Android 驱动和系统开发 2. 解析模拟器GPS模块 （原创）

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。