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);
}
}
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 horizontal dilution
* altitude 546. altitude above sea-level
* altitude units M to indicate meters
* diff height of sea-level above ellipsoid
* diff units M to indicate meters (should be )
* dgps age time in seconds since last DGPS fix
* dgps sid 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。