Android中hw_get_module函数分析
该函数定义在hardware/libhardware/hardware.c文件中，定义如下：
int hw_get_module(const char *id, const struct hw_module_t **module)
{
return hw_get_module_by_class(id, NULL, module);
}
hw_get_module()函数利用HAL层注册信息id，获取相应的模块。
hw_get_module_by_class()函数利用HAL层注册信息id和name，获取相应的模块，主要用于id相同、name不同，即获取相同功能但厂家不同的硬件库。
模块注册信息如下：
这个id是hal层注册时加入的，例如sensor的hal的定义
struct sensors_module_t HAL_MODULE_INFO_SYM = {
.common = {
.tag = HARDWARE_MODULE_TAG,
.version_major = 1,
.version_minor = 0,
.id = SENSORS_HARDWARE_MODULE_ID,
.name = "Sunwave",
.author = "The Android Open Source Project",
.methods = &sensors_module_methods,
},
.get_sensors_list = sensors__get_sensors_list,
};
#define SENSORS_HARDWARE_MODULE_ID "fingerprint"
//hw_module_t是硬件模块结构，是HAL层的灵魂-------------------------------------------------------------------------------------------------
下面分析一下，hw_get_module_by_class()函数。
int hw_get_module_by_class(const char *class_id, const char *inst,
const struct hw_module_t **module)
{
int i = 0;
char prop[PATH_MAX] = {0};
char path[PATH_MAX] = {0};
char name[PATH_MAX] = {0};
char prop_name[PATH_MAX] = {0};
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.
*/
/* First try a property specific to the class and possibly instance */
snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);
if (property_get(prop_name, prop, NULL) > 0) {
if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
goto found;
}
}
/* Loop through the configuration variants looking for a module */
for (i=0 ; i1.
if (inst)
snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
else
strlcpy(name, class_id, PATH_MAX);判断inst是否存在，如果存在，将name 赋值为class_id.inst。主要在多个型号时使用。
例如，class_id =fingerprint，inst = sunwave，则name = fingerprint.sunwave
如果不存在，name=class_id，即fingerprint。
2.
snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);
if (property_get(prop_name, prop, NULL) > 0) {
if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
goto found;
}
}将prop_name赋值为ro.hardware.%name。或者为ro.hardware.fingerprint，或者为ro.hardware.fingerprint.sunwave。
if (property_get(prop_name, prop, NULL) > 0) //查看是否有该属性如果有，查看该so库是否存在。如果存在则加载。
property_get()函数，如果获取到值，返回获取到值prop的大小。
3.
/* Loop through the configuration variants looking for a module */
for (i=0 ; i然后在配置变量中查找，属性值是否存在，配置的变量如下：
static const char *variant_keys[] = {
"ro.hardware", /* This goes first so that it can pick up a different
file on the emulator. */
"ro.product.board",
"ro.board.platform",
"ro.arch"
};如果查找到，则查看name.prop.so库是否存在。例如，fingerprint.sunwave.so。
4.
/* Nothing found, try the default */
if (hw_module_exists(path, sizeof(path), name, "default") == 0) {
goto found;
}如果都不存在，则将prop=default。查看name.defaule.so库是否存在。例如：fingerprint.default.so库。
----------------------------------------------------------------------------------------------------------
然后分析，hw_module_exists()函数。
static int hw_module_exists(char *path, size_t path_len, const char *name,
const char *subname)
{
snprintf(path, path_len, "%s/%s.%s.so",
HAL_LIBRARY_PATH3, name, subname);
if (access(path, R_OK) == 0)
return 0;
snprintf(path, path_len, "%s/%s.%s.so",
HAL_LIBRARY_PATH2, name, subname);
if (access(path, R_OK) == 0)
return 0;
snprintf(path, path_len, "%s/%s.%s.so",
HAL_LIBRARY_PATH1, name, subname);
if (access(path, R_OK) == 0)
return 0;
return -ENOENT;
}/** Base path of the hal modules */
#if defined(__LP64__)
#define HAL_LIBRARY_PATH1 "/system/lib64/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib64/hw"
#define HAL_LIBRARY_PATH3 "/odm/lib64/hw"
#else
#define HAL_LIBRARY_PATH1 "/system/lib/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
#define HAL_LIBRARY_PATH3 "/odm/lib/hw"
#endif该函数会在三个路径判断库是否存在。获取path参数，供load函数使用。
-------------------------------------------------------------------------------------------------------------
最后，分析 load(class_id, path, module);函数
static int load(const char *id,
const char *path,
const struct hw_module_t **pHmi)
{
int status = -EINVAL;
void *handle = NULL;
struct hw_module_t *hmi = NULL;
/*
* 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();
ALOGE("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) {
ALOGE("load: couldn't find symbol %s", sym);
status = -EINVAL;
goto done;
}
/* Check that the id matches */
if (strcmp(id, hmi->id) != 0) {
ALOGE("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 {
ALOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
id, path, *pHmi, handle);
}
*pHmi = hmi;
return status;
}handle = dlopen(path, RTLD_NOW); hmi = (struct hw_module_t *)dlsym(handle, sym);主要有两个函数，
dlopen()函数根据path以指定模式打开指定的动态链接库文件，并返回一个句柄给dlsym()的调用进程。
dlsym()根据动态链接库操作句柄与符号，返回符号对应的地址，不但可以获取函数地址，也可以获取变量地址。
/* Check that the id matches */
if (strcmp(id, hmi->id) != 0) {
ALOGE("load: id=%s != hmi->id=%s", id, hmi->id);
status = -EINVAL;
goto done;
}校核id是否正确。
*pHmi = hmi;赋值hw_module_t结构体指针，即hw_get_module()函数的指针参数。