大家都知道android是基于linux的kernel上的。android可以 运行在intel，高通，nvidia等硬件平台。但是涉及到一些GPU,显卡和一些设备的驱动问题，因为这些驱动都不是开源的，google位了兼容这些设备厂商的驱动源码，提出了硬件抽象层HAL的概念。HAL层对上为framework和native开发提供统一的API接口，为下层驱动的代码提供统一的调用接口。本文主要讲解HAL是如何实现的。
1.HAL的数据结构
HAL的通用写法里面有两个重要的结构体：
1.1 hw_module_t 硬件模块结构体
typedef struct hw_module_t {
/** tag must be initialized to HARDWARE_MODULE_TAG */
uint32_t tag;
uint16_t module_api_version;
#define version_major module_api_version
/**
* version_major/version_minor defines are supplied here for temporary
* source code compatibility. They will be removed in the next version.
* ALL clients must convert to the new version format.
*/
/**
* The API version of the HAL module interface. This is meant to
* version the hw_module_t, hw_module_methods_t, and hw_device_t
* structures and definitions.
*
* The HAL interface owns this field. Module users/implementations
* must NOT rely on this value for version information.
*
* Presently, 0 is the only valid value.
*/
uint16_t hal_api_version;
#define version_minor hal_api_version
/** Identifier of module */
const char *id;
/** Name of this module */
const char *name;
/** Author/owner/implementor of the module */
const char *author;
/** Modules methods */
struct hw_module_methods_t* methods;
/** module's dso */
void* dso;
/** padding to 128 bytes, reserved for future use */
uint32_t reserved[32-7];
} hw_module_t;
该结构体表示 抽象的硬件模块,包含硬件模块的一些基本信息。里面内嵌了一个
typedef struct hw_module_methods_t {
/** Open a specific device */
int (*open)(const struct hw_module_t* module, const char* id,
struct hw_device_t** device);
} hw_module_methods_t;
模块方法的结构体，open的函数指针，用于打开一个硬件设备hw_device_t。开发者需要实现这个open函数。
1.2硬件设备结构体
typedef struct hw_device_t {
/** tag must be initialized to HARDWARE_DEVICE_TAG */
uint32_t tag;
uint32_t version;
/** reference to the module this device belongs to */
struct hw_module_t* module;
/** padding reserved for future use */
uint32_t reserved[12];
/** Close this device */
int (*close)(struct hw_device_t* device);
} hw_device_t;
表示一个硬件抽象设备。这是通用的结构体，开发者可以继承这个结构体添加自己需要的接口。
1.3 获取一个hw_model_t模块
HAL层提供一个方法用户获取一个model，进而同过open方法打开设备device
/**
* Get the module info associated with a module by id.
*
* @return: 0 == success, <0 == error and *module == NULL
*/
int hw_get_module(const char *id, const struct hw_module_t **module);
定义一个全局变量
const struct hw_module_t HAL_MODULE_INFO_SYM={ ...};
用于在hw_get_modules通过解析so时，得到该全局变量。
2.硬件模块库的装载于解析
装载和解析有hw_get_module 完成，它会安按照一定的规则去查找so库，然后解析出全局变量名，得到硬件设备的open函数，最后通过参数返回一个device的指针给调用者。
2.1搜索so的规则;
/** Base path of the hal modules */
#define HAL_LIBRARY_PATH1 "/system/lib/hw"
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"
/**
* There are a set of variant filename for modules. The form of the filename
* is "<MODULE_ID>.variant.so" so for the led module the Dream variants
* of base "ro.product.board", "ro.board.platform" and "ro.arch" would be:
*
* led.trout.so
* led.msm7k.so
* led.ARMV6.so
* led.default.so
*/
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"
};
搜索规则就是按照上面的说明进行。
2.2函数加载解析的过程
（1）调用hw_get_module,通过传给他一个module_id 字符串例如“camera”等。调用hw_get_module_by_class(id, NULL, module);
（2）搜索对应的so并调用load去解析so
int hw_get_module_by_class(const char *class_id, const char *inst,
const struct hw_module_t **module)
{
int status = -EINVAL;
int i = 0;
char prop[PATH_MAX] = {0};
char path[PATH_MAX] = {0};
char 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.
*/
/* 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_PATH2, name);
if (access(path, R_OK) == 0) break;
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;
}
（3）load函数解析so，得到hw_module_t的hw_device_t的函数指针。
/**
* Load the file defined by the variant and if successful
* return the dlopen handle and the hmi.
* @return 0 = success, !0 = failure.
*/
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;
}
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