Android(安卓)传感器hal层分析

Android系统内置对传感器有很多，它们分别是：加速度传感器gsensor(accelerometer)、磁力传感器(magnetic field)、方向传感器(orientation)、陀螺仪(gyroscope)、环境光照传感器(light)、压力传感器(pressure)、温度传感器(temperature)和距离传感器(proximity)等。

Android实现传感器系统代码位置在

framework层： frameworks\base\core\java\android\hardware\目录下

Sensor.java

SensorEventListener.java

SensorListener.java

SensorManager.java

jni :frameworks\base\core\jni

android_hardware_SensorManager.cpp

hal层，这个每个平台的代码位置可能不一样，我现在分析4.4MTK的

mediatek\hardware\sensor

Google为Sensor提供了统一的HAL接口，不同的硬件厂商需要根据该接口来实现并完成具体的硬件抽象层，Android中Sensor的HAL接口定义在：hardware/libhardware/include/hardware/sensors.h

对传感器类型的定义：

传感器模块的定义结构体如下：

该接口的定义实际上是对标准的硬件模块hw_module_t的一个扩展，增加了一个get_sensors_list函数，用于获取传感器的列表。

对任意一个sensor设备都会有一个 sensor_t 结构体，其定义如下：

struct sensor_t {
    /* name of this sensors */
    const char*     name;
    /* vendor of the hardware part */
    const char*     vendor;
    /* version of the hardware part + driver. The value of this field
     * must increase when the driver is updated in a way that changes the
     * output of this sensor. This is important for fused sensors when the
     * fusion algorithm is updated.
     */
    int             version;
    /* handle that identifies this sensors. This handle is used to activate
     * and deactivate this sensor. The value of the handle must be 8 bits
     * in this version of the API.
     */
    int             handle;
    /* this sensor's type. */
    int             type;
    /* maximaum range of this sensor's value in SI units */
    float           maxRange;
    /* smallest difference between two values reported by this sensor */
    float           resolution;
    /* rough estimate of this sensor's power consumption in mA */
    float           power;
    /* minimum delay allowed between events in microseconds. A value of zero
     * means that this sensor doesn't report events at a constant rate, but
     * rather only when a new data is available */
    int32_t         minDelay;

/* reserved fields, must be zero */
void* reserved[8];
};

下面来分析hal流程：

加载JNI时根据模块会找到sensor.c

由上面可以看出来open_sensors是整个加载流程入口

函数操作集合赋值，看到new了一个sensors_poll_context_t类，看看到底这里都做了什么

struct sensors_poll_context_t {
struct sensors_poll_device_t device; // must be first
sensors_poll_context_t();
~sensors_poll_context_t();
int activate(int handle, int enabled);
int setDelay(int handle, int64_t ns);
int pollEvents(sensors_event_t* data, int count);
private:
enum {
hwmsen = 0,
accel,
magnetic,
//gyro,
//light,
//proximity,
//pressure,
numSensorDrivers,
numFds,
};
int handleToDriver(int handle) const {
switch (handle) {
case ID_ACCELEROMETER:
return accel;
case ID_MAGNETIC:
case ID_ORIENTATION:
return magnetic;
case ID_PROXIMITY:
//return proximity;
case ID_LIGHT:
//return light;
case ID_GYROSCOPE:
//return gyro;
case ID_PRESSURE:
break;
//return pressure;
}
return -EINVAL;
}

static const size_t wake = numFds - 1;
static const char WAKE_MESSAGE = 'W';
struct pollfd mPollFds[numFds];
int mWritePipeFd;
SensorBase* mSensors[numSensorDrivers];
};

构造函数

重点来看pollEvents函数

poll是真正实现查询事件

int sensors_poll_context_t::pollEvents(sensors_event_t* data, int count)
{
int nbEvents = 0;
int n = 0;
//ALOGE("pollEvents count =%d",count );

do {
// see if we have some leftover from the last poll()
for (int i=0 ; count && i SensorBase* const sensor(mSensors[i]);
if ((mPollFds[i].revents & POLLIN) || (sensor->hasPendingEvents())) {
int nb = sensor->readEvents(data, count);
if (nb < count) {
// no more data for this sensor
mPollFds[i].revents = 0;
}
//if(nb < 0||nb > count)
// ALOGE("pollEvents count error nb:%d, count:%d, nbEvents:%d", nb, count, nbEvents);//for sensor NE debug
count -= nb;
nbEvents += nb;
data += nb;
//if(nb < 0||nb > count)
// ALOGE("pollEvents count error nb:%d, count:%d, nbEvents:%d", nb, count, nbEvents);//for sensor NE debug
}
}
if (count) {
// we still have some room, so try to see if we can get
// some events immediately or just wait if we don't have
// anything to return

n = poll(mPollFds, numFds, nbEvents ? 0 : -1);
if (n<0) {
ALOGE("poll() failed (%s)", strerror(errno));
return -errno;
}
if (mPollFds[wake].revents & POLLIN) {
char msg;
int result = read(mPollFds[wake].fd, &msg, 1);
ALOGE_IF(result<0, "error reading from wake pipe (%s)", strerror(errno));
ALOGE_IF(msg != WAKE_MESSAGE, "unknown message on wake queue (0x%02x)", int(msg));
mPollFds[wake].revents = 0;
}
}
// if we have events and space, go read them
} while (n && count);
return nbEvents;
}

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