Android 驱动和系统开发 2. 解析模拟器GPS模块

好久没有写技术博客了，恰逢今天还感冒了，这破天气，晚上凉风一吹，就感冒了，要加强锻炼呀。

好了，废话不多说，由于工作需要，我要移植一个虚拟的gps模块，于是乎，我就参考了android模拟器的gps模块的实现方法，只需稍微改动就完成了我的工作了，随后我也会附上我做的模块的代码，这里主要还是来解析下模拟器上的gps模块代码吧。

相信做过android location方面应用的同志都知道，android 模拟器虽然没有真正的GPS功能，但是DDMS可以模拟GPS，通过telnet连接到adb，然后发送GPS数据，再转化成NMEA格式的信号给android系统，就可以模拟出location功能了，相信用过的童鞋都知道，没用过的同志去搜索一下就知道了，这里我就不多说了，我主要还是来分析一下这个模拟的功能是如何实现的，这里还是膜拜一下写android源码的大神们，多看看源码，学到的东西很多呢。

首先，我们直入主题，对于移植系统的人来说（比如说我），关注的是中间部分的代码，android的framework层我们需要改动的很少，最多就是加点log来调试，驱动层呢，因为模拟器没有真实的设备，也不可能利用PC上的资源区模拟，因为PC是没有GPS模块的（除非你的电脑很高级），但是我想还是可以通过网络来得到地理位置的，虽然不是非常的准确，希望google的工程师可以去完善，呵呵，题外话了。说了这么多，我就是想说，android 模拟器中gps模块的功能主要依赖于2个东西，一个是ddms中的geo fix命令，还有一个是hal层中的gps_qemu.c中作为硬件抽象层的处理，把虚拟的数据上报给framework层。

主要层次如下图

好了，思路清晰了，咱就看代码，位于源码目录下/sdk/emulator/gps/gps_qemu.c

首先我们要搞清楚，在andrroid中HAL 的一个位置问题，HAL是为了更好的封装好硬件驱动存在的，主要是一些接口，编译成库文件，给framework中国的jni来调用，我们这里的GPS模块会被编译成gps.goldfish.so文件，在同目录下的Android.mk中有写到

 LOCAL_CFLAGS += -DQEMU_HARDWARE
 LOCAL_SHARED_LIBRARIES := liblog libcutils libhardware
 LOCAL_SRC_FILES := gps_qemu.c
 LOCAL_MODULE := gps.goldfish
 LOCAL_MODULE_TAGS := debug

然后呢，在jni中会这样调用

 static void android_location_GpsLocationProvider_class_init_native(JNIEnv* env, jclass clazz) {
     int err;
     hw_module_t* module;

     method_reportLocation = env->GetMethodID(clazz, "reportLocation", "(IDDDFFFJ)V");
     method_reportStatus = env->GetMethodID(clazz, "reportStatus", "(I)V");
     method_reportSvStatus = env->GetMethodID(clazz, "reportSvStatus", "()V");
     method_reportAGpsStatus = env->GetMethodID(clazz, "reportAGpsStatus", "(III)V");
     method_reportNmea = env->GetMethodID(clazz, "reportNmea", "(J)V");
     method_setEngineCapabilities = env->GetMethodID(clazz, "setEngineCapabilities", "(I)V");
     method_xtraDownloadRequest = env->GetMethodID(clazz, "xtraDownloadRequest", "()V");
     method_reportNiNotification = env->GetMethodID(clazz, "reportNiNotification",
             "(IIIIILjava/lang/String;Ljava/lang/String;IILjava/lang/String;)V");
     method_requestRefLocation = env->GetMethodID(clazz,"requestRefLocation","(I)V");
     method_requestSetID = env->GetMethodID(clazz,"requestSetID","(I)V");
     method_requestUtcTime = env->GetMethodID(clazz,"requestUtcTime","()V");

     err = hw_get_module(GPS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);
     if (err == 0) {
         hw_device_t* device;
         err = module->methods->open(module, GPS_HARDWARE_MODULE_ID, &device);
         if (err == 0) {
             gps_device_t* gps_device = (gps_device_t *)device;
             sGpsInterface = gps_device->get_gps_interface(gps_device);
         }
     }
     if (sGpsInterface) {
         sGpsXtraInterface =
             (const GpsXtraInterface*)sGpsInterface->get_extension(GPS_XTRA_INTERFACE);
         sAGpsInterface =
             (const AGpsInterface*)sGpsInterface->get_extension(AGPS_INTERFACE);
         sGpsNiInterface =
             (const GpsNiInterface*)sGpsInterface->get_extension(GPS_NI_INTERFACE);
         sGpsDebugInterface =
             (const GpsDebugInterface*)sGpsInterface->get_extension(GPS_DEBUG_INTERFACE);
         sAGpsRilInterface =
             (const AGpsRilInterface*)sGpsInterface->get_extension(AGPS_RIL_INTERFACE);
     }
 }

这个函数在android设备启动的时候会被调用来初始化GPS模块的一些东西，主要是来的到GPS模块的一些接口函数，重点看这个函数

 err = hw_get_module(GPS_HARDWARE_MODULE_ID, (hw_module_t const**)&module);

这个函数原型在HAL中的hardware.c中

 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=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
         if (i < HAL_VARIANT_KEYS_COUNT) {
             if (property_get(variant_keys[i], prop, NULL) == 0) {
                 continue;
             }
             snprintf(path, sizeof(path), "%s/%s.%s.so",
                      HAL_LIBRARY_PATH2, name, prop);
             if (access(path, R_OK) == 0) break;

             snprintf(path, sizeof(path), "%s/%s.%s.so",
                      HAL_LIBRARY_PATH1, name, prop);
             if (access(path, R_OK) == 0) break;
         } else {
             snprintf(path, sizeof(path), "%s/%s.default.so",
                      HAL_LIBRARY_PATH1, name);
             if (access(path, R_OK) == 0) break;
         }
     }

     status = -ENOENT;
     if (i < HAL_VARIANT_KEYS_COUNT+1) {
         /* 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;
 }

当我们编译gps模块之后会在/system/lib/hw/下生成一个gps.goldfish.so文件，这个函数就是去寻找这个库文件，然后调用load函数去打开这个库文件，来得到库中的函数接口

 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) != 0) {
         LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
         status = -EINVAL;
         goto done;
     }

     hmi->dso = handle;

     /* success */
     status = 0;

     done:
     if (status != 0) {
         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;
 }

这里我介绍的比较简洁，因为在我之前的博客中已经介绍过这部分的内容了，可以参考这里： http://blog.csdn.net/zhangjie201412/article/details/7225617

好了，回到我们GPS模块的代码上来

之后就会调用

 err = module->methods->open(module, GPS_HARDWARE_MODULE_ID, &device);

来打开设备，来看下HAL中的代码

 static int open_gps(const struct hw_module_t* module, char const* name,
         struct hw_device_t** device)
 {
     struct gps_device_t *dev = malloc(sizeof(struct gps_device_t));
     memset(dev, 0, sizeof(*dev));

     dev->common.tag = HARDWARE_DEVICE_TAG;
     dev->common.version = 0;
     dev->common.module = (struct hw_module_t*)module;
 //    dev->common.close = (int (*)(struct hw_device_t*))close_lights;
     dev->get_gps_interface = gps__get_gps_interface;

     *device = (struct hw_device_t*)dev;
     return 0;
 }

这里只是做了一些初始化，然后把接口函数挂钩一下

 dev->get_gps_interface = gps__get_gps_interface;

这个回调函数很简单

 static const GpsInterface  qemuGpsInterface = {
     sizeof(GpsInterface),
     qemu_gps_init,
     qemu_gps_start,
     qemu_gps_stop,
     qemu_gps_cleanup,
     qemu_gps_inject_time,
     qemu_gps_inject_location,
     qemu_gps_delete_aiding_data,
     qemu_gps_set_position_mode,
     qemu_gps_get_extension,
 };

 const GpsInterface* gps__get_gps_interface(struct gps_device_t* dev)
 {
     return &qemuGpsInterface;
 }

返回qemuGpsInterface结构体，这个机构提中就是一大堆的回调函数。

下面我们按照调用顺序来一个一个介绍这些回调函数。

首先就是qume_gps_init函数

 static int
 qemu_gps_init(GpsCallbacks* callbacks)
 {
     GpsState*  s = _gps_state;

     if (!s->init)
         gps_state_init(s, callbacks);

     if (s->fd < 0)
         return -1;

     return 0;
 }

这里我发现了一个很好玩的东西，这里这个GpsState* s是如何得到全局的实例的呢，是通过_gps_state，而_gps_state的定义是这样的

 typedef struct {
     int                     init;
     int                     fd;
     GpsCallbacks            callbacks;
     pthread_t               thread;
     int                     control[2];
 } GpsState;

 static GpsState  _gps_state[1];

这里我的理解是在全局静态的定义了一个结构体指针，并分配了内存。

为何不在init函数中使用malloc来分配内存，然后使用呢，有点意思，现在还不知道有什么好处，难道只是卖弄吗？

好了，不多说了，接下去看调用的gps_state_init函数

在这之前，我来介绍下GpsState结构体中成员的作用吧

int init:

一个初始化的标志，为1表示初始化了，为0表示未初始化

int fd:

socket读写的文件描述符，如果是真实的硬件的话，应该是串口读写的描述符

callbacks:

这个是从jni传下来的回调函数，得到数据之后就回调

thread:

这个没什么好说的，就是一个线程

int control[2]:

本地使用的socket来进程间通信，会面会讲到。

继续init函数

 static void
 gps_state_init( GpsState*  state, GpsCallbacks* callbacks )
 {
     state->init       = 1;
     state->control[0] = -1;
     state->control[1] = -1;
     state->fd         = -1;

     state->fd = qemud_channel_open(QEMU_CHANNEL_NAME);

     if (state->fd < 0) {
         D("no gps emulation detected");
         return;
     }

     D("gps emulation will read from '%s' qemud channel", QEMU_CHANNEL_NAME );

     if ( socketpair( AF_LOCAL, SOCK_STREAM, 0, state->control ) < 0 ) {
         LOGE("could not create thread control socket pair: %s", strerror(errno));
         goto Fail;
     }

     state->thread = callbacks->create_thread_cb( "gps_state_thread", gps_state_thread, state );

     if ( !state->thread ) {
         LOGE("could not create gps thread: %s", strerror(errno));
         goto Fail;
     }

     state->callbacks = *callbacks;

     D("gps state initialized");
     return;

 Fail:
     gps_state_done( state );
 }

首先书初始化赋值工作，看到没，把init变量赋值为1了。然后调用了qemud_channel_open函数来得到了adb tcp的socket文件描述符。然后调用socketpair创建本地的socket通信对来实现进程间通信，然后创建了线程，赋值回调函数，下图描述了代码执行的流程。

这图有点丑，不过大体思路还是清楚的，可以对照着代码看，这里使用的是event poll技术进行事件的处理，在线程中，把fd和control[1]加入了epoll中，设置为POLLIIN模式，当有事件发生是，就会调用相应的代码，这里的control[1]，在这里做控制作用，只要是控制gps的开始和停止的，所以在线程外面对control[0]进行写操作的话，对应的control[1]就会收到相应的指令，然后采取措施。具体代码如下

 static void
 gps_state_thread( void*  arg )
 {
     GpsState*   state = (GpsState*) arg;
     NmeaReader  reader[1];
     int         epoll_fd   = epoll_create(2);
     int         started    = 0;
     int         gps_fd     = state->fd;
     int         control_fd = state->control[1];

     nmea_reader_init( reader );

     // register control file descriptors for polling
     epoll_register( epoll_fd, control_fd );
     epoll_register( epoll_fd, gps_fd );

     D("gps thread running");

     // now loop
     for (;;) {
         struct epoll_event   events[2];
         int                  ne, nevents;

         nevents = epoll_wait( epoll_fd, events, 2, -1 );
         if (nevents < 0) {
             if (errno != EINTR)
                 LOGE("epoll_wait() unexpected error: %s", strerror(errno));
             continue;
         }
         D("gps thread received %d events", nevents);
         for (ne = 0; ne < nevents; ne++) {
             if ((events[ne].events & (EPOLLERR|EPOLLHUP)) != 0) {
                 LOGE("EPOLLERR or EPOLLHUP after epoll_wait() !?");
                 return;
             }
             if ((events[ne].events & EPOLLIN) != 0) {
                 int  fd = events[ne].data.fd;

                 if (fd == control_fd)
                 {
                     char  cmd = 255;
                     int   ret;
                     D("gps control fd event");
                     do {
                         ret = read( fd, &cmd, 1 );
                     } while (ret < 0 && errno == EINTR);

                     if (cmd == CMD_QUIT) {
                         D("gps thread quitting on demand");
                         return;
                     }
                     else if (cmd == CMD_START) {
                         if (!started) {
                             D("gps thread starting  location_cb=%p", state->callbacks.location_cb);
                             started = 1;
                             nmea_reader_set_callback( reader, state->callbacks.location_cb );
                         }
                     }
                     else if (cmd == CMD_STOP) {
                         if (started) {
                             D("gps thread stopping");
                             started = 0;
                             nmea_reader_set_callback( reader, NULL );
                         }
                     }
                 }
                 else if (fd == gps_fd)
                 {
                     char  buff[32];
                     D("gps fd event");
                     for (;;) {
                         int  nn, ret;

                         ret = read( fd, buff, sizeof(buff) );
                         if (ret < 0) {
                             if (errno == EINTR)
                                 continue;
                             if (errno != EWOULDBLOCK)
                                 LOGE("error while reading from gps daemon socket: %s:", strerror(errno));
                             break;
                         }
                         D("received %d bytes: %.*s", ret, ret, buff);
                         for (nn = 0; nn < ret; nn++)
                             nmea_reader_addc( reader, buff[nn] );
                     }
                     D("gps fd event end");
                 }
                 else
                 {
                     LOGE("epoll_wait() returned unkown fd %d ?", fd);
                 }
             }
         }
     }
 }

好了，android 模拟器的虚拟hal层就介绍到这边，下面来看一下geo fix命令的实现源码，我也是找了好久才找到的，在external/qemu/android/console.c中

 static int
 do_geo_fix( ControlClient  client, char*  args )
 {
     // GEO_SAT2 provides bug backwards compatibility.
     enum { GEO_LONG = 0, GEO_LAT, GEO_ALT, GEO_SAT, GEO_SAT2, NUM_GEO_PARAMS };
     char*   p = args;
     int     top_param = -1;
     double  params[ NUM_GEO_PARAMS ];
     int     n_satellites = 1;

     static  int     last_time = 0;
     static  double  last_altitude = 0.;

     if (!p)
         p = "";

     /* tokenize */
     while (*p) {
         char*   end;
         double  val = strtod( p, &end );

         if (end == p) {
             control_write( client, "KO: argument '%s' is not a number\n", p );
             return -1;
         }

         params[++top_param] = val;
         if (top_param + 1 == NUM_GEO_PARAMS)
             break;

         p = end;
         while (*p && (p[0] == ' ' || p[0] == '\t'))
             p += 1;
     }

     /* sanity check */
     if (top_param < GEO_LAT) {
         control_write( client, "KO: not enough arguments: see 'help geo fix' for details\r\n" );
         return -1;
     }

     /* check number of satellites, must be integer between 1 and 12 */
     if (top_param >= GEO_SAT) {
         int sat_index = (top_param >= GEO_SAT2) ? GEO_SAT2 : GEO_SAT;
         n_satellites = (int) params[sat_index];
         if (n_satellites != params[sat_index]
             || n_satellites < 1 || n_satellites > 12) {
             control_write( client, "KO: invalid number of satellites. Must be an integer between 1 and 12\r\n");
             return -1;
         }
     }

     /* generate an NMEA sentence for this fix */
     {
         STRALLOC_DEFINE(s);
         double   val;
         int      deg, min;
         char     hemi;

         /* format overview:
          *    time of fix      123519     12:35:19 UTC
          *    latitude         4807.038   48 degrees, 07.038 minutes
          *    north/south      N or S
          *    longitude        01131.000  11 degrees, 31. minutes
          *    east/west        E or W
          *    fix quality      1          standard GPS fix
          *    satellites       1 to 12    number of satellites being tracked
          *    HDOP             <dontcare> horizontal dilution
          *    altitude         546.       altitude above sea-level
          *    altitude units   M          to indicate meters
          *    diff             <dontcare> height of sea-level above ellipsoid
          *    diff units       M          to indicate meters (should be <dontcare>)
          *    dgps age         <dontcare> time in seconds since last DGPS fix
          *    dgps sid         <dontcare> DGPS station id
          */

         /* first, the time */
         stralloc_add_format( s, "$GPGGA,%06d", last_time );
         last_time ++;

         /* then the latitude */
         hemi = 'N';
         val  = params[GEO_LAT];
         if (val < 0) {
             hemi = 'S';
             val  = -val;
         }
         deg = (int) val;
         val = 60*(val - deg);
         min = (int) val;
         val = 10000*(val - min);
         stralloc_add_format( s, ",%02d%02d.%04d,%c", deg, min, (int)val, hemi );

         /* the longitude */
         hemi = 'E';
         val  = params[GEO_LONG];
         if (val < 0) {
             hemi = 'W';
             val  = -val;
         }
         deg = (int) val;
         val = 60*(val - deg);
         min = (int) val;
         val = 10000*(val - min);
         stralloc_add_format( s, ",%02d%02d.%04d,%c", deg, min, (int)val, hemi );

         /* bogus fix quality, satellite count and dilution */
         stralloc_add_format( s, ",1,%02d,", n_satellites );

         /* optional altitude + bogus diff */
         if (top_param >= GEO_ALT) {
             stralloc_add_format( s, ",%.1g,M,0.,M", params[GEO_ALT] );
             last_altitude = params[GEO_ALT];
         } else {
             stralloc_add_str( s, ",,,," );
         }
         /* bogus rest and checksum */
         stralloc_add_str( s, ",,,*47" );

         /* send it, then free */
         android_gps_send_nmea( stralloc_cstr(s) );
         stralloc_reset( s );
     }
     return 0;
 }

通过穿进去的经纬度，海拔等信息转化成NMEA格式的gps数据，然后通过socket发出去。

这部分就介绍到这里，之后会更精彩，哈哈。

希望这篇文章对读者有帮助，完全是参考android源码的，对我来说源码是最好的学习途径。

原文地址 http://blog.csdn.net/vv0_0vv/article/details/7998596   ，这篇文章相对其他文章，更深入一点，提到了在gps HAL中用到的socket。