConfig*)dpyConfig.data();
       mWidth = pConfig->resolution.getWidth();
       mHeight = pConfig->resolution.getHeight();
   
       ui::DisplayState* pState = (ui::DisplayState*)dpyState.data();
       if (pState->orientation != ui::ROTATION_0 &&
           pState->orientation != ui::ROTATION_180) {
           // rotate
           std::swap(mWidth, mHeight);
       }
   
       LOG(DEBUG) << "Display resolution is " << mWidth << " x " << mHeight;
   });

3. 從 IAutomotiveDisplayProxyService 擷取硬體 IGraphicBufferProducer (或 HIDL GraphicBufferProducer (HGBP)：

   mGfxBufferProducer = pWindowProxy->getIGraphicBufferProducer(displayId);
   if (mGfxBufferProducer == nullptr) {
       LOG(ERROR) << "Failed to get IGraphicBufferProducer from "
                  << "IAutomotiveDisplayProxyService.";
       return false;
   }

4. 使用 API libbufferqueueconverter 從擷取的 HGBP 取得 SurfaceHolder：

   mSurfaceHolder = getSurfaceFromHGBP(mGfxBufferProducer);
   if (mSurfaceHolder == nullptr) {
       LOG(ERROR) << "Failed to get a Surface from HGBP.";
       return false;
   }

5. 使用 API libbufferqueueconverter，將 SurfaceHolder 轉換為原生視窗：

   mWindow = getNativeWindow(mSurfaceHolder.get());
   if (mWindow == nullptr) {
       LOG(ERROR) << "Failed to get a native window from Surface.";
       return false;
   }

6. 使用原生視窗建立 EGL 視窗介面，然後算繪：

   // Set up our OpenGL ES context associated with the default display
   mDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);
   if (mDisplay == EGL_NO_DISPLAY) {
       LOG(ERROR) << "Failed to get egl display";
       return false;
   }
   ...
   
   // Create the EGL render target surface
   mSurface = eglCreateWindowSurface(mDisplay, egl_config, mWindow, nullptr);
   if (mSurface == EGL_NO_SURFACE) {
       LOG(ERROR) << "eglCreateWindowSurface failed.";
       return false;
   }
   ...

7. 呼叫 IAutomotiveDisplayProxyService::showWindow() 即可在螢幕上顯示算繪的檢視畫面。此服務的優先順序最高，因此一律會從目前的擁有者手中取得螢幕控制權：

   mAutomotiveDisplayProxyService->showWindow();

如需進一步瞭解實作方式，請參閱 $ANDROID_BUILD_TOP/packages/services/Car/evs/sampleDriver/ 中的 service.cpp 和 GlWrapper.cpp。

實作 EVS HAL 時，需要使用下方以粗體顯示的額外程式庫。

cc_binary {
    name: "android.hardware.automotive.evs@1.1-sample",

    vendor: true,

    srcs: [
        ...
    ],

    shared_libs: [
        ...
        "libbufferqueueconverter",
        "android.hidl.token@1.0-utils",
        "android.frameworks.automotive.display@1.0",
        "android.hardware.graphics.bufferqueue@1.0",
        "android.hardware.graphics.bufferqueue@2.0",
    ],

多螢幕支援

列舉顯示裝置並擷取顯示資訊

與相機裝置列舉相同，EVS 架構也提供方法來列舉可用的螢幕。 靜態顯示 ID會編碼類型長 ID、顯示埠資訊 (位於較低位元組) 和 Extended Display IDentification Data (位於較高位元)。IAutomotiveDisplayProxyService::getDisplayIdList() 會傳回實體本機螢幕的顯示 ID 清單，供 EVS 服務使用，而 IEvsEnumerator::getDisplayIdList() 會傳回偵測到的螢幕連線顯示器埠清單。清單中的第一個 ID 一律是主要顯示裝置。

interface IEvsEnumerator extends @1.0::IEvsEnumerator {
    ...
    /**
     * Returns a list of all EVS displays available to the system
     *
     * @return displayIds Identifiers of available displays.
     */
    getDisplayIdList() generates (vec<uint8_t> displayIds);
};

開啟目標顯示裝置

EVS 應用程式會使用目標顯示器埠號碼呼叫 IEvsEnumerator::openDisplay_1_1()：

android::sp<IEvsDisplay> pDisplay = pEvs->openDisplay_1_1(displayId);
if (pDisplay.get() == nullptr) {
    LOG(ERROR) << "EVS Display unavailable. Exiting.";
    return 1;
}

注意：一次只能使用單一螢幕，也就是說，當另一個 EVS 用戶端要求開啟螢幕時，目前的 EVS 用戶端會失去螢幕，即使兩者不相同也是如此。