繼續上一個鏈接內容所講：http://www.cnblogs.com/linhaostudy/p/8430583.html

下面bmp18x sensor為例進行分析。

BMP18x Sensor：

我們以打開bmp180為例，代碼為hardware\qcom\sensors\Bmp180.cpp：

 1 PressureSensor::PressureSensor(char *name)
 2     : SensorBase(NULL, "bmp18x"),
 3       mInputReader(4),
 4       mHasPendingEvent(false),
 5       mEnabledTime(0
 
)
 6 {
 7     mPendingEvent.version = sizeof(sensors_event_t);
 8     mPendingEvent.sensor = SENSORS_PRESSURE_HANDLE;
 9     mPendingEvent.type = SENSOR_TYPE_PRESSURE;
10     memset(mPendingEvent.data, 0, sizeof(mPendingEvent.data));
11 
12     if (data_fd) {
13         strlcpy(input_sysfs_path, SYSFS_CLASS, sizeof
 
(input_sysfs_path));
14         strlcat(input_sysfs_path, name, sizeof(input_sysfs_path));
15         strlcat(input_sysfs_path, "/", sizeof(input_sysfs_path));
16         input_sysfs_path_len = strlen(input_sysfs_path);
17         ALOGI("The pressure sensor path is %s",input_sysfs_path);
18         enable(0
 
, 1);
19     }
20 }
21 
22 
23 PressureSensor::PressureSensor(char *name)
24     : SensorBase(NULL, "bmp18x"),
25       mInputReader(4),
26       mHasPendingEvent(false),
27       mEnabledTime(0)
28 {
29     mPendingEvent.version = sizeof(sensors_event_t);
30     mPendingEvent.sensor = SENSORS_PRESSURE_HANDLE;
31     mPendingEvent.type = SENSOR_TYPE_PRESSURE;
32     memset(mPendingEvent.data, 0, sizeof(mPendingEvent.data));
33 
34     if (data_fd) {
35         strlcpy(input_sysfs_path, SYSFS_CLASS, sizeof(input_sysfs_path));
36         strlcat(input_sysfs_path, name, sizeof(input_sysfs_path));
37         strlcat(input_sysfs_path, "/", sizeof(input_sysfs_path));
38         input_sysfs_path_len = strlen(input_sysfs_path);
39         ALOGI("The pressure sensor path is %s",input_sysfs_path);
40         enable(0, 1);
41     }
42 }

mPendingEvent是一個sensors_event_t結構體，首先對sensor、type等賦值，然後調用enable打開；

 1 /**
 2  *  @brief        This function will enable/disable sensor.
 3  *  @param[in]    handle
 4  *                  which sensor to enable/disable.
 5  *  @param[in]    en
 6  *                  en=1, enable; 
 7  *                  en=0, disable
 8  *  @return       if the operation is successful.
 9  */
10 int PressureSensor::enable(int32_t handle, int en) 
11 {
12     VFUNC_LOG;
13 
14     int res = 0;
15 
16     LOGV_IF(SYSFS_VERBOSE, "HAL:sysfs:echo %d > %s (%lld)",
17             en, pressureSysFs.pressure_enable, getTimestamp());
18     res = write_sysfs_int(pressureSysFs.pressure_enable, en);
19 
20     return res;
21 }

其實就是對SYSFS_CLASS路徑下的文件的enable節點寫值“1”；

SYSFS_CLASS的路徑為（hardware\qcom\sensors\sensors.h）：

1 #define SYSFS_CLASS        "/sys/class/sensors/"

註意，不要忽略了SensorBase(NULL, "bmp18x")，我們看一下它做了什麽？

 1 /*****************************************************************************/
 2 
 3 SensorBase::SensorBase(
 4         const char* dev_name,
 5         const char* data_name,
 6         const struct SensorContext* context /* = NULL */)
 7         : dev_name(dev_name), data_name(data_name), algo(NULL),
 8         dev_fd(-1), data_fd(-1), mEnabled(0), mHasPendingMetadata(0)
 9 {
10         if (context != NULL) {
11                 CalibrationManager& cm(CalibrationManager::getInstance());
12                 algo = cm.getCalAlgo(context->sensor);
13 
14                 /* Set up the sensors_meta_data_event_t event*/
15                 meta_data.version = META_DATA_VERSION;
16                 meta_data.sensor = context->sensor->handle;
17                 meta_data.type = SENSOR_TYPE_META_DATA;
18                 meta_data.reserved0 = 0;
19                 meta_data.timestamp = 0LL;
20                 meta_data.meta_data.what = META_DATA_FLUSH_COMPLETE;
21                 meta_data.meta_data.sensor = context->sensor->handle;
22         }
23 
24         if (data_name) {
25                 data_fd = openInput(data_name);
26         }
27 }

這裏的dev_name為NULL，data_name為bmp18x，設置dev_name和data_name後調用openInput打開設備：

再繼續看一下getInput：

這個函數的作用就是打開"/dev/input"，查找其中的輸入設備名字與傳入參數匹配的那一個，這裏是"bmp18x"，並返回相應的打開句柄。

到這裏sensor就打開了；

Activate、setDelay都是直接調用相應的sensor接口直接調用相應sensor的接口，主要是readEvents：

 1 int PressureSensor::readEvents(sensors_event_t* data, int count)
 2 {
 3     if (count < 1)
 4         return -EINVAL;
 5 
 6     if (mHasPendingEvent) {
 7         mHasPendingEvent = false;
 8         mPendingEvent.timestamp = getTimestamp();
 9         *data = mPendingEvent;
10         return mEnabled ? 1 : 0;
11     }
12 
13     if (mHasPendingMetadata) {
14         mHasPendingMetadata--;
15         meta_data.timestamp = getTimestamp();
16         *data = meta_data;
17         return mEnabled ? 1 : 0;
18     }
19 
20     ssize_t n = mInputReader.fill(data_fd);　　//從輸入設備中讀取數據  
21     if (n < 0)
22         return n;
23 
24     int numEventReceived = 0;
25     input_event const* event;
26 
27 #if FETCH_FULL_EVENT_BEFORE_RETURN
28 again:
29 #endif
30     while (count && mInputReader.readEvent(&event)) {//讀取當前一個event，返回還有的數據大小  
31         int type = event->type;
32         if (type == EV_ABS) {//根據kernel上報的input事件來確定相應的數據值
33             float value = event->value;
34             mPendingEvent.pressure = value * CONVERT_PRESSURE;
35         } else if (type == EV_SYN) {
36             switch (event->code) {
37                 case SYN_TIME_SEC:
38                     mUseAbsTimeStamp = true;
39                     report_time = event->value*1000000000LL;
40                     break;
41                 case SYN_TIME_NSEC:
42                     mUseAbsTimeStamp = true;
43                     mPendingEvent.timestamp = report_time+event->value;
44                     break;
45                 case SYN_REPORT:
46                     if(mUseAbsTimeStamp != true) {
47                         mPendingEvent.timestamp = timevalToNano(event->time);
48                     }
49                     if (mEnabled) {
50                         if (mPendingEvent.timestamp >= mEnabledTime) {
51                             *data++ = mPendingEvent;
52                             numEventReceived++;
53                         }
54                         count--;
55                     }
56                     break;
57             }
58         } else {
59             ALOGE("PressureSensor: unknown event (type=%d, code=%d)",
60                     type, event->code);
61         }
62         mInputReader.next();
63     }
64 
65 #if FETCH_FULL_EVENT_BEFORE_RETURN
66     /* if we didn‘t read a complete event, see if we can fill and
67        try again instead of returning with nothing and redoing poll. */
68     if (numEventReceived == 0 && mEnabled == 1) {
69         n = mInputReader.fill(data_fd);
70         if (n)
71             goto again;
72     }
73 #endif
74 
75     return numEventReceived;
76 }

這裏看一下mInputReader，是一個InputEventCircularReader結構，表示的是一個環形的讀入數據緩沖區：

 1 ssize_t InputEventCircularReader::fill(int fd)
 2 {
 3     size_t numEventsRead = 0;
 4     if (mFreeSpace) {
 5         const ssize_t nread = read(fd, mHead, mFreeSpace * sizeof(input_event));//讀取數據
 6         if (nread<0 || nread % sizeof(input_event)) {
 7             // we got a partial event!!
 8             return nread<0 ? -errno : -EINVAL;
 9         }
10 
11         numEventsRead = nread / sizeof(input_event);
12         // dumpEvents(mHead, numEventsRead);
13         D("nread = %ld, numEventsRead = %d.", nread, numEventsRead);
14         if (numEventsRead) {
15             mHead += numEventsRead;
16             mFreeSpace -= numEventsRead;
17             if (mHead > mBufferEnd) {
18                 size_t s = mHead - mBufferEnd;
19                 memcpy(mBuffer, mBufferEnd, s * sizeof(input_event));
20                 mHead = mBuffer + s;
21             }
22         }
23     }
24 
25     return numEventsRead;
26 }

mCurr表示未讀事件的第一個，初始為緩沖區首部；

Fill函數是把調用read從相應的輸入設備讀入數據mHead位置；

readEvent函數返回mCurr和剩余的數據大小；

next函數讓mCurr移一個input_event；

高通HAL層之bmp18x.cpp