继续上一个链接内容所讲：http://www.cnblogs.com/linhaostudy/p/8430583.html
下面bmp18x sensor为例进行分析。
BMP18x Sensor：
我们以打开bmp180为例，代码为hardware\qcom\sensors\Bmp180.cpp：
PressureSensor::PressureSensor(char *name)
: SensorBase(NULL, "bmp18x"),
mInputReader(),
mHasPendingEvent(false),
mEnabledTime()
{
mPendingEvent.version = sizeof(sensors_event_t);
mPendingEvent.sensor = SENSORS_PRESSURE_HANDLE;
mPendingEvent.type = SENSOR_TYPE_PRESSURE;
memset(mPendingEvent.data, , sizeof(mPendingEvent.data)); if (data_fd) {
strlcpy(input_sysfs_path, SYSFS_CLASS, sizeof(input_sysfs_path));
strlcat(input_sysfs_path, name, sizeof(input_sysfs_path));
strlcat(input_sysfs_path, "/", sizeof(input_sysfs_path));
input_sysfs_path_len = strlen(input_sysfs_path);
ALOGI("The pressure sensor path is %s",input_sysfs_path);
enable(, );
}
} PressureSensor::PressureSensor(char *name)
: SensorBase(NULL, "bmp18x"),
mInputReader(),
mHasPendingEvent(false),
mEnabledTime()
{
mPendingEvent.version = sizeof(sensors_event_t);
mPendingEvent.sensor = SENSORS_PRESSURE_HANDLE;
mPendingEvent.type = SENSOR_TYPE_PRESSURE;
memset(mPendingEvent.data, , sizeof(mPendingEvent.data)); if (data_fd) {
strlcpy(input_sysfs_path, SYSFS_CLASS, sizeof(input_sysfs_path));
strlcat(input_sysfs_path, name, sizeof(input_sysfs_path));
strlcat(input_sysfs_path, "/", sizeof(input_sysfs_path));
input_sysfs_path_len = strlen(input_sysfs_path);
ALOGI("The pressure sensor path is %s",input_sysfs_path);
enable(, );
}
}
mPendingEvent是一个sensors_event_t结构体，首先对sensor、type等赋值，然后调用enable打开；
/**
* @brief This function will enable/disable sensor.
* @param[in] handle
* which sensor to enable/disable.
* @param[in] en
* en=1, enable;
* en=0, disable
* @return if the operation is successful.
*/
int PressureSensor::enable(int32_t handle, int en)
{
VFUNC_LOG; int res = ; LOGV_IF(SYSFS_VERBOSE, "HAL:sysfs:echo %d > %s (%lld)",
en, pressureSysFs.pressure_enable, getTimestamp());
res = write_sysfs_int(pressureSysFs.pressure_enable, en); return res;
}
其实就是对SYSFS_CLASS路径下的文件的enable节点写值“1”；
SYSFS_CLASS的路径为（hardware\qcom\sensors\sensors.h）：
1 #define SYSFS_CLASS "/sys/class/sensors/"
注意，不要忽略了SensorBase(NULL, "bmp18x")，我们看一下它做了什么？
/*****************************************************************************/ SensorBase::SensorBase(
const char* dev_name,
const char* data_name,
const struct SensorContext* context /* = NULL */)
: dev_name(dev_name), data_name(data_name), algo(NULL),
dev_fd(-), data_fd(-), mEnabled(), mHasPendingMetadata()
{
if (context != NULL) {
CalibrationManager& cm(CalibrationManager::getInstance());
algo = cm.getCalAlgo(context->sensor); /* Set up the sensors_meta_data_event_t event*/
meta_data.version = META_DATA_VERSION;
meta_data.sensor = context->sensor->handle;
meta_data.type = SENSOR_TYPE_META_DATA;
meta_data.reserved0 = ;
meta_data.timestamp = 0LL;
meta_data.meta_data.what = META_DATA_FLUSH_COMPLETE;
meta_data.meta_data.sensor = context->sensor->handle;
} if (data_name) {
data_fd = openInput(data_name);
}
}
这里的dev_name为NULL，data_name为bmp18x，设置dev_name和data_name后调用openInput打开设备：
int SensorBase::openInput(const char* inputName) {
int fd = -;
const char *dirname = "/dev/input";
char devname[PATH_MAX];
char *filename;
DIR *dir;
struct dirent *de;
dir = opendir(dirname);
if(dir == NULL)
return -;
strlcpy(devname, dirname, PATH_MAX);
filename = devname + strlen(dirname);
*filename++ = '/';
while((de = readdir(dir))) {
if(de->d_name[] == '.' &&
(de->d_name[] == '\0' ||
(de->d_name[] == '.' && de->d_name[] == '\0')))
continue;
strlcpy(filename, de->d_name, PATH_MAX - strlen(SYSFS_CLASS));
fd = open(devname, O_RDONLY);
if (fd>=) {
char name[];
if (ioctl(fd, EVIOCGNAME(sizeof(name) - ), &name) < ) {
name[] = '\0';
}
if (!strcmp(name, inputName)) {
strlcpy(input_name, filename, PATH_MAX);
break;
} else {
close(fd);
fd = -;
}
}
}
closedir(dir);
ALOGE_IF(fd<, "couldn't find '%s' input device", inputName);
return fd;
}再继续看一下getInput：
getInput
这个函数的作用就是打开"/dev/input"，查找其中的输入设备名字与传入参数匹配的那一个，这里是"bmp18x"，并返回相应的打开句柄。
到这里sensor就打开了；
Activate、setDelay都是直接调用相应的sensor接口直接调用相应sensor的接口，主要是readEvents：
int PressureSensor::readEvents(sensors_event_t* data, int count)
{
if (count < )
return -EINVAL;
//这里两个判断都不会进来
if (mHasPendingEvent) {
mHasPendingEvent = false;
mPendingEvent.timestamp = getTimestamp();
*data = mPendingEvent;
return mEnabled ? : ;
} if (mHasPendingMetadata) {
mHasPendingMetadata--;
meta_data.timestamp = getTimestamp();
*data = meta_data;
return mEnabled ? : ;
} ssize_t n = mInputReader.fill(data_fd); //从输入设备中读取数据
21 if (n < 0)
return n; int numEventReceived = ;
input_event const* event; #if FETCH_FULL_EVENT_BEFORE_RETURN
again:
#endif
while (count && mInputReader.readEvent(&event)) {//读取当前一个event，返回还有的数据大小
int type = event->type;
if (type == EV_ABS) {//根据kernel上报的input事件来确定相应的数据值
float value = event->value;
mPendingEvent.pressure = value * CONVERT_PRESSURE;
} else if (type == EV_SYN) {
switch (event->code) {
case SYN_TIME_SEC:
mUseAbsTimeStamp = true;
report_time = event->value*1000000000LL;
break;
case SYN_TIME_NSEC:
mUseAbsTimeStamp = true;
mPendingEvent.timestamp = report_time+event->value;
break;
case SYN_REPORT:
if(mUseAbsTimeStamp != true) {
mPendingEvent.timestamp = timevalToNano(event->time);
}
if (mEnabled) {
if (mPendingEvent.timestamp >= mEnabledTime) {
*data++ = mPendingEvent;
numEventReceived++;
}
count--;
}
break;
}
} else {
ALOGE("PressureSensor: unknown event (type=%d, code=%d)",
type, event->code);
}
mInputReader.next();
} #if FETCH_FULL_EVENT_BEFORE_RETURN
/* if we didn't read a complete event, see if we can fill and
try again instead of returning with nothing and redoing poll. */
if (numEventReceived == && mEnabled == ) {
n = mInputReader.fill(data_fd);
if (n)
goto again;
}
#endif return numEventReceived;
}
这里看一下mInputReader，是一个InputEventCircularReader结构，表示的是一个环形的读入数据缓冲区：
ssize_t InputEventCircularReader::fill(int fd)
{
size_t numEventsRead = ;
if (mFreeSpace) {
const ssize_t nread = read(fd, mHead, mFreeSpace * sizeof(input_event));//读取数据
if (nread< || nread % sizeof(input_event)) {
// we got a partial event!!
return nread< ? -errno : -EINVAL;
} numEventsRead = nread / sizeof(input_event);
// dumpEvents(mHead, numEventsRead);
D("nread = %ld, numEventsRead = %d.", nread, numEventsRead);
if (numEventsRead) {
mHead += numEventsRead;
mFreeSpace -= numEventsRead;
if (mHead > mBufferEnd) {
size_t s = mHead - mBufferEnd;
memcpy(mBuffer, mBufferEnd, s * sizeof(input_event));
mHead = mBuffer + s;
}
}
} return numEventsRead;
}
mCurr表示未读事件的第一个，初始为缓冲区首部；
Fill函数是把调用read从相应的输入设备读入数据mHead位置；
readEvent函数返回mCurr和剩余的数据大小；
next函数让mCurr移一个input_event；