android从应用到驱动之—camera(2)---cameraHAL的实现

本来想用这一篇博客把cameraHAL的实现和流程都给写完的.搞了半天,东西实在是太多了.这篇先写cameraHAL的基本实现框架,下一篇在具体写camerahal的流程吧.

cameraHAL的实现:

对于初学者来说,最大的疑问是系统是如何调用hardware的.
这里就以camera来举例说明.
调用hardware的程序是cameraservice,我们就去它里面看看它是如何找到hardware的

先把源码贴上来:

/*
**
** Copyright (C) 2008, The Android Open Source Project
**
** Licensed under the Apache License, Version 2.0 (the "License");
** you may not use this file except in compliance with the License.
** You may obtain a copy of the License at
**
**     http://www.apache.org/licenses/LICENSE-2.0
**
** Unless required by applicable law or agreed to in writing, software
** distributed under the License is distributed on an "AS IS" BASIS,
** WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
** See the License for the specific language governing permissions and
** limitations under the License.
*/

#define LOG_TAG "CameraService"
//#define LOG_NDEBUG 0

#include <stdio.h>
#include <sys/types.h>
#include <pthread.h>

#include <binder/IPCThreadState.h>
#include <binder/IServiceManager.h>
#include <binder/MemoryBase.h>
#include <binder/MemoryHeapBase.h>
#include <cutils/atomic.h>
#include <cutils/properties.h>
#include <gui/SurfaceTextureClient.h>
#include <hardware/hardware.h>
#include <media/AudioSystem.h>
#include <media/mediaplayer.h>
#include <surfaceflinger/ISurface.h>
#include <utils/Errors.h>
#include <utils/Log.h>
#include <utils/String16.h>

#include "CameraService.h"
#include "CameraHardwareInterface.h"

namespace android {

// ----------------------------------------------------------------------------
// Logging support -- this is for debugging only
// Use "adb shell dumpsys media.camera -v 1" to change it.
static volatile int32_t gLogLevel = ;

#define LOG1(...) LOGD_IF(gLogLevel >= 1, __VA_ARGS__);
#define LOG2(...) LOGD_IF(gLogLevel >= 2, __VA_ARGS__);

static void setLogLevel(int level) {
    android_atomic_write(level, &gLogLevel);
}

// ----------------------------------------------------------------------------

static int getCallingPid() {
    return IPCThreadState::self()->getCallingPid();
}

static int getCallingUid() {
    return IPCThreadState::self()->getCallingUid();
}

// ----------------------------------------------------------------------------

// This is ugly and only safe if we never re-create the CameraService, but
// should be ok for now.
static CameraService *gCameraService;

CameraService::CameraService()
:mSoundRef(), mModule()
{
    LOGI("CameraService started (pid=%d)", getpid());
    gCameraService = this;
}

void CameraService::onFirstRef()
{
    BnCameraService::onFirstRef();

    if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
                (const hw_module_t **)&mModule) < ) {
        LOGE("Could not load camera HAL module");
        mNumberOfCameras = ;
    }
    else {
        mNumberOfCameras = mModule->get_number_of_cameras();
        if (mNumberOfCameras > MAX_CAMERAS) {
            LOGE("Number of cameras(%d) > MAX_CAMERAS(%d).",
                    mNumberOfCameras, MAX_CAMERAS);
            mNumberOfCameras = MAX_CAMERAS;
        }
        for (int i = ; i < mNumberOfCameras; i++) {
            setCameraFree(i);
        }
    }

    // Read the system property to determine if we have to use the
    // AUDIO_STREAM_ENFORCED_AUDIBLE type.
    char value[PROPERTY_VALUE_MAX];
    property_get("ro.camera.sound.forced", value, "");
    if (strcmp(value, "") != ) {
        mAudioStreamType = AUDIO_STREAM_ENFORCED_AUDIBLE;
    } else {
        mAudioStreamType = AUDIO_STREAM_MUSIC;
    }
}

CameraService::~CameraService() {
    for (int i = ; i < mNumberOfCameras; i++) {
        if (mBusy[i]) {
            LOGE("camera %d is still in use in destructor!", i);
        }
    }

    gCameraService = NULL;
}

int32_t CameraService::getNumberOfCameras() {
    return mNumberOfCameras;
}

status_t CameraService::getCameraInfo(int cameraId,
                                      struct CameraInfo* cameraInfo) {
    if (!mModule) {
        return NO_INIT;
    }

    if (cameraId <  || cameraId >= mNumberOfCameras) {
        return BAD_VALUE;
    }

    struct camera_info info;
    status_t rc = mModule->get_camera_info(cameraId, &info);
    cameraInfo->facing = info.facing;
    cameraInfo->orientation = info.orientation;
    return rc;
}

sp<ICamera> CameraService::connect(
        const sp<ICameraClient>& cameraClient, int cameraId) {
    int callingPid = getCallingPid();
    sp<CameraHardwareInterface> hardware = NULL;

    LOG1("CameraService::connect E (pid %d, id %d)", callingPid, cameraId);

    if (!mModule) {
        LOGE("Camera HAL module not loaded");
        return NULL;
    }

    sp<Client> client;
    if (cameraId <  || cameraId >= mNumberOfCameras) {
        LOGE("CameraService::connect X (pid %d) rejected (invalid cameraId %d).",
            callingPid, cameraId);
        return NULL;
    }

    char value[PROPERTY_VALUE_MAX];
    property_get("sys.secpolicy.camera.disabled", value, "");
    if (strcmp(value, "") == ) {
        // Camera is disabled by DevicePolicyManager.
        LOGI("Camera is disabled. connect X (pid %d) rejected", callingPid);
        return NULL;
    }

    Mutex::Autolock lock(mServiceLock);
    if (mClient[cameraId] != ) {
        client = mClient[cameraId].promote();
        if (client != ) {
            if (cameraClient->asBinder() == client->getCameraClient()->asBinder()) {
                LOG1("CameraService::connect X (pid %d) (the same client)",
                    callingPid);
                return client;
            } else {
                LOGW("CameraService::connect X (pid %d) rejected (existing client).",
                    callingPid);
                return NULL;
            }
        }
        mClient[cameraId].clear();
    }

    if (mBusy[cameraId]) {
        LOGW("CameraService::connect X (pid %d) rejected"
             " (camera %d is still busy).", callingPid, cameraId);
        return NULL;
    }

    struct camera_info info;
    if (mModule->get_camera_info(cameraId, &info) != OK) {
        LOGE("Invalid camera id %d", cameraId);
        return NULL;
    }

    char camera_device_name[];
    snprintf(camera_device_name, sizeof(camera_device_name), "%d", cameraId);

    hardware = new CameraHardwareInterface(camera_device_name);
    if (hardware->initialize(&mModule->common) != OK) {
        hardware.clear();
        return NULL;
    }

    client = new Client(this, cameraClient, hardware, cameraId, info.facing, callingPid);
    mClient[cameraId] = client;
    LOG1("CameraService::connect X");
    return client;
}

void CameraService::removeClient(const sp<ICameraClient>& cameraClient) {
    int callingPid = getCallingPid();
    LOG1("CameraService::removeClient E (pid %d)", callingPid);

    for (int i = ; i < mNumberOfCameras; i++) {
        // Declare this before the lock to make absolutely sure the
        // destructor won't be called with the lock held.
        sp<Client> client;

        Mutex::Autolock lock(mServiceLock);

        // This happens when we have already disconnected (or this is
        // just another unused camera).
        if (mClient[i] == ) continue;

        // Promote mClient. It can fail if we are called from this path:
        // Client::~Client() -> disconnect() -> removeClient().
        client = mClient[i].promote();

        if (client == ) {
            mClient[i].clear();
            continue;
        }

        if (cameraClient->asBinder() == client->getCameraClient()->asBinder()) {
            // Found our camera, clear and leave.
            LOG1("removeClient: clear camera %d", i);
            mClient[i].clear();
            break;
        }
    }

    LOG1("CameraService::removeClient X (pid %d)", callingPid);
}

sp<CameraService::Client> CameraService::getClientById(int cameraId) {
    if (cameraId <  || cameraId >= mNumberOfCameras) return NULL;
    return mClient[cameraId].promote();
}

status_t CameraService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) {
    // Permission checks
    switch (code) {
        case BnCameraService::CONNECT:
            const int pid = getCallingPid();
            const int self_pid = getpid();
            if (pid != self_pid) {
                // we're called from a different process, do the real check
                if (!checkCallingPermission(
                        String16("android.permission.CAMERA"))) {
                    const int uid = getCallingUid();
                    LOGE("Permission Denial: "
                         "can't use the camera pid=%d, uid=%d", pid, uid);
                    return PERMISSION_DENIED;
                }
            }
            break;
    }

    return BnCameraService::onTransact(code, data, reply, flags);
}

// The reason we need this busy bit is a new CameraService::connect() request
// may come in while the previous Client's destructor has not been run or is
// still running. If the last strong reference of the previous Client is gone
// but the destructor has not been finished, we should not allow the new Client
// to be created because we need to wait for the previous Client to tear down
// the hardware first.
void CameraService::setCameraBusy(int cameraId) {
    android_atomic_write(, &mBusy[cameraId]);
}

void CameraService::setCameraFree(int cameraId) {
    android_atomic_write(, &mBusy[cameraId]);
}

// We share the media players for shutter and recording sound for all clients.
// A reference count is kept to determine when we will actually release the
// media players.

MediaPlayer* CameraService::newMediaPlayer(const char *file) {
    MediaPlayer* mp = new MediaPlayer();
    if (mp->setDataSource(file, NULL) == NO_ERROR) {
        mp->setAudioStreamType(mAudioStreamType);
        mp->prepare();
    } else {
        LOGE("Failed to load CameraService sounds: %s", file);
        return NULL;
    }
    return mp;
}

void CameraService::loadSound() {
    Mutex::Autolock lock(mSoundLock);
    LOG1("CameraService::loadSound ref=%d", mSoundRef);
    if (mSoundRef++) return;

    mSoundPlayer[SOUND_SHUTTER] = newMediaPlayer("/system/media/audio/ui/camera_click.ogg");
    mSoundPlayer[SOUND_RECORDING] = newMediaPlayer("/system/media/audio/ui/VideoRecord.ogg");
}

void CameraService::releaseSound() {
    Mutex::Autolock lock(mSoundLock);
    LOG1("CameraService::releaseSound ref=%d", mSoundRef);
    if (--mSoundRef) return;

    for (int i = ; i < NUM_SOUNDS; i++) {
        if (mSoundPlayer[i] != ) {
            mSoundPlayer[i]->disconnect();
            mSoundPlayer[i].clear();
        }
    }
}

void CameraService::playSound(sound_kind kind) {
    LOG1("playSound(%d)", kind);
    Mutex::Autolock lock(mSoundLock);
    sp<MediaPlayer> player = mSoundPlayer[kind];
    if (player != ) {
        player->seekTo();
        player->start();
    }
}

// ----------------------------------------------------------------------------

CameraService::Client::Client(const sp<CameraService>& cameraService,
        const sp<ICameraClient>& cameraClient,
        const sp<CameraHardwareInterface>& hardware,
        int cameraId, int cameraFacing, int clientPid) {
    int callingPid = getCallingPid();
    LOG1("Client::Client E (pid %d)", callingPid);

    mCameraService = cameraService;
    mCameraClient = cameraClient;
    mHardware = hardware;
    mCameraId = cameraId;
    mCameraFacing = cameraFacing;
    mClientPid = clientPid;
    mMsgEnabled = ;
    mSurface = ;
    mPreviewWindow = ;
    mHardware->setCallbacks(notifyCallback,
                            dataCallback,
                            dataCallbackTimestamp,
                            (void *)cameraId);

    // Enable zoom, error, focus, and metadata messages by default
    enableMsgType(CAMERA_MSG_ERROR | CAMERA_MSG_ZOOM | CAMERA_MSG_FOCUS |
                  CAMERA_MSG_PREVIEW_METADATA);

    // Callback is disabled by default
    mPreviewCallbackFlag = CAMERA_FRAME_CALLBACK_FLAG_NOOP;
    mOrientation = getOrientation(, mCameraFacing == CAMERA_FACING_FRONT);
    mPlayShutterSound = true;
    cameraService->setCameraBusy(cameraId);
    cameraService->loadSound();
    LOG1("Client::Client X (pid %d)", callingPid);
}

// tear down the client
CameraService::Client::~Client() {
    int callingPid = getCallingPid();
    LOG1("Client::~Client E (pid %d, this %p)", callingPid, this);

    // set mClientPid to let disconnet() tear down the hardware
    mClientPid = callingPid;
    disconnect();
    mCameraService->releaseSound();
    LOG1("Client::~Client X (pid %d, this %p)", callingPid, this);
}

// ----------------------------------------------------------------------------

status_t CameraService::Client::checkPid() const {
    int callingPid = getCallingPid();
    if (callingPid == mClientPid) return NO_ERROR;

    LOGW("attempt to use a locked camera from a different process"
         " (old pid %d, new pid %d)", mClientPid, callingPid);
    return EBUSY;
}

status_t CameraService::Client::checkPidAndHardware() const {
    status_t result = checkPid();
    if (result != NO_ERROR) return result;
    if (mHardware == ) {
        LOGE("attempt to use a camera after disconnect() (pid %d)", getCallingPid());
        return INVALID_OPERATION;
    }
    return NO_ERROR;
}

status_t CameraService::Client::lock() {
    int callingPid = getCallingPid();
    LOG1("lock (pid %d)", callingPid);
    Mutex::Autolock lock(mLock);

    // lock camera to this client if the the camera is unlocked
    if (mClientPid == ) {
        mClientPid = callingPid;
        return NO_ERROR;
    }

    // returns NO_ERROR if the client already owns the camera, EBUSY otherwise
    return checkPid();
}

status_t CameraService::Client::unlock() {
    int callingPid = getCallingPid();
    LOG1("unlock (pid %d)", callingPid);
    Mutex::Autolock lock(mLock);

    // allow anyone to use camera (after they lock the camera)
    status_t result = checkPid();
    if (result == NO_ERROR) {
        if (mHardware->recordingEnabled()) {
            LOGE("Not allowed to unlock camera during recording.");
            return INVALID_OPERATION;
        }
        mClientPid = ;
        LOG1("clear mCameraClient (pid %d)", callingPid);
        // we need to remove the reference to ICameraClient so that when the app
        // goes away, the reference count goes to 0.
        mCameraClient.clear();
    }
    return result;
}

// connect a new client to the camera
status_t CameraService::Client::connect(const sp<ICameraClient>& client) {
    int callingPid = getCallingPid();
    LOG1("connect E (pid %d)", callingPid);
    Mutex::Autolock lock(mLock);

    if (mClientPid !=  && checkPid() != NO_ERROR) {
        LOGW("Tried to connect to a locked camera (old pid %d, new pid %d)",
                mClientPid, callingPid);
        return EBUSY;
    }

    if (mCameraClient !=  && (client->asBinder() == mCameraClient->asBinder())) {
        LOG1("Connect to the same client");
        return NO_ERROR;
    }

    mPreviewCallbackFlag = CAMERA_FRAME_CALLBACK_FLAG_NOOP;
    mClientPid = callingPid;
    mCameraClient = client;

    LOG1("connect X (pid %d)", callingPid);
    return NO_ERROR;
}

static void disconnectWindow(const sp<ANativeWindow>& window) {
    if (window != ) {
        status_t result = native_window_api_disconnect(window.get(),
                NATIVE_WINDOW_API_CAMERA);
        if (result != NO_ERROR) {
            LOGW("native_window_api_disconnect failed: %s (%d)", strerror(-result),
                    result);
        }
    }
}

void CameraService::Client::disconnect() {
    int callingPid = getCallingPid();
    LOG1("disconnect E (pid %d)", callingPid);
    Mutex::Autolock lock(mLock);

    if (checkPid() != NO_ERROR) {
        LOGW("different client - don't disconnect");
        return;
    }

    if (mClientPid <= ) {
        LOG1("camera is unlocked (mClientPid = %d), don't tear down hardware", mClientPid);
        return;
    }

    // Make sure disconnect() is done once and once only, whether it is called
    // from the user directly, or called by the destructor.
    if (mHardware == ) return;

    LOG1("hardware teardown");
    // Before destroying mHardware, we must make sure it's in the
    // idle state.
    // Turn off all messages.
    disableMsgType(CAMERA_MSG_ALL_MSGS);
    mHardware->stopPreview();
    mHardware->cancelPicture();
    // Release the hardware resources.
    mHardware->release();

    // Release the held ANativeWindow resources.
    if (mPreviewWindow != ) {
        disconnectWindow(mPreviewWindow);
        mPreviewWindow = ;
        mHardware->setPreviewWindow(mPreviewWindow);
    }
    mHardware.clear();

    mCameraService->removeClient(mCameraClient);
    mCameraService->setCameraFree(mCameraId);

    LOG1("disconnect X (pid %d)", callingPid);
}

// ----------------------------------------------------------------------------

status_t CameraService::Client::setPreviewWindow(const sp<IBinder>& binder,
        const sp<ANativeWindow>& window) {
    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    // return if no change in surface.
    if (binder == mSurface) {
        return NO_ERROR;
    }

    if (window != ) {
        result = native_window_api_connect(window.get(), NATIVE_WINDOW_API_CAMERA);
        if (result != NO_ERROR) {
            LOGE("native_window_api_connect failed: %s (%d)", strerror(-result),
                    result);
            return result;
        }
    }

    // If preview has been already started, register preview buffers now.
    if (mHardware->previewEnabled()) {
        if (window != ) {
            native_window_set_scaling_mode(window.get(),
                    NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
            native_window_set_buffers_transform(window.get(), mOrientation);
            result = mHardware->setPreviewWindow(window);
        }
    }

    if (result == NO_ERROR) {
        // Everything has succeeded.  Disconnect the old window and remember the
        // new window.
        disconnectWindow(mPreviewWindow);
        mSurface = binder;
        mPreviewWindow = window;
    } else {
        // Something went wrong after we connected to the new window, so
        // disconnect here.
        disconnectWindow(window);
    }

    return result;
}

// set the Surface that the preview will use
status_t CameraService::Client::setPreviewDisplay(const sp<Surface>& surface) {
    LOG1("setPreviewDisplay(%p) (pid %d)", surface.get(), getCallingPid());

    sp<IBinder> binder(surface !=  ? surface->asBinder() : );
    sp<ANativeWindow> window(surface);
    return setPreviewWindow(binder, window);
}

// set the SurfaceTexture that the preview will use
status_t CameraService::Client::setPreviewTexture(
        const sp<ISurfaceTexture>& surfaceTexture) {
    LOG1("setPreviewTexture(%p) (pid %d)", surfaceTexture.get(),
            getCallingPid());

    sp<IBinder> binder;
    sp<ANativeWindow> window;
    if (surfaceTexture != ) {
        binder = surfaceTexture->asBinder();
        window = new SurfaceTextureClient(surfaceTexture);
    }
    return setPreviewWindow(binder, window);
}

// set the preview callback flag to affect how the received frames from
// preview are handled.
void CameraService::Client::setPreviewCallbackFlag(int callback_flag) {
    LOG1("setPreviewCallbackFlag(%d) (pid %d)", callback_flag, getCallingPid());
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return;

    mPreviewCallbackFlag = callback_flag;
    if (mPreviewCallbackFlag & CAMERA_FRAME_CALLBACK_FLAG_ENABLE_MASK) {
        enableMsgType(CAMERA_MSG_PREVIEW_FRAME);
    } else {
        disableMsgType(CAMERA_MSG_PREVIEW_FRAME);
    }
}

// start preview mode
status_t CameraService::Client::startPreview() {
    LOG1("startPreview (pid %d)", getCallingPid());
    return startCameraMode(CAMERA_PREVIEW_MODE);
}

// start recording mode
status_t CameraService::Client::startRecording() {
    LOG1("startRecording (pid %d)", getCallingPid());
    return startCameraMode(CAMERA_RECORDING_MODE);
}

// start preview or recording
status_t CameraService::Client::startCameraMode(camera_mode mode) {
    LOG1("startCameraMode(%d)", mode);
    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    switch(mode) {
        case CAMERA_PREVIEW_MODE:
            if (mSurface ==  && mPreviewWindow == ) {
                LOG1("mSurface is not set yet.");
                // still able to start preview in this case.
            }
            return startPreviewMode();
        case CAMERA_RECORDING_MODE:
            if (mSurface ==  && mPreviewWindow == ) {
                LOGE("mSurface or mPreviewWindow must be set before startRecordingMode.");
                return INVALID_OPERATION;
            }
            return startRecordingMode();
        default:
            return UNKNOWN_ERROR;
    }
}

status_t CameraService::Client::startPreviewMode() {
    LOG1("startPreviewMode");
    status_t result = NO_ERROR;

    // if preview has been enabled, nothing needs to be done
    if (mHardware->previewEnabled()) {
        return NO_ERROR;
    }

    if (mPreviewWindow != ) {
        native_window_set_scaling_mode(mPreviewWindow.get(),
                NATIVE_WINDOW_SCALING_MODE_SCALE_TO_WINDOW);
        native_window_set_buffers_transform(mPreviewWindow.get(),
                mOrientation);
    }
    mHardware->setPreviewWindow(mPreviewWindow);
    result = mHardware->startPreview();

    return result;
}

status_t CameraService::Client::startRecordingMode() {
    LOG1("startRecordingMode");
    status_t result = NO_ERROR;

    // if recording has been enabled, nothing needs to be done
    if (mHardware->recordingEnabled()) {
        return NO_ERROR;
    }

    // if preview has not been started, start preview first
    if (!mHardware->previewEnabled()) {
        result = startPreviewMode();
        if (result != NO_ERROR) {
            return result;
        }
    }

    // start recording mode
    enableMsgType(CAMERA_MSG_VIDEO_FRAME);
    mCameraService->playSound(SOUND_RECORDING);
    result = mHardware->startRecording();
    if (result != NO_ERROR) {
        LOGE("mHardware->startRecording() failed with status %d", result);
    }
    return result;
}

// stop preview mode
void CameraService::Client::stopPreview() {
    LOG1("stopPreview (pid %d)", getCallingPid());
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return;

    disableMsgType(CAMERA_MSG_PREVIEW_FRAME);
    mHardware->stopPreview();

    mPreviewBuffer.clear();
}

// stop recording mode
void CameraService::Client::stopRecording() {
    LOG1("stopRecording (pid %d)", getCallingPid());
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return;

    mCameraService->playSound(SOUND_RECORDING);
    disableMsgType(CAMERA_MSG_VIDEO_FRAME);
    mHardware->stopRecording();

    mPreviewBuffer.clear();
}

// release a recording frame
void CameraService::Client::releaseRecordingFrame(const sp<IMemory>& mem) {
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return;
    mHardware->releaseRecordingFrame(mem);
}

status_t CameraService::Client::storeMetaDataInBuffers(bool enabled)
{
    LOG1("storeMetaDataInBuffers: %s", enabled? "true": "false");
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) {
        return UNKNOWN_ERROR;
    }
    return mHardware->storeMetaDataInBuffers(enabled);
}

bool CameraService::Client::previewEnabled() {
    LOG1("previewEnabled (pid %d)", getCallingPid());

    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return false;
    return mHardware->previewEnabled();
}

bool CameraService::Client::recordingEnabled() {
    LOG1("recordingEnabled (pid %d)", getCallingPid());

    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return false;
    return mHardware->recordingEnabled();
}

status_t CameraService::Client::autoFocus() {
    LOG1("autoFocus (pid %d)", getCallingPid());

    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    return mHardware->autoFocus();
}

status_t CameraService::Client::cancelAutoFocus() {
    LOG1("cancelAutoFocus (pid %d)", getCallingPid());

    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    return mHardware->cancelAutoFocus();
}

// take a picture - image is returned in callback
status_t CameraService::Client::takePicture(int msgType) {
    LOG1("takePicture (pid %d): 0x%x", getCallingPid(), msgType);

    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    if ((msgType & CAMERA_MSG_RAW_IMAGE) &&
        (msgType & CAMERA_MSG_RAW_IMAGE_NOTIFY)) {
        LOGE("CAMERA_MSG_RAW_IMAGE and CAMERA_MSG_RAW_IMAGE_NOTIFY"
                " cannot be both enabled");
        return BAD_VALUE;
    }

    // We only accept picture related message types
    // and ignore other types of messages for takePicture().
    int picMsgType = msgType
                        & (CAMERA_MSG_SHUTTER |
                           CAMERA_MSG_POSTVIEW_FRAME |
                           CAMERA_MSG_RAW_IMAGE |
                           CAMERA_MSG_RAW_IMAGE_NOTIFY |
                           CAMERA_MSG_COMPRESSED_IMAGE);

    enableMsgType(picMsgType);

    return mHardware->takePicture();
}

// set preview/capture parameters - key/value pairs
status_t CameraService::Client::setParameters(const String8& params) {
    LOG1("setParameters (pid %d) (%s)", getCallingPid(), params.string());

    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    CameraParameters p(params);
    return mHardware->setParameters(p);
}

// get preview/capture parameters - key/value pairs
String8 CameraService::Client::getParameters() const {
    Mutex::Autolock lock(mLock);
    if (checkPidAndHardware() != NO_ERROR) return String8();

    String8 params(mHardware->getParameters().flatten());
    LOG1("getParameters (pid %d) (%s)", getCallingPid(), params.string());
    return params;
}

// enable shutter sound
status_t CameraService::Client::enableShutterSound(bool enable) {
    LOG1("enableShutterSound (pid %d)", getCallingPid());

    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    if (enable) {
        mPlayShutterSound = true;
        return OK;
    }

    // Disabling shutter sound may not be allowed. In that case only
    // allow the mediaserver process to disable the sound.
    char value[PROPERTY_VALUE_MAX];
    property_get("ro.camera.sound.forced", value, "");
    if (strcmp(value, "") != ) {
        // Disabling shutter sound is not allowed. Deny if the current
        // process is not mediaserver.
        if (getCallingPid() != getpid()) {
            LOGE("Failed to disable shutter sound. Permission denied (pid %d)", getCallingPid());
            return PERMISSION_DENIED;
        }
    }

    mPlayShutterSound = false;
    return OK;
}

status_t CameraService::Client::sendCommand(int32_t cmd, int32_t arg1, int32_t arg2) {
    LOG1("sendCommand (pid %d)", getCallingPid());
    int orientation;
    Mutex::Autolock lock(mLock);
    status_t result = checkPidAndHardware();
    if (result != NO_ERROR) return result;

    if (cmd == CAMERA_CMD_SET_DISPLAY_ORIENTATION) {
        // Mirror the preview if the camera is front-facing.
        orientation = getOrientation(arg1, mCameraFacing == CAMERA_FACING_FRONT);
        if (orientation == -) return BAD_VALUE;

        if (mOrientation != orientation) {
            mOrientation = orientation;
            if (mPreviewWindow != ) {
                native_window_set_buffers_transform(mPreviewWindow.get(),
                        mOrientation);
            }
        }
        return OK;
    } else if (cmd == CAMERA_CMD_ENABLE_SHUTTER_SOUND) {
        switch (arg1) {
            case :
                enableShutterSound(false);
                break;
            case :
                enableShutterSound(true);
                break;
            default:
                return BAD_VALUE;
        }
        return OK;
    } else if (cmd == CAMERA_CMD_PLAY_RECORDING_SOUND) {
        mCameraService->playSound(SOUND_RECORDING);
    }

    return mHardware->sendCommand(cmd, arg1, arg2);
}

// ----------------------------------------------------------------------------

void CameraService::Client::enableMsgType(int32_t msgType) {
    android_atomic_or(msgType, &mMsgEnabled);
    mHardware->enableMsgType(msgType);
}

void CameraService::Client::disableMsgType(int32_t msgType) {
    android_atomic_and(~msgType, &mMsgEnabled);
    mHardware->disableMsgType(msgType);
}

#define CHECK_MESSAGE_INTERVAL 10 // 10ms
bool CameraService::Client::lockIfMessageWanted(int32_t msgType) {
    int sleepCount = ;
    while (mMsgEnabled & msgType) {
        if (mLock.tryLock() == NO_ERROR) {
            if (sleepCount > ) {
                LOG1("lockIfMessageWanted(%d): waited for %d ms",
                    msgType, sleepCount * CHECK_MESSAGE_INTERVAL);
            }
            return true;
        }
        if (sleepCount++ == ) {
            LOG1("lockIfMessageWanted(%d): enter sleep", msgType);
        }
        usleep(CHECK_MESSAGE_INTERVAL * );
    }
    LOGW("lockIfMessageWanted(%d): dropped unwanted message", msgType);
    return false;
}

// ----------------------------------------------------------------------------

// Converts from a raw pointer to the client to a strong pointer during a
// hardware callback. This requires the callbacks only happen when the client
// is still alive.
sp<CameraService::Client> CameraService::Client::getClientFromCookie(void* user) {
    sp<Client> client = gCameraService->getClientById((int) user);

    // This could happen if the Client is in the process of shutting down (the
    // last strong reference is gone, but the destructor hasn't finished
    // stopping the hardware).
    if (client == ) return NULL;

    // The checks below are not necessary and are for debugging only.
    if (client->mCameraService.get() != gCameraService) {
        LOGE("mismatch service!");
        return NULL;
    }

    if (client->mHardware == ) {
        LOGE("mHardware == 0: callback after disconnect()?");
        return NULL;
    }

    return client;
}

// Callback messages can be dispatched to internal handlers or pass to our
// client's callback functions, depending on the message type.
//
// notifyCallback:
//      CAMERA_MSG_SHUTTER              handleShutter
//      (others)                        c->notifyCallback
// dataCallback:
//      CAMERA_MSG_PREVIEW_FRAME        handlePreviewData
//      CAMERA_MSG_POSTVIEW_FRAME       handlePostview
//      CAMERA_MSG_RAW_IMAGE            handleRawPicture
//      CAMERA_MSG_COMPRESSED_IMAGE     handleCompressedPicture
//      (others)                        c->dataCallback
// dataCallbackTimestamp
//      (others)                        c->dataCallbackTimestamp
//
// NOTE: the *Callback functions grab mLock of the client before passing
// control to handle* functions. So the handle* functions must release the
// lock before calling the ICameraClient's callbacks, so those callbacks can
// invoke methods in the Client class again (For example, the preview frame
// callback may want to releaseRecordingFrame). The handle* functions must
// release the lock after all accesses to member variables, so it must be
// handled very carefully.

void CameraService::Client::notifyCallback(int32_t msgType, int32_t ext1,
        int32_t ext2, void* user) {
    LOG2("notifyCallback(%d)", msgType);

    sp<Client> client = getClientFromCookie(user);
    if (client == ) return;
    if (!client->lockIfMessageWanted(msgType)) return;

    switch (msgType) {
        case CAMERA_MSG_SHUTTER:
            // ext1 is the dimension of the yuv picture.
            client->handleShutter();
            break;
        default:
            client->handleGenericNotify(msgType, ext1, ext2);
            break;
    }
}

void CameraService::Client::dataCallback(int32_t msgType,
        const sp<IMemory>& dataPtr, camera_frame_metadata_t *metadata, void* user) {
    LOG2("dataCallback(%d)", msgType);

    sp<Client> client = getClientFromCookie(user);
    if (client == ) return;
    if (!client->lockIfMessageWanted(msgType)) return;

    if (dataPtr ==  && metadata == NULL) {
        LOGE("Null data returned in data callback");
        client->handleGenericNotify(CAMERA_MSG_ERROR, UNKNOWN_ERROR, );
        return;
    }

    switch (msgType & ~CAMERA_MSG_PREVIEW_METADATA) {
        case CAMERA_MSG_PREVIEW_FRAME:
            client->handlePreviewData(msgType, dataPtr, metadata);
            break;
        case CAMERA_MSG_POSTVIEW_FRAME:
            client->handlePostview(dataPtr);
            break;
        case CAMERA_MSG_RAW_IMAGE:
            client->handleRawPicture(dataPtr);
            break;
        case CAMERA_MSG_COMPRESSED_IMAGE:
            client->handleCompressedPicture(dataPtr);
            break;
        default:
            client->handleGenericData(msgType, dataPtr, metadata);
            break;
    }
}

void CameraService::Client::dataCallbackTimestamp(nsecs_t timestamp,
        int32_t msgType, const sp<IMemory>& dataPtr, void* user) {
    LOG2("dataCallbackTimestamp(%d)", msgType);

    sp<Client> client = getClientFromCookie(user);
    if (client == ) return;
    if (!client->lockIfMessageWanted(msgType)) return;

    if (dataPtr == ) {
        LOGE("Null data returned in data with timestamp callback");
        client->handleGenericNotify(CAMERA_MSG_ERROR, UNKNOWN_ERROR, );
        return;
    }

    client->handleGenericDataTimestamp(timestamp, msgType, dataPtr);
}

// snapshot taken callback
void CameraService::Client::handleShutter(void) {
    if (mPlayShutterSound) {
        mCameraService->playSound(SOUND_SHUTTER);
    }

    sp<ICameraClient> c = mCameraClient;
    if (c != ) {
        mLock.unlock();
        c->notifyCallback(CAMERA_MSG_SHUTTER, , );
        if (!lockIfMessageWanted(CAMERA_MSG_SHUTTER)) return;
    }
    disableMsgType(CAMERA_MSG_SHUTTER);

    mLock.unlock();
}

// preview callback - frame buffer update
void CameraService::Client::handlePreviewData(int32_t msgType,
                                              const sp<IMemory>& mem,
                                              camera_frame_metadata_t *metadata) {
    ssize_t offset;
    size_t size;
    sp<IMemoryHeap> heap = mem->getMemory(&offset, &size);

    // local copy of the callback flags
    int flags = mPreviewCallbackFlag;

    // is callback enabled?
    if (!(flags & CAMERA_FRAME_CALLBACK_FLAG_ENABLE_MASK)) {
        // If the enable bit is off, the copy-out and one-shot bits are ignored
        LOG2("frame callback is disabled");
        mLock.unlock();
        return;
    }

    // hold a strong pointer to the client
    sp<ICameraClient> c = mCameraClient;

    // clear callback flags if no client or one-shot mode
    if (c ==  || (mPreviewCallbackFlag & CAMERA_FRAME_CALLBACK_FLAG_ONE_SHOT_MASK)) {
        LOG2("Disable preview callback");
        mPreviewCallbackFlag &= ~(CAMERA_FRAME_CALLBACK_FLAG_ONE_SHOT_MASK |
                                  CAMERA_FRAME_CALLBACK_FLAG_COPY_OUT_MASK |
                                  CAMERA_FRAME_CALLBACK_FLAG_ENABLE_MASK);
        disableMsgType(CAMERA_MSG_PREVIEW_FRAME);
    }

    if (c != ) {
        // Is the received frame copied out or not?
        if (flags & CAMERA_FRAME_CALLBACK_FLAG_COPY_OUT_MASK) {
            LOG2("frame is copied");
            copyFrameAndPostCopiedFrame(msgType, c, heap, offset, size, metadata);
        } else {
            LOG2("frame is forwarded");
            mLock.unlock();
            c->dataCallback(msgType, mem, metadata);
        }
    } else {
        mLock.unlock();
    }
}

// picture callback - postview image ready
void CameraService::Client::handlePostview(const sp<IMemory>& mem) {
    disableMsgType(CAMERA_MSG_POSTVIEW_FRAME);

    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->dataCallback(CAMERA_MSG_POSTVIEW_FRAME, mem, NULL);
    }
}

// picture callback - raw image ready
void CameraService::Client::handleRawPicture(const sp<IMemory>& mem) {
    disableMsgType(CAMERA_MSG_RAW_IMAGE);

    ssize_t offset;
    size_t size;
    sp<IMemoryHeap> heap = mem->getMemory(&offset, &size);

    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->dataCallback(CAMERA_MSG_RAW_IMAGE, mem, NULL);
    }
}

// picture callback - compressed picture ready
void CameraService::Client::handleCompressedPicture(const sp<IMemory>& mem) {
    disableMsgType(CAMERA_MSG_COMPRESSED_IMAGE);

    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->dataCallback(CAMERA_MSG_COMPRESSED_IMAGE, mem, NULL);
    }
}

void CameraService::Client::handleGenericNotify(int32_t msgType,
    int32_t ext1, int32_t ext2) {
    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->notifyCallback(msgType, ext1, ext2);
    }
}

void CameraService::Client::handleGenericData(int32_t msgType,
    const sp<IMemory>& dataPtr, camera_frame_metadata_t *metadata) {
    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->dataCallback(msgType, dataPtr, metadata);
    }
}

void CameraService::Client::handleGenericDataTimestamp(nsecs_t timestamp,
    int32_t msgType, const sp<IMemory>& dataPtr) {
    sp<ICameraClient> c = mCameraClient;
    mLock.unlock();
    if (c != ) {
        c->dataCallbackTimestamp(timestamp, msgType, dataPtr);
    }
}

void CameraService::Client::copyFrameAndPostCopiedFrame(
        int32_t msgType, const sp<ICameraClient>& client,
        const sp<IMemoryHeap>& heap, size_t offset, size_t size,
        camera_frame_metadata_t *metadata) {
    LOG2("copyFrameAndPostCopiedFrame");
    // It is necessary to copy out of pmem before sending this to
    // the callback. For efficiency, reuse the same MemoryHeapBase
    // provided it's big enough. Don't allocate the memory or
    // perform the copy if there's no callback.
    // hold the preview lock while we grab a reference to the preview buffer
    sp<MemoryHeapBase> previewBuffer;

    if (mPreviewBuffer == ) {
        mPreviewBuffer = new MemoryHeapBase(size, , NULL);
    } else if (size > mPreviewBuffer->virtualSize()) {
        mPreviewBuffer.clear();
        mPreviewBuffer = new MemoryHeapBase(size, , NULL);
    }
    if (mPreviewBuffer == ) {
        LOGE("failed to allocate space for preview buffer");
        mLock.unlock();
        return;
    }
    previewBuffer = mPreviewBuffer;

    memcpy(previewBuffer->base(), (uint8_t *)heap->base() + offset, size);

    sp<MemoryBase> frame = new MemoryBase(previewBuffer, , size);
    if (frame == ) {
        LOGE("failed to allocate space for frame callback");
        mLock.unlock();
        return;
    }

    mLock.unlock();
    client->dataCallback(msgType, frame, metadata);
}

int CameraService::Client::getOrientation(int degrees, bool mirror) {
    if (!mirror) {
        if (degrees == ) return ;
        else if (degrees == ) return HAL_TRANSFORM_ROT_90;
        else if (degrees == ) return HAL_TRANSFORM_ROT_180;
        else if (degrees == ) return HAL_TRANSFORM_ROT_270;
    } else {  // Do mirror (horizontal flip)
        if (degrees == ) {           // FLIP_H and ROT_0
            return HAL_TRANSFORM_FLIP_H;
        } else if (degrees == ) {   // FLIP_H and ROT_90
            return HAL_TRANSFORM_FLIP_H | HAL_TRANSFORM_ROT_90;
        } else if (degrees == ) {  // FLIP_H and ROT_180
            return HAL_TRANSFORM_FLIP_V;
        } else if (degrees == ) {  // FLIP_H and ROT_270
            return HAL_TRANSFORM_FLIP_V | HAL_TRANSFORM_ROT_90;
        }
    }
    LOGE("Invalid setDisplayOrientation degrees=%d", degrees);
    return -;
}

// ----------------------------------------------------------------------------

static const int kDumpLockRetries = ;
static const int kDumpLockSleep = ;

static bool tryLock(Mutex& mutex)
{
    bool locked = false;
    for (int i = ; i < kDumpLockRetries; ++i) {
        if (mutex.tryLock() == NO_ERROR) {
            locked = true;
            break;
        }
        usleep(kDumpLockSleep);
    }
    return locked;
}

status_t CameraService::dump(int fd, const Vector<String16>& args) {
    static const char* kDeadlockedString = "CameraService may be deadlocked\n";

    const size_t SIZE = ;
    char buffer[SIZE];
    String8 result;
    if (checkCallingPermission(String16("android.permission.DUMP")) == false) {
        snprintf(buffer, SIZE, "Permission Denial: "
                "can't dump CameraService from pid=%d, uid=%d\n",
                getCallingPid(),
                getCallingUid());
        result.append(buffer);
        write(fd, result.string(), result.size());
    } else {
        bool locked = tryLock(mServiceLock);
        // failed to lock - CameraService is probably deadlocked
        if (!locked) {
            String8 result(kDeadlockedString);
            write(fd, result.string(), result.size());
        }

        bool hasClient = false;
        for (int i = ; i < mNumberOfCameras; i++) {
            sp<Client> client = mClient[i].promote();
            if (client == ) continue;
            hasClient = true;
            sprintf(buffer, "Client[%d] (%p) PID: %d\n",
                    i,
                    client->getCameraClient()->asBinder().get(),
                    client->mClientPid);
            result.append(buffer);
            write(fd, result.string(), result.size());
            client->mHardware->dump(fd, args);
        }
        if (!hasClient) {
            result.append("No camera client yet.\n");
            write(fd, result.string(), result.size());
        }

        if (locked) mServiceLock.unlock();

        // change logging level
        int n = args.size();
        for (int i = ; i +  < n; i++) {
            if (args[i] == String16("-v")) {
                String8 levelStr(args[i+]);
                int level = atoi(levelStr.string());
                sprintf(buffer, "Set Log Level to %d", level);
                result.append(buffer);
                setLogLevel(level);
            }
        }
    }
    return NO_ERROR;
}

}; // namespace android

void CameraService::onFirstRef()看这个函数:

void CameraService::onFirstRef()
{
    BnCameraService::onFirstRef();

    if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
                (const hw_module_t **)&mModule) < ) {
        LOGE("Could not load camera HAL module");
        mNumberOfCameras = ;
    }
    else {
        mNumberOfCameras = mModule->get_number_of_cameras();            //获取camera设备的个数.对应下文中的获取camera个数函数.
        if (mNumberOfCameras > MAX_CAMERAS) {
            LOGE("Number of cameras(%d) > MAX_CAMERAS(%d).",
                    mNumberOfCameras, MAX_CAMERAS);
            mNumberOfCameras = MAX_CAMERAS;
        }
        for (int i = ; i < mNumberOfCameras; i++) {
            setCameraFree(i);
        }
    }    

    // Read the system property to determine if we have to use the
    // AUDIO_STREAM_ENFORCED_AUDIBLE type.
    char value[PROPERTY_VALUE_MAX];
    property_get("ro.camera.sound.forced", value, "");
    if (strcmp(value, "") != ) {
        mAudioStreamType = AUDIO_STREAM_ENFORCED_AUDIBLE;
    } else {
        mAudioStreamType = AUDIO_STREAM_MUSIC;
    }
}

找了半天也就是它这个函数里边有hw_get_module()这个函数,一看名字就知道是获取hardware的,不找它找谁啊。那么onFirstRef()函数又是何时调用的？
onFirstRef（）属于其父类RefBase，该函数在强引用sp新增引用计数时调用，什么意思？就是当 有sp包装的类初始化的时候调用。这里在frameworks/base/services/camera/libcameraservice/CameraService.h 中class CameraService :中有定义

  // these are initialized in the constructor.
        sp<CameraService>               mCameraService;  // immutable after constructor
        sp<ICameraClient>               mCameraClient;
        int                             mCameraId;       // immutable after constructor
        int                             mCameraFacing;   // immutable after constructor
        pid_t                           mClientPid;
        sp<CameraHardwareInterface>     mHardware;       // cleared after disconnect()
        int                             mPreviewCallbackFlag;
        int                             mOrientation;     // Current display orientation
        bool                            mPlayShutterSound;

很明显是这里来初始化的,当然这里不是重点,如果全部都写的话,那基本上就写不完了.

找到了hw_get_module()这个函数,让我们看它的具体实现.
hardware/libhardware/hardware.c
代码如下:

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <hardware/hardware.h>

#include <cutils/properties.h>

#include <dlfcn.h>
#include <string.h>
#include <pthread.h>
#include <errno.h>
#include <limits.h>

#define LOG_TAG "HAL"
#include <utils/Log.h>

/** Base path of the hal modules */
#define HAL_LIBRARY_PATH1 "/system/lib/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"

/**
 * There are a set of variant filename for modules. The form of the filename
 * is "<MODULE_ID>.variant.so" so for the led module the Dream variants
 * of base "ro.product.board", "ro.board.platform" and "ro.arch" would be:
 *
 * led.trout.so
 * led.msm7k.so
 * led.ARMV6.so
 * led.default.so
 */

static const char *variant_keys[] = {
    "ro.hardware",  /* This goes first so that it can pick up a different
                       file on the emulator. */
    "ro.product.board",
    "ro.board.platform",
    "ro.arch"
};

static const int HAL_VARIANT_KEYS_COUNT =
    (sizeof(variant_keys)/sizeof(variant_keys[]));

/**
 * Load the file defined by the variant and if successful
 * return the dlopen handle and the hmi.
 * @return 0 = success, !0 = failure.
 */
static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status;
    void *handle;
    struct hw_module_t *hmi;

    /*
     * load the symbols resolving undefined symbols before
     * dlopen returns. Since RTLD_GLOBAL is not or'd in with
     * RTLD_NOW the external symbols will not be global
     */
    handle = dlopen(path, RTLD_NOW);
    if (handle == NULL) {
        char const *err_str = dlerror();
        LOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }

    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
    hmi = (struct hw_module_t *)dlsym(handle, sym);
    if (hmi == NULL) {
        LOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }

    /* Check that the id matches */
    if (strcmp(id, hmi->id) != ) {
        LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
        status = -EINVAL;
        goto done;
    }

    hmi->dso = handle;

    /* success */
    status = ;

    done:
    if (status != ) {
        hmi = NULL;
        if (handle != NULL) {
            dlclose(handle);
            handle = NULL;
        }
    } else {
        LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }

    *pHmi = hmi;

    return status;
}

int hw_get_module_by_class(const char *class_id, const char *inst,
                           const struct hw_module_t **module)
{
    int status;
    int i;
    const struct hw_module_t *hmi = NULL;
    char prop[PATH_MAX];
    char path[PATH_MAX];
    char name[PATH_MAX];

    if (inst)
        snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
    else
        strlcpy(name, class_id, PATH_MAX);

    /*
     * Here we rely on the fact that calling dlopen multiple times on
     * the same .so will simply increment a refcount (and not load
     * a new copy of the library).
     * We also assume that dlopen() is thread-safe.
     */

    /* Loop through the configuration variants looking for a module */
    for (i= ; i<HAL_VARIANT_KEYS_COUNT+ ; i++) {
        if (i < HAL_VARIANT_KEYS_COUNT) {
            if (property_get(variant_keys[i], prop, NULL) == ) {
                continue;
            }
            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH2, name, prop);
            if (access(path, R_OK) == ) break;

            snprintf(path, sizeof(path), "%s/%s.%s.so",
                     HAL_LIBRARY_PATH1, name, prop);
            if (access(path, R_OK) == ) break;
        } else {
            snprintf(path, sizeof(path), "%s/%s.default.so",
                     HAL_LIBRARY_PATH1, name);
            if (access(path, R_OK) == ) break;
        }
    }

    status = -ENOENT;
    if (i < HAL_VARIANT_KEYS_COUNT+) {
        /* load the module, if this fails, we're doomed, and we should not try
         * to load a different variant. */
        status = load(class_id, path, module);
    }

    return status;
}

int hw_get_module(const char *id, const struct hw_module_t **module)
{
    return hw_get_module_by_class(id, NULL, module);
}

让我们看看它的流程:

int hw_get_module(const char *id, const struct hw_module_t **module)
    int hw_get_module_by_class(const char *class_id, const char *inst,const struct hw_module_t **module)
        static int load(const char *id,const char *path,const struct hw_module_t **pHmi)
                /* Check that the id matches */
                if (strcmp(id, hmi->id) != ) {
                    LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
                    status = -EINVAL;
                    goto done;
                }
                ...

可以知道,真正来寻找hardware的桥梁是这个ID,在if (strcmp(id, hmi->id) != 0)中,id是frameworks/base/services/camera/libcameraservice/CameraService.cpp中直接赋值的
如下:

  if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
                (const hw_module_t **)&mModule) < ) {
        LOGE("Could not load camera HAL module");
        mNumberOfCameras = ;
    }

而hmi->id中的这个id很明显是hardware中应该定义的了.
我们看hmi是怎么得来的.

    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;
    hmi = (struct hw_module_t *)dlsym(handle, sym);

不用跟踪这个函数就能看出来.hmi一定是从sym中来获取的.所以这里我们也就知道hardware中一定要有这个结构体.这也是实现一个hardware必须要做的事情,这里在hardware.h中也有说明:

/**
 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
 * and the fields of this data structure must begin with hw_module_t
 * followed by module specific information.
 */

即hardware中一定要有这个叫HAL_MODULE_INFO_SYM的结构体.这也是实现一个hardware的第一步:Step-1:实现一个名字为HAL_MODULE_INFO_SYM的结构体,这个结构体必须以hw_module_t开头

好吧,来看一下camera的hardware中是怎么定义的.

看一下common是不是结构体hw_module_t
hardware/libhardware/include/hardware/camera.h

好了,既然hardware要有结构体,那么必须要给他初始化.自定义的函数也得给实现了.
上文注释已经写出来了.这里只是粘贴一下函数的实现.

所以也就自然而然的调用到了实现hardware的第二步,Step-2:
open函数的实现及作用.还是看cameraHAL中对其的实现.

层层包装啊,再看HAL_camera_device_open:

static int HAL_camera_device_open(const struct hw_module_t* module,
                                  const char *id,
                                  struct hw_device_t** device)
{
    int cameraId = atoi(id);
    if (cameraId <  || cameraId >= HAL_getNumberOfCameras()) {
        return -EINVAL;
    }
    if (g_cam_device) {                                //如果系统第二次打开摄像头,则此时g_cam_device已经填充完毕,可以不再填充了.
        if (obj(g_cam_device)->getCameraId() == cameraId) {
            goto done;
        } else {
            LOGE("Cannot open camera %d. camera %d is already running!",
                    cameraId, obj(g_cam_device)->getCameraId());
            return -ENOSYS;
        }
    }
    g_cam_device = (camera_device_t *)malloc(sizeof(camera_device_t));
    if (!g_cam_device)
        return -ENOMEM;
    g_cam_device->common.tag     = HARDWARE_DEVICE_TAG;
    g_cam_device->common.version = ;
    g_cam_device->common.module  = const_cast<hw_module_t *>(module);
    g_cam_device->common.close   = HAL_camera_device_close;
    g_cam_device->ops = &camera_device_ops;
    LOGI("%s: open camera %s", __func__, id);
    g_cam_device->priv = new CameraHardwareSec(cameraId, g_cam_device);
done:
    *device = (hw_device_t *)g_cam_device;
    LOGI("%s: opened camera %s (%p)", __func__, id, *device);
    return ;
}

这里我们知道.open的作用就是打开指定ID号的摄像头以及填充device结构体,供上层直接调用我们HAL的具体函数比如takePicture(),startPreview()等等.
但是应该怎么去填充这个结构体呢?
还是先看hardware/libhardware/include/hardware/hardware.h怎么说吧.

/**
 * Every device data structure must begin with hw_device_t
 * followed by module specific public methods and attributes.
 */

它说每一个设备都必须以hw_device_t开始,后面跟着methods和attributes.

那我们就在HAL中定义一个static的结构体,按着上边赋值完毕后返回这个指针就成了.看HAL

static camera_device_t *g_cam_device;            //定义设备的结构体具体如下.
typedef struct camera_device {
    hw_device_t common;                            //以hw_device_t开始
    camera_device_ops_t *ops;                    //紧跟着methods
    void *priv;                                 //私有数据.
} camera_device_t;

看看是怎么填充的:

g_cam_device->common.tag     = HARDWARE_DEVICE_TAG;        //hardware.h中定义的.具体设备tag必须这样定义.
g_cam_device->common.module  = const_cast<hw_module_t *>(module);    //和module模块建立联系
g_cam_device->common.close   = HAL_camera_device_close;        //设备关闭的函数
g_cam_device->ops = &camera_device_ops;                        // 这个里边包含的就是那些操作摄像头所需要的具体实现.讲HAL的运行流程时细讲.
g_cam_device->priv = new CameraHardwareSec(cameraId, g_cam_device);    //真正实现hardware初始化摄像头的函数,讲HAL的运行流程时细讲.

这里先看看camera_device_ops的具体实现,也牵扯到了HAL实现的第三步,Step-3:具体设备的函数实现.

#define SET_METHOD(m) m : HAL_camera_device_##m        //相当于m : HAL_camera_device_m
static camera_device_ops_t camera_device_ops = {
        SET_METHOD(set_preview_window),                //展开后相当于set_preview_window : HAL_camera_device_set_preview_window,即HAL_camera_device_set_preview_window是set_preview_window的具体实现
        SET_METHOD(set_callbacks),                    //展开后相当于set_callbacks : HAL_camera_device_set_callbacks
        SET_METHOD(enable_msg_type),                //同上
        SET_METHOD(disable_msg_type),
        SET_METHOD(msg_type_enabled),
        SET_METHOD(start_preview),
        SET_METHOD(stop_preview),
        SET_METHOD(preview_enabled),
        SET_METHOD(store_meta_data_in_buffers),
        SET_METHOD(start_recording),
        SET_METHOD(stop_recording),
        SET_METHOD(recording_enabled),
        SET_METHOD(release_recording_frame),
        SET_METHOD(auto_focus),
        SET_METHOD(cancel_auto_focus),
        SET_METHOD(take_picture),
        SET_METHOD(cancel_picture),
        SET_METHOD(set_parameters),
        SET_METHOD(get_parameters),
        SET_METHOD(put_parameters),
        SET_METHOD(send_command),
        SET_METHOD(release),
        SET_METHOD(dump),
};

先不说HAL中调用函数的具体实现,看看是service怎么调用的.
以start_preview为例.

camera.start_preview()    //应用程序
        public native final void startPreview();                                                   //此处调用JNI frameworks/base/core/java/android/hardware/Camera.java
            static void android_hardware_Camera_startPreview(JNIEnv *env, jobject thiz)            //JNI frameworks/base/core/jni/android_hardware_Camera.cpp
                sp<Camera> camera = get_native_camera(env, thiz, NULL);                        //获取cameraClient    frameworks/base/core/jni/android_hardware_Camera.cpp
                camera->startPreview();                                                        //执行cameraClient的函数    frameworks/base/libs/camera/Camera.cpp.
                    status_t CameraService::Client::startPreview();                            //service中函数的实现    frameworks/base/services/camera/libcameraservice/CameraService.cpp
                        status_t CameraService::Client::startCameraMode(camera_mode mode)     //service中的调用    frameworks/base/services/camera/libcameraservice/CameraService.cpp
                            status_t CameraService::Client::startPreviewMode()                //service最终调用    frameworks/base/services/camera/libcameraservice/CameraService.cpp
                                 status_t startPreview()                                    //调用到hardware的接口    frameworks/base/services/camera/libcameraservice/CameraHardwareInterface.h
                                    mDevice->ops->start_preview(mDevice);                    //调用到HAL层的start_preview具体实现HAL_camera_device_start_preview.   device/magiclab/common/libcamera/SecCameraHWInterface_zoom.cpp

OK,函数调用到这里也就完成了应用程序调用hardware内具体设备函数的流程.HAL的实现其实也就是实现上面每个函数.使它们协同合作而已.

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