作者：刘昊昱 

博客：http://blog.csdn.net/liuhaoyutz

Android版本：2.3.7_r1

Linux内核版本：android-goldfish-2.6.29

一、硬件抽象层核心数据结构

Android硬件抽象层有三个核心数据结构,分别是hw_module_t , hw_module_methods_t, hw_device_t。定义在hardware/libhardware/include/hardware/hardware.h文件中：

 40/** 41 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM 42 * and the fields of this data structure must begin with hw_module_t 43 * followed by module specific information. 44 */ 45typedef struct hw_module_t { 46    /** tag must be initialized to HARDWARE_MODULE_TAG */ 47    uint32_t tag; 48 49    /** major version number for the module */ 50    uint16_t version_major; 51 52    /** minor version number of the module */ 53    uint16_t version_minor; 54 55    /** Identifier of module */ 56    const char *id; 57 58    /** Name of this module */ 59    const char *name; 60 61    /** Author/owner/implementor of the module */ 62    const char *author; 63 64    /** Modules methods */ 65    struct hw_module_methods_t* methods; 66 67    /** module's dso */ 68    void* dso; 69 70    /** padding to 128 bytes, reserved for future use */ 71    uint32_t reserved[32-7]; 72 73} hw_module_t; 74 75typedef struct hw_module_methods_t { 76    /** Open a specific device */ 77    int (*open)(const struct hw_module_t* module, const char* id, 78            struct hw_device_t** device); 79 80} hw_module_methods_t; 81 82/** 83 * Every device data structure must begin with hw_device_t 84 * followed by module specific public methods and attributes. 85 */ 86typedef struct hw_device_t { 87    /** tag must be initialized to HARDWARE_DEVICE_TAG */ 88    uint32_t tag; 89 90    /** version number for hw_device_t */ 91    uint32_t version; 92 93    /** reference to the module this device belongs to */ 94    struct hw_module_t* module; 95 96    /** padding reserved for future use */ 97    uint32_t reserved[12]; 98 99    /** Close this device */100    int (*close)(struct hw_device_t* device);101102} hw_device_t;

40-44行，注意这段说明文字，硬件抽象层HAL由一个一个的模块组成，Android规定，每一个模块都是一个命名为HAL_MODULE_INFO_SYM的自定义结构体，并且该结构体的第一个成员必须为hw_module_t类型的变量，其它成员变量根据需要由开发者设置。

82-85行，注意这段说明文字，每个设备对应一个自定义结构体，该结构体的第一个成员必须为hw_device_t，其它成员根据需要由开发者设置。

例如，sensor模块对应的结构体定义在hardware/libhardware/include/hardware/sensors.h文件中：

344/**345 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM346 * and the fields of this data structure must begin with hw_module_t347 * followed by module specific information.348 */349struct sensors_module_t {350    struct hw_module_t common;351352    /**353     * Enumerate all available sensors. The list is returned in "list".354     * @return number of sensors in the list355     */356    int (*get_sensors_list)(struct sensors_module_t* module,357            struct sensor_t const** list);358};sensor设备对应的结构体如下：392/**393 * Every device data structure must begin with hw_device_t394 * followed by module specific public methods and attributes.395 */396struct sensors_poll_device_t {397    struct hw_device_t common;398399    /** Activate/deactivate one sensor.400     *401     * @param handle is the handle of the sensor to change.402     * @param enabled set to 1 to enable, or 0 to disable the sensor.403     *404     * @return 0 on success, negative errno code otherwise405     */406    int (*activate)(struct sensors_poll_device_t *dev,407            int handle, int enabled);408409    /**410     * Set the delay between sensor events in nanoseconds for a given sensor.411     * It is an error to set a delay inferior to the value defined by412     * sensor_t::minDelay. If sensor_t::minDelay is zero, setDelay() is413     * ignored and returns 0.414     *415     * @return 0 if successful, < 0 on error416     */417    int (*setDelay)(struct sensors_poll_device_t *dev,418            int handle, int64_t ns);419420    /**421     * Returns an array of sensor data.422     * This function must block until events are available.423     *424     * @return the number of events read on success, or -errno in case of an error.425     * This function should never return 0 (no event).426     *427     */428    int (*poll)(struct sensors_poll_device_t *dev,429            sensors_event_t* data, int count);430};

对于三星公司的crespo(Nexus S的开发代号)，其sensor模块的真正实现代码定义在device/samsung/crespo/libsensors/sensors.cpp文件中：

108static struct hw_module_methods_t sensors_module_methods = {109        open: open_sensors110};111112struct sensors_module_t HAL_MODULE_INFO_SYM = {113        common: {114                tag: HARDWARE_MODULE_TAG,115                version_major: 1,116                version_minor: 0,117                id: SENSORS_HARDWARE_MODULE_ID,118                name: "Samsung Sensor module",119                author: "Samsung Electronic Company",120                methods: &sensors_module_methods,121        },122        get_sensors_list: sensors__get_sensors_list,123};

而在open_sensors函数中，对相应设备对应的sensors_poll_device_t结构进行了赋值：

305/** Open a new instance of a sensor device using name */306static int open_sensors(const struct hw_module_t* module, const char* id,307                        struct hw_device_t** device)308{309        int status = -EINVAL;310        sensors_poll_context_t *dev = new sensors_poll_context_t();311312        memset(&dev->device, 0, sizeof(sensors_poll_device_t));313314        dev->device.common.tag = HARDWARE_DEVICE_TAG;315        dev->device.common.version  = 0;316        dev->device.common.module   = const_cast(module);317        dev->device.common.close    = poll__close;318        dev->device.activate        = poll__activate;319        dev->device.setDelay        = poll__setDelay;320        dev->device.poll            = poll__poll;321322        *device = &dev->device.common;323        status = 0;324325        return status;326}

poll__close、poll__activate、poll__setDelay、poll__poll等函数也是在该文件中实现。

二、Android如何使用硬件抽象层

硬件抽象层的作用是对上层Application Framework屏蔽Linux底层驱动程序，那么Application Framework与硬件抽象层通信的接口是谁呢？答案是hw_get_module函数，该函数定义在hardware/libhardware/hardware.c文件中：

120int hw_get_module(const char *id, const struct hw_module_t **module)121{122    int status;123    int i;124    const struct hw_module_t *hmi = NULL;125    char prop[PATH_MAX];126    char path[PATH_MAX];127128    /*129     * Here we rely on the fact that calling dlopen multiple times on130     * the same .so will simply increment a refcount (and not load131     * a new copy of the library).132     * We also assume that dlopen() is thread-safe.133     */134135    /* Loop through the configuration variants looking for a module */136    for (i=0 ; i

 34/** 35 * There are a set of variant filename for modules. The form of the filename 36 * is ".variant.so" so for the led module the Dream variants 37 * of base "ro.product.board", "ro.board.platform" and "ro.arch" would be: 38 * 39 * led.trout.so 40 * led.msm7k.so 41 * led.ARMV6.so 42 * led.default.so 43 */ 44 45static const char *variant_keys[] = { 46    "ro.hardware",  /* This goes first so that it can pick up a different 47                       file on the emulator. */ 48    "ro.product.board", 49    "ro.board.platform", 50    "ro.arch" 51}; 52 53static const int HAL_VARIANT_KEYS_COUNT = 54    (sizeof(variant_keys)/sizeof(variant_keys[0]));

 56/** 57 * Load the file defined by the variant and if successful 58 * return the dlopen handle and the hmi. 59 * @return 0 = success, !0 = failure. 60 */ 61static int load(const char *id, 62        const char *path, 63        const struct hw_module_t **pHmi) 64{ 65    int status; 66    void *handle; 67    struct hw_module_t *hmi; 68 69    /* 70     * load the symbols resolving undefined symbols before 71     * dlopen returns. Since RTLD_GLOBAL is not or'd in with 72     * RTLD_NOW the external symbols will not be global 73     */ 74    handle = dlopen(path, RTLD_NOW); 75    if (handle == NULL) { 76        char const *err_str = dlerror(); 77        LOGE("load: module=%s\n%s", path, err_str?err_str:"unknown"); 78        status = -EINVAL; 79        goto done; 80    } 81 82    /* Get the address of the struct hal_module_info. */ 83    const char *sym = HAL_MODULE_INFO_SYM_AS_STR; 84    hmi = (struct hw_module_t *)dlsym(handle, sym); 85    if (hmi == NULL) { 86        LOGE("load: couldn't find symbol %s", sym); 87        status = -EINVAL; 88        goto done; 89    } 90 91    /* Check that the id matches */ 92    if (strcmp(id, hmi->id) != 0) { 93        LOGE("load: id=%s != hmi->id=%s", id, hmi->id); 94        status = -EINVAL; 95        goto done; 96    } 97 98    hmi->dso = handle; 99100    /* success */101    status = 0;102103    done:104    if (status != 0) {105        hmi = NULL;106        if (handle != NULL) {107            dlclose(handle);108            handle = NULL;109        }110    } else {111        LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",112                id, path, *pHmi, handle);113    }114115    *pHmi = hmi;116117    return status;118}