Android Camera 通过V4L2与kernel driver的完整交互过程

http://blog.chinaunix.net/uid-26215986-id-3552456.html

原文地址：Android Camera 通过V4L2与kernel driver的完整交互过程 作者：xinyuwuxian

Android Camera 通过V4L2与kernel driver的完整交互过程

之前在 Android Camera 的执行流程 http://blog.chinaunix.net/uid-26765074-id-3499537.html 这篇文章中已经详细介绍了
Android Camera app到调用open打开camera 设备的完成过程，但是只是到此为止，并未对较底层的操作控制等做任何分析，
今天就要进入这个环节了

这里就直接从open那个方法开始了，先说一下这个open是怎么被调用的

实例化Camera Hal接口 hardware，hardware调用initialize()进入HAL层打开Camear驱动。
CameraHardwareInterface中initialize()定义在以下路径:frameworks/base/services/camera/libcameraservice/CameraHardwareInterface.h

代码如下：

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;
        }
#ifdef OMAP_ENHANCEMENT_CPCAM
        initHalPreviewWindow(&mHalPreviewWindow);
        initHalPreviewWindow(&mHalTapin);
        initHalPreviewWindow(&mHalTapout);
#else
        initHalPreviewWindow();
#endif
        return rc;
    }
此处通过module->method->open()方法真正打开Camera设备,

其中module的定义在以下路径:frameworks/base/services/camera/libcameraservice/CameraService.h

class CameraService :
    public BinderService<CameraService>,
    public BnCameraService
{

class Client : public BnCamera
    {
    public:
        ......

private:

.....

};

camera_module_t *mModule;

};

此处还必须找到camera_module_t 的定义,以更好的理解整个运行流程,通过追根溯源找到了camera_module_t 定义,

camera_module_t的定义在以下路径:hardware/libhardware/include/hardware/camera.h

typedef struct camera_module {
    hw_module_t common;
    int (*get_number_of_cameras)(void);
    int (*get_camera_info)(int camera_id, struct camera_info *info);
} camera_module_t;
其中包含get_number_of_cameras方法和get_camera_info方法用于获取camera info

另外hw_module_t common;这个选项十分重要,此处应重点关注,因为是使用hw_module_t结构体中的open()方法打开设备文件的

继续找到hw_module_t 结构体的定义.在以下路径:hardware/libhardware/include/hardware/hardware.h

1. typedef struct hw_module_t {
2. /** tag must be initialized to HARDWARE_MODULE_TAG */
3. uint32_t tag;
4. /** major version number for the module */
5. uint16_t version_major;
6. /** minor version number of the module */
7. uint16_t version_minor;
8. /** Identifier of module */
9. const char *id;
10. /** Name of this module */
11. const char *name;
12. /** Author/owner/implementor of the module */
13. const char *author;
14. /** Modules methods */
15. struct hw_module_methods_t* methods;
16. /** module's dso */
17. void* dso;
18. /** padding to 128 bytes, reserved for future use */
19. uint32_t reserved[32-7];
20. } hw_module_t;

1. typedef struct hw_module_methods_t {
2. /** Open a specific device */
3. int (*open)(const struct hw_module_t* module, const char* id,
4. struct hw_device_t** device);
5. } hw_module_methods_t;

这里的这个open方法才是我们真正调用的open方法，那么这里只是定义，他又是在哪里实现的呢？？
这个问题我们还是需要往前面追溯啊，当然这在上一篇文章中也没有提到，不卖关子了，其实重点就是我们调用了这个module->methods->open(module, mName.string(),(hw_device_t **)&mDevice)方法
但是这个module是在哪里初始化的呢？看看CameraService类里的这个方法吧，路径：frameworks/base/services/camera/libcameraservice/CameraService.cpp

1. void CameraService::onFirstRef()
2. {
3. BnCameraService::onFirstRef();
4. if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
5. (const hw_module_t **)&mModule) < 0) {
6. LOGE("Could not load camera HAL module");
7. mNumberOfCameras = 0;
8. }
9. else {
10. mNumberOfCameras = mModule->get_number_of_cameras();
11. if (mNumberOfCameras > MAX_CAMERAS) {
12. LOGE("Number of cameras(%d) > MAX_CAMERAS(%d).",
13. mNumberOfCameras, MAX_CAMERAS);
14. mNumberOfCameras = MAX_CAMERAS;
15. }
16. for (int i = 0; i < mNumberOfCameras; i++) {
17. setCameraFree(i);
18. }
19. }
20. }

不错，这个module正是通过这个hw_get_module方法获得的，其实他是通过方法中的CAMERA_HARDWARE_MODULE_ID作为flag最终找到已经定义好的module，那么这个已经定义好的module又在哪呢，是什么样子的呢？
这里我就直接放在这里，不在拐弯抹角了，方法路径：hardware/ti/omap4xxx/camera/CameraHal_Module.cpp

1. static int camera_device_open(const hw_module_t* module, const char* name,
2. hw_device_t** device);
3. static int camera_device_close(hw_device_t* device);
4. static int camera_get_number_of_cameras(void);
5. static int camera_get_camera_info(int camera_id, struct camera_info *info);
6. static struct hw_module_methods_t camera_module_methods = {
7. open: camera_device_open
8. };
9. camera_module_t HAL_MODULE_INFO_SYM = {
10. common: {
11. tag: HARDWARE_MODULE_TAG,
12. version_major: 1,
13. version_minor: 0,
14. id: CAMERA_HARDWARE_MODULE_ID,
15. name: "TI OMAP CameraHal Module",
16. author: "TI",
17. methods: &camera_module_methods,
18. dso: NULL, /* remove compilation warnings */
19. reserved: {0}, /* remove compilation warnings */
20. },
21. get_number_of_cameras: camera_get_number_of_cameras,
22. get_camera_info: camera_get_camera_info,
23. };

这里还是很关键的，通过id：CAMERA_HARDWARE_MODULE_ID作为识别码找到这个module，get module完成任务，大家可以看到，这个定义好的module实现了methods中的open方法，
实现了camera_get_number_of_cameras和camera_get_camera_info，当然还包括了其他一些变量的初始化
这里开始我们找到了我们真正需要的open方法，万里长征走完一大步了，现在就去看看这个open方法干了些什么吧

1. /* open device handle to one of the cameras
2. *
3. * assume camera service will keep singleton of each camera
4. * so this function will always only be called once per camera instance
5. */
6. int camera_device_open(const hw_module_t* module, const char* name,
7. hw_device_t** device)
8. {
9. int rv = 0;
10. int num_cameras = 0;
11. int cameraid;
12. ti_camera_device_t* camera_device = NULL;
13. camera_device_ops_t* camera_ops = NULL;
14. android::CameraHal* camera = NULL;
15. android::CameraProperties::Properties* properties = NULL;
16. android::Mutex::Autolock lock(gCameraHalDeviceLock);
17. CAMHAL_LOGI("camera_device open");
18. if (name != NULL) {
19. cameraid = atoi(name);
20. num_cameras = gCameraProperties.camerasSupported();
21. if(cameraid > num_cameras)
22. {
23. LOGE("camera service provided cameraid out of bounds, "
24. "cameraid = %d, num supported = %d",
25. cameraid, num_cameras);
26. rv = -EINVAL;
27. goto fail;
28. }
29. if(gCamerasOpen >= MAX_SIMUL_CAMERAS_SUPPORTED)
30. {
31. LOGE("maximum number of cameras already open");
32. rv = -ENOMEM;
33. goto fail;
34. }
35. camera_device = (ti_camera_device_t*)malloc(sizeof(*camera_device));
36. if(!camera_device)
37. {
38. LOGE("camera_device allocation fail");
39. rv = -ENOMEM;
40. goto fail;
41. }
42. camera_ops = (camera_device_ops_t*)malloc(sizeof(*camera_ops));
43. if(!camera_ops)
44. {
45. LOGE("camera_ops allocation fail");
46. rv = -ENOMEM;
47. goto fail;
48. }
49. memset(camera_device, 0, sizeof(*camera_device));
50. memset(camera_ops, 0, sizeof(*camera_ops));
51. camera_device->base.common.tag = HARDWARE_DEVICE_TAG;
52. camera_device->base.common.version = 0;
53. camera_device->base.common.module = (hw_module_t *)(module);
54. camera_device->base.common.close = camera_device_close;
55. camera_device->base.ops = camera_ops;
56. camera_ops->set_preview_window = camera_set_preview_window;
57. #ifdef OMAP_ENHANCEMENT_CPCAM
58. camera_ops->set_buffer_source = camera_set_buffer_source;
59. #endif
60. camera_ops->set_callbacks = camera_set_callbacks;
61. camera_ops->enable_msg_type = camera_enable_msg_type;
62. camera_ops->disable_msg_type = camera_disable_msg_type;
63. camera_ops->msg_type_enabled = camera_msg_type_enabled;
64. camera_ops->start_preview = camera_start_preview;
65. camera_ops->stop_preview = camera_stop_preview;
66. camera_ops->preview_enabled = camera_preview_enabled;
67. camera_ops->store_meta_data_in_buffers = camera_store_meta_data_in_buffers;
68. camera_ops->start_recording = camera_start_recording;
69. camera_ops->stop_recording = camera_stop_recording;
70. camera_ops->recording_enabled = camera_recording_enabled;
71. camera_ops->release_recording_frame = camera_release_recording_frame;
72. camera_ops->auto_focus = camera_auto_focus;
73. camera_ops->cancel_auto_focus = camera_cancel_auto_focus;
74. camera_ops->take_picture = camera_take_picture;
75. camera_ops->cancel_picture = camera_cancel_picture;
76. camera_ops->set_parameters = camera_set_parameters;
77. camera_ops->get_parameters = camera_get_parameters;
78. camera_ops->put_parameters = camera_put_parameters;
79. camera_ops->send_command = camera_send_command;
80. camera_ops->release = camera_release;
81. camera_ops->dump = camera_dump;
82. #ifdef OMAP_ENHANCEMENT_CPCAM
83. camera_ops->reprocess = camera_reprocess;
84. camera_ops->cancel_reprocess = camera_cancel_reprocess;
85. #endif
86. *device = &camera_device->base.common;
87. // -------- TI specific stuff --------
88. camera_device->cameraid = cameraid;
89. if(gCameraProperties.getProperties(cameraid, &properties) < 0)
90. {
91. LOGE("Couldn't get camera properties");
92. rv = -ENOMEM;
93. goto fail;
94. }
95. camera = new android::CameraHal(cameraid);
96. if(!camera)
97. {
98. LOGE("Couldn't create instance of CameraHal class");
99. rv = -ENOMEM;
100. goto fail;
101. }
102. if(properties && (camera->initialize(properties) != android::NO_ERROR))
103. {
104. LOGE("Couldn't initialize camera instance");
105. rv = -ENODEV;
106. goto fail;
107. }
108. gCameraHals[cameraid] = camera;
109. gCamerasOpen++;
110. }
111. return rv;
112. fail:
113. if(camera_device) {
114. free(camera_device);
115. camera_device = NULL;
116. }
117. if(camera_ops) {
118. free(camera_ops);
119. camera_ops = NULL;
120. }
121. if(camera) {
122. delete camera;
123. camera = NULL;
124. }
125. *device = NULL;
126. return rv;
127. }

看看这么长的代码，open的任务还是比较中的，没办法，能者多劳嘛，红色部分是最重点的部分
从这里可以知道，这里就像一个控制中心，上传调用到这里被分发出去，实现各自的操作，我们就以startPreview为例进行分析

1. int camera_start_preview(struct camera_device * device)
2. {
3. int rv = -EINVAL;
4. ti_camera_device_t* ti_dev = NULL;
5. LOGV("%s", __FUNCTION__);
6. if(!device)
7. return rv;
8. ti_dev = (ti_camera_device_t*) device;
9. rv = gCameraHals[ti_dev->cameraid]->startPreview();
10. return rv;
11. }

这里每open一个device就会相应的创建并且初始化一个CameraHal 对象，定义在：hardware/ti/omap4xxx/camera/CameraHal.cpp
并且把这个对象保存在gCameraHals这个数组中，正因为这样这里camera_start_preview才可以通过这个数据检索对象调用方法
现在我们就看看这个startPreview()方法是怎样实现的

1. /**
2. @brief Start preview mode.
3. @param none
4. @return NO_ERROR Camera switched to VF mode
5. @todo Update function header with the different errors that are possible
6. */
7. status_t CameraHal::startPreview() {
8. LOG_FUNCTION_NAME;
9. // When tunneling is enabled during VTC, startPreview happens in 2 steps:
10. // When the application sends the command CAMERA_CMD_PREVIEW_INITIALIZATION,
11. // cameraPreviewInitialization() is called, which in turn causes the CameraAdapter
12. // to move from loaded to idle state. And when the application calls startPreview,
13. // the CameraAdapter moves from idle to executing state.
14. //
15. // If the application calls startPreview() without sending the command
16. // CAMERA_CMD_PREVIEW_INITIALIZATION, then the function cameraPreviewInitialization()
17. // AND startPreview() are executed. In other words, if the application calls
18. // startPreview() without sending the command CAMERA_CMD_PREVIEW_INITIALIZATION,
19. // then the CameraAdapter moves from loaded to idle to executing state in one shot.
20. status_t ret = cameraPreviewInitialization();
21. // The flag mPreviewInitializationDone is set to true at the end of the function
22. // cameraPreviewInitialization(). Therefore, if everything goes alright, then the
23. // flag will be set. Sometimes, the function cameraPreviewInitialization() may
24. // return prematurely if all the resources are not available for starting preview.
25. // For example, if the preview window is not set, then it would return NO_ERROR.
26. // Under such circumstances, one should return from startPreview as well and should
27. // not continue execution. That is why, we check the flag and not the return value.
28. if (!mPreviewInitializationDone) return ret;
29. // Once startPreview is called, there is no need to continue to remember whether
30. // the function cameraPreviewInitialization() was called earlier or not. And so
31. // the flag mPreviewInitializationDone is reset here. Plus, this preserves the
32. // current behavior of startPreview under the circumstances where the application
33. // calls startPreview twice or more.
34. mPreviewInitializationDone = false;
35. ///Enable the display adapter if present, actual overlay enable happens when we post the buffer
36. if(mDisplayAdapter.get() != NULL) {
37. CAMHAL_LOGDA("Enabling display");
38. int width, height;
39. mParameters.getPreviewSize(&width, &height);
40. #if PPM_INSTRUMENTATION || PPM_INSTRUMENTATION_ABS
41. ret = mDisplayAdapter->enableDisplay(width, height, &mStartPreview);
42. #else
43. ret = mDisplayAdapter->enableDisplay(width, height, NULL);
44. #endif
45. if ( ret != NO_ERROR ) {
46. CAMHAL_LOGEA("Couldn't enable display");
47. // FIXME: At this stage mStateSwitchLock is locked and unlock is supposed to be called
48. // only from mCameraAdapter->sendCommand(CameraAdapter::CAMERA_START_PREVIEW)
49. // below. But this will never happen because of goto error. Thus at next
50. // startPreview() call CameraHAL will be deadlocked.
51. // Need to revisit mStateSwitch lock, for now just abort the process.
52. CAMHAL_ASSERT_X(false,
53. "At this stage mCameraAdapter->mStateSwitchLock is still locked, "
54. "deadlock is guaranteed");
55. goto error;
56. }
57. }
58. ///Send START_PREVIEW command to adapter
59. CAMHAL_LOGDA("Starting CameraAdapter preview mode");
60. ret = mCameraAdapter->sendCommand(CameraAdapter::CAMERA_START_PREVIEW);
61. if(ret!=NO_ERROR) {
62. CAMHAL_LOGEA("Couldn't start preview w/ CameraAdapter");
63. goto error;
64. }
65. CAMHAL_LOGDA("Started preview");
66. mPreviewEnabled = true;
67. mPreviewStartInProgress = false;
68. return ret;
69. error:
70. CAMHAL_LOGEA("Performing cleanup after error");
71. //Do all the cleanup
72. freePreviewBufs();
73. mCameraAdapter->sendCommand(CameraAdapter::CAMERA_STOP_PREVIEW);
74. if(mDisplayAdapter.get() != NULL) {
75. mDisplayAdapter->disableDisplay(false);
76. }
77. mAppCallbackNotifier->stop();
78. mPreviewStartInProgress = false;
79. mPreviewEnabled = false;
80. LOG_FUNCTION_NAME_EXIT;
81. return ret;
82. }

在我的理解看来上面标注的部分是这个方法的关键，这个地方可是会让初次研究这里的人晕头转向的，因为我就在这里犯了错误，走岔道了，下面会说明到底是怎么走岔道的：
先说一下吧，这里调用mCameraAdapter对象的sendCommand方法

1. status_t BaseCameraAdapter::sendCommand(CameraCommands operation, int value1, int value2, int value3, int value4) {
2. status_t ret = NO_ERROR;
3. struct timeval *refTimestamp;
4. BuffersDescriptor *desc = NULL;
5. CameraFrame *frame = NULL;
6. LOG_FUNCTION_NAME;
7. switch ( operation ) {
8. case：
9. ...............
10. case CameraAdapter::CAMERA_START_PREVIEW:
            {
                CAMHAL_LOGDA("Start Preview");
                if ( ret == NO_ERROR )
                {
                    ret = setState(operation);
                }

    if ( ret == NO_ERROR )
                {
                    ret = startPreview();
                }

    if ( ret == NO_ERROR )
                {
                    ret = commitState();
                }else{
                    ret |= rollbackState();
                }
                break;
            }

11. }
12. }

1. status_t BaseCameraAdapter::setState(CameraCommands operation)
2. {
3. status_t ret = NO_ERROR;
4. LOG_FUNCTION_NAME;
5. const char *printState = getLUTvalue_translateHAL(operation, CamCommandsLUT);
6. mLock.lock();
7. switch ( mAdapterState )
8. {
9. case INTIALIZED_STATE:
10. ............
11. case LOADED_PREVIEW_STATE:
12. switch ( operation )
13. {
14. case CAMERA_START_PREVIEW:
15. CAMHAL_LOGDB("Adapter state switch LOADED_PREVIEW_STATE->PREVIEW_STATE event = %s",
16. printState);
17. mNextState = PREVIEW_STATE;
18. break;
19. }
20. }
21. }

1. status_t BaseCameraAdapter::startPreview()
2. {
3. status_t ret = NO_ERROR;
4. LOG_FUNCTION_NAME;
5. LOG_FUNCTION_NAME_EXIT;
6. return ret;
7. }

就是这里了，所以我用可很醒目的颜色标注出来，很多人会想当然的理解，你不是调用了startPreview方法嘛，那就是他了啊！可是这里为什么什么动作都没做呢？？
于是就卡在这里不知所措了，那个纠结啊
现在就来解开这个谜团吧！！！

这个我们还是要往前追溯了，追溯到哪里呢？？那就从这里开始吧
mCameraAdapter->sendCommand(CameraAdapter::CAMERA_START_PREVIEW);
这个方式是在CameraHal的startPreview() 方法中被调用的
所以我要知道这个mCameraAdapter对象原型是什么啊，他从哪里而来，原来他是CameraHal这个类的一个成员，定义在：hardware/ti/omap4xxx/camera/inc/CameraHal.h
CameraAdapter *mCameraAdapter;
这里必须打破砂锅追到底，找到CameraAdapter 这个类的定义，他的定义同样这这个.h文件中

1. /**
2. * CameraAdapter interface class
3. * Concrete classes derive from this class and provide implementations based on the specific camera h/w interface
4. */
5. class CameraAdapter: public FrameNotifier, public virtual RefBase
6. {
7. protected:
8. enum AdapterActiveStates {
9. INTIALIZED_ACTIVE = 1 << 0,
10. LOADED_PREVIEW_ACTIVE = 1 << 1,
11. PREVIEW_ACTIVE = 1 << 2,
12. LOADED_CAPTURE_ACTIVE = 1 << 3,
13. CAPTURE_ACTIVE = 1 << 4,
14. BRACKETING_ACTIVE = 1 << 5,
15. AF_ACTIVE = 1 << 6,
16. ZOOM_ACTIVE = 1 << 7,
17. VIDEO_ACTIVE = 1 << 8,
18. LOADED_REPROCESS_ACTIVE = 1 << 9,
19. REPROCESS_ACTIVE = 1 << 10,
20. };
21. public:
22. typedef struct
23. {
24. CameraBuffer *mBuffers;
25. uint32_t *mOffsets;
26. int mFd;
27. size_t mLength;
28. size_t mCount;
29. size_t mMaxQueueable;
30. } BuffersDescriptor;
31. enum CameraCommands
32. {
33. CAMERA_START_PREVIEW = 0,
34. CAMERA_STOP_PREVIEW = 1,
35. CAMERA_START_VIDEO = 2,
36. CAMERA_STOP_VIDEO = 3,
37. CAMERA_START_IMAGE_CAPTURE = 4,
38. CAMERA_STOP_IMAGE_CAPTURE = 5,
39. CAMERA_PERFORM_AUTOFOCUS = 6,
40. CAMERA_CANCEL_AUTOFOCUS = 7,
41. CAMERA_PREVIEW_FLUSH_BUFFERS = 8,
42. CAMERA_START_SMOOTH_ZOOM = 9,
43. CAMERA_STOP_SMOOTH_ZOOM = 10,
44. CAMERA_USE_BUFFERS_PREVIEW = 11,
45. CAMERA_SET_TIMEOUT = 12,
46. CAMERA_CANCEL_TIMEOUT = 13,
47. CAMERA_START_BRACKET_CAPTURE = 14,
48. CAMERA_STOP_BRACKET_CAPTURE = 15,
49. CAMERA_QUERY_RESOLUTION_PREVIEW = 16,
50. CAMERA_QUERY_BUFFER_SIZE_IMAGE_CAPTURE = 17,
51. CAMERA_QUERY_BUFFER_SIZE_PREVIEW_DATA = 18,
52. CAMERA_USE_BUFFERS_IMAGE_CAPTURE = 19,
53. CAMERA_USE_BUFFERS_PREVIEW_DATA = 20,
54. CAMERA_TIMEOUT_EXPIRED = 21,
55. CAMERA_START_FD = 22,
56. CAMERA_STOP_FD = 23,
57. CAMERA_SWITCH_TO_EXECUTING = 24,
58. CAMERA_USE_BUFFERS_VIDEO_CAPTURE = 25,
59. #ifdef OMAP_ENHANCEMENT_CPCAM
60. CAMERA_USE_BUFFERS_REPROCESS = 26,
61. CAMERA_START_REPROCESS = 27,
62. #endif
63. #ifdef OMAP_ENHANCEMENT_VTC
64. CAMERA_SETUP_TUNNEL = 28,
65. CAMERA_DESTROY_TUNNEL = 29,
66. #endif
67. CAMERA_PREVIEW_INITIALIZATION = 30,
68. };
69. enum CameraMode
70. {
71. CAMERA_PREVIEW,
72. CAMERA_IMAGE_CAPTURE,
73. CAMERA_VIDEO,
74. CAMERA_MEASUREMENT,
75. CAMERA_REPROCESS,
76. };
77. enum AdapterState {
78. INTIALIZED_STATE = INTIALIZED_ACTIVE,
79. LOADED_PREVIEW_STATE = LOADED_PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
80. PREVIEW_STATE = PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
81. LOADED_CAPTURE_STATE = LOADED_CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
82. CAPTURE_STATE = CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
83. BRACKETING_STATE = BRACKETING_ACTIVE | CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE ,
84. AF_STATE = AF_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
85. ZOOM_STATE = ZOOM_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
86. VIDEO_STATE = VIDEO_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
87. VIDEO_AF_STATE = VIDEO_ACTIVE | AF_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
88. VIDEO_ZOOM_STATE = VIDEO_ACTIVE | ZOOM_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
89. VIDEO_LOADED_CAPTURE_STATE = VIDEO_ACTIVE | LOADED_CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
90. VIDEO_CAPTURE_STATE = VIDEO_ACTIVE | CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
91. AF_ZOOM_STATE = AF_ACTIVE | ZOOM_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
92. BRACKETING_ZOOM_STATE = BRACKETING_ACTIVE | ZOOM_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
93. LOADED_REPROCESS_STATE = LOADED_REPROCESS_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
94. LOADED_REPROCESS_CAPTURE_STATE = LOADED_REPROCESS_ACTIVE | LOADED_CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
95. REPROCESS_STATE = REPROCESS_ACTIVE | CAPTURE_ACTIVE | PREVIEW_ACTIVE | INTIALIZED_ACTIVE,
96. };
97. public:
98. ///Initialzes the camera adapter creates any resources required
99. virtual int initialize(CameraProperties::Properties*) = 0;
100. virtual int setErrorHandler(ErrorNotifier *errorNotifier) = 0;
101. //Message/Frame notification APIs
102. virtual void enableMsgType(int32_t msgs,
103. frame_callback callback = NULL,
104. event_callback eventCb = NULL,
105. void *cookie = NULL) = 0;
106. virtual void disableMsgType(int32_t msgs, void* cookie) = 0;
107. virtual void returnFrame(CameraBuffer* frameBuf, CameraFrame::FrameType frameType) = 0;
108. virtual void addFramePointers(CameraBuffer *frameBuf, void *buf) = 0;
109. virtual void removeFramePointers() = 0;
110. //APIs to configure Camera adapter and get the current parameter set
111. virtual int setParameters(const CameraParameters& params) = 0;
112. virtual void getParameters(CameraParameters& params) = 0;
113. //Registers callback for returning image buffers back to CameraHAL
114. virtual int registerImageReleaseCallback(release_image_buffers_callback callback, void *user_data) = 0;
115. //Registers callback, which signals a completed image capture
116. virtual int registerEndCaptureCallback(end_image_capture_callback callback, void *user_data) = 0;
117. //API to send a command to the camera
118. virtual status_t sendCommand(CameraCommands operation, int value1=0, int value2=0, int value3=0, int value4=0) = 0;
119. virtual ~CameraAdapter() {};
120. //Retrieves the current Adapter state
121. virtual AdapterState getState() = 0;
122. //Retrieves the next Adapter state
123. virtual AdapterState getNextState() = 0;
124. // Receive orientation events from CameraHal
125. virtual void onOrientationEvent(uint32_t orientation, uint32_t tilt) = 0;
126. // Rolls the state machine back to INTIALIZED_STATE from the current state
127. virtual status_t rollbackToInitializedState() = 0;
128. // Retrieves the current Adapter state - for internal use (not locked)
129. virtual status_t getState(AdapterState &state) = 0;
130. // Retrieves the next Adapter state - for internal use (not locked)
131. virtual status_t getNextState(AdapterState &state) = 0;
132. protected:
133. //The first two methods will try to switch the adapter state.
134. //Every call to setState() should be followed by a corresponding
135. //call to commitState(). If the state switch fails, then it will
136. //get reset to the previous state via rollbackState().
137. virtual status_t setState(CameraCommands operation) = 0;
138. virtual status_t commitState() = 0;
139. virtual status_t rollbackState() = 0;
140. };

看一下我标出的这是红色部分啊，为什么我要把它们标注成红色呢？？
懂C++面向对象思想应该都知道virtual这个关键字是干什么的，
如果一个类的方法被定义为virtual，如果该类的子类实现了同样的方法，则这个方法被调用的时候，会忽略父类的实现，而直接调用子类的实现，前提是方法名，包括变量类型，个数必须一致
那么这里有没有类继承了CameraAdapter 这个类，并且实现了其中的一些虚拟函数呢？？答案是肯定的，我可是吃了苦头才发现的
不过也是赖自己，这是只有定义是没有实现的，肯定是由子类来实现这是方法的，不，还是不赖自己，让我吃苦头的是这里是双层继承的，双层继承啊，我怎么知道
不卖关子了，那么谁继承了CameraAdapter 这个类呢？ 先给路径：hardware/ti/omap4xxx/camera/inc/BaseCameraAdapter.h

1. class BaseCameraAdapter : public CameraAdapter
2. {
3. public:
4. BaseCameraAdapter();
5. virtual ~BaseCameraAdapter();
6. ///Initialzes the camera adapter creates any resources required
7. virtual status_t initialize(CameraProperties::Properties*) = 0;
8. virtual int setErrorHandler(ErrorNotifier *errorNotifier);
9. //Message/Frame notification APIs
10. virtual void enableMsgType(int32_t msgs, frame_callback callback=NULL, event_callback eventCb=NULL, void* cookie=NULL);
11. virtual void disableMsgType(int32_t msgs, void* cookie);
12. virtual void returnFrame(CameraBuffer * frameBuf, CameraFrame::FrameType frameType);
13. virtual void addFramePointers(CameraBuffer *frameBuf, void *y_uv);
14. virtual void removeFramePointers();
15. //APIs to configure Camera adapter and get the current parameter set
16. virtual status_t setParameters(const CameraParameters& params) = 0;
17. virtual void getParameters(CameraParameters& params) = 0;
18. //API to send a command to the camera
19. virtual status_t sendCommand(CameraCommands operation, int value1 = 0, int value2 = 0, int value3 = 0, int value4 = 0 );
20. virtual status_t registerImageReleaseCallback(release_image_buffers_callback callback, void *user_data);
21. virtual status_t registerEndCaptureCallback(end_image_capture_callback callback, void *user_data);
22. //Retrieves the current Adapter state
23. virtual AdapterState getState();
24. //Retrieves the next Adapter state
25. virtual AdapterState getNextState();
26. // Rolls the state machine back to INTIALIZED_STATE from the current state
27. virtual status_t rollbackToInitializedState();
28. protected:
29. //The first two methods will try to switch the adapter state.
30. //Every call to setState() should be followed by a corresponding
31. //call to commitState(). If the state switch fails, then it will
32. //get reset to the previous state via rollbackState().
33. virtual status_t setState(CameraCommands operation);
34. virtual status_t commitState();
35. virtual status_t rollbackState();
36. // Retrieves the current Adapter state - for internal use (not locked)
37. virtual status_t getState(AdapterState &state);
38. // Retrieves the next Adapter state - for internal use (not locked)
39. virtual status_t getNextState(AdapterState &state);
40. //-----------Interface that needs to be implemented by deriving classes --------------------
41. //Should be implmented by deriving classes in order to start image capture
42. virtual status_t takePicture();
43. //Should be implmented by deriving classes in order to start image capture
44. virtual status_t stopImageCapture();
45. //Should be implmented by deriving classes in order to start temporal bracketing
46. virtual status_t startBracketing(int range);
47. //Should be implemented by deriving classes in order to stop temporal bracketing
48. virtual status_t stopBracketing();
49. //Should be implemented by deriving classes in oder to initiate autoFocus
50. virtual status_t autoFocus();
51. //Should be implemented by deriving classes in oder to initiate autoFocus
52. virtual status_t cancelAutoFocus();
53. //Should be called by deriving classes in order to do some bookkeeping
54. virtual status_t startVideoCapture();
55. //Should be called by deriving classes in order to do some bookkeeping
56. virtual status_t stopVideoCapture();
57. //Should be implemented by deriving classes in order to start camera preview
58. virtual status_t startPreview();
59. //Should be implemented by deriving classes in order to stop camera preview
60. virtual status_t stopPreview();
61. //Should be implemented by deriving classes in order to start smooth zoom
62. virtual status_t startSmoothZoom(int targetIdx);
63. //Should be implemented by deriving classes in order to stop smooth zoom
64. virtual status_t stopSmoothZoom();
65. //Should be implemented by deriving classes in order to stop smooth zoom
66. virtual status_t useBuffers(CameraMode mode, CameraBuffer* bufArr, int num, size_t length, unsigned int queueable);
67. //Should be implemented by deriving classes in order queue a released buffer in CameraAdapter
68. virtual status_t fillThisBuffer(CameraBuffer* frameBuf, CameraFrame::FrameType frameType);
69. //API to get the frame size required to be allocated. This size is used to override the size passed
70. //by camera service when VSTAB/VNF is turned ON for example
71. virtual status_t getFrameSize(size_t &width, size_t &height);
72. //API to get required data frame size
73. virtual status_t getFrameDataSize(size_t &dataFrameSize, size_t bufferCount);
74. //API to get required picture buffers size with the current configuration in CameraParameters
75. virtual status_t getPictureBufferSize(size_t &length, size_t bufferCount);
76. // Should be implemented by deriving classes in order to start face detection
77. // ( if supported )
78. virtual status_t startFaceDetection();
79. // Should be implemented by deriving classes in order to stop face detection
80. // ( if supported )
81. virtual status_t stopFaceDetection();
82. virtual status_t switchToExecuting();
83. virtual status_t setupTunnel(uint32_t SliceHeight, uint32_t EncoderHandle, uint32_t width, uint32_t height);
84. virtual status_t destroyTunnel();
85. virtual status_t cameraPreviewInitialization();
86. // Receive orientation events from CameraHal
87. virtual void onOrientationEvent(uint32_t orientation, uint32_t tilt);
88. // ---------------------Interface ends-----------------------------------
89. status_t notifyFocusSubscribers(CameraHalEvent::FocusStatus status);
90. status_t notifyShutterSubscribers();
91. status_t notifyZoomSubscribers(int zoomIdx, bool targetReached);
92. status_t notifyMetadataSubscribers(sp<CameraMetadataResult> &meta);
93. //Send the frame to subscribers
94. status_t sendFrameToSubscribers(CameraFrame *frame);
95. //Resets the refCount for this particular frame
96. status_t resetFrameRefCount(CameraFrame &frame);
97. //A couple of helper functions
98. void setFrameRefCount(CameraBuffer* frameBuf, CameraFrame::FrameType frameType, int refCount);
99. int getFrameRefCount(CameraBuffer* frameBuf, CameraFrame::FrameType frameType);
100. int setInitFrameRefCount(CameraBuffer* buf, unsigned int mask);
101. static const char* getLUTvalue_translateHAL(int Value, LUTtypeHAL LUT);
102. .................
103. .................
104. }

这里我只列出了一部分代码，不过大家清楚了，BaseCameraAdapter 继承CameraAdapter，不过这里还没完呢，看看这个类中定义的方法
那么多的virtual 方法，后来自己才发现的，他还是被别的类继承了，而且其中的很多方法被子类重新实现了
所以实现上上面调用的startPreview方法其实不是BaseCameraAdapter.cpp中实现的那个startPreview方法
那挺调用的startPreview方法在哪里呢，自然是继承了BaseCameraAdapter 类的那个子类实现的startPreview
现在就把这个罪魁祸首拉上来，先看定义：hardware/ti/omap4xxx/camera/inc/V4LCameraAdapter/V4LCameraAdapter.h

1. /**
2. * Class which completely abstracts the camera hardware interaction from camera hal
3. * TODO: Need to list down here, all the message types that will be supported by this class
4. Need to implement BufferProvider interface to use AllocateBuffer of OMX if needed
5. */
6. class V4LCameraAdapter : public BaseCameraAdapter
7. {
8. public:
9. /*--------------------Constant declarations----------------------------------------*/
10. static const int32_t MAX_NO_BUFFERS = 20;
11. ///@remarks OMX Camera has six ports - buffer input, time input, preview, image, video, and meta data
12. static const int MAX_NO_PORTS = 6;
13. ///Five second timeout
14. static const int CAMERA_ADAPTER_TIMEOUT = 5000*1000;
15. public:
16. V4LCameraAdapter(size_t sensor_index);
17. ~V4LCameraAdapter();
18. ///Initialzes the camera adapter creates any resources required
19. virtual status_t initialize(CameraProperties::Properties*);
20. //APIs to configure Camera adapter and get the current parameter set
21. virtual status_t setParameters(const CameraParameters& params);
22. virtual void getParameters(CameraParameters& params);
23. // API
24. virtual status_t UseBuffersPreview(CameraBuffer *bufArr, int num);
25. virtual status_t UseBuffersCapture(CameraBuffer *bufArr, int num);
26. static status_t getCaps(const int sensorId, CameraProperties::Properties* params, V4L_HANDLETYPE handle);
27. protected:
28. //----------Parent class method implementation------------------------------------//看看人家这里说的很清楚，这是父类的方法
29. virtual status_t startPreview();
30. virtual status_t stopPreview();
31. virtual status_t takePicture();
32. virtual status_t stopImageCapture();
33. virtual status_t autoFocus();
34. virtual status_t useBuffers(CameraMode mode, CameraBuffer *bufArr, int num, size_t length, unsigned int queueable);
35. virtual status_t fillThisBuffer(CameraBuffer *frameBuf, CameraFrame::FrameType frameType);
36. virtual status_t getFrameSize(size_t &width, size_t &height);
37. virtual status_t getPictureBufferSize(size_t &length, size_t bufferCount);
38. virtual status_t getFrameDataSize(size_t &dataFrameSize, size_t bufferCount);
39. virtual void onOrientationEvent(uint32_t orientation, uint32_t tilt);
40. //-----------------------------------------------------------------------------
41. private:
42. class PreviewThread : public Thread {
43. V4LCameraAdapter* mAdapter;
44. public:
45. PreviewThread(V4LCameraAdapter* hw) :
46. Thread(false), mAdapter(hw) { }
47. virtual void onFirstRef() {
48. run("CameraPreviewThread", PRIORITY_URGENT_DISPLAY);
49. }
50. virtual bool threadLoop() {
51. mAdapter->previewThread();
52. // loop until we need to quit
53. return true;
54. }
55. };
56. //Used for calculation of the average frame rate during preview
57. status_t recalculateFPS();
58. char * GetFrame(int &index);
59. int previewThread();
60. public:
61. private:
62. //capabilities data
63. static const CapPixelformat mPixelformats [];
64. static const CapResolution mPreviewRes [];
65. static const CapFramerate mFramerates [];
66. static const CapResolution mImageCapRes [];
67. //camera defaults
68. static const char DEFAULT_PREVIEW_FORMAT[];
69. static const char DEFAULT_PREVIEW_SIZE[];
70. static const char DEFAULT_FRAMERATE[];
71. static const char DEFAULT_NUM_PREV_BUFS[];
72. static const char DEFAULT_PICTURE_FORMAT[];
73. static const char DEFAULT_PICTURE_SIZE[];
74. static const char DEFAULT_FOCUS_MODE[];
75. static const char * DEFAULT_VSTAB;
76. static const char * DEFAULT_VNF;
77. static status_t insertDefaults(CameraProperties::Properties*, V4L_TI_CAPTYPE&);
78. static status_t insertCapabilities(CameraProperties::Properties*, V4L_TI_CAPTYPE&);
79. static status_t insertPreviewFormats(CameraProperties::Properties* , V4L_TI_CAPTYPE&);
80. static status_t insertPreviewSizes(CameraProperties::Properties* , V4L_TI_CAPTYPE&);
81. static status_t insertImageSizes(CameraProperties::Properties* , V4L_TI_CAPTYPE&);
82. static status_t insertFrameRates(CameraProperties::Properties* , V4L_TI_CAPTYPE&);
83. static status_t sortAscend(V4L_TI_CAPTYPE&, uint16_t ) ;
84. status_t v4lIoctl(int, int, void*);
85. status_t v4lInitMmap(int&);
86. status_t v4lInitUsrPtr(int&);
87. status_t v4lStartStreaming();
88. status_t v4lStopStreaming(int nBufferCount);
89. status_t v4lSetFormat(int, int, uint32_t);
90. status_t restartPreview();
91. int mPreviewBufferCount;
92. int mPreviewBufferCountQueueable;
93. int mCaptureBufferCount;
94. int mCaptureBufferCountQueueable;
95. KeyedVector<CameraBuffer *, int> mPreviewBufs;
96. KeyedVector<CameraBuffer *, int> mCaptureBufs;
97. mutable Mutex mPreviewBufsLock;
98. mutable Mutex mCaptureBufsLock;
99. mutable Mutex mStopPreviewLock;
100. CameraParameters mParams;
101. bool mPreviewing;
102. bool mCapturing;
103. Mutex mLock;
104. int mFrameCount;
105. int mLastFrameCount;
106. unsigned int mIter;
107. nsecs_t mLastFPSTime;
108. //variables holding the estimated framerate
109. float mFPS, mLastFPS;
110. int mSensorIndex;
111. // protected by mLoc

大家看到了V4LCameraAdapter 又继承了BaseCameraAdapter，双层继承，实现了父类的一些方法
所有这里算是媳妇熬着婆了，终于找到了我们想要的startPreview
不过看到终于进入了V4LCameraAdapter 这个类，我知道，离成功已经很近了，V4L2就是直接去和driver谈判的
那么我们就看看V4LCameraAdapter 这个类中的startPreview方法吧，路径：ardware/ti/omap4xxx/camera/V4LCameraAdapter/V4LCameraAdapter.cpp

1. status_t V4LCameraAdapter::startPreview()
2. {
3. status_t ret = NO_ERROR;
4. LOG_FUNCTION_NAME;
5. Mutex::Autolock lock(mPreviewBufsLock);
6. if(mPreviewing) {
7. ret = BAD_VALUE;
8. goto EXIT;
9. }
10. for (int i = 0; i < mPreviewBufferCountQueueable; i++) {
11. mVideoInfo->buf.index = i;
12. mVideoInfo->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
13. mVideoInfo->buf.memory = V4L2_MEMORY_MMAP;
14. ret = v4lIoctl(mCameraHandle, VIDIOC_QBUF, &mVideoInfo->buf);
15. if (ret < 0) {
16. CAMHAL_LOGEA("VIDIOC_QBUF Failed");
17. goto EXIT;
18. }
19. nQueued++;
20. }
21. ret = v4lStartStreaming();
22. // Create and start preview thread for receiving buffers from V4L Camera
23. if(!mCapturing) {
24. mPreviewThread = new PreviewThread(this);
25. CAMHAL_LOGDA("Created preview thread");
26. }

不错，这条语句就是我一直找寻的，真是众里寻他千百度，蓦然回首，那句就在灯火阑珊处
这样，其他的事情就全部由v4l2去做了，这些过程会单独分一章去学习
还有就是上面绿的部分，同样要分一章学习，很重要

待续。。。。。。。。。。。