android4.0 camera hal 移植

先从Camera Hal提供给Camera Service 的接口（CameraHardwareInterface.h）开始看起。

从注释可以看出，基本的流程是没有变的。

/**

* CameraHardwareInterface.h defines the interface to the

* camera hardware abstraction layer, used for setting and getting

* parameters, live previewing, and taking pictures.

*

* It is a referenced counted interface with RefBase as its base class.

* CameraService calls openCameraHardware() to retrieve a strong pointer to the

* instance of this interface and may be called multiple times. The

* following steps describe a typical sequence:

*

* -# After CameraService calls openCameraHardware(), getParameters() and

* setParameters() are used to initialize the camera instance.

* CameraService calls getPreviewHeap() to establish access to the

* preview heap so it can be registered with SurfaceFlinger for

* efficient display updating while in preview mode.

* -# startPreview() is called. The camera instance then periodically

* sends the message CAMERA_MSG_PREVIEW_FRAME (if enabled) each time

* a new preview frame is available. If data callback code needs to use

* this memory after returning, it must copy the data.

*

* Prior to taking a picture, CameraService calls autofocus(). When auto

* focusing has completed, the camera instance sends a CAMERA_MSG_FOCUS notification,

* which informs the application whether focusing was successful. The camera instance

* only sends this message once and it is up to the application to call autoFocus()

* again if refocusing is desired.

*

* CameraService calls takePicture() to request the camera instance take a

* picture. At this point, if a shutter, postview, raw, and/or compressed callback

* is desired, the corresponding message must be enabled. As with CAMERA_MSG_PREVIEW_FRAME,

* any memory provided in a data callback must be copied if it's needed after returning.

*/

但是在具体的实现上，做了改动。一个C++的风格，一个C的风格。现在Hal层也越来越像linux了。

在froyo的Hal层，主要是去实现一个继承自CameraHardwareInterface抽象类的子类，在这个类里去实现父类的（纯）虚函数。

但是在ICS里，CameraHarwareInterface类的方法实现变了，现在是通过hw_module_t结构体获得hw_device_t的结构体，而这个结构体里包括Hal层的各方法回调。也就是froyo里的sensor架构。

看一下initialize函数：

status_t initialize(hw_module_t *module)
    {
        LOGI("Opening camera %s", mName.string());
        int rc = module->methods->open(module, mName.string(),
                                       (hw_device_t **)&mDevice);
        if (rc != OK) {
            LOGE("Could not open camera %s: %d", mName.string(), rc);
            return rc;
        }
        initHalPreviewWindow();
        return rc;
    }

有了这个，也就可以摒弃froyo中的openCameraHardware()接口了。

/** factory function to instantiate a camera hardware object */
extern "C" sp<CameraHardwareInterface> openCameraHardware();

再看一下两个版本之间，CameraHardwareInterface类中各方法实现的变化。以startPreview为例。

froyo：

/**
     * Start preview mode.
     */
    virtual status_t    startPreview() = 0;

ICS：

status_t startPreview()
    {
        LOGV("%s(%s)", __FUNCTION__, mName.string());
        if (mDevice->ops->start_preview)
            return mDevice->ops->start_preview(mDevice);
        return INVALID_OPERATION;
    }

越来越像linux风络了。

另外，ICS的CameraHardwareInterface又添加了一些private的成员。

data_callback函数，也多加了一个参数*metadata。

typedef void (*data_callback)(int32_t msgType,
                            const sp<IMemory> &dataPtr,
                            camera_frame_metadata_t *metadata,
                            void* user);