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;}