MTK Android P Sensor架构（一）

需求场景：

本来如果只是给传感器写个驱动并提供能读取温湿度数据的节点，是一件比较轻松的事情，但是最近上层应用的同事要求我们按照安卓标准的流程来，这样他们就能通过注册一个服务直接读取传感器事件数据了。这样做的好处就是第三方的应用也能正常读取温湿度的数据并展示。

正文：

网上分析安卓9.0 sensor相关的资料不多，下面找到了一位大神对安卓9.0整个sensor框架总结的流程图：

虽然流程比较粗糙，但是也有助于我们跟踪代码。这里重点说一下，sensor架构中的HAL层分为两部分：

• 安卓官方实现部分：

hardware/libhardware/modules/sensors

• 芯片产商实现部分（MTK平台）:

vendor/mediatek/proprietary/hardware/sensor

一般来讲，在适配一款新的sensor，改动只会涉及vendor层到kernel层，再往上都是安卓标准的，但是为了了解整个流程怎么走的，参考这位大神的博客，在这里我也稍微介绍一下framework层的部分。

代码路径：

frameworks\base\services\java\com\android\server\SystemServer.java

private void startBootstrapServices() {
 ...
 mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
            TimingsTraceLog traceLog = new TimingsTraceLog(
                    SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
            traceLog.traceBegin(START_SENSOR_SERVICE);
            startSensorService(); /* 调用JNI接口 */
            traceLog.traceEnd();
        }, START_SENSOR_SERVICE);
 ...
}

system_server启动之后会通过JNI接口启动sensorService。

代码路径：

frameworks\base\services\core\jni\com_android_server_SystemServer.cpp

static void android_server_SystemServer_startSensorService(JNIEnv* /* env */, jobject /* clazz */) {
    char propBuf[PROPERTY_VALUE_MAX];
    property_get("system_init.startsensorservice", propBuf, "1");
    if (strcmp(propBuf, "1") == 0) {
        SensorService::instantiate();
    }
 
}
 
/*
 * JNI registration.
   */
   static const JNINativeMethod gMethods[] = {
   /* name, signature, funcPtr */
   { "startSensorService", "()V", (void*) android_server_SystemServer_startSensorService },
   { "startHidlServices", "()V", (void*) android_server_SystemServer_startHidlServices },
   };
 

从上面可以发现，最后调用到

android_server_SystemServer_startSensorService

函数，里面会判断属性

system_init.startsensorservice

是否为1，然后才会真正去启动

SensorService

服务。所以这里涉及到第一个改动，设置

system_init.startsensorservice

属性，这里我是直接在

build/make/tools/buildinfo.sh

里面写死为1。

用SensorService::instantiate()方式创建的sensorservice实例后，调用里面的SensorService::onFirstRef方法。

代码路径：

frameworks\native\services\sensorservice\SensorService.cpp

void SensorService::onFirstRef() {
    ALOGD("nuSensorService starting...");
    SensorDevice& dev(SensorDevice::getInstance()); /* 创建并获取SensorDevice实例 */
 ...
 
 if (dev.initCheck() == NO_ERROR) {
     sensor_t const* list;
     ssize_t count = dev.getSensorList(&list); /* 通过SensorDevice，并调用到vendor层去获取sensor的数目 */
     if (count > 0) {
         ssize_t orientationIndex = -1;
         bool hasGyro = false, hasAccel = false, hasMag = false;
         uint32_t virtualSensorsNeeds =
                (1<<SENSOR_TYPE_GRAVITY) |
                (1<<SENSOR_TYPE_LINEAR_ACCELERATION) |
                (1<<SENSOR_TYPE_ROTATION_VECTOR) |
                (1<<SENSOR_TYPE_GEOMAGNETIC_ROTATION_VECTOR) |
                (1<<SENSOR_TYPE_GAME_ROTATION_VECTOR);
 
         for (ssize_t i=0 ; i<count ; i++) {
             bool useThisSensor=true;
 
             switch (list[i].type) {
                 case SENSOR_TYPE_ACCELEROMETER:
                     hasAccel = true;
                     break;
                 case SENSOR_TYPE_MAGNETIC_FIELD:
                     hasMag = true;
                     break;
                 case SENSOR_TYPE_ORIENTATION:
                     orientationIndex = i;
                     break;
                 case SENSOR_TYPE_GYROSCOPE:
                 case SENSOR_TYPE_GYROSCOPE_UNCALIBRATED:
                     hasGyro = true;
                     break;
                 case SENSOR_TYPE_GRAVITY:
                 case SENSOR_TYPE_LINEAR_ACCELERATION:
                 case SENSOR_TYPE_ROTATION_VECTOR:
                 case SENSOR_TYPE_GEOMAGNETIC_ROTATION_VECTOR:
                 case SENSOR_TYPE_GAME_ROTATION_VECTOR:
                     if (IGNORE_HARDWARE_FUSION) {
                         useThisSensor = false;
                     } else {
                         virtualSensorsNeeds &= ~(1<<list[i].type);
                     }
                     break;
             }
             if (useThisSensor) {
                 registerSensor( new HardwareSensor(list[i]) );
             }
         }
 
         // it's safe to instantiate the SensorFusion object here
         // (it wants to be instantiated after h/w sensors have been
         // registered)
         SensorFusion::getInstance();
 
         if (hasGyro && hasAccel && hasMag) {
             ...
         }
 
         if (hasAccel && hasGyro) {
             ...
         }
 
         if (hasAccel && hasMag) {
             ...
         }
 
         ...
     }
 }
}
 

我这次主要是增加温湿度传感器的功能，上面的流程中没有过多涉及温湿度的，有兴趣的可以参考大神的博客自行分析。不过这里重点关注一下SensorDevice这个类，它是连接上层应用和HAL层的中间枢纽：

代码路径：

frameworks\native\services\sensorservice\SensorDevice.cpp

SensorDevice::SensorDevice()
        : mHidlTransportErrors(20), mRestartWaiter(new HidlServiceRegistrationWaiter()) {
    if (!connectHidlService()) {
        return;
    }
 float minPowerMa = 0.001; // 1 microAmp
 
 checkReturn(mSensors->getSensorsList(
        [&](const auto &list "&") {
            const size_t count = list.size();
 
            mActivationCount.setCapacity(count);
            Info model;
            for (size_t i=0 ; i < count; i++) {
                sensor_t sensor;
                convertToSensor(list[i], &sensor);
                // Sanity check and clamp power if it is 0 (or close)
                if (sensor.power < minPowerMa) {
                    ALOGE("Reported power %f not deemed sane, clamping to %f",
                          sensor.power, minPowerMa);
                    sensor.power = minPowerMa;
                }
                mSensorList.push_back(sensor);
 
                mActivationCount.add(list[i].sensorHandle, model);
 
                checkReturn(mSensors->activate(list[i].sensorHandle, 0 /* enabled */));
            }
        }));
 
 mIsDirectReportSupported =
        (checkReturn(mSensors->unregisterDirectChannel(-1)) != Result::INVALID_OPERATION);
}

在SensorDevice构造函数中，通过调用connectHidlService()和安卓部分的HAL层服务建立连接。连接后，就可以调用已经在HAL层注册的sensor设备了，比如这里就调用getSensorsList()来获取sensor设备列表，并放回sensor的数目。然后就是通过mSensors->activate()来"激活"sensor设备，而每个sensor具体的activate()函数由驱动工程师实现。

激活sensor设备后，就可以开始获取sensor的数据了，在SensorService中会通过poll机制去查询底层sensor的数据：

代码路径：

frameworks\native\services\sensorservice\SensorService.cpp

bool SensorService::threadLoop() {
    ...
 SensorDevice& device(SensorDevice::getInstance());
 
 const int halVersion = device.getHalDeviceVersion();
 do {
     ssize_t count = device.poll(mSensorEventBuffer, numEventMax);
     if (count < 0) {
         ALOGE("sensor poll failed (%s)", strerror(-count));
         break;
     }
 
     ...
 } while (!Thread::exitPending());
 
 ALOGW("Exiting SensorService::threadLoop => aborting...");
 abort();
 return false;
}

整个threadLoop函数里面内容挺多的，但是目前只关注读取数据的poll部分。可以看到device就是SensorDevice的一个实例，前面我们讲到上层都是通过SensorDevice和HAL层连接，这里也不例外，也是调用到了SensorDevice中的poll函数，这里我给出这个调用的流程：

1、frameworks\native\services\sensorservice\SensorDevice.cpp
SensorDevice::poll()
 2、vendor\mediatek\proprietary\hardware\sensor\sensors-1.0\sensors.cpp
 poll__poll()
  3、vendor\mediatek\proprietary\hardware\sensor\sensors-1.0\SensorManager.cpp
  SensorManager::pollEvent()
   4、vendor\mediatek\proprietary\hardware\sensor\sensors-1.0\SensorContext.cpp
   sensors_poll_context_t::pollEvent

上面简陋的流程展示了从framework层一路调用到vendor层：

int sensors_poll_context_t::pollEvent(sensors_event_t* data, int count) {
    int nbEvents = 0;
    int n = 0;
    int averageCount = 0, loop = 0, loopcount = 0;
    int backupcount = count, backuploop = 0;
 do {
    loopcount++;
    computeCountForEachFd(count, &averageCount, &loop);
    backuploop = loop;
    for (int i = 0; count && loop && i < numFds; i++) {
        SensorBase* const sensor(mSensors[i]);
  if (mPollFds[i].revents & POLLIN || sensor->pendingEvent()) {
   int nb = sensor->readEvents(data, averageCount);
            ...
        }
    }
    // try to see if we can get some events immediately or just wait if
    // we don't have anything to return, important to update fd revents
    // which sensor data pending in buffer and aviod one sensor always
    // occupy poll bandwidth.
    n = TEMP_FAILURE_RETRY(poll(mPollFds, numFds, nbEvents ? 0 : -1));
    if (n < 0) {
        ALOGE("poll() failed (%s)", strerror(errno));
        return -errno;
    }
 } while (n && count);
 return nbEvents;
}

这里面我们重点关注三点

（1） mPollFds的定义如下

struct pollfd mPollFds[numFds];

其中，

struct pollfd {
 int fd;        /* 文件描述符 */
 short events; /* 等待的事件 */
 short revents; /* 实际发生了的事件 */
};

所以mPollFds就是用来监听代表每个sensor是否有数据上报的文件描述符

enum {
    accel,
    magnetic,
    gyro,
    light,
    proximity,
    pressure,
    humidity,
 temperature,
    stepcounter,
    pedometer,
    activity,
    situation,
    scpfusion,
    apfusion,
    bio,
    wakeupset,
    numFds,
};
 

如果想自定义一种sensor就需要给这个枚举类型增加值

（2） mSensors的定义如下：

SensorBase* mSensors[numFds];

SensorBase是一个基类，所有的sensor类都继承于它，比如我这次实现的湿度传感器：

class HumiditySensor : public SensorBase {
 private:
     int mEnabled;
     sensors_event_t mPendingEvent;
     SensorEventCircularReader mSensorReader;
     int64_t mEnabledTime;
     char input_sysfs_path[PATH_MAX];
     int input_sysfs_path_len;
     int mDataDiv;
     int64_t m_hmdy_last_ts = 0;
     int64_t m_hmdy_delay = 0;
 
     void processEvent(struct sensor_event const *event);
 
 public:
        HumiditySensor();
     virtual ~HumiditySensor();
     virtual int readEvents(sensors_event_t* data, int count);
     virtual int setDelay(int32_t handle, int64_t ns);
     virtual int enable(int32_t handle, int enabled);
     virtual int batch(int handle, int flags, int64_t samplingPeriodNs, int64_t maxBatchReportLatencyNs);
     virtual int flush(int handle);
     virtual int getFd() {
         return mSensorReader.getReadFd();
     };
};

从类的声明来看，定义了很多函数，比如readEvents、enable和batch等等，这些最终都会和底层驱动联系起来，后面再细说。

（3）在sensors_poll_context_t的构造函数中会对上面两点讲到的数组进行初始化：

sensors_poll_context_t::sensors_poll_context_t()
{
 ...
 mSensors[humidity] = new HumiditySensor(); /* 分配一个Humidity传感器的类 */
    mPollFds[humidity].fd = mSensors[humidity]->getFd(); /* 获取对应sensor的字符描述符 */
    mPollFds[humidity].events = POLLIN; /* 等待POLLIN类型的事件 */
    mPollFds[humidity].revents = 0;
 ...
}

再回到上面的

sensors_poll_context_t::pollEvent()

函数，通过

mPollFds[i].revents

判断到如果发生了POLLIN事件，证明可以获取数据了，就调用对应sensor的readEvents()

函数去获取。接下来我们就进入到sensor设备对应的HAL层里面了，现在以湿度sensor为例：

代码路径：

vendor\mediatek\proprietary\hardware\sensor\sensors-1.0\Humidity.cpp

int HumiditySensor::readEvents(sensors_event_t* data, int count) {
    if (count < 1)
        return -EINVAL;
 
    ssize_t n = mSensorReader.fill();
    if (n < 0)
        return n;
    int numEventReceived = 0;
    struct sensor_event const* event;
    
    while (count && mSensorReader.readEvent(&event)) {
        processEvent(event);
        if (event->flush_action <= FLUSH_ACTION) {
            ...
        }
        mSensorReader.next();
    }
    return numEventReceived;
 
}

我们可以看到读取数据实际又是统一通过

SensorEventCircularReader

这个类来操作：

代码路径：

vendor\mediatek\proprietary\hardware\sensor\sensors-1.0\SensorEventReader.cpp

SensorEventCircularReader::SensorEventCircularReader(size_t numEvents)
    : mBuffer(new struct sensor_event[numEvents * 2]),
      mBufferEnd(mBuffer + numEvents),
      mHead(mBuffer),
      mCurr(mBuffer),
      mFreeSpace(numEvents) {
    mReadFd = -1;
    mWriteFd = -1;
}

构造函数里面分配了Buffer来存储接收的数据

ssize_t SensorEventCircularReader::fill() {
    size_t numEventsRead = 0;
    if (mFreeSpace) {
        const ssize_t nread = TEMP_FAILURE_RETRY(read(mReadFd, mHead, mFreeSpace * sizeof(struct sensor_event)));
        if (nread < 0 || nread % sizeof(struct sensor_event)) {
            return 0;
        }
 
        ...
    }
    
    return numEventsRead;
 
}

fill顾名思义就是往分配的buffer里面填充数据，通过我们熟悉的read()函数来获取数据。

ssize_t SensorEventCircularReader::readEvent(struct sensor_event const** events) {
    *events = mCurr;
    ssize_t available = (mBufferEnd - mBuffer) - mFreeSpace;
    return available ? 1 : 0;
}

readEvent()

只是判断buffer中是否有数据，然后就是调用

mSensorReader.next()

获取下一个buffer。再回到

HumiditySensor::readEvents()

在读取到数据后会调用

processEvent()

去处理数据：

void HumiditySensor::processEvent(struct sensor_event const *event) {
    mPendingEvent.relative_humidity = (float) event->word[0] / mDataDiv;
}
 

mPendingEvent.relative_humidity就是最终上报给上层应用的值了。

至此，framework层到vendor层的流程就分析完了，后面我们会分析kernel层的sensor框架。